Vox AC30 blowing F2 fuse

[RS_Guitar]

Hi Everyone,
New member, sorry if asking any stupid questions.
I have the following problem with my Vox amp:

Vox AC30/6 TB
1997

Powers up but no sound and no standby light coming on

Blowing fuse F2(500ma) even after changing the GZ34

Then noticed on the board that resistor R73 looked blown also(pictures included).

Looking for any suggestions if I can fix it, not sure if I change the resistor it will be ok to retry or something else has caused all this.
Thank you very much everyone.

[pdf64]

I think those are the Marshall designed / built AC range http://bmamps.com/Schematics/vox/Vox..._power_amp.pdf
R73 needs to be replaced. Use a metal film or oxide type with a flame retardant coating. Mount if off the board a bit, as per R71. Any charred PCB around it needs to be ground out, as it may be conductive.
F2 is downstream of the GZ34, so a bad one won't blow it.
The lack of anything to protect the PT from a shorted rectifier seems to be rather an oversight, especially given the horrid standby implementation (don't use it).

When R73 is sorted out, power it up with no rectifier or power tubes.
If ok, fit the rectifier and try again.
If ok, fit the power tubes. Assume that the EL84 fed by R73 is bad.
It's probably time for a new set of them.

[RS_Guitar]

Thank you very much pdf64, I really appreciate your help and quick response.
I will buy some new power tubes this weekend and resistor and go for it, it's definitely worth a try for me.
I'll let you know how I make out.

[RS_Guitar]

Thank you very much for the reply and information pdf64.
I was able to fix it over the weekend. I changed the resistor with 100R 1/2w(the rest were good according to meter) and all the tubes with new ones.

[nevetslab]

Your 5AR4 Rectifier tube is still wired incorrectly. Vox may never have corrected that, and yours is still taking the output current thru the heater of the tube at Pin 2. Need to move that Red lead over to Pin 8.

This isn't the source of your problem here, but it does need correcting.

Also, one of the changes I made on all the AC30/6-TB amps in our rental inventory was to remove the plate wires of the rectifier tube from the standby switch, re-connecting them to directly to the Pwr Xfmr secondary. Instead, switch the cathode circuit of the output stage to ground for the Mute function of Standby. The DC Power lamp will still remain lit, but you won't be killing rectifier tubes as the stock circuit has a nasty habit of doing.

[pdf64]

Your 5AR4 Rectifier tube is still wired incorrectly. Vox may never have corrected that, and yours is still taking the output current thru the heater of the tube at Pin 2. Need to move that Red lead over to Pin 8..

Are you sure that the Marshall built era had this problem? I thought it came in with the next era.

EDIT - Oh dear, apologies, the inside chassis photo here seems to confirm that the Marshall built ones had that error. http://www.voxshowroom.com/uk/amp/30tbx_hood.html
Definitely worth moving the dc output over to terminal #8.

Also, one of the changes I made on all the AC30/6-TB amps in our rental inventory was to remove the plate wires of the rectifier tube from the standby switch, re-connecting them to directly to the Pwr Xfmr secondary. Instead, switch the cathode circuit of the output stage to ground for the Mute function of Standby. The DC Power lamp will still remain lit, but you won't be killing rectifier tubes as the stock circuit has a nasty habit of doing.

I think I know what you mean, that's the way to do it.
But a face value, it would leave the power tubes unbiased!
Rather the switch should interrupt the ground return of the power tube cathode bias circuit.
In the meantime, or instead of that mod, just don't use standby.

[nevetslab]

That's what I meant to say. I am lifting the ground connection to the the cathode network, so there's no plate current flowing in any of the output tubes. Too many users of the amp (rentals) to know NOT to use the S/B switch, so this has worked out great. No pops or issues switching in and out of S/B. MUTE is with Ground LIFTED off the cathode network.

[RS_Guitar]

"Your 5AR4 Rectifier tube is still wired incorrectly. Vox may never have corrected that, and yours is still taking the output current thru the heater of the tube at Pin 2. Need to move that Red lead over to Pin 8."

Did not know about this, thank you very much.