1965 Vox AC50 Cathode Bypass Resistor Dropping Too Much Voltage

[twilightofthedogs]

Hey all, I've been fiddling with my AC50 which generally is in good working order, but the first stage has always been something a little fishy about the first stage 12au7. I've been trying to sort this and I discovered that on the normal channel I was dropping about 9v at the cathode bypass for the normal channel at pin 3. The schematic is pretty fuzzy, but it sure looks like its supposed to be 2v to me. The other half of V1 seems to be in spec. The voltages at the plates of both half of the tubes seem to be about right, and the plate resistors and cathode bypass resistors and caps all seem to be on spec. Any idea what might be causing this?

[The Dude]

Have you tried a different tube?

[twilightofthedogs]

Thanks Dude. I should have mentioned that I have tried different tubes with no change.

[Enzo]

First off, does the amp work OK and sound good? If so I am not sure we have a problem. If nothing is burning up and it works, let it live.

[Chuck H]

The first stage is using VERY common values for the plate and cathode resistors. And you indicate that those resistors are in spec.?. If you're reading a nine volt drop across the cathode resistor AND you've tried a known good tube in the circuit then I suspect there must be dirt in the test.

[Jon Snell]

If there is 9volts on the cathode there must be 5volts +ve on the grid. This is not correct. If that is the case, replace the grid leak resistor as it has probably gone high value.

[pdf64]

2V across the 4k7 ECC82 bias resistor seems unrealistically low, the drawing has obviously been damaged, it probably originally read 9.2V.
Consider that the 100V dropped across its 47k anode load resistor indicates about 2mA anode (= cathode) current, and 2mA through 4k7 would result in over 9V.
They’re wonderful amps, and isn’t that OT a monster?
Check for coupling caps leaking DC, most significantly those to the power valve grids. eg lift the legs feeding the grids, power up, and if the VDC doesn’t settle down to a very low level, say less than 10mV, then replace them, using 630V new ones.

[Chuck H]

Oh crap. I had a brain fart and was looking at the circuit in the "brilliant" channel. Oops. So, what Pete said^^^^^^^^^^^^^^^^

[twilightofthedogs]

Thanks all. It's quite possible that there is nothing wrong with that particular voltage reading. Due to the fuzzy schemo its quite possible that 9v is correctish, just more than i am typically used to seeing. And i am certainly not up enough on the theory/math to look at it an calculate exactly what it should be, but judging from PDF64s post it looks about right. The amp basically sounds good and has all the typical rehab stuff done to it (filter caps, a few replaced resitors, etc). Too me the the normal channel sounds a little oversaturated/fizzy when its going full blast. Its not entirely unpleasant distortion, but it also doesn't strike me as sounding "correct." Also i get squealing sometimes when the volume is up and treble are turned up all the way.

The amp has gone through a lot over it's life. Someone "converted" it to a hundred watt amp by adding 2 more el34s. The power transformer was also replaced at some point but seems to be putting out the correct voltages. I've spent years fiddling with it and getting it back to original spec. Mercifully they didn't change out the original OT!

[twilightofthedogs]

I thought you all might get a kick out these photos i found floating around online of the amp when i first got it way back in 2012. It is much changed now. I am still as baffled now as I was then.

[Dave H]

It's quite possible that there is nothing wrong with that particular voltage reading. Due to the fuzzy schemo its quite possible that 9v is correctish, just more than i am typically used to seeing.

It's fine as Pete said. It's because it's a 12AU7. You are probably more used to seeing 12AX7 voltages.