DR bad PT?

[Raybob]

Have in a '80 Silver Face DR that was partially blackfaced by someone, and has major distortion at all volume levels. Tried different choke and different OT, no help. Replaced electrolytic filters and bias caps, and replaced many resistors beyond 10%, some still up to 6~7% off. Both R100 for 5V CT were toasted and replaced. Voltage is 370-0-370 on PT, not 330-0-330 as spec. I'm wondering if some of the coils can be shorted inside PT. With signal in, bias probe connected, as volume is raised, power tube current goes down, not up. Wouldn't that be possible PT short?

[Enzo]

First and foremost, this is a guitar amp, not rocket surgery, nothing in it is critical. For decades, Fender made their amps with 20% resistors, electrolytic caps had - honest - tolerances like +80/-20%. SO a resistor off by 10 % is not a problem in the slightest.

Was that 370AC with the tubes all installed and the amp in the run setting (not standby)? I don;t worry about high voltage. If it were a problem, ALL of them with that transformer would have the problem.

You know what causes those 100 ohm resistors to burn up? SHorted power tubes. You reported throwing a ton of parts at this amp - resistors, caps transformer, choke. But THE most likely cause of a problem in an amp - the tubes - you don't mention. Did you try different tubes in it?

Are all the tube heaters running?


Your bias probe is intended to measure DC current at idle. When you measure current with it while playing, you are trying to make this measure with a lot of AC content. DOn;t worry aboout it, at least not from that reading.

Power transformers do fail once in a while, but they are absolutely the last thing on the list.

Your amp sounds bad. Did you try through different speakers? Did you make sure not to use the extension speaker jack, only use the main speaker jack? I assume you already checked the screen resistors on the power tubes, so: do you have good B+ voltage on BOTH screen and plate on BOTH power tubes? Pins 3 and 4. Is there good bias VOLTAGE on pin 5 of each? Move to the phase inverter tube. Is there B+ on BOTH pins 1 and 6? Is it about the same on each? Is it a 12AT7 like it should be? The cathodes - pins 3 and 8 are wired together. The schematic says 90v there. I don;t care if it is only 80 or up to 110 or something, but what I donlt want to see is something real low like 40 volts. That would mean one side was not running.

[Raybob]

First and foremost, this is a guitar amp, not rocket surgery, nothing in it is critical. For decades, Fender made their amps with 20% resistors, electrolytic caps had - honest - tolerances like +80/-20%. SO a resistor off by 10 % is not a problem in the slightest.

Was that 370AC with the tubes all installed and the amp in the run setting (not standby)? I don;t worry about high voltage. If it were a problem, ALL of them with that transformer would have the problem.

370 is unloaded. B+ was up close to 470. Put in 5U4 and now have B+ of 438.

You know what causes those 100 ohm resistors to burn up? SHorted power tubes. You reported throwing a ton of parts at this amp - resistors, caps transformer, choke. But THE most likely cause of a problem in an amp - the tubes - you don't mention. Did you try different tubes in it?

Yes, new JJs. No difference.

Are all the tube heaters running?

All heaters running.

Your bias probe is intended to measure DC current at idle. When you measure current with it while playing, you are trying to make this measure with a lot of AC content. DOn;t worry aboout it, at least not from that reading.

Power transformers do fail once in a while, but they are absolutely the last thing on the list.

Your amp sounds bad. Did you try through different speakers?

Chassis was brought in with no speaker. Using my bench speaker that works fine with any other amp.

Did you make sure not to use the extension speaker jack, only use the main speaker jack?

Not using ext jack.

I assume you already checked the screen resistors on the power tubes, so: do you have good B+ voltage on BOTH screen and plate on BOTH power tubes? Pins 3 and 4.

Both screen resistors were over 10% off and are changed now.

Is there good bias VOLTAGE on pin 5 of each?

Yes.

Move to the phase inverter tube. Is there B+ on BOTH pins 1 and 6? Is it about the same on each?

189 and 204, using 5U4.

Is it a 12AT7 like it should be?

Wasn't but is now.

The cathodes - pins 3 and 8 are wired together. The schematic says 90v there. I don;t care if it is only 80 or up to 110 or something, but what I donlt want to see is something real low like 40 volts. That would mean one side was not running.

74 at cathodes, using 5U4.

Each thing I've done has made the wave look better but not right. Signal looks good up to PI. Looks good at PI cathode. Starts looking bad at PI grid attached to .1 cap. As I increase volume, wave doesn't peak out, top of wave becomes pointy, bottom looks like big wide curve. At clipping, top and bottom are clipped at major angle on scope.

[Enzo]

OH for science, disconnect your NFB so it doesnl;t confuse anything.

Have you ANY output transformer sitting around? I have one from an old fender Bassman I use for testing. You might disconnect the plate leads from the power tube sockets and clip in some other OT for testing. See if the OT might have had a shorted winding.

Also, go over to RG's Geofex site and look up his simple transformer tester. SImple and cheap to make - and use.

I'd beieve thios was a bad OT before I;d think the PT were involved.

[Raybob]

OH for science, disconnect your NFB so it doesnl;t confuse anything.

One of the first things I did. It's on a jumper wire now.

Have you ANY output transformer sitting around? I have one from an old fender Bassman I use for testing. You might disconnect the plate leads from the power tube sockets and clip in some other OT for testing. See if the OT might have had a shorted winding.

Also, go over to RG's Geofex site and look up his simple transformer tester. SImple and cheap to make - and use.

I'd beieve thios was a bad OT before I;d think the PT were involved.

My first thought was OT and I put a board across top of chassis, wired up a new Heyboer DR OT. No help. Did the same with a new choke. Guess I'll try wiring a new PT. Have one sitting next to chassis (New Heyboer for DR), leads are long enough to reach everything. Just thought I'd try posting here for ideas before I unsolder 10 or so, connections.

[Enzo]

I still have no idea how the PT could be responsible, but in the absence of a scope to follow the signal, why not.

But really, instead of disconnecting all that stuff, is it not just the high voltage you suspect? If so, then connect your other PT to the mains and then leave the original one wired except for the HV winding, then clip in your new HV wires. Might save you some effort.


If you lack a scope, I bet you have another amp around. Make a "signal tracer." They make signal tracers as a complete unit, but really all it is is an amplifier with a probe for an input. You can make any amplifier into a signal tracer with the addition of an input probe. And all the probe is is a cap in series to block DC, and a high value resistor to terminate the cap on the tracer side of the cap. SO a guitar cord without a plug on one end is the basis. Put a clip lead on the shield, so you can ground it to the chassis of what you are working on. Connect the center wire to the cap and to a 1 meg resistor to ground. Make the cap high enough voltage to handle anything in your amp.

The cap value isn't important, I grab a 0.047uf 630v cap from my bench (I always have them handy) but a 0.1uf or a 0.02uf would work too.

One end of the cap is wired to the cord, the other end is free, and it is what you probe the circuit with. You can apply a signal to the amp input, then listen to it stage by stage through the UUT on your tracer. UUT is "unit under test." This lets you hear what things sound like, and you can determine just where in the circuit the sound goes to crap.

The other end of that process is signal injection. You can have a signal source, and again using a cap for DC isolation, touch that signal to points in the circuit. You start at the output stage and work back. Touch a signal to the grid of a power tube and you should hear it out the UUT speaker. Then move back to the grid of the phase inverter, then step back stage by stage to hear where it changes from sounding OK to crappy.

[Raybob]

I have a scope. Signal from input is fine up to PI, fine at PI cathode, gets pointy top of wave, big round curve bottom of wave, starting at PI pin 7, on through to speaker.

I'm suspecting PT because of the high voltage, possibly coils shorted together, lowering current capability somehow. I'm guessing that's why current at power tubes goes down as volume goes up.

Edit: Just re-read the part of clipping in the new PT. If I leave the old PT connected, then I'm also connecting the new PT to those inactive coils in other PT. Guess I can unsolder one leg of each green, yellow, and red wires.

[Enzo]

Not sure I follow, if you disconnect the old winding and clip the new transformer in its place, it has no connection to the other coils, other than the primary. The original PT continues to run the heaters and bias. But no harm in doing it totally.


If you think the PT is crapping out, and have a scope, then scope the B+ at each node, see if it is acting funny or not.

And you maesured the 74v at the PI cathodes. Measure the grids. YOu cannot do this to ground. Measure voltage from the cathodes to each grid. Ought to get about 1.8v. Or go to the other end of the 470 ohm cathode resistor and just measure from there to the grid - across each grid resistor. That should give around zero volts. If you try to measure from grid to ground, your meter impedance will confuse the readings considerably. Or verify the grid resistors by measureing resistance from pins 2 and 7 down to 3 and 8.

That 0.1uf cap from the PI grid to the 47 ohm tail, is it OK?

[Alex R]

I have a scope. Signal from input is fine up to PI, fine at PI cathode, gets pointy top of wave, big round curve bottom of wave, starting at PI pin 7, on through to speaker.

just a little point to add - it's normal for the PI plates to feed the power tube grids with a wave that scopes like that, the PI is splitting the wave, remember.

[Raybob]

Not sure I follow, if you disconnect the old winding and clip the new transformer in its place, it has no connection to the other coils, other than the primary. The original PT continues to run the heaters and bias. But no harm in doing it totally.


If you think the PT is crapping out, and have a scope, then scope the B+ at each node, see if it is acting funny or not.

Wired in other PT, lower voltages but same problems.

And you maesured the 74v at the PI cathodes. Measure the grids. YOu cannot do this to ground. Measure voltage from the cathodes to each grid. Ought to get about 1.8v. Or go to the other end of the 470 ohm cathode resistor and just measure from there to the grid - across each grid resistor. That should give around zero volts. If you try to measure from grid to ground, your meter impedance will confuse the readings considerably.

Got 1.4 on each, 1.56 across 470 ohm.

That 0.1uf cap from the PI grid to the 47 ohm tail, is it OK?

Tried a sub there, but it was fine too.

Wave at speaker is real flat at bottom, angled up to right. Gets real big as volume goes up. Top seems to clip at severe angle but not until volume is up to about 6.

[Raybob]

Tried moving signal to PI input. Now at both PI plates, wave appears ok, but is waving up and down on scope, about every second. Odd... for me at least.

[Alex R]

that's just hum probably, picked up from whatever you're using to inject the signal.

[Raybob]

Problem was bad tube socket, the one I had bias king plugged into. Put it in other socket and current now goes up when volume goes up. Duh.

[Zer09]

Yikes. That is one expensive tube socket. Sometimes the Gerald Weber method doesn't work. This is exactly why if I don't find something after a few minutes of poking around it automatically goes on a scope. Just reading this made me tense and made my head ache from bad memories.