Weird OT problem

[tubedogsmith]

Last nite I replaced a blown output trannie in an older vox AC30 with a Dyna clone A470S from Triode. when I fired it up I got a super distorted low volume tone. the amp had a 4/8/16 ohm switch in it and as you turned down from 16 to 8 it got way quieter and there was no sound at all at 4 ohms. The amp voltages were about 20vdc too low as well. After double checking cords, speakers, switches, etc... I tacked in another OT of the same model and it worked fine, voltages were up where they should be, all is good, so I pulled the first one and installed the second. It worked great, correct voltages everywhere when I fired it up and tested it at different volumes and impedence settings for about a half hour. I closed shop and went to bed. This afternoon when I went in to button it all up the amp is now doing the same thing again it was with the first replacement. Low volume, voltages down about 20vdc,very distorted, and the 16 ohm setting is loudest, 8ohm half that and 4 ohm nothing.
I've not seen anything like this before, any ideas??

[Steve Conner]

I'm going to guess the problem is completely unrelated to the OT. It sounds like your power tube bias is off and they are drawing way too much current. Maybe your cathode bypass capacitor is shorted, or maybe you accidentally shorted a cathode pin to ground when installing the new OT.

Look to see if the power tubes are red plating when the problem happens. Also check the power tube cathode voltage with respect to ground.

[tubedogsmith]

Steve, thanks for the reply. No redplating and 10.5v on the output tubes cathodes which seems a little low. I'm guessing it's not an OT problem either now.

[Mr. Droopy-Drawers]

This is where a good bench loaded with test equipment comes in handy. Trace a signal up to the power tubes, and if all seems well, the problem is in your output stage. You need about 22VAC grid-to-grid to get 17-watts output, if I remember my numbers rightly. Do you have enough driving voltage? Then the problem is in the output stage. Remove the output tubes and test the OPT for impedance by applying exactly 1VAC on the secondary of say the 8-ohm load setting. Then we'll say you get 25VAC measured Plate-to-Plate on the primary. Well, that's equal to about a 5K primary impedance, and the OPT should be good. I have had AC30 OPT's go short, so be aware. The Dyna clone ought to better, but just be aware.
Anyway, check some numbers, and they'll point you in the right direction. You just have to know what the numbers are trying to tell you. Measure anything you can think of, and you'll get to the bottom of this.

[Alex R]

...but first check the value of and connections to the cathode resistor and its bypass cap - all the connections between the cathodes and ground. Those resistors heat up in AC30s and go off value/melt their joints, sometimes, and the caps are often wrong too, in intermittent ways.

It's my view that the incendiary AC30 is anyway better biased a little cooler, ie with a bigger resistor.