Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: spunko on March 06, 2024, 10:44:12 pm

Title: VOX AC30C2 - Top Boost has less output/volume than Normal channel.
Post by: spunko on March 06, 2024, 10:44:12 pm

Hola amigos. I having some trouble with this VOX AC30C2 and I need your valuable help.
Like the tittle says, the top boost channel has less volume than the normal channel. I know is hard to explain, also I don't have a scope to see an image, and the only way to explain would be Top boost at 8, and Normal at 5 to have them sound even.

I already tried some basic stuff and the result was the same:
 - changed tubes.
 - swaped input PCBs - changed the wires from top boost to normal and viceversa. The result was expected, low volume on Normal input.
 - tested without power tubes and PI tube, sent preamp signal to another power amp, same result

I checked for DC voltages and I think some are not present on V1 (pins 3, 7 and 8) compared to V2 and V3.
Here is what I got. What would you suggest to check next?
Thanks.
(https://i.imgur.com/fG5uyIg.png)

https://el34world.com/charts/Schematics/Files/Vox/Vox_ac30c2.pdf

Title: Re: VOX AC30C2 - Top Boost has less output/volume than Normal channel.
Post by: sluckey on March 06, 2024, 10:54:41 pm

I don't trust your voltage reading. Check again.

Title: Re: VOX AC30C2 - Top Boost has less output/volume than Normal channel.
Post by: spunko on March 07, 2024, 03:10:11 am

I have typed everything wrong  :BangHead:  Thanks for pointing it out.  :worthy1:

Rechecked V1/2/3. First reading was taken without power tubes. I have added a seccond reading with all tubes installed.

(https://i.imgur.com/RmXdjKs.png)

Title: Re: VOX AC30C2 - Top Boost has less output/volume than Normal channel.
Post by: sluckey on March 07, 2024, 07:52:51 am

Those voltages look fine. No problems.

Title: Re: VOX AC30C2 - Top Boost has less output/volume than Normal channel.
Post by: spunko on March 07, 2024, 10:34:32 am

Thank you very much. I was looking for voltages on the schematic and couldn’t find them to compare.
Pots were super dirty and scratchy. Cleaned them, resit all conectors and tested again and  now it has life and it sounds loud. Maybe one conector wasn’t pushed all the way. I did the chop stick test from the start but nothing happened.

I don’t know if I’m going crazy but I still fell like it lacks some volume. It sounds loud, but not as loud as the normal channel, like if normal is on 5, top boost is on 4.
What could I check next?

Title: Re: VOX AC30C2 - Top Boost has less output/volume than Normal channel.
Post by: sluckey on March 07, 2024, 10:38:14 am

I think you may be chasing rabbits. Don't follow down the hole!   :icon_biggrin:

Title: Re: VOX AC30C2 - Top Boost has less output/volume than Normal channel.
Post by: pdf64 on March 07, 2024, 06:58:51 pm

The V3 LTP anode voltages seem somewhat different to each other, how about if the valve is swapped out?

Title: Re: VOX AC30C2 - Top Boost has less output/volume than Normal channel.
Post by: AlNewman on March 07, 2024, 09:16:58 pm

Shouldn't there be a differential between pins 8 and 7 of V2?

Title: Re: VOX AC30C2 - Top Boost has less output/volume than Normal channel.
Post by: sluckey on March 07, 2024, 10:15:44 pm

Quote from: AlNewman on March 07, 2024, 09:16:58 pm

Shouldn't there be a differential between pins 8 and 7 of V2?

Yes. Probably about a volt.