Vox AC30 1964 TREBLE - LOW OUTPUT

[GREG HODY]

we have a Vox AC30 circa 1964 with low output.
It used to scream but suddenly lost about half its volume.
The voltages to the the preamp and output stage are within 10% of what the schematic says they should be. We have 4 new output tubes & sockets and everything has been verified wired correct. The speakers (blue ones) are wired in series for 16 ohms and are on the right tap of the output transformer.
We noticed that the dc resistance between the center tap of the output transformer going to the +320 volts and either side going to the output tubes are not equal. One side measued 75 ohms and the other side 105 ohms with respect to the center tap. Makes me wonder if the transformer has a problem. The unit had a treble option added recently but was working fine until being played hard one night where the volume went up and down several times and is now half the volume it once was. My buddy likes to jam with the volume cranked up full so I'm thinking that stressed out something in the amp. Anybody have an AC30 working correctly and can check the resistance on the primary side of the output transformer? Any ideas?

[Jazz P Bass]

Ac 30 opt

From my experience the two windings "should" be close to each other resistance wise.
If you have an low level AC source you could try coming in from the secondary side & reading each primary winding.
I've used my signal generator already.
Again, they should be the same.
My gut reaction is your tranny is toast.

[pdf64]

Just happened to have my 63 AC30 on the bench at the moment. The woden OT taps measure 83 and 103 to the CT. So it's not certain, best to use the test described in the above post.
Have you check all the screen grid resistors and that all the el84s are operational, loose socket contacts, or conductive tracking on the tube sockets?
A new set of el84s should be tried before condeming the OT. Peter.

[GREG HODY]

reply to tests-AC 30

We did what Jazz P Bass suggested with the generator. The waveform was driven by a signal generator and small (10W) power amp into the 16 ohm winding. The input was 20 volts pk-pk and the output on the tube side was 80 volts pk-pk on the oscilloscope. Many thanks to Jazz & pdf64 for your suggestions and help. The voltage output on the tube side referenced to center tap should have been a lot higher than 80 volts because the turns ratio for an output transformer is more than 10 to 1 with respect to the speaker side. The next step would be to borrow a Z meter and check the primary. Our service manual says 4000 ohms anode to anode which is 2000 ohms from the center tap to each set of tubes. A Z meter measures the impedance with 1 khz ac signal that gives a reading of combined resistance and inductive reactance. The jury is still out on this one. Anybody have any further comment? It would be easy if we knew for sure if the transformer is dead. It plays, but at half-volume compared to before it went. Anybody have any other suggestions of where to look? For those who noticed the electrolytic capacitor across the 47 ohm cathode resistor on the output tubes this is cathode bypass and the symptom of a bad cap is reduced volume. We have a new cap in this unit recently serviced.

[Jazz P Bass]

OPT turns ratio

I just tested working a EL84 push pull output transformer from a Mesa.
I am getting a 10 - 0.55 turns ratio. That's primary to secondary.
The DC resistance is 130 ohms , outside taps to center.
Hope this helps.

[Mars]

Have you tried swopping pre-amp tubes around? If one half of your phase splitter is dead, you would get half power for example. All the pre-amp tubes are identical 12ax7 types.

I would also double check the coupling caps going from the phase splitter to the input grids of the el84's. If they go dodgy, the volume suffers. I usually change them to new ones of same value when I service an amp. The old WIMA ones develops cracks and moisture affects the electrical qualities negatively after 40 years of service.