1960s JMI AC10 Twin - I'm stuck...

[Yeatzee]

Howdy friends, been a minute! I've got a friend's early 60s Vox AC10 on the bench and I'm a little lost hoping I can get some advice from those much less sleep deprived than myself! (baby #2 born recently)

The ef86 / vibrato channel has a lot more idle noise than I feel like it should, particularly at healthy volumes AND it's not a consistent even noise.

i=pwVq4ruujESyExof

The amp has been recapped and a good number of resistors replaced. I've tried everything to track down the issue but I'm at a brick wall hoping for directions around it!

Things I've tried:
3 different ef86 tubes
2 different Ecf82 tubes
Cleaned all of the tube sockets and pots with deoxit
Replaced every component in yellow (or lifted one lead and temp'd in a replacement for testing)



Nothing has improved the noise in this channel. I tried scoping just the raw idle noise in the preamp to see if I could detect it that way but it's so quiet at that stage that my scope can't really pick it up well enough to conclude anything.

I did also try a trick BGG told me about, which is to use a 0.1uF film cap to ground and touch it to components (discharging between each one) to see where the noise is killed to try and pinpoint it. This pointed me to this section



Touching the cap to any of these components kills it. So I lifted a leg of each cap and resistor one by one and tacked in a replacement and nothing fixed it....

Any ideas? Perhaps the volume pot itself? I did also swap the ef86 cathode resistor and cap to be safe. Maybe it's something really dumb, but I'm stuck and time is not on my side. Thanks for the help!

[Latole]

Check or replace C4,C5,C6
Or it is a filter caps issue. Are they new ?

[shooter]

0.1uF film cap to ground

did you try this at pin 9 of V1?

[Yeatzee]

0.1uF film cap to ground

did you try this at pin 9 of V1?

Yeah I tried grounding out the grids, no change.

[Yeatzee]

Check or replace C4,C5,C6
Or it is a filter caps issue. Are they new ?

Guess I can lift those and see as well! All electrolytics are new, normal channel has no noise issues.

[Yeatzee]

No real discernable improvement. I guess there's a chance it's cumulative, and that any one component isn't making such a big difference that it's super obvious on its own? I'm at a loss, half ready to just shotgun everything temporarily and see what happens.

[Latole]

Some wire dress may induce some noise.
Can you move the wires wich are close to the tremolo circuit ?

[astronomicum]

I would try lifting R6. That will disconnect the LFO. May help narrow it down. I know you cleaned the pots but there is DC on the Tremolo pot (based on the schematic). I would try putting a jumper across it and see.

[Yeatzee]

I would try lifting R6. That will disconnect the LFO. May help narrow it down. I know you cleaned the pots but there is DC on the Tremolo pot (based on the schematic). I would try putting a jumper across it and see.

R6 330k removed, no change so it's not part of that network. Yeah the Trem pot when dimed increases the gain and ends up with about 50vdc there.

[Yeatzee]

Some wire dress may induce some noise.
Can you move the wires wich are close to the tremolo circuit ?

These are all solid core wire so no real cable management is possible without risking breaking something

[astronomicum]

I would try lifting R6. That will disconnect the LFO. May help narrow it down. I know you cleaned the pots but there is DC on the Tremolo pot (based on the schematic). I would try putting a jumper across it and see.

R6 330k removed, no change so it's not part of that network. Yeah the Trem pot when dimed increases the gain and ends up with about 50vdc there.

With R6 lifted, you now know it is not coming from the LFO. Jumping out the pot will tell you if the pot is creating the noise.

[tubeswell]

These are all solid core wire so no real cable management is possible without risking breaking something

But you may already have a finely fractured solid core wire or part which is causing the problem, so...


I think you could (carefully) chopstick the amp (when turned on) to see if any of the connections/wires/parts around the apparent noise-emitting area result in static/hash/popping when tapped/prodded - but only do this if you're confident working on live gear and check you're well-insulated


Another potential issue you could check is whether there's good tension on the V1 tube socket pin clamps - and sub in another tube there

[Latole]

Some wire dress may induce some noise.
Can you move the wires wich are close to the tremolo circuit ?

These are all solid core wire so no real cable management is possible without risking breaking something

Can you post some good pictures ?

[Yeatzee]

Details in the video (use time stamps it's towards the final quarter).

Long story short I subbed everything out of the amp, swapped tubes disconnected the Trem etc etc. In the end I thought I narrowed it down to the amplitude pot because I had already subbed out the 150k and 0.1uF cap previously but it turned out to be the 150k resistor.... I temp'd in power metal films everywhere when I was testing but I only had two 150k resistors both AB CC's. As fate would have it, one of the two was bad like the original.... What fun!

[astronomicum]

I had already subbed out the 150k and 0.1uF cap previously but it turned out to be the 150k resistor

Glad you found the issue 

[Yeatzee]

To close out the series, hopefully this helps settle the discussion re: AC10 input wiring which is pretty odd. I did a comparison of stock AC10 with its 220k's vs 100k parallel resistors on each 220k (roughly 68k final value) vs removing one of the switch tip to ground connections making one input high and one input "low" (more like normal in this amp):

i=ju9DTqigmOxihyMj

I also compared the wattage with and without the 100R plate resistors these had to see if that's really weighing it down or not.