Designing a stereo tube guitar amp

[John Fisher]

I have a request from someone to build a stereo tube guitar amp head. Probably around 15 watts X 15 watts using el 84's.
This amp will have quite a few bells and whistles. (chanel switching on each channel) effects loops and so forth. It will be quite an amp as it will have 2 of them in one.
I realize that the more complicated the amp, the more that potential problems are compounded in the design. Hums oscillations and so forth. One thing I am particularly concerned about is the transformer placement. For example amp 1 will be on the left and amp 2 on the right of the chassis. Lets just say that the input for amp 1 will be on the left side followed by the tone stack and then the second amp input will be basically starting in the middle of the chassis followed by it's tone stack and so forth.
I was thinking of having 2 power trannys and 2 OT's.
The power tranny for amp number 1 will be basically in the middle of the chassis away from it's input but then the problem will be that it is close to the input of the second amp. Being that the power trannys will be completely mounted on the top of the chassis, will that still possibly effect the input of the second amp?
One factor would also be the size of the chassis as although I don't want a huge chassis it would have to be big enough to work properly considering the above problems mentioned.
I hope you can understand where I am coming from.
Thanks for reading the long post.

[Alex R]

Many questions arise; one is - why have separate power transformers?

[Chuck H]

+1

If it were my project I would build it like two seperate amps without power supplies and then use a third chassis for a shared power supply. I'd place the power supply at the bottom of the chassis and hang the two amps side by side.

I was once asked to do a stereo amp with some special features. Designing it was a real PITA because my customer wanted it compact also. But he didn't want channel switching or effects loops. If your guy is also asking for tube reverb I'd resign the project or tell him he has to have two amps to do what he wants. Period.

[MWJB]

You're going to have to put up with a larger chassis, fact of life.

Why use 2 PTs? Just use 1 PT & 2 OT's, forget the PT in the middle of the chassis, lay it out like a regular amp PT at one end, inputs at the other, mount the OTs near the PT but ensure that laminations are at 90 degrees to each other.

You're essentialy building a 2 channel amp, each channel with it's own PI & OT.

[John Fisher]

OK! I thought about using only one power supply but then I thought it would be nice to somehow balance the weight of the amp head better but that may not apply at this moment. What if I was to put the one power supply at the far right side and the 2 OT's in the middle or possibly a little more to the left of the middle. To me function would be first priority and weight ballance second.
Thanks!

[MWJB]

The OT's should be as far away from the inputs as possible, I'd expect them to sit between the power & preamp/PI tubes. So I guess that they would end up pretty close to the PT.

[mooreamps]

I've thought of this on and off.. My take on this would be to have a stereo input jack feeding independent preamps inside ; and then feeding both signals into a twin tube power amp ; either SE or P/P should not matter ; to take advantage of some of the stereo outputs I've seen on some "Chorus" type pedals... ; thus maintaining the "stereo" effect.. I've not had any requests as yet to set something up like this.... But I am working on a request to have a guitar setup for a balanced output ; feeding into a stereo cable into a stereo input on an amp into a balanced input on the first gain stage ; thus using the common mode rejection to attenuate environmental noise......


-g

[imaradiostar]

I think Gary has a good idea there- I would try a little twist on it. I'd make the preamp section mono- one Fender (clean) channel, one Marshall (master volume) channel and possibly some other channel. At this point I'd favor something in between the two as far as gain level but that's beside the point! These would be switched to a send that's capable of driving stereo effects. From the return to the speakers it would be a stereo amp- allowing the full glory of stereo verb, chorus, etc.

I'm interested to see how this turns out. I've thought quite a bit about building something like this. I used to use an old stereo amp with 2 EL84's per side as my main amp. I miss that rig- lots of interesting sounds and my TC Electronic chorus sounded huge.

Jamie

[mooreamps]

I just looked at a few photos of the TC Chorus module you mentioned. Apparently it has "two" output jacks for stereo, implying two input jacks for the amp.

But I have a question. What type of player mostly uses the chorus effects : Rock, Jazz, or Blues ??

-g

[imaradiostar]

What type of player mostly uses the chorus effects : Rock, Jazz, or Blues ?

That's a bit like asking what kind of person most likes to eat pizza- someone from Italy, New York or Chicago. There are wonderful recorded examples of chorus effects in each of the above genres. I don't really understand why you asked.

Chorus is traditionally thought to sound better after distortion effects. Since the original poster mentioned channel switching it makes sense that he'd have an effect loop to allow delay and modulation effects to be placed after the distortion preamp. It's simple to use a mono send to a mono delay to a stereo chorus like the TC chorus or a line 6 modulation modeller. You can easily make the amp with stereo loop sends and returns though I doubt you'll need both sends in many cases.

jamie

[mooreamps]

That's a bit like asking what kind of person most likes to eat pizza- someone from Italy, New York or Chicago. There are wonderful recorded examples of chorus effects in each of the above genres. I don't really understand why you asked.

jamie

I tend to have three different layouts for each type of player...

-g

[Gibsonman63]

Interesting thread. Delay would be really good in this arrangement as well or even delay with reverb on the delayed signal. For simplicity, I would think a single pre-amp with stereo effect returns would be the ticket.

I beleive that it was tube works that produced a combo in the 90's with a single slot 19" rack space to mount a rackmount effect right in the amp.

[imaradiostar]

I tend to have three different layouts for each type of player...

-g

That's a good reason. I prefer not to draw tonal lines for genres but I can see how that works for some people. I like to have a little more flexibility.

For example, Eric Johnson tends to run the gamut of the aforementioned styles and his tones could better be described as marshall lead, marshall dirty rhythm, marshall plus fuzz face, and clean fender. All of those tones are useful in blues, jazz and rock depending on the song.

jamie

[imaradiostar]

I hope you can understand where I am coming from.
Thanks for reading the long post.

I've thought a bit more about your idea and I've had some thoughts.

Are you an experienced amp builder? Have you built a high gain preamp before?

You may find it easier to start with a proven design for your preamp such as a traditional Fender/Marshall channel switcher like a bogner shiva. The schematic can be found easily on the internet and built relatively simply with a turret or eyelet board. This would give you very usable dirty and clean channels and an effect send.

As for the power section you could do two rather simple things. First, assume it's completely stereo from the effect return out. You could build two simple triode gain stages (like the return path in the shiva or many other amps) to a stereo volume control driving two 18 watt marshall or vox AC15 output sections. That assumes you want the tone of a long tailed (schmidt) phase inverter. A simpler design that would use the same filament current but would allow for other interesting options would be to use a pair of 6u8's or 6gh8a's. The pentodes would amplifiy the return signal and the triodes would serve as split load phase inverters. EL84's require such tiny input signals that the output tubes would likely distort long before you'd get the phase inverter to distort. It would lower the tube count and make for an interesting circuit. You'd have 3 12ax7's for the preamp/effect send and either 2 6u8's or 3 12ax7's for the effect return and phase inverter.

Jamie

[jrfrond]

If it were me, I'd just purchase a Roland Jazz Chorus, either JC-77 or JC-120. Lightweight, sounds good, and it's all done for you.

I'm not trying to rain on anyone's parade but that amp will have a lot of iron and a big chassis. There's also a lot of potential for grounding and noise issues with a dual-mono tube amp like that, and it might need to be prototyped first to work out the kinks. Being a stereo amp, you'd need to pay close attention to channel phase relationship. You cannot add ONE extra gain stage, or you'll have phase issues. As far as the power transformer is concerned (if you were to use a single supply), you would need one with a large enough VA rating to support all of the tubes and heater, plus a low voltage supply (or two, or three) for the channel switching and possible onboard EFX.

Sounds like a nice idea, but COULD turn out to be a troublesome boat anchor. Just a caveat.....