Need help identifying this 30 watt EL84 amp circuit

[jman72]

Hey Everyone,

My buddy was given this amp by a friend of his several years ago (custom built just for my friend, REALLY nice, beautiful build), but he's having some issues with it and wanted me to take a look and see if I could figure out what was wrong. Naturally, knowing what the actual circuit is based on would be helpful, but I can't contact the builder and I'm not very familiar with non-Fender circuits. Here's what I know about the amp:

-MAYBE AC30 based???
-30 Watts (4 EL84 power tubes, three 12ax7 preamp tubes), diode rectified
-Two inputs/channels (Top Boost and Normal)
-Cut, Bass, Treble controls; separate Top Boost and Normal volume controls
-Separate 4 & 8 ohm output jacks

That's pretty much it. Here are some various shots of the amp that might help identify the circuit. If you think you know what amp/circuit this might be based on, I'd love to hear about it.

 

[Dacious]

Can't really tell but I'd say someone's based a AC30 build off a Laney VC30 amp shell.

The toroidal transformer lacking a 5 volt winding means someone's made it with a diode rectifier.

Without more info it's hard to say which one but they're not miles apart.

 

[Wally]

in order for someone here to analyze that circuit, we need to see the circuit much as one would see it on the bench. That is, we have to see the input jacks, the pots, the circuit board and the tube sockets all in one shot. Same with the output end of the board.

 

[schmee]

Maybe a Dr Z Maz 38 clone? Hard to say.

 

[rdjones]

The choke is a Fender part, 125C1A equivalent.
The output transformer is from the HotRod Deluxe.
I'm not familiar with any traditional style tube guitar amp(s) with a toroid power transformer.
The controls appear to be Vox-like.

I'd say it is its own custom thing, a mish-mash of various design elements.
I would have expected something that was Vox inspired to include an EF86 or other pentode preamp channel which this does not have.

 

[jman72]

Thanks for the input everyone. Yes, it looks like it might have some elements from a few different amps rolled together. I guess I'll just have to figure it out flying without a schematic.

In any event, I get sound from the amp (it sounds great), but when it's turned up a bit and I hit a low E or G chord, I get this growing, fizzy rumbling that sizzles out as the notes die out. The problem is not as bad when I connect an extension speaker cab (but still there), so I think that the vibrations from the speaker are causing something to make intermittent contact in the circuit (it is a combo amp)- either a loose ground or a cold solder joint. When I tap on the top of the cabinet (particularly near the tone/volume knobs), I can tell that something is loose and it makes/loses contact and I get a loud grounding issue hum and crackles. At least I've figured out where the problem lies.

I'll have to pull the chassis back out and chopstick around to see where the loose connection/bad solder joint is and fix it.

 

[Wally]

It could be a tube or a dirty contact in a tube socket. Check the simple stuff first.

 

[jman72]

I think I found the culprit. The main ground bus wire that goes through all the pots and the input jacks (and ultimately to the chassis) is not soldered correctly at the solder lug of the normal channel input jack (you can see it clearly in the pic below). When the amp vibrates, this is causing the intermittent grounding issue.



I cleaned it up and re-soldered it. The problem now seems to be fixed. Thanks for everyone's help!

 

[gusfinley]

At first glance it looks like an AC30-ish amp with Top Boost and Normal channels. The Top Boost circuit only uses 3 triodes so many people use the spare triode for a single-triode "normal" channel and run the outputs into either end of the phase inverter to mix them in-phase.

It would probably be very similar to the current Vox Handwired series AC15HW1(X) or AC30HW2(X).

I'm glad you got it sorted!