As far as surface mount goes... my eyes hurt just looking at board layout on the schematic. I am a moron when it comes to SMD work but I somehow get it right. I even use an old vintage 10 watt pencil and always use chip qwik. I have yet to get into hot air station territory. But seriously a tiny transistor is a bit hard not to F up the pads. I find chip qwik gets the device off the board fastest. It cost money but it's a god send. Try it out on crap boards that don't matter. It's not that hard to do. Oh check out reverse tweezers for soldering in a tiny component. Takes a bit of practice.

I would not be too concerned about "a few tenths of a volt" offset.
Here is the SM if you want to dive in.

Do you have a schematic for the unit? I don't think the output transistor is completely shorted but who knows? I say this as the fuse is working and powers up without blowing. Either way it doesn't have an output transformer so no need to have a speaker or load connected. Remove the load or speaker and then take voltages. Post schematic to make sense of the issue at hand. I imagine a smaller transistor could fail and the unit would work but with issues.

hey folks! i'm a tube amp tech, first time posting here, excited to learn some more about solid state circuits!

i've got a Roland JC-40 on the bench with DC on the outputs, just a few tenths of a volt on one channel and it still sorta works, but the other has pretty much the full rail across it. power transformer still puts out 15-0-15 unloaded, speakers both measure 4.2DCR, no popped caps or signs of overheating.

haven't done any testing yet beyond that, but a search shows that this may be a fairly common issue for this amp, so i wanted to post and ask if anyone is familiar with typical failure states for this amp and has any suggestions for what to check or upgrade.

my general plan is to get all the output transistor voltages and power supply node voltages and go from there, but i'm also not very knowledgeable about SS diagnostics beyond that. i do have the schematic from a previous post on this site. wondering if there are any general guides to SS amp diagnostics you all would recommend?

also wondering what your general approach would be to an SMD amp like this... would you likely actually get into replacing passives on the component level? or would one typically only replace power devices and e-caps, and if there's a more complex failure, is it a better idea to just get an authorized service center to order board replacements?

thanks in advance!

Quote from: silverfox on April 08, 2026, 06:40:46 PMHmmm, 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.

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.

I did a quick test of the DSP1, and found the hum is only from its power amp, and not its pre-amp or its DSP, because when I connected the SEND output using a jack, which disconnects the preamp output to the power amp, to an external power amp, the Crater still produced the same hum but the expernal amplifier does not and it works perfectly with the Crafter pre-amp and DSP.

Since it's a 50Hz hum, it's either from mains leakage or from the transformer EM radiation. Robdean found changing C57 and C58 made no difference, so it could be the transformer needs shielding or the power-amp design is bad.

If the problem is the power amp, then there are 3 solutions :

1) Investigate why the power amp is producing hum with no input, and change it's design.

2) Replace the power amp with a third party one if 1) fails. There are plenty cheap modules avalible.

3) Replace the power supply with a more stable switch mode power supply, if shielding fails.


I was an electronics design engineer but I don't have a lot of free time, soI will investtigae the above options later on.

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.

Thanks for the tip. I've downloaded it.

Schematic is in the first post of that thread you linked.
There are a lot of things that can create hum. His problem is not necessarily the same as yours so you should go through that whole thread and see if the checks suggested get you anywhere.