Volume drop after redplating tubes.

[92lx50]

That circled diode looks like it could be the bias supply rectifier diode. If so it is probably the cause of your original problem and is definitely a serious problem now if it is "Bad". However, there is another single diode and what looks like burned resistors on the same PC board. That could also be the bias supply too. Do not power the amp on with the output tubes installed until this bias supply is verified good / repaired.

Tell us what you mean by "Bad" with respect to that diode. Is it open, sorted or other?

With the power tubes removed I recommend that you measure the voltage at each power tube pin 5. When the bias supply is working properly you will find a negative voltage with respect to system ground at each tube. It will be in the order of -40 to -55V. Please tell us what you find and post a schematic in this thread for reference.

Hi Tom,

I checked the voltages at pin 5 as instructed and they were all within that range. I also tested the 4 diodes that are grouped together and a couple of the were under .500. Would this be a problem?

Also, I replaced the one diode that was in question and it measures over .500 one way and then when I switch the leads around, it starts at 1.7 or so and climbs till it hits 2.0, and then it zero's out.

Any suggestions on where to go from here?

[Audiotexan]

I agree with g1 and, upon reviewing the pics again, I not longer think that the resistors I circled in green in post #5 are burnt to the point that they look damaged.
Tom

It's odd, and yes, they could measure good, but the main reason they caught my attention was because of the rest of the picture having pretty good color recognition (eg: what appears to be a 4.8k 5% near the three 22uf, 450v caps.) and those two in addition to the discoloration didn't look anything like Red in the first digit.

In fact, closer to purple or even brown to these eyes in that lighting. The only thing I can say for sure about the discolored one's is that they're 5%'ers (Gold band on the left end). lol

Anyways...back to lurking!

[Tom Phillips]

I checked the voltages at pin 5 as instructed and they were all within that range....

Good. That indicates that the bias supply is working and that it is connected to each power tube.

...I also tested the 4 diodes that are grouped together and a couple of the were under .500. Would this be a problem?...

That’s not a problem. Common rectifier diodes should test less that 0.6V in the forward direction and “open” in the reverse direction. You can check new diodes before they are installed to determine the exact readings that your meter gives.

...Also, I replaced the one diode that was in question and it measures over .500 one way and then when I switch the leads around, it starts at 1.7 or so and climbs till it hits 2.0, and then it zero's out.

I not sure what you mean by “zero’s out” but that is odd behavior. Sometimes there is residual voltage in a circuit from charged capacitors that will cause odd readings when you try to make measurements on components that are mounted in the circuit. However, it you changed the diode and now it measures good, then all is well and we move on.

...Any suggestions on where to go from here?

If the amp was on my bench, I’d proceed with power tube installation and test. The preferred procedure is to set the bias to the far cold end of the range and re-verify the proper voltage at pin 5 of each power tube before installing the power tubes. The coldest bias is produced by the most negative bias voltage. There are no voltage reading shown on the schematic but I'd expect your bias supply to be capable of producing at least -55V and very likely up into the -60s.

Once you start down this path you will need to adjust the optimum bias after the tubes warm up and verify that the tubes are all OK and reasonable matched. In your opening post you said “...The amp powers up fine and biased...”. I’m not sure what you did and what you knowledge and equipment availability is with respect to setting the bias. The stock amp does not have bias sense resistors. Let us know what you plan to do to check and set bias and we will go from there.

[92lx50]

Good. That indicates that the bias supply is working and that it is connected to each power tube.


That’s not a problem. Common rectifier diodes should test less that 0.6V in the forward direction and “open” in the reverse direction. You can check new diodes before they are installed to determine the exact readings that your meter gives.


I not sure what you mean by “zero’s out” but that is odd behavior. Sometimes there is residual voltage in a circuit from charged capacitors that will cause odd readings when you try to make measurements on components that are mounted in the circuit. However, it you changed the diode and now it measures good, then all is well and we move on.


If the amp was on my bench, I’d proceed with power tube installation and test. The preferred procedure is to set the bias to the far cold end of the range and re-verify the proper voltage at pin 5 of each power tube before installing the power tubes. The coldest bias is produced by the most negative bias voltage. There are no voltage reading shown on the schematic but I'd expect your bias supply to be capable of producing at least -55V and very likely up into the -60s.

Once you start down this path you will need to adjust the optimum bias after the tubes warm up and verify that the tubes are all OK and reasonable matched. In your opening post you said “...The amp powers up fine and biased...”. I’m not sure what you did and what you knowledge and equipment availability is with respect to setting the bias. The stock amp does not have bias sense resistors. Let us know what you plan to do to check and set bias and we will go from there.

I will proceed as you stated. I have all new tubes that I have tested already in another amp just to make sure I didn't get a dud. As far as bias equipment, I have a bias probe that I have used on my Marshall's and a few other amps.

Would you know what wattage I should use to replace those hanky looking resistors? I know the value just not sure what wattage.

[Tom Phillips]

...Would you know what wattage I should use to replace those hanky looking resistors? I know the value just not sure what wattage.

I would use 1/2 watt resistors.

[92lx50]

Alright. New tubes installed, plate voltage 466, tubes biased to 32ma +or- 1 ma, each tube checked and rock solid stable.

To recap, screen grid resistors, capacitors on power board and diode that was in question replaced.

Here is a video instead of trying to explain. Pretty much the same thing since the tubes redplated.

https://www.youtube.com/watch?v=VKr9GOL84Gs

[Tom Phillips]

Since you said, in the video, that the low volume / distorted signal problem exits when you inject signal into the FX return then I suggest that you troubleshoot the V4B stage and the phase inverter next. Start by measuring the plate and cathode voltages of each of those stages.

[The Dude]

Also check those 2 flyback diodes from the OT windings to ground for leakage. You might even temporarily unsolder them to see if there's any improvement.

[92lx50]

Since you said, in the video, that the low volume / distorted signal problem exits when you inject signal into the FX return then I suggest that you troubleshoot the V4B stage and the phase inverter next. Start by measuring the plate and cathode voltages of each of those stages.

Wasn't quite sure what you meant so I measured the voltage on the pins of v4 and v5.

v4
pin 1=1.0
2=9.4
3=16.7
6=0
7=0
8=1.5

v5
pin 1=4.1
2=0
3=27.6
6=0
7=15.5
8=27.6

[92lx50]

Also check those 2 flyback diodes from the OT windings to ground for leakage. You might even temporarily unsolder them to see if there's any improvement.

Could you circle those diodes for me? I'm not sure which ones they are.

[The Dude]

I can't spot them in the picture, but they are the ones marked 5kv on the schematic and they go from the OT wires (blue and brown) to ground. You should be able to find them by finding the wires. The diodes will not be shorted, or you'd be blowing fuses. They might however have enough leakage to lower your output. I have seen it happen before. You can either check them on a high resistance scale and look for conduction both directions, or simply unsolder them temporarily and see if you level comes up.

[92lx50]

I can't spot them in the picture, but they are the ones marked 5kv on the schematic and they go from the OT wires (blue and brown) to ground. You should be able to find them by finding the wires. The diodes will not be shorted, or you'd be blowing fuses. They might however have enough leakage to lower your output. I have seen it happen before. You can either check them on a high resistance scale and look for conduction both directions, or simply unsolder them temporarily and see if you level comes up.

Ok, I see which ones. I already cut those. The were going from pin 3 to pin 8 to ground on the 2 outside tube sockets. I guess the were taken out by the redplating tubes.

[g1]

On your V4 & V5 readings, the plate voltages seem to be missing. Pins 1&6 of both tubes should have hundreds of volts. You may need to dig in harder with your probes.
For V4, the cathode voltages seem to show that the plate voltages must be there, but for V5 try again.

[92lx50]

On your V4 & V5 readings, the plate voltages seem to be missing. Pins 1&6 of both tubes should have hundreds of volts. You may need to dig in harder with your probes.
For V4, the cathode voltages seem to show that the plate voltages must be there, but for V5 try again.

Gotcha. Went back and I got v4 pin 1=336 and pin 6=231

For v5 pin 1=4 and pin 6=205

[g1]

Then check the resistor that pin1 of V5 connects to (68K or 100K, can't tell which from schemtic). At pin1 you should have voltage similar to that of pin 6.