vox ac30 tb 1965 problems

[ztigandre]

hello

just bought a ac30 tb from 1965. it plays ok at lower volumes,but when i turn it up more than 25 % it starts to distort heavily.

a friend of mine that has a similar amp says it is way to loud at 25 %..

that volume is normal at 75 % on other amps.

the tubes power tubes are checked and the pre-amp tubes are good.

the amp is original but the caps have been replaced.

it also have new celestion blues so they should be fine..

it doesnt seem to have any mods but i think something has been done
wich causes this problem..

the rectifier tube is also ok..good

it also has a lot of kracling noise when played for more than 25 min.
espescially at higher volumes..

what can the problem be?? bias adjustment??

[Robert M. Martinelli]

Hi,

1965 AC30s like yours are cathode-biased amps, so no bias adjustment is present and/or necessary.

Since you stated that everything ( tubes, caps ) looks good but the speakers have been replaced, I would check the speakers' connection/impedance. The output transformer ( if original ) has two output taps, 8 and 16 Ohms. You could wire two 8 Ohms speakers in series ( 16 Ohms ) and connect them to the 16 Ohm OT tap, or two 16 Ohms speakers in parallel and wire them to the 8 Ohms tap.

An impedance mismatch would reduce output power, increase distortion and, if on the low side, it would tend to overload the OT and output tubes.

Hope this helps

Best regards

Bob

[trevorus]

Hi,

1965 AC30s like yours are cathode-biased amps, so no bias adjustment is present and/or necessary.

Since you stated that everything ( tubes, caps ) looks good but the speakers have been replaced, I would check the speakers' connection/impedance. The output transformer ( if original ) has two output taps, 8 and 16 Ohms. You could wire two 8 Ohms speakers in series ( 16 Ohms ) and connect them to the 16 Ohm OT tap, or two 16 Ohms speakers in parallel and wire them to the 8 Ohms tap.

An impedance mismatch would reduce output power, increase distortion and, if on the low side, it would tend to overload the OT and output tubes.

Hope this helps

Best regards

Bob

If I'm not mistaken here, the lower impedance would not be as detrimental to the tubes as would be a higher impedance. I can't remember for sure though.

[Robert M. Martinelli]

Hi Trev,
well, you know tube amps are quite forgiving, but....yes, you're right, usually an impedance mismatch on the high side is dangerous for the tubes, but on AC30s the +B voltage ( about 320 VDC ) is not enough to damage them if the impedance mismatch is on the high side. OTOH Power loss and bad linearity are the results of a mismatch on either side, and the fact that the speakers have been changed, together with the amp being not as loud as it should be and the "early" distortion made me think about a mismatch....I could be wrong of course, but that's my best guess with the info I had at my disposal...

Regards
Bob