VOX AC30 - Bad distortion

[mightyatom]

Hi all,
First time poster, long time follower of this forum.

I have a 64 Vox AC30 - non top boost - with thin, harsh clipping distortion on the vib/trem channel (along with volume loss). Because of the the circuit path this in turn affects the other 2 channels, adding a little of that bad distortion to them too.

I've checked as much of the vib/trem circuit as i can make out on a schematic (can't seem to find a really clear one that's easy to read the compontent values), tested for bad carbon comps and coupling caps. The resistors seem roughly within spec, I've clipped new caps across the old ones to see if that makes any difference but it doesn't. I've cleaned the switches for vib and trem. I've been through every solder joint on all 3 boards and can't find a dry joint. I've tested all the electrolytics with my Atlas ESR60 meter, all were ok except the 250uf/25v cap in the EL84 bias circuit, that's now replaced.

I've also, cleaned all the tube sockets, subbed in other tubes right through the amp, cleaned the jacks, checked the speaker connections, played the amp through different speakers but nothing makes any difference.

The problem is obviously with the vib/trem channel but I noticed when playing an ipod through the channel when I switch off the amp (and the caps are discharging) that the fault disappears as the sound fades for a few seconds. I get about a second of clean, distortion free, full volume great sounding audio!

Does this narrow the fault exclusively to an electrolytic somewhere? Any suggestions would be very much welcome.

Schematic: http://www.voxamps.com/downloads/circuits/ac301960.jpg

Thanks in advance!

[pdf64]

'I have a 64 Vox AC30 - non top boost - with thin, harsh clipping distortion on the vib/trem channel (along with volume loss). Because of the the circuit path this in turn affects the other 2 channels, adding a little of that bad distortion to them too.'
The above doesn't seem quite right to me.
The Bright / Normal channels, and Vibrato channel don't share any circuit path (other than ground, they have different B+ branches), they even go into different 'sides' of the long tail pair.
So, to clarify, is the same distortion present on all 3 channels? If so, is it to the same degree, or seems worse on eg the Vibrato channel.
It's probably just one circuit stage which has it's operating conditions askew, maybe a cathode bypass cap or coupling cap leaking excessive dc; if so, the Vdc around that stage should indicate a problem.
Pete

[mightyatom]

OK Pete, thanks very much for the reply. The distortion is very bad on the vib/trem channel - both inputs - along with the volume being much reduced, the other 2 channels have the correct volume but some of this same bad distortion mixed in with their sound to a lesser degree. I'll check for DC around the vib/trem stage.
What do you think of the sound and volume clearing up as the amp is switched off?

[Enzo]

You are stuck in the it must be a bad part mode.

Your meters can tell you cap ESR and whether it is shorted, and measure its value, but they can't tell you that the cap won't leak like a screen door at high voltage.

Isolate the problem. Start by looking at voltages. If the other two channels sound good, then the common circuits are probably OK, but compare the phgase inverter pin voltage from th schematic to your amp. The schematic also shops voltages all over the various 12AX7s, compare to yours. All are type 12AX7 except V8, which should be 12AU7, is it? and its voltages?

[mightyatom]

Thanks Enzo, I'll take a look in few hours and report back.

[mightyatom]

Hi Enzo,
OK, some voltage readings from the 12ax7's (and 12au7 in V8).

The first voltage will always be the voltage on the schematic - from what i can read...
V1
pin 1 - 170vdc / mine = 185vdc
pin 6 - 170vdc / mine = 185vdc

V2 (PI)
pin 1 - 230vdc / mine = 166vdc
pin 6 - 230vdc / mine = 18vdc gradually dropping with lots of trem / oscillation noise when probe touches pin.

V7
pin 1 - 140vdc? (hard to read) / mine = 160vdc
pin 6 - 200vdc / mine = 231vdc

V8 (mod)
pin 1 - 75vdc / mine = 54vdc
pin 6 - 75vdc / mine = 55vdc

V9 (osc)
pin 1 - 120vdc / mine = 98vdc
pin 6 - 195vdc / mine = 224vdc

So, there's some that are off the mark but I guess the main problem lies with the PI voltages. Could this be related to C6 & C9 the .15/400v caps?

Thank you

[Enzo]

Guitar amps are not precision circuits, anything close is usually fine. The only problem voltage in your list is pin 6 of V2. That side of the tube is not working. C9? ANything is possible, but if that cap even went to dead short, it still ought to leave 100v on that plate. And at the same time it would also put that 100v on the grid of the power tube. I suspect that is not happening. I'd be checking R19 to see if it still is 100k.

[mightyatom]

Enzo - problem found! R18 / 100K resistor was at 285k and R19 was completely open. I've just subbed in 2 new ones and the distortion has disappeared and volume and tone returned! Thanks so much, I love this forum!

[pdf64]

It's worth checking all resistors, they can be regarding as life expired by now.
Pete