Vox Korg-Era AC30 Repair Help

[Matt D]

Hi Guys,
My bandmate asked me to look at his Korg-Era AC30TB. Over time it has developed a loud buzz/hum and you can hear the tremolo ticking, especially in the top boost input. I changed all the filter caps and replaced all the tubes with known good ones hoping the problem would be something easy but no dice. The circuit is the AC30-60-00 which may be different from the AC30-60-02 schematics commonly available online.

I can't find a layout for this amp that relates the tubes on the schematics to where they physically are on the amp, but I'm guessing V11 is the preamp tube closest to the 6 input jacks and V6 (phase inverter) is closest to the power tubes.

Here are my voltages in the preamp section:

V11 - 12AX7 (Trem?)

A1= 201v A2= 106-143v
G1= 13v G2= 0v
K1= 21v K2= 2v

V10 - 12AX7 (Vib/Trem Input?)

A1= 206 A2=143
G1= 8 G2= 0
K1= 14v K2= 1v

V9 12AU7 (Vibe mixer?)

A1= 70v A2= 73v
G1, G2 = 0v
K1= 3v K2= 3v

V8 12AX7 (Normal/Brilliant Input?)

A1= 184v A2= 181v
G1, G2 = 0v
K1, K2= 2v

V7 12AX7 (Gain Stage/Cathode follower)

A1= 283v A2 = 181v
G1= 175v G2= 0v
K1= 175v K2= 1v

V6 (Phase Inverter)

A1= 240v A2= 246v
G1= 34v G2= 35v
K1= 52v K2= 52v

The anode of V11 oscillates between 106v and 148v. I guessing this is normal for the functioning of the tremolo.

I have a B+ of 335 at the rectifier tube. 330v on the plates, 322v on the screens and 10v on the cathodes of the power tubes. When I remove what I think is V11 the ticking stops when I remove V8 the buzz/hum stops. Any suggestions? I want to get this thing off the bench so I can build something fun!

[Matt D]

When all volumes are turned down the amp is quiet. The Vib/Trem channel is quiet when turned up on it's own. The buzz Normal channel strangely gets quieter in the first part of the rotation of the pot and louder in the second part of the rotation. No Trem ticking in the normal channel. The Top Boost Channel is where most of the problem seems to be. Ticking and a very loud buzz.

Plugging a cable into the tremolo footswitch jack causes the ticking to stop.

None of the main coupling caps seem to be leaking DC. There is no DC on the pots. The only place I found DC where I didn't expect it was 5V between C39 and R40 and 5V between C40 and R41.

I hooked up and audio probe and touching the probe to any part of the signal path between the Brilliant channel input jack and the grid of the first triode caused the amp to oscillate wildly. I reflowed all the solder joints on the jacks and their associated ribbon cable. Then reflowed just about every other solder joint on the pcb. No luck. Starting to suspect some kind of grounding issue but all the grounds seem good.

[Stevem]

It sounds like the shorting switch in one of the input jacks in not shorting as it should when nothing is plugged in to it, so I would use a clip lead to place the needed short across them one by one to pin it down.

I would pull the preamp tubes out one by one starting with V1 ( first preamp tube) to pin down where the hum issue starts at if its not from the output stage.
Also if you have less than 190 MV of AC ripple voltage on pins 1and 6 of the first preamp tube, than your hum is likely not due to bad or leaky power supply filters!
Get this issue sorted out first before moving on to the tremolo issue!

[Matt D]

Thanks for the reply Steve. Looks like it is an input jack! I don't know why I didn't check that earlier.