Looking for advice on a stereo power amp build

[LeftyStrat]

I'm putting together a rig to jam with an old drummer friend of mine. We've done something similar in the past. I play bass and guitar (sometimes heavily processed), and use loopers. Very spacey experimental stuff, but fun nonetheless when we hit upon something that approaches pleasing noise.

I've decided to put together a small rack with an all tube stereo power amp, and maybe a couple of preamps, and my TSR-24 digital fx unit, combined with a couple of lightweight pine TL-806 cabs made heavy by original EVM12L's.

The thing is, I don't exactly want a pristine stereo power amp. I've got a couple of original A-470's I plan on using, and I want to be able to run multiple tubes (anything that likes a 4k3 primary @ around 450V). I do want it to handle bass guitar, so I'm thinking heavy filtering and SS rectification.

Here are my thoughts:
- Switch for defeating negative feedback.
- Pentode/Triode/UL switch.
- Dual ganged pots for a stereo James tone stack.
- LTP w/ larmar ppi master.
- 70 watts per channel with EL34's.

Has anyone done something similar? Any advice?

[trane34]

I just got started down that rabbit hole with a bm/183 rack build. I used to own a THD bivalve which had a baxandall stack. Are they all about the same? I could see how that'd be useful for different instruments. I ended up moving on because it was just too weird/unusual for me. I kept messing with the tone controls all the time and never got the same sound twice. But that also could have been because I put every kind of tube imaginable in there.

Do you have a PT picked out? I'm using a O'netics 100 watt pt for mine that should work for your build. 10 amps on the heaters. 700vac@400ma. Plus 5 vac and 15.3 windings for switching. I've attached their wiring schem if you want to take a look at it.

[LeftyStrat]

I just got started down that rabbit hole with a bm/183 rack build. I used to own a THD biflex which had a baxandall stack. Are they all about the same? I could see how that'd be useful for different instruments. I ended up moving on because it was just too weird/unusual for me. I kept messing with the tone controls all the time and never got the same sound twice. But that also could have been because I put every kind of tube imaginable in there.

Do you have a PT picked out? I'm using a O'netics 100 watt pt for mine that should work for your build. 10 amps on the heaters. 700vac@400ma. Plus 5 vac and 15.3 windings for switching. I've attached their wiring schem if you want to take a look at it.

Bax and James are similar. Instead of tone shaping, I'm looking for something to deal with the room. I would still have tone shaping in the preamp. I'll check out that PT.

[passfan]

Thats going to be a tall order for tubes. Even the ST series was 35 watts per channel if memory serves me. Easy to get 75 watts out of a guitar amp with all the gain stages. Not so with a stereo tube amp. It's just a PI and a power section looking for 1 volt input (which is what most cd players put out) Pretty much the reason why tubes fell to solid state back in the late 60's when transistors became more commonly used. Google search it. I'm sure something will come up and keep in mind that a 35 watt stereo amp is still going to be loud.

[Leo_Gnardo]

Those A-470's are fine transformers BUT really don't have the oomph you'll be seeking for bass guitar. Dyna's stereo 70 used them, a terrific sounding stereo amp, but even with a juiced-up power supply, the low end just goes to hell at volume. There just isn't enough steel in the core to support it.

Take a look at the amps Sunn turned out in the late 60's - early 70's. Sunn simply used Dyna transformers & circuitry for their output sections. The smaller guitar amps were pretty good, but had a "crushing" compression once you jammed into the 40-50W range. Not bad for guitar if you like that sort of thing. (I had a Sorado in the shop for rebuild 2 years ago. When cranked up, sounded like early 70's Stephen Stills solos.)

OTOH Sunn's PA-120 used 4 x KT88 or 6550 through the same OT you find in Dyna MkIII, and delivered beyond 100W clean, with good bass response. Do a build like that and your rack won't be "light" by any means, but you'll have clean low bass at volume. If you need loud and clean, there's no compromise, you need power. Two amps like that in a rack and you'll need king kong arms to move it.

Then there's the speakers. The Thiele design lets you get the max low response but still limited by the fact they're EV 12's. You might want to consider Eminence Kappa Pro 12's which in my experience deliver satisfying bottom end with bass. Or go to 15's.

For a guitar rig, you can "go with what you got."

For bass, I'm afraid you'll find the low end lacking, especially when you and your drummer friend start slammin' the volume.

[LeftyStrat]

Thanks for all the replies. I failed to mention that this drummer is really good at matching the volume of other musicians, so I don't have to be really loud.

I may consider using something else for bass, and concentrate on making this amp just for guitar. I played an Ampeg Micro VR recently and was really impressed with the sound. I tried several other heads into that 2x10 cabinet, and all sounded good. I may just do a tube preamp like the Alembic FB-2, and use a cheap class D amp for the power, and grab one of those cabs.

[Leo_Gnardo]

You're lucky to have a drummer with that rare talent!

And those Ampeg 2x10's deliver an uncanny volume of bass.

Luv my Alembic pre. You could build one channel, and if you want to fancy it up, a master volume, then cathode-follower driving a small output transformer to make sure you get no ground loop buzz.

[LeftyStrat]

You're lucky to have a drummer with that rare talent!

And those Ampeg 2x10's deliver an uncanny volume of bass.

Luv my Alembic pre. You could build one channel, and if you want to fancy it up, a master volume, then cathode-follower driving a small output transformer to make sure you get no ground loop buzz.

Yeah, this guy has been teaching drums for probably 20 years. He's one of those guys who consider themselves musicians before drummers. Though he would probably correct me and say 'percussionist.'

For the Alembic pre, I thought about building two channels and voicing one side for guitar and the other for bass, then use a third tube to provide two CF's for each channel. Do you find it boomy for guitar? Don't have the schematic in front of me, but as I recall it has pretty large value cathode bypass caps.

[Leo_Gnardo]

Boomy for guitar? Well maybe a bit, but it's much the same as most Fender input/EQ sections. Gotta keep the bass dialed well down to stay out of the boomy range esp. when playing at high volume. And I put 12AY7's in mine to trim the gain back some. But your own preamp - you're building it, so you can make the component choices to deliver the responses you want. I've seen a couple of Alembic pre's where parts were changed just for this reason. Heck you could put a couple extra switches on the faceplate so you can have a wider choice of tonal colors available - indulge yourself - drill baby drill!

There's a pedal steel player who wants me to build him a preamp soon, so we'll both be going at it.

[DonMoose]

My only suggestion is to get the iron as close as practical to the front panel - if not ON the front panel - for mounting/travel security.