Univox U-45B Low, Distorted Output

[telecaster]

The amp has low, distorted output, and the B+ seems low. I changed the filter caps (C4, C10, C11) to 10uF @ 450V, and R15 to a 10K 2W. I also changed the first gain stage by deleting the 4.7M R4 and adding a 1.5K resistor and 22uF bypass cap in the cathode circuit. With a scope and signal generator I get around a 10x gain through the first stage (seems ok). Back to the low B+ voltage - if I read my tube manual right, a 6X4 with 500V AC in at 50mA should give ~250V out to the filter. With all the tubes plugged, the voltage at C11 is 188 volts. Pulling either one of the 6BM8's raises the voltage to 202V, pulling both raises it to 312V. With all tubes plugged in, the current through R15 (10K now) is .5 mA (C10=67V, other end of R15 is 62V). That means 8mA through R16, so the screen grids of the 6BM8's are drawing 7.5 mA. With all that as a backdrop, are the 6BM8's bad? I never measured screen current in a pentode power amp, not sure what's too much. Appreciate any insight.

[soundguruman]

Half the rectifier isn't working?
Half the PT is open?

[woodyc]

Your B+ should be around 250V with about 50ma total to the plates and 5ma total to the screens. I don't know what the problem is, but I'd be tempted to check the cathode resistor and the voltage at the tube grids -- may be bad coupling caps or a shorted bypass cap.

[telecaster]

Attached schematic and additional info

soundguruman - the Power transformer is giving just over 500V AC on the secondary, so it looks good. I didn't think about the rectifier tube being bad, thanks. I'll have to order one to test since it's an odd tube (to me anyway).

woodyc - the output tubes common cathode resistor (R14) measures correctly at ~250 ohms. There is 16 volts DC across it, so the cathode current(s) equal 64mA. There isn't any DC leakage on the 6BM8 control grids, they are at a few millivolts DC, I can recheck the exact values.

[Dave Curtis, dB AudioTech]

Change the cathode cap on the 6BM8s. I ended up changing all the caps in mine, plus quite a few resistors, as well. Works great now. Those old Japanese oil caps don't fare so well after 45 years or so...

[telecaster]

Fixed! The Voltage issue turned out to be a bad connection at the rectifier tube from the PT. I couldn't find a 6X4, so I was about to wire in two diodes when I discovered the one wire was loose. Strangely, when probing good contact was made. After the voltages were returned to normal, all the coupling caps were leaky, I went ahead and replaced them all as advised. I also followed the advice and did all cathode bypass caps and the 6BM8 bias resistor (went with 470 ohms) plus my prior changes to the series caps in the power supply and the input stage mod. The amp sounds good now, nice breakup when pushed, and a decent tremelo. My thanks to all who responded - you all were helpful!

[soundguruman]

Fixed! The Voltage issue turned out to be a bad connection at the rectifier tube from the PT. I couldn't find a 6X4, so I was about to wire in two diodes when I discovered the one wire was loose. Strangely, when probing good contact was made. After the voltages were returned to normal, all the coupling caps were leaky, I went ahead and replaced them all as advised. I also followed the advice and did all cathode bypass caps and the 6BM8 bias resistor (went with 470 ohms) plus my prior changes to the series caps in the power supply and the input stage mod. The amp sounds good now, nice breakup when pushed, and a decent tremelo. My thanks to all who responded - you all were helpful!

IT'S ALWAYS GREAT TO TAKE SOMETHING COOL, AND MAKE IT WORK AGAIN.
And as I remember, that Univox has a nice grind to it. Yeahhhhhhhh, Baby!