Finishing Organ Amp Conversion

David E H · May 17, 2009

Hey guys, I've had a lot of questions lately about a lot of projects, so I apologize if I've been flooding the forum.

I finally finished wiring up a clone of a famous amp based on a Hammond AO-35 chassis. I've never built an amp before, and I'm too cheap to buy a kit right now, so I invested my time into this. It's a little bit above me, but oh well.

I went with point to point wiring on this thing, which may have cause more bad than good. The chassis is very small, but I ended up fitting everything in. I finished wiring everything up and triple checked that it was all correct.

Now, when I turn it on, everything powers up fine. (However, I forgot to put my variac to use the first time...) The tubes light naturally and everything appears to run as it should.

When standby is switched, and everything is engaged it all works, however I'm getting some nasty tone and oscillation. I understand that my rat's nest wiring is to account for a lot of the oscillation, but I'm also getting really premature, farty, distortion. There's also a lot of "bumble bee buzzing" with aggressive playing. I'm sort of having a feeling that there could be more to blame than just the wiring....perhaps there's a cold joint or the schematic is wrong? Or maybe I need to double check the resistor values themselves....

The speaker is functioning properly.....

Hmmm...any ideas?

jimytheassassin · May 17, 2009

Congrats on your amp build! I read your issue, and though I'm also working on my own first build (thus a beginner), did you forget to ground the input? It looks as though the yellow wire is still floating.. or is that an optical illusion?

David E H · May 17, 2009

Congrats on your amp build! I read your issue, and though I'm also working on my own first build (thus a beginner), did you forget to ground the input? It looks as though the yellow wire is still floating.. or is that an optical illusion?

I knew someone was going to mention that. Yeah, it's floating.....it's not the problem though. It was connected to the star ground at one point, but I removed it just to see how it was sound if it was just grounded by the chassis.

Best of luck on your build.

Belva · May 18, 2009

Nice build. And I'm sure normh,WRG or amp surgeon can help you find what's wrong. Brings up a question. I see organ amps frequently. Problem is you need a preamp to go with it. Why not use a processor or even something like Behringer's boost pedal? PB100 I think. Preamp/booster with a tone stack. Sounds almost plug & play?

jimytheassassin · May 18, 2009

Thanks.. Ya I'm actually doing a Phono Amp conversion. I don't know if you have any good books, but Morgan Jones: Building valve amplifiers: is a good read for things like this. I think you're right that the "rats nest" could be a big source of your oscillation issue. A couple more things appear immediately to me, like the neutral side of your AC is dang near on the chassis, the "star ground" is right above the Output transformer (probably not the quietest place), and the output plug is haphazardly kinda tossed in there. I think you may eventually run into problems with the gauge of wire between your AC chord and the fuse/transformer etc due to heat. Perhaps this is all stuff you're aware of, but there's for sure no shortage of possible howlers in there. Although you tried to right angle everything, there are still plenty of points where the AC and DC are parallel (like the B+ to the pre amp passing over the rectifier). Too bad you couldn't have used that cap can instead of putting the Filter caps over the PT. But all in all, not very much room in there so it's a tough job indeed.

Distortion.. dunno, maybe it's all related here. You did test the carbon comp resistors though right? They're a possible source of noise, but distortion is probably due to something else like that .68

WRGKMC · May 18, 2009

If its working thats half the battel but you're gonna have to do a clean up. All the connecting wires need to be against the chassis to absorb noise. This is a key factor that cannot be overlooked. Having the wires agains the chassis may cut the emf down 50%. a wire hanging free in space acts as an antenna transmitting any noise to the other components. when its against the chassis a large amount is absorbed by the grounded frame. Placing a metal lid on the bottom may help a littel for overall AC noise coming from the exterior, but it wont do alot for oscillation created by wires suspended within the box.

So what I would do is rewire all the wires against the chassis. It shouldnt be too bad, Some that are too short you may have to replace. Others it just involves unsoldering one end and rerouting. Try not to cross wires over eachother. For heater filiment wires you can twist the heater wires in a brade. It tends to choke a littel AC noise a littel more and cancel the emf and also makes them easy to identify.

David E H · May 18, 2009

Alright, thank you all for the advice....I guess I will just have to reroute the wires.....This will show some ignorance, but I didn't know it was that important to do so. I always thought it was just an audiophile recommendation. Will this also reduce premature, farty, distortion?

WRGKMC · May 18, 2009

Dont know. I would get the thing tidyed up first and see what you get. Keeping wires near the frame has a firm foundation in minimising noise. Its also become a clean build showoff thing which is secondary to the units function in my book but a clean build does make it easier to troubleshoot later too which is something you may still need to do if you still have issues.

The clean wiring thing in radios was a must. The movement of a single wire could change a frequency or throw the RF amp stages out of alignment with changes in capacitence. Obviously amplifiers aren't used for tuning in radio stations but keeping hum, distortion, crosstalk and RF interference minimal using the frame for at least half the wires is a must. Inputs to amp stages in many amps also use shielded wire to keep the signal path quiet if the wires being routed over boards or longer distances from say a preamp to a power amp in the same case. In your build you'll be fine so long as the wires are near the frame.

David E H · May 18, 2009

Success! I rearranged the preamp wires so that they were cornered against the chassis and it got rid of all the oscillation and most of the noise. And it turns out the speaker really was to blame for the farting distortion. I've turned it up all the way with very little noise, and only natural distortion.

I used a couple other 12" speakers in parallel that are alright, but a little bright, and I also plugged into the speaker in my Traynor combo which was okay, but sorta lame. So I think I'm going to build either a 2x10 or 2x12 cabinet.(Any speaker suggestions?)

Anyways, thank you all for your help. It has been invaluable to getting this project thus far. Here's some chassis pics:

jimytheassassin · May 19, 2009

Awesome! I love happy endings. The speakers may seem less bright when they're in a cabinet and behind grill cloth. Plus you can always put in a beam blocker for extra measure. I don't know what you were hoping to achieve sound wise, but I'm intrigued by a Matchless cab 1-10/1-8 design. Anyways, cheers

WRGKMC · May 19, 2009

Good deal. You'll get maximum power and sound quality from a cab with matching impediance. if you're wanting a loud amp, get a speaker with a high SPL. High SPL with double the wattage rating of the amp would most likely be the ideal setup for the amp and give maximum tone and natureal distortion.

David E H · May 19, 2009

Good deal. You'll get maximum power and sound quality from a cab with matching impediance. if you're wanting a loud amp, get a speaker with a high SPL. High SPL with double the wattage rating of the amp would most likely be the ideal setup for the amp and give maximum tone and natureal distortion.

Okay....I'm searching around for some good speakers for this amp....the tone knob is kind weird and it jumps from super bassy to trebly with just a slight turn. I don't know if it's my fault in wiring, or if it's just the nature of the tone stack.

At this point, I think I'm going for an open backed 2x12 pine cabinet. I have some friends at a wood working shop who can build one pretty cheap. As far as the drivers, I'm looking at Tone Tubbies which are ceramic, Webers, and I'm also considering investing in vintage speakers. I'm not sure yet, though.

jimytheassassin · May 20, 2009

Pots have response curves,(B) Linear being best for tone, and (A) logarithmic being best for volume. If you use a logarithmic pot for tone, it will do as you describe. might want to check that. As to vintage speakers, you can find them for cheap usually. Just like old used tubes, I wouldn't pay too much for "snake oil", but it's your call to decide.

David E H · May 20, 2009

Pots have response curves,(B) Linear being best for tone, and (A) logarithmic being best for volume. If you use a logarithmic pot for tone, it will do as you describe. might want to check that. As to vintage speakers, you can find them for cheap usually. Just like old used tubes, I wouldn't pay too much for "snake oil", but it's your call to decide.

Ah...gotcha with the pots....I'll have to order another one at some point and try it. As with the speakers, I figure I'll probably just buy new to be safe at this point. Thanks.

WRGKMC · May 20, 2009

I dont know if that head can drive two twelves very well. My suggestions for 12" begin at about 35 to 50 tube watts. Anything lower may sound a bit dark and woofey. A pair of 10" may be just the ticket for getting a nice cranked sound. As far as the Tone control goes I'd leave it alone till you get your cabinet set. Depending on the speakers, they will likely fill in that midrange you're lacking, and the high end will come in habdy for speaker breakup. Once you get a cab built then some final tine tweaking can be done to match the cab.

Parts Express has some decent prices on guitar speakers.

http://www.parts-express.com/speakers.cfm

They also list the SPL levels. They do have some Alnico speakers that run lower wattage that might be what you're needing. I bought a set of 4 x 10 Alnico Jensons to run with my Blackface Bassman which wound up being a perfect match. Eminance also makes some Alnicos that are 35w and have a bit louder output. They also have a full line of custom speakers that are decently priced and worth checking out. I dont know if webbers come in a lower wattage version. My thoughts are from reading their specs is you'll need higher wattage like 100w to get them sounding good. I never heard of Tone Tubies. Maybe they're new or made by someone else and given their own name. Ceramic speakers can sound great for clean stuff but when pushed dont break up as smoothly as an alnico can. Alnicos are noted for a good match with lower watt tube amps, but it still comes down to prices and what you may want. I thought I'd just share a few of these intems to open up some other possible angles you might want to try.

David E H · May 20, 2009

I dont know if webbers come in a lower wattage version. My thoughts are from reading their specs is you'll need higher wattage like 100w to get them sounding good. I never heard of Tone Tubies. Maybe they're new or made by someone else and given their own name. Ceramic speakers can sound great for clean stuff but when pushed dont break up as smoothly as an alnico can. Alnicos are noted for a good match with lower watt tube amps, but it still comes down to prices and what you may want. I thought I'd just share a few of these intems to open up some other possible angles you might want to try.

Hey, thanks. Yeah, I'm aware of the ceramic/alnico difference, and would prefer alnico. If you're thinking that 10" is the way to go, what wattage should I be looking at for each speaker? I'm looking to match 8 ohms, so I'll be needing 16, or 4 ohms speakers.

I found these....they seem to get decent reviews. Would your guess be that they're ceramic?

http://warehousespeakers.com/products.php?cat=8&PHPSESSID=3fcdccaf19ce63e9bd380d92171f4d07

jimytheassassin · May 21, 2009

That rig won't push more than 17+ watts, so you'd be safe with anything that meets or exceeds the rating as a pair, or individually. The closer you are to the amplifiers output rating, the more likely you are to hear speaker breakup at the power peak. For some people, this is another desirable form of distortion. If you do go for the lower limit, ribbed speakers will allow you to push for later breakup. Cheers.

David E H · May 21, 2009

I also found these:

http://accessories.musiciansfriend.com/product/Jensen-P10R-25-Watt-10-Replacement-Speaker?sku=665011&src=3WFRWXX&ZYXSEM=0&CAWELAID=26048013

Consume · May 21, 2009

Good deal. You'll get maximum power and sound quality from a cab with matching impediance. if you're wanting a loud amp, get a speaker with a high SPL. High SPL with double the wattage rating of the amp would most likely be the ideal setup for the amp and give maximum tone and natureal distortion.

I have a quesiton.

Where do you guys come up with the concept of having a speaker that is twice the amplifiers power rating?

As a soundman in training, 20 years ago, I was taught that the Amplifier's output should be twice the speaker's value for maximum clean signal, but in reality the amp and the speakers RMS levels should be matched as closely together as possible.

To the OP:

Congrats on building your first amp!

Have you considered using muiltiple smaller speakers?

I'm just curious - since you went to the trouble of building your own amp, you ought to also look into different speaker cabinet designs to go with it.

David E H · May 21, 2009

To the OP:

Congrats on building your first amp!

Have you considered using muiltiple smaller speakers?

I'm just curious - since you went to the trouble of building your own amp, you ought to also look into different speaker cabinet designs to go with it.

Hey, thanks so much man. It's a good feeling to finally have one under my belt. As with "multiple smaller speakers," do you mean multiple drivers in one cab, or separate smaller cabs?

I've thought about being creative with designs, but I'm looking to create this into a solid gigging rig. My other amps are 1968 Super Reverb and a solid state Traynor, so I'll finally have a tube amp I am comfortable to gig, leave home with, bring to college, etc....

I'm open to any design suggestions and creating my own tone, so if you have any ideas, let me know. I really only know the basics of cabinet building, and thankfully somebody else will be building it for me.

jimytheassassin · May 21, 2009

I have a quesiton.

Where do you guys come up with the concept of having a speaker that is twice the amplifiers power rating?

As a soundman in training, 20 years ago, I was taught that the Amplifier's output should be twice the speaker's value for maximum clean signal, but in reality the amp and the speakers RMS levels should be matched as closely together as possible.

I've read a lot online journals, and some books, but I'm still learning. I don't particularly agree with the double watt recommendation, but it depends on what someone wants to achieve in their sound character. Plus, all this has to be taken with a grain of salt in respect to how these things are all measured by the sound industry at large. SPL levels, Wattage ratings, frequency response curves.. all of this is suspect until you actually hear something perform, and while they're good indicators, there is still no way of knowing how TRUE the specs are. While the amps may be rated for peak power, Tube amps can push further into distortion (%THD) than S.S, which means the average power rises higher relative to it's counterpart at "full volume". Thus a tube amp can demand more of a speaker, more continuously. Wattage is still wattage (as power and heat are concerned), but once you exceed the rating, you can get into thermal and mechanical tolerance issues which could blow a speaker. AND depending on the amp maker, sometimes they rate their amps at less than the actual output (i.e. Marshall). If you go above the rating, you can increase the "headroom" of clean "speaker" output. It may also darken your tone and remove "sparkle" if you go to an extreme. But all in all, it's entirely subjective and defined by our own individual ears.

Anyways, this is what I've come to learn from all of my reading, but there will always be room for different opinions in the sound world. Your scientific reasoning may be accurate on the subject. But when it comes to guitar amps, the debate about tone and how to "get it" will continue to rage.

David E H · May 21, 2009

...when it comes to guitar amps, the debate about tone and how to "get it" will continue to rage.

Sounds like that could be your thesis. Thanks for the info.

WRGKMC · May 21, 2009

""I have a quesiton.

Where do you guys come up with the concept of having a speaker that is twice the amplifiers power rating?

As a soundman in training, 20 years ago, I was taught that the Amplifier's output should be twice the speaker's value for maximum clean signal, but in reality the amp and the speakers RMS levels should be matched as closely together as possible""

From the way this is written It sounds like you're saying the amp wattage should be twice as much as the speaker, Think you remember this backwards, it the other way around. Speakers sound best (cleanest) when running at 75% of their maximum wattage. The responce is linear and clean and there enough wattage left to be a protection buffer.

As far as where it comes from? First in my case is my own experience, which is quite long and matches most others in the business. Second, I used to do pro audio repair for a living when I first got my electronics degree. I've done more repairs than I can count on Hi Fi and pro audio components an PAs. Also did repairs for Music Shops repairing amps. Nearly every case where theres blown speakers its because ther speaker wattage was equal or lower then the amp driving it. (Or the amp went DC and pumped straight AC through the voice coil) Next, The better manufacturers often match their components with higher wattage speakers. Some may be 50% higher, many are 50% higher. This maximises clean headroom. Fourth, I've blown alot of speakers myself and know exactly how they sound when they go and what they went. (Blew my first speaker in scienc class show and tell in grade school. Had an AC transformer that pushed AC through a speaker to demonstrate sound. It cooked the speaker but I got an A for the presentation). All in all it deals with overheating the voice coil.

You can run an amp with matching RMS voltage, It usually sounds dam good for guitar. The problem is with the peak rms with certain speakers. Low end bass can make the speaker fart out and sound nasty. You're also riding the edge of the speakers tollerence. Over time the voice coil glue can deteriorate and crack or get air pockets. After that it doesnt take much for the exposed wire to overheat and blow the speaker voice coil.

Over wattage can be a big issue too. It takes a minimum wattage to push a speaker into its linear frequency responce range. Speakers sound best when they're running in their 75% range. A 300w speaker being run by a 10w amp is going to sound pretty limp and have a much lower SPL then something in its range.

For a practical example I have a small Ampeg amp from the 70s. It has a light weight 10" Utah speaker in it. Its probibly a 25w speaker. The amp is very loud, crisp and responsive. I have about 20 - 10" speakers I've collected over the years of various wattages and builds and have tried nearly all of them out. None sound anywheres near as nice as the lightweight paper design of the original. I bought some Jenson Alnicos for another project and put one in it and it was the closest match for the original so far. The exception was the Jensons have a slightly lower SPL than the stock speaker. Its probibly because the stock speakers spider and cone were pretty flexable and broken in that made the difference.

So to sum it up, Practical experience in my case backs up the science I've studied over the years 100%. The only things that vary is the tone in the end. Having good tone based on the cabinet, amp, and someones ears trump all other considerations so long as those considerations are safe.

Consume · May 21, 2009