Twin Reverb repair

[anteriorl5]

Have a 1965 Twin on the bench. It sounded good until heating up, then started to fail. Replaced all the screen grid resistors and found one that fell apart when removing it. Thing is, I checked preamp voltages before the work on the power tube sockets ad everything was close to spec. Now I get 450 volts on the plates of the phase inverter 12AT7. Since the 100K and 82K resistors test good, I'm wondering how this can be. Did I short the tube checking voltages and it is not drawing current? The tube lights up and I do get a very distorted tone testing the inputs with a guitar.

[shooter]

here's a good start
http://el34world.com/Forum/index.php?topic=23286.0
without hunting I'm not sure, this an AB763 circuit or one earlier ?

did you make sure there was bias volts before heating up the tubes?
450v indicates it's not conducting

[anteriorl5]

AB763 circuit, mostly original. I will check bias voltage later today.

[Latole]

Best is to check bias current ( at cathode ) not voltage.
If no current check bias voltage and tubes,

[2deaf]

Turn the amp off.  Measure resistance from V6 (PI) pins 3/8 to ground.

[anteriorl5]

Amp off, resistance from pin 3-8 to ground is 656K.

[sluckey]

Amp off, resistance from pin 3-8 to ground is 656K.

That's the problem. Should be roughly 22.5K. Only 3 resistors in series to ground, 470Ω, 22K, and 100Ω. So, you have a bad resistor or a bad connection.

Is this a Reissue amp with printed circuit board?

[Latole]

Amp off, resistance from pin 3-8 to ground is 656K.

That's the problem. Should be roughly 22.5K. Only 3 resistors in series to ground, 470Ω, 22K, and 100Ω. So, you have a bad resistor or a bad connection.

Is this a Reissue amp with printed circuit board?

You are right.  Higher resistors= less tube conduction = higher plate voltage

[anteriorl5]

No circuit board, thankfully. Original blackface Twin from 1965.

[anteriorl5]

Going to order parts. I'm seeing lots of questionable solder joints, all the 100K resistors have drifted higher, a few bad ceramic caps, etc. This will take a little time.
Thanks for the help!

[sluckey]

This should help. I don't suspect the 3 resistors as much as I suspect solder joints, especially the joint to the brass plate ground.

[dude]

Possible the tube if solder joints and resistors check out ok, lose pin connections...?

[Latole]

Possible the tube if solder joints and resistors check out ok, lose pin connections...?

No IMO. Resistors reading is 25 times higher as they should be.

[2deaf]

I'm not betting on the ground connection nor the 100 Ohm resistor because the 820 Ohm NFB resistor offers an alternate path to ground.

Once all of the bets are placed, I suppose the OP could measure the resistance to ground from each side of the 470r, 22K, and 100r resistors.

[shooter]

fwiw;
the 65 SR I fixed had the 100 ohm loose original solder.  3 other original joints in TS and working well.
I did have to "probe", visually near impossible to detect

[mresistor]

I've recently worked on two Silverface SR's, a '72 and '73, and each one had a ground wire on the brass plate pop up (come unsoldered) .

[anteriorl5]

Got around to working on the TR finally...turns out there was a 220K resistor where there should have been a 22K. Easy switchout. Voltages are good now and the amp works. Time for a basic service, cleaning sockets and pots, then a few ceramic caps need replaced.