[_Knighthawk_]

it may not be relevant, but if an amp I just built made that sound, I would be disconnecting the lead/leads from the input jacks as a first step in figuring it out.

Happy to give that a shot. What is the general idea/thinking behind this approach?

 

[Lincoln]

Happy to give that a shot. What is the general idea/thinking behind this approach?

looking for messed up input jacks? Eliminating one area that has caused me trouble in the past.
Pulling the first preamp tube would be next if the noise remains the same.

 

[_Knighthawk_]

looking for messed up input jacks? Eliminating one area that has caused me trouble in the past.
Pulling the first preamp tube would be next if the noise remains the same.

Both good ideas, Lincoln. Thanks.

I'm going to have some time to play around tomorrow (wanted to get to it tonight but we had company out of the blue).

 

[_Knighthawk_]

So, good news and bad news.

Good news first: I put a 470R resistor in series with the diode (as per @Paul Running) and it seems to have tamed both the hum and the negative voltage. I am now getting between -28VDC and -30VDC. I won't try to do better than that!

The bad news: I swapped the power tubes and it's not the tubes. I'm still getting some red plating on the second power tube (which I believe on would call V5 if one were to annotate the 5G9 Fender drawing). Not sure what might be causing this - it's odd.

Is it possible that the artificial CT that I am running off of that socket might be causing the issue? The 470 R screen resistor is almost spot-on and seem solid on the meter. The voltage is a bit high - between 390 and 405 VDC. So, yes, higher than the 368 VDC called for in the schem.

 

[_Knighthawk_]

looking for messed up input jacks? Eliminating one area that has caused me trouble in the past.
Pulling the first preamp tube would be next if the noise remains the same.

@Lincoln -> I have observed that the hum coming off the depth pot (when increased from zero) does not occur (or is very significantly diminished) when I plug a guitar into one of the inputs. Absent a guitar in the input, the cycling hum reappears.

 

[Alan Small]

 

[nonreverb]

So, good news and bad news.

Good news first: I put a 470R resistor in series with the diode (as per @Paul Running) and it seems to have tamed both the hum and the negative voltage. I am now getting between -28VDC and -30VDC. I won't try to do better than that!

The bad news: I swapped the power tubes and it's not the tubes. I'm still getting some red plating on the second power tube (which I believe on would call V5 if one were to annotate the 5G9 Fender drawing). Not sure what might be causing this - it's odd.

Is it possible that the artificial CT that I am running off of that socket might be causing the issue? The 470 R screen resistor is almost spot-on and seem solid on the meter. The voltage is a bit high - between 390 and 405 VDC. So, yes, higher than the 368 VDC called for in the schem.

The bias voltage might still be too low. You should have a range up to around -40V with voltages @ ~ 400V.

 

[_Knighthawk_]

The bias voltage might still be too low. You should have a range up to around -40V with voltages @ ~ 400V.

So, bad coupling cap...or what are you thinking might be at the root of this?

 

[Paul Running]

So, bad coupling cap

If the coupling caps to the grids of the power tubes leak, the bias voltage will offset in a positive direction; maybe check the output caps for leakage.

 

[nonreverb]

So, bad coupling cap...or what are you thinking might be at the root of this?

Yes possible. Have you measured the bias voltage at the individual tube pins?
Are you using Sovtek 6V6's by chance?

 

[_Knighthawk_]

I did measure the -voltage at the pins. I will do it again and publish them here (they weren't inconsistent with the drawing).

I'm using Tungsol 6V6GTs.

Doesn't everyone?

 

[_Knighthawk_]

Yes possible. Have you measured the bias voltage at the individual tube pins?

Negative voltage on pin 5 is -28.5VDC while on standby (same on both V4 and V5). It really jumps around between -21 and - 31VDC when running (standby off).

 

[nonreverb]

Negative voltage on pin 5 is -28.5VDC while on standby (same on both V4 and V5). It really jumps around between -21 and - 31VDC when running (standby off).

Running is the important measurement as the current draw will pull the voltage down if the tube is conducting excessively.

 

[_Knighthawk_]

Are you suggesting that the tube IS conducting excessively?

Why does it jump around so much? It had been my understanding that the meter actually reflected mean values - which is why I find it curious that it jumps around so much.

 

[nonreverb]

Are you suggesting that the tube IS conducting excessively?

Why does it jump around so much? It had been my understanding that the meter actually reflected mean values - which is why I find it curious that it jumps around so much.

Does it jump around on both or just the one that's red plating?

 

[_Knighthawk_]

Does it jump around on both or just the one that's red plating?

Both.

 

[nonreverb]

Both.

Well that's shitty.
As for the meter, it only displays mean AC values. It will follow actual DC readings.
Does this phenomenon occur at the bias rectifier circuit as well?
What brand of meter is it?

 

[nonreverb]

Here's something to try. Remove the power tubes and see if the display is still jumping.

 

[Mark Brown]

@nonreverb thanks for dedicating your time to this kind of thing over and over again. I learn so much just following along with the advice you give to people freely.

Keep it up buddy. We all appreciate it.

Someday, promise, I will help others as I have been helped. Just got to learn a thing or two first.

 

[_Knighthawk_]

The meter is an 'Ideal'.