1948 Fender Champion 800 no output

[davohilts]

Hello,
A friend gave me an old Fender Champion 800 he has had for some time. He said that it had very low output and suspected the tubes. I decided to fully service the amp and pulled the chassis out along with the speaker baffle. The amp has the output transformer mounted to the speaker with wires connecting the OT to the chassis and the OT is connected to the speaker. (See pics) The amp was serviced last in the '60's and two 20mfd filter caps were installed at that time, everything else looks original.
This is such a simple amp and it is driving me nuts because I can't figure out what the problem is. Here is my mystery:
I've installed a virtual center tap and removed the grounds on the heaters and connected the virtual center tap to the cathode resistor off the 6V6. I replaced the 2-20mfd filter caps with 3-10mfd caps and wired up the amp like the Champ 5C1 schematic.
I replaced the 25v25mfd cathode bypass cap with a new one, I replaced the way out of tolerance coupling caps on the 6SJ7 tube with new ones. I removed the .05 mfd cap going to ground off the fuse assuming this is not necessary with the new grounded cord I installed.
Now when I power it up I get unusual voltages. The amp's cord was dangerous so I didn't test voltages prior to servicing. I get 460V off the 5Y3 and 426V on Pin 4 of the 6V6, 460V on the plate (p3) and 74V across the cathode (pin 8). On the 6SJ7 tube I get 192V on the plate (p8), 27V on Pin 6, -.75V on pin 4.
Tracing the signal I can hear an increase from the input to pin 8 of the 6SJ7 and can hear the signal increase with the volume control on Pin 4. When I move to pin 3 of the 6V6 I get no signal and also no output thru the speaker.
Does anyone know what the heck is going on? I'm stumped.
Thanks,

Dave

[Enzo]

Dave, seriously, resist the urge to modify the amp before you fix it. I know it is tempting to upgrade everything "as long as I am already in there". but what it really does is add multiple more opportunities to cloud the issues or even cause new trouble. The heater wiring for example will have no bearing on super low output.

Have you checked the cathode resistor of the 6V6? 74v on that cathode ought to shut the tube off pretty well. That is 74v of bias. But 74v across 500 ohms gives me 148ma tube current. And it would be dissipating 11 watts. That will be one VERY hot resistor. But since your plate voltages are so high, I suspect the tube is drawing little current. So explore that.

The picture is apparently original, not current, so have you now replaced EVERY cap on it, other than the death cap? What I see in the photo all pretty much need replacement.

The transformer is probably fine, but easy to check. Take out the tube, and disconnect one speaker lead, so the transformer has no load on either winding. Then go over to RG's Geofex web site and look up him transformer tester, a very simple device. and apply it to the transformer primary leads.

[davohilts]

Dave, seriously, resist the urge to modify the amp before you fix it. I know it is tempting to upgrade everything "as long as I am already in there". but what it really does is add multiple more opportunities to cloud the issues or even cause new trouble. The heater wiring for example will have no bearing on super low output.

Have you checked the cathode resistor of the 6V6? 74v on that cathode ought to shut the tube off pretty well. That is 74v of bias. But 74v across 500 ohms gives me 148ma tube current. And it would be dissipating 11 watts. That will be one VERY hot resistor. But since your plate voltages are so high, I suspect the tube is drawing little current. So explore that.

The picture is apparently original, not current, so have you now replaced EVERY cap on it, other than the death cap? What I see in the photo all pretty much need replacement.

The transformer is probably fine, but easy to check. Take out the tube, and disconnect one speaker lead, so the transformer has no load on either winding. Then go over to RG's Geofex web site and look up him transformer tester, a very simple device. and apply it to the transformer primary leads.

Thanks for the help Enzo! I know I broke a major rule. I thought wow, this is such a simple circuit and all of the caps are gonners why not just totally rebuild it. Well I got my answer. Crap! I'll take your advice and test the OT. I'm getting 336VAC on each leg of the 5Y3 off the Power Transformer, does that seem right? Anyway, thanks for the help. I've got my work cut out for me.
Dave

[g1]

Measure the resistance from 6V6 socket pin8 to ground. Should be around 500 ohms. There must be a bad connection somewhere between those 2 points, or the resistor is bad. As Enzo said, there's no way that resistor can really be dissipating 11W without frying. So something is wrong in that part of the circuit.

[davohilts]

Thanks g1!
Another piece of the puzzle. I will check it out! That makes a lot of sense!
Dave

[davohilts]

Thanks Enzo and g1 for the advice. I tested the resistance from P6 of the 6V6 to ground and it was infinite. By disconnecting the heater virtual center tap from the 500r cathode resistor and ground them both I get 530 ohms from P3 to ground and magically I have a working amp that is nice and quiet.
I get 293 ohms from the center tap of OT to P3 with 8.9V across it and a plate voltage of 333V. So PC=V/R or 8.9/293=.03mA x 333PV = 10 watts PD.
All seems well.
thanks guys!