Vox AC15C1 distorted

[mikeskory]

I've been working on an AC15C1 this week. The sound is distorted. I can see and hear the distortion at the speaker. I can also see if at pin 2 of each EL84. I thought it was generating at the EL34s. When I measured the voltage drop at V4 EL84 it was 180mv. So I changed all 4 grid resistors. the ones at pin 9 and pin 2. One appeared to have a broken lead. The voltage drops appears ok then so I calculated the watts of dissipation. 13.7w at V4 and 13.5 at V5. But...the distortion was untouched. I clipped in an old output trans but the distortion was the same. This seems like a staright forward curcuit but I can't seem to isolate the problem. I'll take some troubleshooting guidance. I have the right schemo which I'll upload in the next email.

[Enzo]

What does the distortion look like? Clipping? crossover distortion? Symmetrical or not? Is the phase inverter clipping? Where along the signal path does the distortion first appear?

[mikeskory]

Vox AC15C1 distorted

What does the distortion look like? Clipping? crossover distortion? Symmetrical or not? Is the phase inverter clipping? Where along the signal path does the distortion first appear?

Here is the schematic

Vox_AC15C1.pdfVox_AC15C1.pdf

It seems to appear on the output of V3 12AX7. I post again this afternoon. Had to leave for a while.

I EMAILED a better schemo because I could not get it to upload.

[Enzo]

Yeah, that is an 11MB file size, way too large for this forum.

V3 is your phase inverter, sounds like that stage is clipping to me - well LOOKS like, rather than sounds from here.

Is the distortion present at C32, as it enters V3?

Also note the signal path before C32 runs from the V1, V2 circuits through a bit of solid state land ther around the reverb. Into U1a and back out U5a. Make sure the ICs have good power rails right at the chips, and see if they are clipping. My sneaking suspicion is that the phase inverter itself is clipping. That is where a Peavey Classic 30 always clips first.

[mikeskory]

I'm home now but I stopped at work to re-check a few things. There's no signal at Pin 2 of V3. If I crank up the reverb and both vibrato controls, I can get a little hashiness...just enough to see that I'm twisting the controls. Could not find C32 but I think it's C42 and misnamed on the schemo. But anyway, no signal at R55 or pin 1 of U5.
Should I see a good sign wave at pin 2 and pin 7 of V3?

The +27v and -27v are at D5 and D2.

Monday I'll post more measurements to make sure the +27/-27v is not missing from the rest of the ICs AND pics of the distortion.

What does V3 do other then the effects? Should the amp play with V3 out of the socket?

[The Dude]

The amp won't make sound with V3 removed. It's the phase inverter and drives the output tubes. Have you tried tube substitution? It's not uncommon to see defective stock tubes in these amps.

[mikeskory]

@the Dude:
yes I replaced the V3. The amp had all 12ax7s when it arrived.

@the dune and @enjo:
I'm home but I want to understand how this flows before Monday. The tone generator signals from the 2 input jacks go to V1. Normal to pin 2, TB to pin 7. The signals are amplified independently in this tube.

Output from V1 pin 6 (normal) goes to U1. The output of V1 Pin 1 (TB) goes into V2 at pin 2. Is V2 only used for the TB sound??

Eventually the sound gets to V3 Pin 2. I should then have a good clean sine at V3 Pin 2 and a smaller signal at V3 pin 7. The outputs of V3 are 1 & 6. They should be crisp with the same amplitude but out of phase, Is this correct?

[mikeskory]

I wanted to send you guys screen shots of my scope reading at V3 plates. But they are too big to upload. Is there another way to get the scope shots to you? Enzo I'm just going to email them to you via GMAIL.

[Dave H]

I wanted to send you guys screen shots of my scope reading at V3 plates. But they are too big to upload. Is there another way to get the scope shots to you?

Why are they too big? Are they in bmp format? You could save them in png format. Paint will do it. I edit screen shots in XnView before saving as png

[mikeskory]

Vox V3 Jpegs

Here are the scope shots showing the distortion at the plates of V3
Thank you Corliss

[The Dude]

Honestly, those don't look that bad, for a guitar amp, to me. The first is a little odd, but IMO, not something that you would hear as an awful distortion. It almost looks like more of a scope triggering error. What is the P-P signal level there? If you turn the input down, does that little extra loop on the first shot go away? Have you tried the amp with a known good speaker- maybe we're chasing a ghost?

[mikeskory]

Honestly, those don't look that bad, for a guitar amp, to me. The first is a little odd, but IMO, not something that you would hear as an awful distortion. It almost looks like more of a scope triggering error. What is the P-P signal level there? If you turn the input down, does that little extra loop on the first shot go away? Have you tried the amp with a known good speaker- maybe we're chasing a ghost?

I know. After looking at them thats what I thought. But here is what it looks like at the EL 84 V4 and V5 anodes. I've soldered the sockets, changed the EL84s and replaced 4 grid resistors. R65/R77 and R76/R83. I'll check those tomorrow morning to make sure I didn't grab the wrong values.

Mike

[mikeskory]

Just to follow up, we put this Vox amp to bed last week. Original complaint was low volume/distorted sound and it was blowing heater fuses on occasion. We replaced 2 EL84, the resistors R65/R77 and R76/R83, 2 4700/16v C17/C25 caps. Checked the bias. Customer came, was happy and paid. Thanks for the input.