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Winter amp project


Casey4s

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I decided to start another amp project for this winter.

 

I like the tone stack in this schematic so I adapted this idea to my needs.

 

Stock preamp Schematic:

 

023.jpg

 

 

This is my version of the preamp:

 

This still needs some massaging, and I will totaly redraw this before beginning. I streamlined a few things, but I am adding a (pre PI) Master Volume and Fender style reverb to the preamp. I am also going to add a tone stack bypass for a grittier sound.

 

018.jpg

 

 

Any comments ?

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I like the tone stack in this schematic so I adapted this idea to my needs.

I have a V4B (bass amplifier). Very similar preamp. I really like the way this tone control system works.

 

I'm not sure how the midrange control will act without the inductor. You should sim that, if you have the software.

 

There is a LOT of gain available if you use a switch to add a bypass cap on the cathode of the second triode.

 

Post PI MV does some nice things. Why do you prefer yours before the PI?

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I have a V4B (bass amplifier). Very similar preamp. I really like the way this tone control system works.


I'm not sure how the midrange control will act without the inductor. You should sim that, if you have the software.


There is a LOT of gain available if you use a switch to add a bypass cap on the cathode of the second triode.


Post PI MV does some nice things. Why do you prefer yours before the PI?

 

 

I have a VT22 which is essentially the same as a V4. I don't know where to get that inductor so I am going to see how this works out. I went pre PI because it is simpler to do with a single gang pot.

 

I took that 6K11 out of the circuit as well as the 12DW7 so I can utilize 9A type tubes for simplicity.

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I went pre PI because it is simpler to do with a single gang pot.

Not sure why a double pot is a deal-breaker for you, but so be it. All the PPIMVs I've seen with a single pot are massive tonesucks. If you have to use a single, you're absolutely right to put it before the PI.

 

The sections of the 6K11 and the 12DW7 that you're replacing with V3 in your drawing are both low mu triodes. A 12AU7 would match up better.

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Not sure why a double pot is a deal-breaker for you, but so be it. All the PPIMVs I've seen with a single pot are massive tonesucks. If you have to use a single, you're absolutely right to put it before the PI.


The sections of the 6K11 and the 12DW7 that you're replacing with V3 in your drawing are both low mu triodes. A 12AU7 would match up better.

 

Hmmmm this is why I am asking for constructive suggestions from you guys. I just started the reasearch and drawings yesterday so I will reconsider the MV. Thanks.

 

 

I have been pouring through tube manuals looking for the design data for the 12DW7 & 6K11. I am already pretty familiar with the DW, I just couldn't remember which side was what. RCA specs are what I like or the good Sylvania or Tung-Sol manuals, I finall found it in a 50's Sylvania, with a 70's annex page. I looked in RC30 and RC19 first then RC25 then RC15, RC14 and nothing from RCA. They show it as disc. but I can't find that they ever made one. Same stroy with Tung-Sol manuals.

 

I haven't tried looking for the 6K11 design pages :facepalm: .yet.

 

 

I knew I would probably use a 5751 in place of the DW, as a compromise or a 12BH7, 12AY/AZ7 so I am flexable. I have a {censored} load of 9A type tubes to pick from and I need to use a few up. I do have some DWs here but I waned something more common thus the 9A pinouts.

 

Part of this project for me is to put to use some of the stuff I have lying around. So the idea is to builld a cool useable amp with mostly what I have lying around, and most of my stuff is NOS tubes.I

 

I haven't even looked at what transformers I have someplace so I can't start lying out my power supply.

 

The output stage is basic generic Fender/Marshall/ etc (Williamson variations) so I haven't bothered to put a lot of thought there yet other than the tubes for the output and rectifier, which is where I actually started with this build.

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The output stage is basic generic Fender/Marshall/ etc (Williamson variations) so I haven't bothered to put a lot of thought there yet other than the tubes for the output and rectifier, which is where I actually started with this build.

The iron you have to choose from is gonna have a lot to do with your choice of finals. I'm a big fan of 6V6s but they have more of a limit on the max B+ you can use and are happier looking into a higher load impedance than 6L6s, 5881s, or EL34s. And of course you can only extract about 20~25 watts from a pair. But that's enough for me.

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You're probably aware of most of the online resources. But in case you haven't run across this, Duncan Amps is a good place to start for tube data.

http://tdsl.duncanamps.com/tubesearch.php

There are links to other sites when you get the results for a particular tube type, so it's easy to browse for the data that's organized in a way that suits you best.


They also have a tonestack calculator/simulator.

http://www.duncanamps.com/tsc/

It's a free download and allows you to see the results of changing values on different components in several different tone control networks. Very handy, imho.



The iron you have to choose from is gonna have a lot to do with your choice of finals. I'm a big fan of 6V6s but they have more of a limit on the max B+ you can use and are happier looking into a higher load impedance than 6L6s, 5881s, or EL34s. And of course you can only extract about 20~25 watts from a pair. But that's enough for me.

 

I like the stuff in a book as a desk reference, but yes I just downloaded their tone stack calculator and the power supply calculator which I can really use now.

 

I have eight pairs of NOS Tung-Sol 5881's and I am determined to use a pair. I also want to used a Mullard GZ37 I have had for 10 years. So I might have to buy iron to fit the needs. The design limits on the real 5881's is 360V PV which should be pretty easy to stay between 350 and 375.

 

005.jpg

 

 

I have built a lot of 6V6 stuff because I like the creaminess of the 6V6's. This amp I don't want a lot of power anyway I am targeting around 28-30 watts at the speaker. This should be easy with a pair of 5881's.

 

If I do find that inductor before build time I am going to incorporate it in the stack like the original schematic.

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So I might have to buy iron to fit the needs. The design limits on the real 5881's is 360V PV which should be pretty easy to stay between 350 and 375.

The conduction curves for a 5881 are almost identical to those for a 6L6GC. I suspect they were made as a "ruggedized" version of a 6L6 for industrial/military applications.

 

The 360v max on the plate is probably a very conservative specification. Fender used 5881s and 6L6s somewhat interchangeably on the Blonde and Brown series amps, with the same iron. In some cases, they had 450v on the plates of the 5881s.

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The 5881 was originaly a Hi Fi tube by Tung-Sol to compete with RCA's 6L6GB. The GB is a 19 watt tube and the 5881 is a 23 watt tube but was compact so that the Military used it as a standard for the 6L6WGB spec. It was used in B52's up until the 1970's. These were also used in some of the MacIntosh HiFi /stereos as well as Fisher and a few others in the 50's 60' and 70's.

 

The GZ37 http://www.recordcrate.net/diy/2A3/GZ37.pdf

 

002-1.jpg

 

004.jpg

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Post PI MV does some nice things. Why do you prefer yours before the PI?

 

Nice tip! :thu: I will do that.

 

I am also going to add a Fender tweed style Presence control, but then also add a NFB lift which puts the amp right in your face (on most amps I've tried this on) so I am going for it. Of course as soon as you lift the NFB it's ByBy presence.

 

I am thinking dual bias meaning a protected switch to go from Fixed bias to self adjusting (adjustable) bias for the early Tweed vibe.

 

I am going with dwell and mix for the reverb, I am not a fan of a reverb tone control except on an outbaord.

 

I am also considering driving the reverb with an EF86 in place of the 12AT7 that is usually there. I have a couple of Mullard books here so I'll see what it would take.

 

I am going to start searching for that Ampeg inductor or equivilant. There has to be a source someplace, some of those Ampeg switches would be nice too.

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I am also going to add a Fender tweed style Presence control, but then also add a NFB lift which puts the amp right in your face (on most amps I've tried this on) so I am going for it. Of course as soon as you lift the NFB it's ByBy presence.

There are some options regarding changing from NFB to free running. What you can do depends on how the NFB is applied.

 

If you look at the Narrow Panel Super, they're applying the NFB directly to the cathode of the last stage before the Cathodyne PI. The presence control dials in a cap to shunt the highs in the NFB to ground. In this circuit, it's easiest to kill the NFB by opening the loop. The presence control will still have an effect but it won't be as strong, because it's only reducing the amount of cathode degen.

 

Contrast that approach to the Brown Vibrasonic. Here they've added a 1500 ohm resistor to the bottom of the tail. This is where they apply the NFB and presence control. You can safely shunt the presence cap and turn the presence control into a "balls" control to adjust how tight or free the output stage is. 1500 ohms is a small enough portion of the total cathode resistance that you won't throw the bias of this stage out of whack when you bring that node all the way to ground.

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I am thinking about a version of this basic narrow panel power section.

 

 

http://schematicheaven.com/fenderamps/bassman_5f6a_schem.pdf

 

 


Contrast that approach to the Brown Vibrasonic. Here they've added a 1500 ohm resistor to the bottom of the tail. This is where they apply the NFB and presence control. You can safely shunt the presence cap and turn the presence control into a "balls" control to adjust how tight or free the output stage is. 1500 ohms is a small enough portion of the total cathode resistance that you won't throw the bias of this stage out of whack when you bring that node all the way to ground.

 

 

Another good idea.

 

 

I am thinking 3 pos rotory switches for some of this stuff but they eat a lot of space on the back of the control panel. I would really like to find a few toggles with (on-on-on) and eliminate the rockers that Ampeg used.

 

 

thanks

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I don't know where to get that inductor so I am going to see how this works out.

 

I'm inclined to agree with SYK. You might talk to the guy at Fliptops? But you could just as easily use several inductors to make up the taps of the one. I.e. the original was an 800mh inductor with taps for 300mh and 100mh. To get your total 800mh with three separate inductors you would need a 400mh, 300mh, and a 100mh in series. Give or take on the values depending on availability.

 

Btw, those 6K11's last forever. If you're worried about adding an oddball plinker to the mix the purchase of one would probably be your last. ;)

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I'm inclined to agree with SYK. You might talk to the guy at Fliptops? But you could just as easily use several inductors to make up the taps of the one. I.e. the original was an 800mh inductor with taps for 300mh and 100mh. To get your total 800mh with three separate inductors you would need a 400mh, 300mh, and a 100mh in series. Give or take on the values depending on availability.


Btw, those 6K11's last forever. If you're worried about adding an oddball plinker to the mix the purchase of one would probably be your last.
;)

 

I was told on an Ampeg forum that mercury Magnetics has a reproduction of that inductor. I just eMailed them so I won't get a reply for a day or so.

 

I am just more comfortable with 9A tubes or octal preamps, I don't want to deal with that 12 pin socket etc. if I don't have to. I have hundreds of 9A tubes so I might as well use some of em.

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I went through my iron yesterday and I have a Hammond 1650P OT and a Hammond 372 series hand wound clone. I haven't tested it yet but it is either 300 0 300 or 350 o 350 but it should be doable for my intended use.

 

I am still searching for the inductors I need and having no luck. I am thinking in terms of a 400 + 300 + 100 mH wired seperatly. I really don't think I will find an Ampeg direct replacement.

 

If anyone has more expierience with inductor sourcing I would appreciate any insight.

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Looks like a cool build. Good to see you back around these parts as I always enjoyed reading your build posts and learned quite a lot from them when I was first getting interested in amp building.

 

 

Yeah I took a 2 year leave of absence from this place to get my sanity back. I stay away from the AMP forum most of the time because of all the childishness over there. Too bad because there are some cool guys there but just too many butt heads. Where as here in DIY mostT of the guys are more laid back.

 

I hope all has been going well for you since I was here last.

 

Have you built anything recently?

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Yeah I took a 2 year leave of absence from this place to get my sanity back. I stay away from the AMP forum most of the time because of all the childishness over there. Too bad because there are some cool guys there but just too many butt heads. Where as here in DIY mostT of the guys are more laid back.


I hope all has been going well for you since I was here last.


Have you built anything recently?

 

 

Heh, thats why I usually go to the amp forum but I dont take much of the goings on all that seriously. Next time I read a thread about mismatching impedance I might scream though.

 

Only really built a few stomp boxes I wanted to try which are now in bits because I dicked about with them and couldnt be bothered to put them back together. I've modded my valveking to actually be a good amp. It had too much gain and a useless tonestack which has now been fixed. I made a madamp A15mkII kit too. Cant remember if you were still about then.

 

Currently I'm in the midst of designing a 2.5ch preamp. I'll be finishing the schematic and (hopefully) the layout up tonight. I'll be sure to make a thread here on it. When I'm happy with how it sounds I've been asked to build it up as a 20W 2x6V6 amp for a friend. He plays folk/rock sort of stuff with his lady who plays viola. He originally wanted 50W but it would have been overkill for what he's doing. After playing my 20Wer all out he decided that was more his thing. It'll run a much smaller bill too! Beyond that I've seen some cool little 5W OTs for 12BH7/ECC99 that I want to try out soon and I have a MM bassman-135 PT that I Was given sat on my desk calling my name too.

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Heh, thats why I usually go to the amp forum but I dont take much of the goings on all that seriously. Next time I read a thread about mismatching impedance I might scream though.


Only really built a few stomp boxes I wanted to try which are now in bits because I dicked about with them and couldnt be bothered to put them back together. I've modded my valveking to actually be a good amp. It had too much gain and a useless tonestack which has now been fixed. I made a madamp A15mkII kit too. Cant remember if you were still about then.


Currently I'm in the midst of designing a 2.5ch preamp. I'll be finishing the schematic and (hopefully) the layout up tonight. I'll be sure to make a thread here on it. When I'm happy with how it sounds I've been asked to build it up as a 20W 2x6V6 amp for a friend. He plays folk/rock sort of stuff with his lady who plays viola. He originally wanted 50W but it would have been overkill for what he's doing. After playing my 20Wer all out he decided that was more his thing. It'll run a much smaller bill too! Beyond that I've seen some cool little 5W OTs for 12BH7/ECC99 that I want to try out soon and I have a MM bassman-135 PT that I Was given sat on my desk calling my name too.

 

 

 

Cool...

 

Glad to see you have been busy, I haven't built anyting in 2 years so I am anxious to build this over the winter.

 

Yeah make sure you post a thread with your build stuff. :thu:

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I am still searching for the inductors I need and having no luck. I am thinking in terms of a 400 + 300 + 100 mH wired seperatly. I really don't think I will find an Ampeg direct replacement.


If anyone has more expierience with inductor sourcing I would appreciate any insight.

No experience with this one, but you might contact Ampeg/LOUDtechnologies. They're currently producing a remake of the SVT in its all-tube glory. They use a similar inductor. It has a different part number than the one used on the V4 and V4B, but I think that's because of the way it mounts. I'm fairly certain it has the same inductances.

 

Maybe it's available from them as a replacement part? :idk:

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Thanks SYK,

You always seem to have alot of good insight and suggestions. I just might cotact Ampeg and see what's up. The one that Mercury has is $90. Thats as much as my iron cost me. I am going to try the small coils in series to get the values I need if I can't get the part from Ampeg. If it screws up I can always disable it and spend the $90. I just might use a rotary selector to switchthis stuff, I really hate rockers on an amp.

 

 

I worked on my power supply this afternon. My PT measures 325-0-325 at 77 ohms with 250 MA on the mains. The power Trans and the OT both weigh 8 lbs, this is going to be one really heavy sucker when I am done, like my VT-22.

 

004-1.jpg

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Remember to treat them as you would any choke/transformer. Arrange them to avoid magnetic core coupling. For a one off I might just go the extra mile and individually shield them with mu-metal.

 

 

Yeah the fields my be invisible but they are real enough.

 

I set a brand new PT on fire on turn up when I had it so close the speaker magnet that there was only about 1/8" of air gap between the two. Sure tought me a lesson...LOL

 

I always alternatly turn my magnetics to keep the fields from comming together.

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I worked on my power supply this afternon. My PT measures 325-0-325 at 77 ohms with 250 MA on the mains. The power Trans and the OT both weigh 8 lbs, this is going to be one really heavy sucker when I am done, like my VT-22.

That analysis doesn't seem like it represents what you'll actually see for loads on the amp. And I don't think you want to connect the CT of the OT to "A" without any filtering at that point. You get a reasonable amount of power supply rejection in a push-pull circuit, but no filtering on the supply feeding the CT is pushing it too far, imho.

 

Where it says "C = Plates" what is that? Is that actually the supply for the screen grids on your 5881s? I'm a bit lost here.

 

Usually, the order of chain is:

supply for plate of finals (to CT of OT)

screen supply

PI supply

preamp.

 

There's a reason for this. The change in current on the plates and screens are large compared to the change in currents in the preamp tubes. We decouple the earlier stages with resistors and capacitors to get less of the signal from the output stage having an effect on the supply for the earlier stages. Some designs take it a step farther and have no more than two stages in the preamp sharing the same supply before using a resistor and cap to decouple the stages before it.

 

You might want to consider adding up all the currents including the idle current of the 5881 plates and their screen grids as well as the PI and preamp. Usually, calculate the plate current of the finals @ 70% power you want to extract from them at whatever the plate supply voltage you'll be using.

 

Using the total current, you can predict the voltage drop across the rectifier by consulting the graphs from the manufacturer. Start from there, then work toward the preamp requirements.

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Thanks for the compliment. I've always loved working with electronics. Tubes in particular. Anything I can do to help my brothers who share that love is time well spent.
:)

$90 would be a deal-breaker for me, too. I'd be grumbling at $25 but probably pay that, reluctantly. Here's hoping Ampeg has some good news for you.


A rotary would work, or we could probably sort out the switching with a DPDT on-on-on minitoggle if you prefer.


That analysis doesn't seem like it represents what you'll actually see for loads on the amp. And I don't think you want to connect the CT of the OT to "A" without any filtering at that point. You get a reasonable amount of power supply rejection in a push-pull circuit, but no filtering on the supply feeding the CT is pushing it too far, imho.


Where it says "C = Plates" what is that? Is that actually the supply for the screen grids on your 5881s? I'm a bit lost here.

Usually, the order of chain is:

supply for plate of finals (to CT of OT)

screen supply

PI supply

preamp.


There's a reason for this. The change in current on the plates and screens are large compared to the change in currents in the preamp tubes. We decouple the earlier stages with resistors and capacitors to get less of the signal from the output stage having an effect on the supply for the earlier stages. Some designs take it a step farther and have no more than two stages in the preamp sharing the same supply before using a resistor and cap to decouple the stages before it.


You might want to consider adding up all the currents including the idle current of the 5881 plates and their screen grids as well as the PI and preamp. Usually, calculate the plate current of the finals @ 70% power you want to extract from them at whatever the plate supply voltage you'll be using.


Using the total current, you can predict the voltage drop across the rectifier by consulting the graphs from the manufacturer. Start from there, then work toward the preamp requirements.

 

Yes that is my fault, it should say the Screens of the 5881's. :facepalm:

 

I was mimiking a 5F6A supply and power section. This is the first time I have used the Duncan software. I wanted to start with a familiar design for the supply and tweak the RC network to get my desired voltages.

 

The software doesn't let me lay it out the way I see it.

 

Thanks for the input...

 

 

EDIT:

 

I can't access the help files with the program, so I had to "wing it" somewhat. For the "load resistance" it asks for I assume it wants the primary impedance of the OT, in this case 6.6K or are they asking for the "Plate Resistance" of the tubes ?

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