Two 20 watt amps in one chassis - layout/grounding

[pjd3]

Hello,

Thats it. I'm doing it, and have begun collecting components and parts.

I'm looking by years end to have built a little 20 watt stereo tube amp to take advantage of the nice sounding stereo pedals that are out plus, it just seems like a really cool thing to try, and to have.

I'll probably just use a twin reverb chassis as I see a couple of 18-20 watt amps should fit in quite fine, according to some layout drafts I did. Not sure about tube rect vs solid state yet but, the jury is still out on that.

Are there any "global" considerations that come to mind in terms of say, grounding techniques or transformer/tube/sectional proximity that come to mind should you prospect on doing something like this? It seems to me there is plenty of room in a twin reverb chassis to keep signal away from power and to locate tubes and transformers as not to interfere with each other or cause too much heat build up in a particular place. These would be 20 watt amps, something like two single channel AB763 or maybe two plexi preamps, but no reverb or tremelo.

Anything you would be notably concerned about in layout or build? Oh, I should mention that I am intending to have two fully independent amps, separate knobs and switches for each amp, even separate power transformers and output transformers. I would normally think of having a common PT or dual ganged pots for all controls but, not for this one. Everything independent.

Thanks everyone, I really have to do this.

Best,
Phil D

[TUBEDUDE]

Best to build them as parallel pairs that share no components. Though a beefy supply well controlled should supply the power needed with little interaction. I would build them as mirror images of each other with the power supplies in the center of the chassis. The inputs on each end.

[pjd3]

Hey thanks TUBEDUDE,

I have followed that recommendation right up to the "power supply in the middle and inputs on the end".

Otherwise, I have it planned out to be two completely separate amps including the power transformers, with the only common components being the AC cord that would branch off to two separate switches/fuses/PT primary coil. One of the benefits I found with that is if anything, including a power transformer poops the bed, there will still be one complete amp left, with 2 speakers as a bonus. Although, the intention, at least for myself is a final stereo digital reverb output L and R to each separate amp input.

For reasons that live in just notions and assumptions, pretty much, was to place the PT's in far corners of the back of the chassis to keep the lions share of weight away from the chassis center and to keep the input volumes beside each other. But I most certainly will consider the benefits that may come from having inputs start at the ends with power section in the middle.
I can actually picture either being a plausible layout if done right. I definitely want the whole shebang to be mirror imaged. That right there made the couple of rough layout drafts go much better.

Thanks! it's very interesting to hear what folks have to say about this crazy idea that I can't wait to make a reality.

Best,

Phil

[didit]

Hello --

Recommend looking into at the construction/layout designs of latter generations of the mid-size tube stereo amps of early 1960s. The Dynaco ST-35 would be an excellent place to start, and then scan the rest for additional ideas. Sharing a power supply is fine, and can be central toward the chassis rear. This will save some weight and space, and simplify overall. With small attention to borrowing sensible design principles the inter-channel coupling will be negligible.

To modernize, consider building high tension power derived around quality toroidal. Potentially do filament power from some flavour of cleanly regulated electrically "noiseless" SMPS, perhaps one designed/suited for medical applications, which again saves weight along with DC current.

This does presume you're after a relatively modern amp sonic profile similar to a AB763 Deluxe Reverb vs seeking to replicate older Tweedy types.

Best ..

[cdemike]

I built an amp like this and second TUBEDUDE and didit's suggestions. The amp I built had a single shared toroidal transformer feeding to completely parallel, dissimilar amps (a Brown Princeton and an AC15-ish preamp each feeding an ECC99 push-pull output section). Beyond the PT, there are no shared components in the signal path (including output transformers). It has parallel B+ rails as well, but from a shared four-section can transformer which predictably introduced significant noise issues. I was originally happy with noise levels, but over time there was enough hum in the Vox preamp that it's gone through several iterations in an effort at cutting noise levels down. I haven't really succeeded in terms of noise levels, and I think the issue ultimately stem from the issues you've already identified -- layout and grounding.

Based on my experience with that build, I'd recommend, in addition to the suggestions above, that you use only single-section capacitors (no multi-section can types). It also seems to me that the increased complexity of things happening inside one chassis raises the potential for chassis currents considerably, so I'd wondering if, for example, the Dynaco ST-35 and other larger stereo amps had a strict star grounding scheme. I can't track down a layout for the ST-35 showing details on grounding, but I wonder what their approach was. Based on my experience, though, I'd suggest an overkill approach to grounding.

[didit]

Hello --

Suggestion was about overall placement, not so much the detailed layout. That era of Dynaco amps all had a PCB based construction. Would be straightforward tracing the grounding, though perhaps unhelpful in the end. They were successful using a "can" for primary HT DC power filtering. That said, recommendation on using discrete filter capacitors is wise. While taking more space and adding some potential costs it's easier to tweak in what will inevitably be somewhat of a one-off prototype.

Best .. Ian

[maxkracht]

+1 for using a toroid. I'm not sure what benefits there would be for having separate power transformers, but you can stack toroids on top of each other. You could also do one transformer with seperate windings. Something like this would work if you're ok using a SS rectifier. https://www.antekinc.com/as-3t325-300va ... ansformer/

[pjd3]

Thanks everyone, very interesting suggestions, especially on the toroidal power transformer. I hadn't thought of that as have never used one in a build.

The reason for separate power transformers was based on how I initially pictured the usability of the front panel in terms of the control knob layout. The preference was to have the input volumes next to each other for rapid balance and control of the channel or amp volumes, say for a quick volume adjustment live - each amp's volume would be right next to each other with the rest of the controls splaying out in mirror image - this would entail a minimum of 14 knobs on the front panel, alone). Should this be the case, it stood that the input jacks should be close to the first input tube, which would be all at the center/front of the amp. That being the case I would want the power transformer(s) as far away from the center front as possible, suggesting the ends of the rear chassis. That made the prospect of separate power transformers at each rear end of the chassis a plausible possibility, if not too economical. But, I'm not trying to save pennies on the dollar and I precariously made and excuse that it would keep the weight at the ends of the chassis which would offer more support (less middle chassis sag, while also being that if one power transformer was to fail (not common, but possible) then there would still be a whole amp to be available for the gig, just mono now instead of stereo. Plus, I became interested in the "Musical Power Systems" line of transformers which has offerings for PT's that are speced out fine for 18-20 watt amps while half the price of brand name offereings.

I may be a victim of "symmetry blindness" as there was something enticing about the Looks of this layout - Not an optimum basis for a layout but, if it could be made to work for a quiet well-behaved amp, I'd consider it. This amp for me would be a stereo pedal platform with the advantage of a plexi clean basic tone, which I rather like even for "Fenderish" type chord comping.

But, since this is just in the conception phase, I will be smart to look at recommendations from those with more experience than I (which is probably all of you)!

Thank you for giving me good ideas to consider. I'm not finished yet.

Best,
Phil D

[pjd3]

I like the presence of the two High voltage coils on that toroidal transformer.

Makes it a great contender right there - and plenty of 6.3vac current available.

Thanks for the link

Phil D.

[TUBEDUDE]

Phil you can have centered vol controls. Just have the input stages on each end. Your volume controls will be after several gain stages anyway so they're at line level now. Using shielded cable you should still get low hum.

[pjd3]

Alright, lets see if I've got this straight in my head.

With plexi preamps, I can place the first input tube stages at the ends of the amp, power section at mid/rear, then run the amplified signal from each end to the normal and bright 1Meg pots that would be located in the middle front of the chassis? I would have provided enough gain by then to effectively minimize any residual potential power section noise? The normal and bright signals will see V2A next. If shieled cables and that amount of gain would be enough to keep noise effectively acceptable then the single toroidal is looking like a good idea. As long as V2A far enough away from power section components and is seeing a large enough signal for starters, then there may be hope perhaps? I personally don't have the experience to know it that is the case but, I could very well be convinced by someone who has trodded a path like this. I would like to keep the controls working from the middle to the outer. Just seems so much more accessible, quick and usable. And the less money and components is surely a boon. I'm seriously considering this.

Thank you for complicating my life with sensibility !!!! Of course, I still might cave and cower in ungrounded fear and revert to my initial idea. I do think that could work fine but, at the expense of, expense and volume.

Thanks everyone for chipping in your ideas.
Best,
Phil D

[Stevem]

To have more front panel real estate you might consider stacked pots for two of the tone controls.

To get a nice leg up on a two channel amp layout in one chassis you might want to check out the schematic of a big stereo tube amp like a HH Scott 272.

[wpaulvogel]

I’d just build it as a HiFi amp with increased gain and long tailed phase inverters. Use the HiFi approach for the chassis even if you used two power transformers. Crank it up for grit.

[TUBEDUDE]

Or separate pre and power chassis. I did a stereo pre (originally for my Betterbacker) and coupled it with a stereo KT66 power amp for my T. V.

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Never mind Phil. I see from your other post this must fit in a Twin combo cabinet.

[pjd3]

Ha amazing. Those are definitely cool, if not a bit fugly.

I happened to find twin reverb blank chassis on Ebay for around 50 bucks, thought that was pretty good, and in doing a full size layout drafts on paper, it looked like it had quite enough dimension to fit two amps.

The other aspect to this was also to make for a very quick set up, where you want to get to the gig right after work with 30 minutes to set up, and to have a great sound at the same time and amp, pedal board, guitar and bag, maybe a single trip for car to stage. So the twin like form factor just seemed like it would meet those ends. And full set of channels would make it versatile as a stereo combo or two indendent amps say, it you had two very different guitars that benefited from two amp settings. Plus cant lie, I have a picture of a mirror image dual amp panel that just seems very attractive, even a kind of conversation piece. Its fun when people look at something like the RCA dog and wonder, WTF am I looking at! Good gig break conversation. I'm just finishing up on a Celtic themed baritone 28 5/8" guitar that is going to make people wonder whats wrong with me. I"m quite OK with that. More to talk about.

So yeah, this is a matter of "The old dog needs to get to the gig set up fast and sound good". And maybe have a weird looking cool one off amp.

Thanks everyone, good stuff.
Best,
Phil D