Seeking repair thoughts vintage VOX AC-100 guitar amp...

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ron_swanson

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I have a vintage MkII VOX AC-100 amp ( this is the version of the amp I have... The VOX Showroom - Vox AC-100 MkII Fixed Biased Amp Head - A Look Under the Hood ) on the bench that's exceeded my troubleshooting experience. Hoping someone here with more guitar tube amp repair knowledge might point me in a proper direction.

Initially, the amp was clean and strong until the volume hits about 1/3 - 1/2 after which it goes into an ugly thumping / distorted sound mostly effecting the lower ( guitar ) frequencies. Not quite motor boating sound, but something in that sound range.

After replacing all of the electrolytic caps ( power & preamp which were likely original from the 1960's as they were all UK made ) with new caps, unfortunately, the same issue persists.

Notes...

Someone in the past has re-designed the bias section that incudes a trimmer that allows for the -35V negative bias to be adjusted which the original OEM design did not. Not sure why that was necessary, but it seems to be doing the job.

After measuring numerous test points, all of the noted schematic voltages are within spec with the exception of the voltages around V3 ( phase inverter ) which is WAY off.

Schematic...

ac-100_help.png

Test points ( TP ) are noted in yellow. General area in question within a red dotted rectangle. RED test points are outside of expected voltages by over 100V. The 'M' in each test point box is the actual measured voltages. The voltage below is the schematic voltages. Hope that makes sense...

Most of the resistors in the red dotted rectangle section in question were either resistor values ( by code bands ) not indicated on the schematic ( more redesigning? ) or above value specification when measured physically. I replaced all of these since they all likely original components from the 1960's to match the schematic with modern carbon film 5% resistors.

After the replacement and triple checking my work, the initial thumping / distorted sound remained unchanged and the voltages at TP10A / TP10B only slightly improved - both test points now measure 235V respectively and TP11 remains unchanged. The measured voltages above where those BEFORE replacing the resistors in this area.

Video of scope going from less than 1/2 volume to 1/2 the back down again.

View attachment IMG_2616.MOV

So, with the above in mind, I'm stumped of where to go next. Any pointers will be greatly appreciated.

Thanks in advance.
 

Tubetec

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Sounds like maybe you have a bad coupling cap somewhere , check the grids for even a slightly positive voltage , if you find even a few hundred mV dc ,disconect the coupling cap from the following grid and check again with the meter . Ive also found cathode followers in pre driver stages that appeared to measure ok DC voltage wise but the cathode resistor had gone very high and it sounded like crap
 

Bo Deadly

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It seems to me C11 and / or C12 must be leaking current because the sum of the current through the PI anode resistors does not match the current through the cathode resistor.
 

NOON

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It's not just electrolytic caps that go leaky with time, a lot of other caps used for coupling will also start turning into resistors. Also check the value of resistors, some of them can drift quite far from spec, especially if subjected to moisture or excessive heat.
 

ron_swanson

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Update...

Following the advice above as best I could, at this point all of the capacitors have been replaced with new ( electrolytic and others ) as well as about 1/2 of the pre-amp tag board resistors. I found one grid follower resistor (R10) that was coded as an incorrect value ( Meaning, not matching the schematic value. Supposed to be 56k, but was an way high out of spec 68k ) and another (R12) that was just outside of spec. Replaced both.

Fired up the amp again and while it's a little better overall sound-wise, it's still punky when the volume is above 1/3 to 1/2 and above. Measurements remain mostly unchanged. Test Point 10A/B still reading 140Vdc below expected readings and test point 11 reading ~3x what it should be.

See updated attachment.

ac-100_help_2.png

I added the coupling capacitor mV reading ( in blue) to the schematic.

@Tubetec.... I'm not sure I understand your instructions here...

>>> if you find even a few hundred mV dc ,disconect the coupling cap from the following grid and check again with the meter

So, with all of the above in mind, still scratching my head...

Thank in advance!

Cheers, Greg
 

rock soderstrom

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You have surely already exchanged a few tubes (V3 first) as a test?

Edit: clean the V3 socket with contact cleaner, resolder the socket with an installed dummy tube, rebend the contacts for a snug fit or change the socket for a new one.

I found one grid follower resistor (R10) that was coded as an incorrect value ( Meaning, not matching the schematic value. Supposed to be 56k, but was an way high out of spec 68k )
R10 is the cathode resistor of the cathode follower (V2b) that drives the tone network. Just for your information, this has nothing to do with your problem.
 
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ron_swanson

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rock soderstrom

I have swapped V1 with V3 - same difference. I don't regularly keep an inventory of tubes on hand and because of the current tube price increases in general I'm trying to avoid having to purchase any if that can be avoided. But, may have no choice. TBD.

>>> Edit: clean the V3 socket with contact cleaner, resolder the socket with an installed dummy tube, rebend the contacts for a snug fit or change the socket for a new one.

Ok. Will do. Can I ask why soldering with a dummy tube installed is suggested? I never heard of something like this.

I understand R10 isn't directly part of my problem, but as noted above; the code on that resistor was incorrect compared to the schematic and almost 2x the indicated value.

Thanks for your input! Much appreciated.
 

mad.ax

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Swapping V1 & V3 is not a conclusive test. ECC82 is a dual triode. In V1 position only the first triode (Va) is used. Therefore the second triode could be dead or worn out, and you wouldn't notice... For V3 you need two good triodes, ideally you even need them to be matched for the phase inverter to be working well.

Your issue is definitely in the phase inverter. Since all the components around are good, that leaves the tube. (Can't hurt to check/resolder the socket, but your measurements shows taht both halves are drawing current, hence the contacts must be ok.)

Test advised by Tubetec. DC voltmeter on pin 2, then pin 7. Have you disconnected C8 and re-measured those 72mV? What about the readings on C13?

As for why the dummy tube, that's because on most sockets, the pins can move a little, so if you insert a tube before soldering, then you can be sure that every pins will stand in the right position.
 
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ron_swanson

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mad.ax

Ok, I'll jump for a couple of ECC82's...

>>> Have you disconnected C8 and re-measured those 72mV?

No. I didn't understand that step. So I'm clear, I unsolder C8 from one side or the other and then remeasure the previously measured 72mV point again?

>>> What about the readings on C13?

Argh! Missed that one...

Thanks for the 'dummy tube' info. Makes sense.

Cheers!
 

rock soderstrom

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It is always better to test with known 'good' tubes.

All contacts in the socket must be clean so that the tube has good electrical connectivity. Dirt, dust and oxidation are your enemies, which can lead to high contact resistance.

The opposite is also possible if over the years the amp has been serviced with some strange "contact cleaner". This plus dust and dirt can lead to connectivity in places you don't want it. Therefore, clean everything with alcohol first.

I had some problems with sockets in vintage tube amps, sometimes only a new socket helps.

I agree that the phaseinverter is the problem, which could also be from increasing turret board conductivity. Some old Fender amps have this problem of the eyelet board becoming electrically conductive due to moisture. No idea if the old Vox amps tend to do this as well. I hope not...
 
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ron_swanson

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rock soderstrom

Hoping a socket replacement is not required as the OEM installed sockets are riveted to the chassis rather than screwed in place. Not a total dealbreaker if need be, but prefer not to be drilling parts out.

I'll work on cleaning things up while I wait for the 12AU7's I just ordered to arrive.

Thanks!
 

Telegraph-Hill-2021

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rock soderstrom

Hoping a socket replacement is not required as the OEM installed sockets are riveted to the chassis rather than screwed in place. Not a total dealbreaker if need be, but prefer not to be drilling parts out.

I'll work on cleaning things up while I wait for the 12AU7's I just ordered to arrive.

Thanks!
Had alot of Voxes on my desk. Mostly 50&30. You do not need to change the sockets. You can remove the single pins. Most of the time one fork was missing or broken giving some headache but don't think that is the issue here. Measure the DC voltage on C11 and C12 on the EL34 side. Check all the 1 Meg Ohm Resistors. Sometimes you need to solder them out at one side to get good readings. Good luck.

Cheers,
Oli
 

abbey road d enfer

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After measuring numerous test points, all of the noted schematic voltages are within spec with the exception of the voltages around V3 ( phase inverter ) which is WAY off.
This schematic of the AC1 shows that yours is not too way off.
It's hard to tell what's causing the issue you have, but I would start checking if it happens with the input tube disconnected.
Do the treble and bass controls affect the problem?
 
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rock soderstrom

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This schematic of the AC1 shows that yours is not too way off.
It's hard to tell what's causing the issue you have, but I would start checking if it happens with the input tube disconnected.
Do the treble and bass controls affect the problem?
Uhh, same circuit with different voltages. Which one is the right one?
 

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ron_swanson

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Uhh, same circuit with different voltages. Which one is the right one?

rock soderstrom

Argh.... I have 3 AC-100 schematic versions in hand that I choose from, now with this one 4.

I choose the one that most closely matched the amp I have in hand, but this new schematic after a quick look seems to suggest that my voltages are correct overall, and in the PI section? What made this more difficult, is this amp has had 'redesigns' in the past. As mentioned above, for example, the bias section is completely reworked. It works as required and is adjustable so a perfect adjustable -35v is possible. I left that section alone. Other components were not close to any of the schematics I have including this new one. So, none of the schematics I have in hand matched exactly to where I found this amp. This new schematic though, might be the one I really needed / wanted. Sigh... I'll need to do a close look-see. Thanks!

So, assuming based on this new schematic, my 235V PI ( V3 - Pins 1 and 6 ) voltages are correct, still something with the tube, tube socket? If so, this makes the issue at hand even more perplexing in my mind. All voltages correct, but nasty noise. I'm still planning on giving all sockets a good, proper cleaning, re- solder of V3 at a minimum, and re-tensioning all as well as swapping out the preamp tubes with new tubes at least for testing purposes. Also checking the coupling cap voltages after being disconnected as suggested earlier in this thread.

Question...., when I was desoldering to measure and / or replacing the old resistors, many of them would bubble up with some sort of liquid. Would this suggest that this amp has been subjected to moisture in the past? If so, should I consider proactively replacing all of the carbon comp resistors? The remaining carbon comps at the moment measure in tolerance. Not a fan of shot gunning repairs, but maybe in this case justified?

abbey road d enfer

>>> Could be the one in post #1 has been done with a ECC83 in the PI, and the last one with an ECC82

Are you suggesting that my amp physically may have a ECC83 in V3 or that the schematic in post #1 was created incorrectly with ECC83 measurements? I have checked my amp's physical tube complement. V1 & V3 = ECC82. V2 = ECC83. All appear to be original Mullards or at least vintage tubes of some past vintage.

Cheers!
 
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abbey road d enfer

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Are you suggesting that my amp physically may have a ECC83 in V3 or that the schematic in post #1 was created incorrectly with ECC83 measurements? I have checked my amp's physical tube complement. V1 & V3 = ECC82. V2 = ECC83.
The voltages you report are consistent with an ECC82 in the PI.
BTW, setting bias with a voltage is some kind of inadequate.
It's really the current that should be monitored. It is quite easy on this amp because of the 1r resistors in series with each plate.
With the values you published, each EL34 dissipates 22W at idle, which is dangerously close to the 25W maximum.
 
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