Peavey Special 212 Consolidated design-theory-hack questions.

[silverfox]

Pleased to make the acquaintance of, Y'all.

Rather than spread this project over several threads, I thought I would consolidate the discussion, particularly as it is all related, in the end. Here is a link to the amp schematic in question: https://el34world.com/charts/Schematics/files/Peavey/Peavey_transtube_special_212_orig_1995_schematic.pdf

Electronic experience: Electronics and computer tech from 1979 to present. Built tube amps a couple of times. Basic diagnostic skills. Electronic theory, but weak on Op Amps- Which is what bought me here.

Objective: At this stage, I'm believing I'd like to build a stereo version of the power amp section of the Special, for use with my Fractal Audio AX8; An amp modeler. I picked one up in the Pawn shop knowing full well many of the controls on the front were damaged, and needed replacement. It's odd how the Pawn dealer will be reluctant to sell at a reduced price, a piece of broken equipment he was scammed on when pawned for next to nothing. Originally, I was after the speakers, but the power amp section sounded really nice to my ears.

Questions:

Looking at the schematic, I think I see, but I'm not sure how this power amp works. It appears to resemble a tube amp phase inverter circuit, or AB tube output section.

What does VR18a,b change in the circuit to obtain the so called Transtube effect?

This doesn't appear to be very complicated to build. It may even be most cost effective to pick up another for about $100, cannibalize the circuit boards, and solder up a stereo version?

Something odd here; When I cranked this amp up, testing it out, it didn't sound like 130 watts. Perhaps, 40 at best. And for sure, I don't believe this  is going to put out 200 watts. I was unable to locate the transistor data sheet, although I didn't spend much time; to determine if the power transistors, combined output, were even in that power rating. C107 on the power amp input could be bad, and I know the Dynamics pot is faulty. Another consideration is to drive the power amp with the output from a mixer. I'm not sure what signal level is needed to drive the power amp to max output.

To get around the faulty Dynamics pot for now, I'm considering simply bridging the two legs of the related 10k pots with 5k resistors, to get a better idea as to the usability of this amp with my Fractal Audio amp emulator. Mid point setting on Dynamics sounded good enough for now.

For now, that about sums up my questions and reasons for joining the forum. I was originally pointed to the site after searching the Special and landing on a, "Post", here that discussed the amp.

Thanks for any assistance in this project. Regards,

Silverfox.

[g1]

Are you positive the schematic matches your amp?  Is there 6 large power transistors mounted on the heatsink?
There are many versions of transtube, some are lower power.  The 200W rating is only when second speaker cab. is in use.
 
The power amp topology is fairly standard, complimentary output style.  Using complimentary type outputs (NPN and PNP) means no phase inversion is necessary.
The 2 sides of VR18 are affecting feedback networks.

[silverfox]

Are you positive the schematic matches your amp?  Is there 6 large power transistors mounted on the heatsink?

Yes. There are 6 transistors. Thanks for the response regarding the topology of the output circuit. I'll have to research that.

Best regards,

Silverfox.

[aquataur]

If the T-Dynamics pot is faulty, then this is the first thing to address if you feel like it has no power.

The T-Dynamics shifts the workload between preamp and power amp. It throttles the preamp back and pumps the power amp up and vice versa. This makes for a loose vs. compressed tone.

Those amps normally are as loud as hell with the control CW.

Unfortuntately, the pot is a specialty. This does not happen for the first time. Look up the Peavey forum for a fix.
Bridging the pots with resistors mid-way does not do the trick.

[phatt]

Hi Silverfox,
              Be aware, Stereo sounds great in a small room or on recorded music it shines.
Reality check; on stage it adds little and can just disappear in the mix.
If you want big spectral sound on stage,,, get a Leslie speaker which delivers a far more realistic stereo sound on stage.
Phil.

[silverfox]

If the T-Dynamics pot is faulty, then this is the first thing to address if you feel like it has no power.

The T-Dynamics shifts the workload between preamp and power amp. It throttles the preamp back and pumps the power amp up and vice versa. This makes for a loose vs. compressed tone.

Those amps normally are as loud as hell with the control CW.

Unfortuntately, the pot is a specialty. This does not happen for the first time. Look up the Peavey forum for a fix.
Bridging the pots with resistors mid-way does not do the trick.

Thanks for the information. The pots are wired so that one value increases, and the other decreases. I've contacted Peavey regarding parts. They seem to be willing to sell them to me. However, if I can't get that specialty dual pot, why wouldn't bridging those two circuits with say, a 7.4k, and 2.5k resistor work. That would be the half way point between the dynamics setting?

Another question: I presently only use the power amp in, with an amp modeler, so perhaps, due to the signal level of the modeler output, that is why the amp didn't get as loud as expected. I played through a Peavey 1-12 tube amp before and that was loud. I mean shake the winders loud! The neighbor down the street wandered over to ask me to turn it down as his windows were shaking.

Thanks for the information,

Silverfox.

[silverfox]

Hi Silverfox,
              Be aware, Stereo sounds great in a small room or on recorded music it shines.
Reality check; on stage it adds little and can just disappear in the mix.
If you want big spectral sound on stage,,, get a Leslie speaker which delivers a far more realistic stereo sound on stage.
Phil.

Yes. I'm realizing I don't need stereo. Thanks.

Silverfox.

[silverfox]

I'm curious to know if the, Presence, Resonance, and Dynamics circuit could be added to a tube based power amp I've built. Reproduce the Op Amp filter circuit and add it to the input of the power amp.

Silverfox.

[aquataur]

Presence and Resonance come into the bargain. They are just modifications of the feedback loop on a tube amp. On a SS amp, it is an OPA with a mixed voltage/current feedback. You`ll see it in the Peavey schematics. I don´t know if T-Dynamics makes sense on a tube amp.

The first thing that comes to mind is POWER SCALING. It is a circuit developed or made popular by Kevin O´Connor, which throttles back the Anode voltage using a power MOSFET, but proportionally balanced the bias voltage too.

This can give you a hard working power amp at bedroom levels.

Both modifications are explained in depth in his books, but you can also buy the kits with the right HV parts.

[silverfox]

Power Scaling. Yes, I had forgotten that design. I spent quite a bit of time on a tube amp forum, and that was mentioned a number of times back in, oh what was it, 2013 or there about. I didn't realize the design also varied the bias as well. Hmmm, with the preamp output and the power amp input being altered at the same time, I wonder if a sort of power scaling is at work in this amp?

Something to consider. I built a tube power amp, however, it only occasionally fits well with the amp modeler I use. Takes a lot of EQ-ing to get a good sound. The Peavey, when I put it back together may work well with much less fiddling around. When I purchased the Peavey, for $100, at the Pawn Shop, I was only after the Sheffield speakers. But the amp sounded amazing. I'm in the process of ordering the pots needed to repair the amp.

I'll report back.

Silverfox.

[aquataur]

Hmmm, with the preamp output and the power amp input being altered at the same time, I wonder if a sort of power scaling is at work in this amp?

No, not technically. It would be the preamp input BTW.
It may have a comparable aural effect though.

Tube amp does not mean good sound automatically, as much as SS does not mean bad sound automatically.

I bought the very same amp, blue teal, for the rehearsal room. It is very versatile and sounds good in all situations, but people prefer to take the Marshall. Only for its name. It is a SS yoke and has a preamp tube in it just to be a tube amp
It sounds lousy but that's how people are knitted.

[aquataur]

As fate would have it, I was called that part of the rehearsal space was to be cleared. I liberated the Special 212 from the bondage of those unworthy.

I found that they had no clue what to do with all the controls. Also, they have trodded over the footswitch cable, and excerted the connector to some heavy stress.

It took me a few hours to find the crack in one of the tracks.
Where are the times when connectors on amps were indestructible...

Apart from that, the interior of the amp looks very tidy, almost empty compared to what you expect by glancing at the schematic.

It would not be exactly my wettest dream to have to exchange one of the pots, but it is doable.

The 'verb pan is defective, output coil is ruptured. This is one of the accutronics that use plugs. I ordered a replacement from MOD. This is soldered.

These amps appear very rugged throughout (it is a silver stripe btw.)

I can understand that people shy away from them, because maybe one is asked to read a manual , but I dare saying that if you had to play a gig without anything else on you, having one of those amps would not be the worst thing that could happen. You'd have to harness the thing though...