Vox AC30 / 6 TB spike noise problem.
« on: June 15, 2018, 02:34:38 PM »
Hi everyone,

I am currently servicing an AC30 / 6TB wich is kind of impertinent.

Long story made short, it's a double side PCB and it had an electric arc between +HT and a tube heaters power raíl through a capacitor leg  (C46) . It blew the choke and the fuse.

After it, the amp has been recapped and a new 30H 100mA choke has been fitted. Also, I have mount this part of the circuit out of the PCB (P2P) and added another fuse to protect the choke.
Also, I have added a Pot to balance the power tubes. Without the inverting tube (V6) you can tune the power tubes to be dead quiet using "unmatched tubes".

With inverting tube fitted it has an audible "spike noise" wich seems to be generated in the "Normal Channel" between C11, R58 and the tube. The spikes are not before R58 , and no spikes on either side of R57 (Brillant Channel) . With the tube pulled out and R58 lifted, spikes are also there. Even with the tube socket pulled out spikes are there...

The noise varíes with the Normal potentiometer. The noise is there with the potentiometer at Min and at Max, but dissappears at mid position, it seems to cancell out.

When in Stand-By mode, the AC line of the tube heaters is a nice and beautiful sine wave, but when is turned ON, the wave gets squeezed.

Has anyone experienced symptoms of conductive PCB's..??


AC30 Top Boost schematics :
Pre :
Power amp & PSU :
« Last Edit: June 15, 2018, 03:51:12 PM by NadMk »


Re: Vox AC30 / 6 TB spike noise problem.
« Reply #1 on: June 15, 2018, 02:35:43 PM »
Filament heaters in StanBy


Re: Vox AC30 / 6 TB spike noise problem.
« Reply #2 on: June 15, 2018, 02:36:45 PM »
AC line when running...

Re: Vox AC30 / 6 TB spike noise problem.
« Reply #3 on: February 12, 2019, 04:48:58 AM »
Hello NadMk,

I just found your post and was wondering if you found a solution in the meantime? I seem to have the same kind of problem with my old Vox, not so much the heater filaments but the noise in the normal channel in front of the phase inverter.



Re: Vox AC30 / 6 TB spike noise problem.
« Reply #4 on: February 12, 2019, 08:48:44 AM »
The only experience I have with conductive PCBs so to speak is the old black fiberboards in old amps. When they get a little moisture in them, you can put your meter with one probe to ground and another touching the board itself and see 200+ volts standing on the board. In the amp, it manifest as parasitic oscillations, pops, crackles and all around tom foolery.  In my cases, replacing the board with a new garolite turret board fixed the issues but I only carried over know working signal path caps. I replace all resistors with new resistors of the same type that were in the amp (IE if it was a metal film, I used metal film, If it was a carbon comp, I used carbon comp). I couldnt tell the difference in tone when I was done, but the amp was clean.

To marquis5000, First thing Id try is a tube tap to see if any of them are microphonic. If that yields nothing, start pulling tubes from V1 and stop when the noise disappears or before the power tubes. DO NOT PULL JUST ONE OF THE POWER TUBES.  Concentrate on the area around the tube that made the noise go away. Try chopsticking the amp if none of the above yield any results. Concentrate on the area that made the most noise. You'll probably find a Plate resistor or coupling cap that's the issue. I consider those higher wear parts in tube amps.


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