Speaker out jack...

[Jazzer2020]

[ATTACH=CONFIG]n32083044[/ATTACH] [ATTACH=CONFIG]n32083045[/ATTACH] Maybe someone who really knows their amp wiring can explain what is going on here?

I see two different jacks being used on the Main speaker out and the External speaker out.

I see the Output tran wiring feeding to the jacks this way:

Green is tied off (capped). Yellow is going to Main speaker and is tied across to External speaker.

Ground is going to External speaker.

Two leads on Main speaker jack are tied together.

 

Not sure what this wiring indicates?

 

Here are three [ATTACH=CONFIG]n32083043[/ATTACH] photos.

The first jack near the black switch is the Main speaker.

The second jack over is the External speaker jack.

 

 

[WRGKMC]

 

Thanks very much WRG, I actually understood everything you wrote!

 

I have a few more questions and will also fill you in a bit more on my situation.

 

You talked about the "loudness and gain/tone curves of the two speakers wont be the same however".

This was with an internal 8 ohm and an external 16 ohm speaker.

 

What about if I changed the internal to 16 ohm and used an external 16 ohm speaker, to deliver 8 ohms?

Would the loudness and gain/tone curves of the two speakers be the same as the current internal 8 ohm speaker?

 

OK here's my situation.

My 'internal' speaker is actually external. My amp has been built as a head and external speaker cab.

This gives me great flexibility as I can change my 'internal' speaker by just unplugging the cab and plugging in another.

 

So for me to go with two 16 ohm speakers is not a big deal at all (except for the cost of course).

 

Ideally I would like to play with the 8 ohm 'internal' most of the time and have the ability to go with two speakers

some of the time.

 

It sounds like a transformer swap would be the most cost-effective way to go.

This way I could use the existing 'internal' 8 ohm speaker and my external 8 ohm cab.

 

I spoke with a guy at Valvetrain in Florida. He recommends the CLASSICTONE # 40-18006: Output, 40W - 4/8/16 Ohm.

Would you agree with that choice?

 

Speaking about 'classictone', would a transformer swap affect the tone of the amp at all?

 

Just took a look at photos of my output transformer and the 40-18006.

Mine has three leads: black, yellow, green I assume for ground, 4 ohm, 8 ohm

40-18006 has four leads: black, yellow, green, violet for ground, 4 ohm, 8 ohm and 16 ohm

 

So... why not just keep mine and add a switch to toggle between 4 ohm and 8 ohm?

 

 

1. - Yes my comments were about using an 8 and 16 ohms speaker.

 

2. - If both speakers are 16 ohm then you should be fine, 16+16 in parallel = 8 ohm total.

 

3. - The Internal and External jacks are likely to be exactly the same. They are probably wired in parallel unless the builder told you otherwise

 

4. - Classic Tone or Mercury Magnets for a transformer would be fine. If the builder recommends the classic tone I'd go with that. They've either tried them or use them in their amps.

 

5. - A swap from a single tap to a multi tap transformer from the same manufacturer will most likely sound identical. All they do is add taps and extra winds for the additional impedances. The quality of the build and materials should be identical, so should the sound quality.

 

I doubt most people could detect a difference between most transformers using their ears. The sound difference would come indirectly from the re-biasing of the power tubes to match the new transformers primary resistance (if there is any)

 

So... why not just keep mine and add a switch to toggle between 4 ohm and 8 ohm?

 

 

You cant assume that wire is a tap. It might be but you have to know the type of transformer it is. Please post the number on the transformer so my advice is accurate!!!!

 

If your output transformer is this one..... http://www.classictone.net/40-18038.pdf Then your amp is wired for 4 not 8 ohms. The yellow and black are the 4 ohms tap so connecting two 8 ohms speakers or one 4 ohm is essential to having an impedance match.

 

If this is some other transformer , the yellow and black wires may be 8 ohms. The green wire that's wrapped up may simply be a housing ground that isn't used because the transformer is bolted to the chassis. and the ground is simply redundant.

 

You're only guessing unless you know for a fact what transformer it is. Different manufacturers use their own color codes so you should never assume anything. You have to check their specs to know what the wires are.

 

Power transformers tend to have some similar colors. The twisted pair of green wires for example tend to be the heater tap. Black, White Brown are usually AC primary used universally. The rest can be any color. They can even have stripes that are different colors. You usually need a color code chart from the manufacturer to know which wires are which, (and/or you measure the outputs with a meter)

 

If you're lucky, that will be a tap - the thing that is most important to know is, you can only use one tap at a time.

 

IF you wire one jack for one impedance and the other jack for another impedance you cant use both jacks together. Yopu''l blow the transformer is it has two loads connected to two different taps.

 

The transformer load is taken from the ground to ONE tap or the other ONLY.

 

If one jack is wired for 4 ohms, and the other 8 ohms, and you want to connect Two 8 ohm speakers then you need to have both speakers in parallel. If 2 speakers are in a single cab then they are simply wired in parallel. If you have two separate cabs, then one cab needs an extension jack from the second cab, and the first plugs into the 4 ohm out.

 

Same goes for the 8 ohm out. You can connect a single 8 ohm speaker to that jack or a cab with 2X4 ohm in series to equal 8 or 2X16 in parallel to equal 8. If you have two cabs, you connect the first 16 ohm to the 8 ohm jack, and that cab must have an extension jack in parallel to plug in the second 16 ohm cab for a total of 8 ohms.

 

You cant run a single 16 ohm cab on either the 1 or 8 ohm transformer tap however.

 

Again, if this is a Classictone transformer 40-18038 that's in the amp now, then its wired for 4 ohms and you should be running a pair of 8 ohm speakers now. Running a single 8 isn't as good on a 4 ohm tap. You could wire the second jack for the 8 ohm tap and change the labeling on each to say 4 ohm for the yellow wire, 8 ohm for the green wire.

 

Wiring it only requires cutting the jumper which connects both jacks in parallel then soldering the green to where the jumper used to connect to the external jack. The black and yellow wires stay where they are. Then label the internal 4 ohms and the extension 8 ohms.

 

This is only if you post the transformer type you have in there now so I can confirm it. Don't want to get your hopes up otherwise.

I can draw you a diagram if need be, I just don't want you to make mistakes and cause damage.

[Jazzer2020]

Thanks very much WRG for your comprehensive reply!

 

 

 

3. - The Internal and External jacks are likely to be exactly the same. They are probably wired in parallel unless the builder told you otherwise

 

I didn't send the best photos, but if you look closely (and this could be important) you will see that the two jacks are different.

One is one-pole the other is two-pole. Yes they are wired in parallel.

 

 

You cant assume that wire is a tap. It might be but you have to know the type of transformer it is. Please post the number on the transformer so my advice is accurate!!!!

 

Well from all the photos I took a few months ago I can't see a number (there might be one). I will wait until I hear back from the builder on this.

 

If your output transformer is this one..... http://www.classictone.net/40-18038.pdf Then your amp is wired for 4 not 8 ohms. The yellow and black are the 4 ohms tap so connecting two 8 ohms speakers or one 4 ohm is essential to having an impedance match.

 

First off, I don't think it is mine for a couple of reasons. The Classictone trans have their name on the top, my tran does not have any labeling.

Also this tran is for 20 W output. My amp is 25 W output.

 

Secondly, my amp has an Eminence Legend GB128 50 W 8 ohm speaker.

So I think it is safe to say that the builder would not have wired the tran for an 4 ohm internal speaker.

 

If this is some other transformer , the yellow and black wires may be 8 ohms. The green wire that's wrapped up may simply be a housing ground that isn't used because the transformer is bolted to the chassis. and the ground is simply redundant.

 

You're only guessing unless you know for a fact what transformer it is. Different manufacturers use their own color codes so you should never assume anything. You have to check their specs to know what the wires are.

 

Yes we will have to wait and see what the tran is exactly. Because the yellow wiring is interesting. I was expecting green instead.

 

If you're lucky, that will be a tap - the thing that is most important to know is, you can only use one tap at a time.

 

Yes I hope and am expecting the green to be a tap. If it is, I may be able to wire her somehow to allow for a second 8 ohm speaker.

 

Again, if this is a Classictone transformer 40-18038 that's in the amp now, then its wired for 4 ohms and you should be running a pair of 8 ohm speakers now. Running a single 8 isn't as good on a 4 ohm tap. You could wire the second jack for the 8 ohm tap and change the labeling on each to say 4 ohm for the yellow wire, 8 ohm for the green wire.

 

As I mentioned above, it is very safe to say it is not a 40-18038 for all the reasons I mentioned.

The amp is currently wired for 8 ohms.

 

Wiring it only requires cutting the jumper which connects both jacks in parallel then soldering the green to where the jumper used to connect to the external jack. The black and yellow wires stay where they are. Then label the internal 4 ohms and the extension 8 ohms.

 

This is only if you post the transformer type you have in there now so I can confirm it. Don't want to get your hopes up otherwise.

I can draw you a diagram if need be, I just don't want you to make mistakes and cause damage.

 

 

What I'm hoping for is the following:

 

Having the tran default to 8 ohm tap for the 'internal' speaker (as is currently).

Then having a switch that changes the tran wiring to the 4 ohm tap so that BOTH the 'internal' and the

external speakers can be 8 ohm and plugged in together.

 

If I could achieve this, would my tone/volume suffer any hit going from 8 ohm down to 4 ohm output?

 

 

I just read that the Fender Hot Rod Deluxe does exactly what I am looking for.

It has Internal and External speaker jacks.

You can just leave the Internal at 8 ohm load and play with one speaker.

 

You can add a second 8 ohm speaker; just plug it into the Ext. jack (so now you have 2 speakers).

The wiring is set up to change the output tran to 4 ohm.

 

Or you can just use an External speaker, 4 ohm, by plugging in a 'dummy plug' (no speaker attached) into the Internal speaker jack.

 

 

[WRGKMC]

 

What I'm hoping for is the following:

 

Having the tran default to 8 ohm tap for the 'internal' speaker (as is currently).

Then having a switch that changes the tran wiring to the 4 ohm tap so that BOTH the 'internal' and the

external speakers can be 8 ohm and plugged in together.

 

If I could achieve this, would my tone/volume suffer any hit going from 8 ohm down to 4 ohm output?

 

 

 

No it doesn't affect tone or volume. An impedance match is an impedance match. Doesn't matter if it's 4, 8, 16, or 32 ohms. So long as the speaker and amp impedance match then you'll have optimal power transfer from the tubes to the speaker.

 

Its only when they aren't matched when you have problems.

 

Think of it in these terms. In a car you have an engine, Transmission and Tires. The engine is the tubes that provide power. The transmission is the transformer and the tires the speakers.

 

The gear ratio in the transmission is for a certain tire size. If I make the tire smaller (increase impedance) the engine races at high RPM to get the car up to the same speed as normal tires. Positives: It will have quick pick. Negatives: It will burn too much gas running at the same speed as normal tires, the engine may blow because of the high RPM's or it will burn up and overheat because the car isn't moving forward fast enough to force air into the radiator to cool it.

 

Vice versa, if I have a car with tires too large, the engine has more internal stress trying to get those tires spinning up to speed. Positives: once up to speed the engine will run at a lower RPM and burn less. Negatives, Its acceleration will suck, pushing harder on the pedal forces gas inside but the valves aren't opening quick enough to let the exhaust out leaving carbon in the engine. You burn the clutch up trying to get enough speed to engage and the brakes will burn up trying to stop the larger tires.

 

The transmission is balanced with a proper gear ratio between the engine and tires. If I install larger or smaller tires, I have to change the transmissions gear ratio to transfer the right amount of power so I don't overwork or underwork the engine.

 

 

Like mechanics, electronics is based on Physics. It even uses the same mathematical formulas. They just swap values. The transformer is a transmission that needs to be balanced between the tubes and speaker, otherwise bad things happen. Just like the car, you may not blow the engine immediately but it will occur when pushing it to its limits.

 

As far as the speaker wattage goes, its like the weight limit on a tire. So long as its above the minimum you're safe. In the case of the amp, 22W RMS and above is safe, you wont blow the voice coil.

 

The other item which I'll throw in there is cars have efficiency rating called miles per gallon. Speakers have an efficiency rating too called SPL. (Sound Pressure Level) This tells you how well a speaker converts watts to moving air. (Decibels) Higher SPL ratings on speakers make the speakers louder.

 

The wattage is more like a fuse rating. So long as its above the wattage it wont blow. It has absolutely nothing to do with loudness. Changing from a low wattage speaker to a Higher wattage speaker wont make an amp louder. A higher wattage speaker can take more power IF the head can produce it, That's all. Of course higher wattage speakers can be built with stiffer and more durable materials to handle higher wattages. This can work against the amp if the wattage of the speaker dwarfs the wattage of the head. When the materials are too heavy duty, the speaker never gets to its sweet spot in volume level which is usually between 50~75% of max wattage where the cone produces its maximum frequency response.

 

Your 50W speaker is a good match for the head power wise. Adding additional speakers simply adds more bass frequencies with the additional cabinets resonating. The actual loudness when checked with a decibel meter will only change if the speakers have a higher SPL. If the additional speaker has a lower SPL, the volume can actually decrease when an extension is connected.

 

This is all rudimentary stuff of course. Main point is to be safe and avoid burning the amp up needlessly. There's plenty of room to work within specs to achieve proper sound quality without having to work outside specs.

 

 

 

 

 

[WRGKMC]

 

I didn't send the best photos, but if you look closely (and this could be important) you will see that the two jacks are different.

One is one-pole the other is two-pole. Yes they are wired in parallel.

 

All tube amps short the output when the jacks are removed this is safer then leaving an open connection which will blow the tubes out in no time. The one jack is switched to connect hot wire to ground when the jack is removed.

 

I just read that the Fender Hot Rod Deluxe does exactly what I am looking for.

It has Internal and External speaker jacks.

You can just leave the Internal at 8 ohm load and play with one speaker.

 

You can add a second 8 ohm speaker; just plug it into the Ext. jack (so now you have 2 speakers).

The wiring is set up to change the output tran to 4 ohm.

 

There's nothing different about the deluxe transformer, other then its wattage. Its either single tap or multi tap just like any other tube amp.

 

 

Or you can just use an External speaker, 4 ohm, by plugging in a 'dummy plug' (no speaker attached) into the Internal speaker jack.

 

This is where ignorance becomes dangerous. You shouldn't believe everything you read on the net because most of it is posted by people with no formal electronics education.

 

A dummy plug would use a fixed resistor. Its not an active inductance with magnetic flux working like a speakers voice coil provides.

 

The biggest part of this is you'll cut the power in half. The resistor is going to consume wattage just like the speaker does but it wont be moving any air like a second speaker would so all you hear is a dip in volume when its connected.

 

There are other reasons why this is bad. A dummy load may prevent the amp from being damaged but it does not provide the same kind of a load a speaker does so the signal is mutated. You must know amps outputs is rated in Impedance and is measured in AC ohms which varies. This is NOT the same thing as a DC resistor that's measured in ohms which does not vary.

 

This gets complex quickly unless you study the science but the signals coming from an amp vary in frequency (obviously because they are musical tones which vary in pitch) The way a magnetic coil reacts to AC signals (the voice coil) is the coil increases changes with frequency. There's usually and increase at the resonant frequency and higher frequencies as the coil resists changes as the frequencies increase.

 

This chart shows fixed resistance compared to a speakers variable resistance.

 

 

 

The speaker is rated for an impedance. This is an active average inductive resistance which acts like a shock absorber to the tubes and transformer and varies in resistance as the signals amplitude and frequency change. There is a 90^o phase difference between the current and voltage - the voltage leads the current by 90^o http://hyperphysics.phy-astr.gsu.edu/hbase/electric/phase.html

 

When you use a fixed DC resistance, there is no variance and no phase shift. You'll convert power to heat but the load has no inductance so it does things to the waves. Here's a clip from Wiki which explains some of this.

[h=3]Impedance phase angle[edit][/h] Impedance variations of the load with frequency translate into variation in the phase relationship between the amplifier's voltage and current outputs. For a resistive load, usually (but not always) the voltage across the amplifier's output devices is maximum when the load current is minimum (and the voltage is minimum across the load) and vice versa, and as a result the power dissipation in those devices is least. But due to the complex and variable nature of the driver/crossover load and its effect on the phase relationship between the voltage and current, the current will not necessarily be at its minimum when the voltage across the output devices is maximum - this results in increased power dissipation in the amplifier output stage which manifests as heating in the output devices. The phase angle varies most near resonance in moving coil loudspeakers. If this point is not taken into consideration during the amplifier design, the amplifier may overheat causing it to shut down, or cause failure of the output devices. See Power factor for more detail.

[h=3]Damping issues[edit][/h] A loudspeaker acts as a generator when a coil is moving in a magnetic field. When the loudspeaker coil moves in response to a signal from the amplifier, the coil generates a response that resists the amplifier signal and acts as a "brake" to stop the coil movement. This is the so-called back EMF. The braking effect is critical to speaker design, in that designers leverage it to ensure the speaker stops making sound quickly and that the coil is in position to reproduce the next sound. The electrical signal generated by the coil travels back along the speaker cable to the amplifier. Well-designed amplifiers have low output impedance so that this generated signal has little effect on the amplifier.

Characteristically, solid state amplifiers have had much lower output impedances than tube amplifiers. So much so that differences in practice between a 16 ohm nominal impedance driver and a 4 ohm nominal impedance driver have not been important enough to adjust for. Damping factor (ratio of output impedance (amplifier) to input impedance (driver voice coil)) are adequate in either case for well-designed amplifiers.

Tube amplifiers have sufficiently higher output impedances that they normally included multi-tap output transformers to better match to the driver impedance. Sixteen ohm drivers (or loudspeakers systems) would be connected to the 16-ohm tap, 8 ohm to the 8 ohm tap, etc.

This is significant since the ratio between the loudspeaker impedance and the amplifier's impedance at a particular frequency provides damping (i.e., energy absorption) for the back EMF generated by a driver. In practice, this is important to prevent ringing or overhang which is, essentially, a free vibration of the moving structures in a driver when it is excited (i.e., driven with a signal) at that frequency. This can be clearly seen in waterfall measurement plots. A properly adjusted damping factor can control this free vibration of the moving structures and improve the sound of the driver.

https://en.wikipedia.org/wiki/Electrical_characteristics_of_dynamic_loudspeakers

 

What this all boils down too (see I was trying to keep it simple but its nearly impossible to do so without giving some factual reasoning) is a DC resistance wont dampen and wont act like a shock absorber. Instead of the sound being linear to the ears, you have some frequencies which will sound weaker then others. Between this and the fact you're cutting the on speakers wattage in half its a stupid way of trying to fix an impedance issue using an improper method. A dummy speaker with an active coil is the only real method you can use and again, it produces no sound so you loose volume using any kind of dummy load.

 

 

[Jazzer2020]

[ATTACH=CONFIG]n32084228[/ATTACH]

 

 

Thanks again WRG for helping out. Lot's of good stuff.

 

 

 

All tube amps short the output when the jacks are removed this is safer then leaving an open connection which will blow the tubes out in no time. The one jack is switched to connect hot wire to ground when the jack is removed.

 

This is good to know. I wonder why then we are always precautioned never to turn on an amp with no speaker connected?

 

 

This is where ignorance becomes dangerous. You shouldn't believe everything you read on the net because most of it is posted by people with no formal electronics education.

 

A dummy plug would use a fixed resistor. Its not an active inductance with magnetic flux working like a speakers voice coil provides.

 

The biggest part of this is you'll cut the power in half. The resistor is going to consume wattage just like the speaker does but it wont be moving any air like a second speaker would so all you hear is a dip in volume when its connected.

 

I didn't pull this off the net from some guy. I took it straight from Fender's HRD User Manual.

 

"Main speaker:

One of the following speaker load

configs must always be connected

when unit is ON.

 

1. Int. 8 ohm Ext. (no speaker)

2. Int. 8 ohm Ext. 8 ohm

3. Int. Open plug (no speakers connected) Ext. 4 ohm "

 

 

I just looked up the schematic to the Fender Hot Rod Deluxe and understand the wiring for both speaker jacks.

 

They basically have the Ext. jack as a two-way switch. When the jack is open, it connects to the tran 8 ohm tap.

When it is closed (plug inserted) it connects to the tran 4 ohm tap.

The hot lead on the Ext jack is connected to the Int jack hot lead (parallel).

 

The Int. jack is closed (with no plug inserted) and goes to ground.

When a plug is inserted it connects to the Ext. speaker wiring.

 

 

I am however confused with the wiring for my amp's speaker jacks.

First off the plugs are swapped from Fender's (two-way switch on the Int. jack and one-way on the Ext. jack).

Also I don't understand the wiring.

I am attaching a photo above, maybe you can figure out what's going on?

 

BTW, still waiting for a reply from the builder on the transformer connections. He's slow.

[Jazzer2020]

It's been almost two weeks and still no reply from the builder. Unfortunately this has been the case most of the time when I try to communicate with him.

 

I have a few more questions about this transformer connection.

 

I went back to one of my earliest e-mails with the builder (many months ago). I had asked him about wiring the Ext. speaker jack for a line out and this is what he wrote: [TABLE]

[TR]

[TD]"Once it's wired for line out then you'd no longer be able to run a second speaker through there. The original design of this is to run another speaker cab through there like another 8ohm speaker."[/TD]

[/TR]

[/TABLE]

Given what he wrote, the question begs to be asked.

 

How can the Main speaker out and Ext. speaker out be wired so that you get:

 

1. 8 ohm impedance for Main speaker only AND

2. 8 ohm impedance when you have BOTH the Main speaker and Ext. speaker plugged in (both 8 ohm speakers)?

 

given that only ONE output transformer tap is being used?

 

Second question. Do they have transformers that can switch automatically from 8 ohm to 4 ohm

depending on what is being plugged into the jacks?

 

In other words, can the transformers sense the impedance of the speakers and toggle automatically between 4 and 8 ohm?

If they can't I am totally baffled.

 

 

 

 

 

 

 

 

 

 

 

[Mr.Grumpy]

How can the Main speaker out and Ext. speaker out be wired so that you get:

 

1. 8 ohm impedance for Main speaker only AND

2. 8 ohm impedance when you have BOTH the Main speaker and Ext. speaker plugged in (both 8 ohm speakers)?

 

given that only ONE output transformer tap is being used?

It can't be done. At least not in any practical, sensible way.

 

Second question. Do they have transformers that can switch automatically from 8 ohm to 4 ohm

depending on what is being plugged into the jacks?

NO.

 

In other words, can the transformers sense the impedance of the speakers and toggle automatically between 4 and 8 ohm?

NO.

 

The Hot Rod Deluxe you mentioned earlier in this thread performs this "magic" via a switching output jack. When a plug is inserted into the extension speaker jack, a contact is physically moved from the 8 ohm tap to the 4 ohm tap on the transformer. The transformer can't "sense" or adjust anything, it's just a couple of coils of wire. The actual switching is done by the jack.

 

If you want to record, then either buy a decent recording microphone, or a speaker-level direct box like the Hughes & Kettner "Red Box" D.I., these are both $99 solutions and don't require any modifications to your custom built boutique amp head.

 

 

 

[Jazzer2020]

It can't be done. At least not in any practical, sensible way.

 

NO.

 

NO.

 

The Hot Rod Deluxe you mentioned earlier in this thread performs this "magic" via a switching output jack. When a plug is inserted into the extension speaker jack, a contact is physically moved from the 8 ohm tap to the 4 ohm tap on the transformer. The transformer can't "sense" or adjust anything, it's just a couple of coils of wire. The actual switching is done by the jack.

 

If you want to record, then either buy a decent recording microphone, or a speaker-level direct box like the Hughes & Kettner "Red Box" D.I., these are both $99 solutions and don't require any modifications to your custom built boutique amp head.

 

 

 

Thanks Grumpy for your reply. I appreciate it.

I'm going to get to the bottom of this sooner or later (hopefully sooner).

 

Right now, recording is not a priority, and that is why I told the builder just go ahead with the stock Ext. speaker jack wiring.

 

I will continue to do research on this because I am baffled.

 

My photos of my amp's insides clearly shows an output transformer with two taps (yellow, green + ground).

I'm assuming the taps are for 4 ohm and 8 ohm.

On the jack connection side you can only see two wires going into the jacks, the yellow and the black.

 

I can't for the life of me understand how you can get both 8 ohm (with only Main) and 4 ohm (when you use Main + Ext. cab jack)

with this wiring.

It just doesn't make sense to me.

 

 

 

[Mr.Grumpy]

I can't for the life of me understand how you can get both 8 ohm (with only Main) and 4 ohm (when you use Main + Ext. cab jack)

with this wiring.

.

You don't. Even with both jacks connected it's still an 8 ohm output, HOWEVER, since this is a lower-powered Fender clone, the speaker transformer will tolerate a 4 ohm load even though it's not matched impedance. The impedance mismatch will make your speaker transformer run a bit warmer than it would with a properly-matched 8 ohm load, but the transformer can handle it. The problem with an impedance mis-match between the speaker transformer and speaker load is that with a mis-match, some of the electrical energy intended for the speaker gets 'absorbed' by the transformer windings and turns into heat. With a 25 watt amp, the speaker transformer can handle this extra heat. No big deal.

 

You couldn't get away with mis-matched output and cab impedance with a higher power amp like a Marshall stack or even a Twin Reverb - the speaker transformer will get hot and eventually fail.

 

You're making too big a deal out of this. If you want two cabs, hook them up and PLAY. If your transformer gets too hot, it will warn you by giving off that lovely "hot electrical" smell. Try this: play your amp with just the 'proper' 8 ohm cab connected. Play at your usual volume for at least 10 or 15 minutes. Carefully see if you can feel the speaker transformer to see how warm it is. Now plug in your second cab and repeat the experiment, and see if the speaker transformer is noticeably warmer or hot. As long as the output (speaker) transformer isn't too hot to touch, you should be OK. Bonus points if you have (or can borrow) one of those non-contact infrared thermometers to read the actual transformer temp.

 

[Jazzer2020]

 

...You're making too big a deal out of this. If you want two cabs, hook them up and PLAY. If your transformer gets too hot, it will warn you by giving off that lovely "hot electrical" smell. Try this: play your amp with just the 'proper' 8 ohm cab connected. Play at your usual volume for at least 10 or 15 minutes. Carefully see if you can feel the speaker transformer to see how warm it is. Now plug in your second cab and repeat the experiment, and see if the speaker transformer is noticeably warmer or hot. As long as the output (speaker) transformer isn't too hot to touch, you should be OK. Bonus points if you have (or can borrow) one of those non-contact infrared thermometers to read the actual transformer temp.

 

OK I tried out your test.

 

Results: Int. speaker alone, usual volume for 15 minutes, not too warm (hardly).

Int. speaker + Ext. speaker, usual volume for 15 minutes, just barely warmer (if any).

 

I don't think I'm making a big deal out of this at all. This is a very expensive boutique amp we are talking about.

We are not talking about some $800-900 cheesy amp.

 

After doing a fair bit more research last night on Fender amps I found a bunch with wiring the same or very similar to mine.

In all the ones that were using just the 8 ohm tap, they stipulated in their manuals that if an extension speaker were to be used do the following:

1. Unplug the internal speaker (8 ohm)

2. Plug in 16 ohm speakers to the internal AND external jacks.

 

Given the transformer that is in my amp (two-tap, probably 4 ohm/8 ohm) and given the e-mail that the builder wrote me

earlier this year about using the extension speaker jack (regular wiring), it is my current opinion that he goofed with the

jack/transformer wiring.

 

Hopefully he won't ignore me too much longer so I can get to the bottom of this.

 

 

 

 

 

 

[Jazzer2020]

Well I finally was able to make contact with the builder.

 

So here's the deal.

He tries to wire faithfully to original Fenders and so it's wired so that when you plug in an Ext. Spkr. (8 ohm)

the two speakers are in parallel and pulling 4 ohm from the 8 ohm amp design.

 

He also said (as I suspected by looking at the wiring) that the output transformer has 4 and 8 ohm taps.

 

So I will look into getting the jacks wired for 8 ohm when Main is used and 4 ohm when Main + Ext are used

(using the 4 ohm tap of the transformer).

 

 

 

 

 

 

 

 

[onelife]

...

The Hot Rod Deluxe you mentioned earlier in this thread performs this "magic" via a switching output jack. When a plug is inserted into the extension speaker jack, a contact is physically moved from the 8 ohm tap to the 4 ohm tap on the transformer. The transformer can't "sense" or adjust anything, it's just a couple of coils of wire. The actual switching is done by the jack...

 

Here's the drawing for the output stage of the Hot Rod Deluxe.

 

 

You can see where the 4 ohm tap and the 8 ohm tap on the output transformer are connected to switching jacks. This is what allows you to plug one or two 8 ohm speakers in and have the proper impedance.

[Jazzer2020]

 

Here's the drawing for the output stage of the Hot Rod Deluxe.

 

You can see where the 4 ohm tap and the 8 ohm tap on the output transformer are connected to switching jacks. This is what allows you to plug one or two 8 ohm speakers in and have the proper impedance.

 

Thanks onelife.

 

Yes I found this wiring diagram a few weeks ago too.

That's when I discovered the way to get 4 or 8 ohms depending on what is plugged into the speaker jacks.

 

It's quite clever. First it has a safety circuit to ground out if no speaker is attached to Main.

Then it shunts the Main to 8 ohm if only the Main is plugged in.

And of course shunts to 4 ohm if both Main and Ext are plugged in together.

 

 

But here's the thing that confuses me.

 

On the one hand the builder stated he wants to stay as faithful to the original Fender designs... that's why he wired the speaker jacks the way he did.

 

The original Fender Princetons have same wiring as he has

for the the Main and Ext speakers.

 

But then on the other hand, he goes ahead and supes up the amp to take it from 15W to 25W, changing from 6V6's to 6L6's.

 

So if he can make such drastic changes to the original design, you'd think he could make a slight mod to the speaker out jacks?

 

Allowing for 8 ohm/ 4 ohm switching.

He already put an output transformer with these taps.

 

I think the original didn't even have a 4 ohm tap!!

 

Why put your boutique amp at risk of damaging power tubes etc. just for the sake of keeping as close to the

original design as possible? Doesn't make sense to me.

 

 

One other thing. Is there a way to test the ohm's coming from an output transformer?

The wiring on my tran is Green, Yellow and Black.

 

I understand that manufacturers can use any coloring code they choose, but of all the wiring

diagrams I've looked at over the last few weeks, none have had Yellow for 8 ohm.

They have all been Green for 8 ohm, when a 4 ohm / 8 ohm tran was used.

 

[onelife]

...

Doesn't make sense to me.

 

Yes but it is what it is and now you know what you have to work with.

 

You can choose to modify your amp to properly accomodate an external 8 ohm load by incorporate switching jacks like the ones in the HRD - which would be my choice,

[Jazzer2020]

 

Yes but it is what it is and now you know what you have to work with.

 

You can choose to modify your amp to properly accomodate an external 8 ohm load by incorporate switching jacks like the ones in the HRD - which would be my choice,

 

 

Yes I'll be modifying the jack wiring. Probably get my tech to do it. Funny thing is that the jacks are already there, don' t even need new ones.

Just swap the Main with the Ext jack. Right now the Ext is single pole and the Main is double pole.

 

 

[onelife]

I still advise a switching jack for the Main Speaker Out, especially with the amp being seperate from the speaker, just in case the amplifier gets powered up without the speaker attached. As has been mentioned previously, it's better to short the output of a tube amp than to have an open circuit.

 

btw, it's sometimes easier to get one's head around the concept of impedance and loading electronic circuits if one thinks of the load being a conductance as well as an impedance. For example, adding a second 8 ohm speaker in parallel - which is the scenario we've been discusding - doubles the conductance of the load on the output transformer.

[Mr.Grumpy]

...the jacks are already there, don' t even need new ones.

NO! ....a "double pole" jack (which isn't the correct term) is not the same as a switching jack. You'll have to replace both jacks to implement "output tap switching" via the external speaker jacks like the Hot Rod Deluxe. An alternate solution is to use standard jacks wired in parallel and use a toggle switch to select the output impedance.

[Jazzer2020]

NO! ....a "double pole" jack (which isn't the correct term) is not the same as a switching jack. You'll have to replace both jacks to implement "output tap switching" via the external speaker jacks like the Hot Rod Deluxe. An alternate solution is to use standard jacks wired in parallel and use a toggle switch to select the output impedance.

 

OK thanks Grumpy. I have learned a heck of a lot about this stuff since I started the thread, but I guess I still have a fair bit to learn.

I was just going by what I saw inside my amp (wiring) and looking at the Fender Hot Rod schematic.

 

So the Ext Spk jack in the Hot Rod is a switching jack? It looks like when nothing is plugged into it, the circuit goes to the 8 ohm tap.

But when a plug is plugged into the jack, the Tip closes/connects to the 4 ohm tap.

 

 

[Jazzer2020]

I still advise a switching jack for the Main Speaker Out' date=' especially with the amp being seperate from the speaker, just in case the amplifier gets powered up without the speaker attached. As has been mentioned previously, it's better to short the output of a tube amp than to have an open circuit.[/quote']

 

OK. When I look at the current Main jack, I see the output being shorted to ground with nothing plugged into it.

(The Main jack has two hot leads, the Ext jack has just one hot lead).

 

btw, it's sometimes easier to get one's head around the concept of impedance and loading electronic circuits if one thinks of the load being a conductance as well as an impedance. For example, adding a second 8 ohm speaker in parallel - which is the scenario we've been discusding - doubles the conductance of the load on the output transformer.

 

OK thanks for that!

 

[Jazzer2020]

Time for an update in this saga...

 

I finally heard back from the builder a few weeks ago and learned the model of the output transformer.

It is a 4/8 ohm transformer as suspected.

However the wiring color is non-standard (the yellow is 8 ohm and green is 4 ohm).

 

When I asked him why he wired the speakers the way he did (in parallel, with no option to get to the 4 ohm tap) he said he tries to stay as faithful to the original Fender Princeton wiring as possible.

 

Well this didn't make any sense to me on several levels.

First off, the model amp he made for me is a 'suped-up' Princeton.

It takes 6L6's instead of 6V6's and has a 22W output rather than 12W (or 15W).

So he has already deviated big-time from the original Princeton wiring.

 

Secondly, why install an Ext. output jack that will overload the output transformer if an Ext. speaker is plugged in (with the Main)?

 

 

I have been in touch with my local tech guy and went over a possible mod with him, suggesting

the Fender Hot Rod Ext speaker jack switching.

Plugging in an 8 ohm speaker into the Ext speaker jack, changes to the 4 ohm tap of the transformer

for both the Ext. and the Main speakers.

 

He advised against going with the Hot Rod wiring. He said the Ext. jack has signal going through it all the time, even with nothing plugged into it.

He said the pass through contacts tend to fail regularly and cause issues. Worst case is a load disconnection and accompanying potential output transformer damage. Other issues include compromise in tone.

 

He is currently making two suggestions:

 

1. A Mesa One jack for each output. This avoids the above problems however it provides no short protection when nothing is plugged in.

 

2. One jack, a toggle and a custom Y-speaker cable for two cab applications. That can provide short protection when nothing is plugged in and is the most fail safe way.

 

 

I would prefer the Mesa One solution, it seems cleaner. However it would be nice to somehow also have short protection.

 

Does anyone have thoughts on these two solutions and possibly a third that would offer the best of both worlds?

[Jazzer2020]

This morning I thought of another possible solution. It's clean and rather simple.

 

Take a DPST switch. Cut the wire currently going from the 8 ohm transformer tap.

Attach one end of the cut wire (the end that was going to the transformer) to one

pole of the switch.

 

Connect the other end of the cut wire to the DPST output.

 

Connect the second pole of the DPST switch to the 4 ohm transformer tap.

 

This way you have:

 

1. Quick access to a single Main speaker output (8 ohm).

2. Quick access to a dual Main + Ext. speaker output (4 ohm).

3. Maintain the existing short protection wiring.

 

(I just noticed that Mr.Grumpy made this suggestion on Nov. 14.)