VOX AC100CPH Losing Volume Help Please?

[ncusack]

Hello Everyone,

I have an AC100CPH on the bench that is losing volume after about 25 minutes of being played. This screams cold solder joints to me and after cracking the head open to take a look it's easy to see the solder work in the amp is less than stellar. What I would like to know is if anyone here has experience with a similar issue that could point to a good starting point with this repair.

Cheers,

Neill

[stratele52]

I have no experience on this amp but lot of tubes amps.

Loosing volume ; do you check with DC meter the B+ path, where B+ is low ot no B+.

In my pinion you loose some or all B+ . If you can find after 25 minute where B+ is   and where you loose it . There should be a bad resistor or solder or connector between good and bad B+.

[ncusack]

Ok so far I have power the amp up and had the HT fuse blow. I replaced the fuse and removed the power tubes and was able to get appropriate voltage measurements to the preamp tubes and what would be the power tube plates. I installed the power tubes again and left standby on this time and found that one power tubes filament wont heat up. I check the tube sockets and all of them work as they should so it has to be a bad tube. I don't have a quad set of EL34's on hand to check but my assumption right now is that this tube has been whats been causing the issues. I would just like to know if there is anything else I should check before trying a new set of power tubes?

Cheers

Neill

[stratele52]

Be sure you have heater voltage at this tube , 6.3 volts AC . If yes and tube not glow , try good one to see if glow. If no, you have to fix that before.
If good tube glow , the other one are bad.

Look socket's contact if it is  loose or rusty or bad solder there. 

You can use your amp with only 2 tubes on

[archaos]

Hello Everyone,

I have an AC100CPH on the bench that is losing volume after about 25 minutes of being played. This screams cold solder joints to me and after cracking the head open to take a look it's easy to see the solder work in the amp is less than stellar. What I would like to know is if anyone here has experience with a similar issue that could point to a good starting point with this repair.

Cheers,

Neill

Do you mean you completely lost volume or you got a volume drop ?
I had that kind of issue (volume drop) with 2 different TB AC30s. Each time it came from the slope resistor (100K) tied to the tone stack (cathode follower, 2nd preamp tube).
I went for metal film or metal oxyde resistor instead of the stock CC or carbon film resistor, & was done.