Volume/Tone pot interaction affecting voltage & creating noise

[yello]

I ended up rotating the OT 180 degrees and the problem reduced by about 50%. I then chopsticked wires in that area, and got the OT primary wires into a better position, and the problem is fixed. Filmosounds are very tight, and wires are all running close, so getting the OT primary wires tight to the chassis, close to the el84 sockets but away from other things as much as possible did the trick. There is still a plate voltage drop when both the volume and tone pots are all the way up.

So, now I am wondering if it is worth increasing the stock 2.2k el84 grid stoppers to 22k? Maybe that is the ticket for addressing the plate voltage drop?

Or needed to add screen stopper resistors? Stock it doesn't have them, and it is likely a good idea.

Or replace the volume pot, if that is related to the voltage drop?

I'm doing my best to answer my own questions to some degree by gathering info, and trying things. I will likely try the 22k grid stoppers first, then add screen stoppers. Then assess if volume pot replacement is needed, I'm leaning towards not but we will see.

[Bieworm]

Grid stoppers won’t affect the plate voltage imho.. they will prevent the crossover distortion. Screen resistors will drop the screen voltage a little. Plate voltage will be tweaked with dropping B+, or change the cathode resistor

[yello]

I ended up changing the el84 grid stoppers to 22k. Works well. And added a 1k screen resistor for each el84. That dropped the voltage on the screens by about 5 volts, so now screens are slightly lower than plates, instead of being 3v higher.

Left the volume pot, works fine.

I am wondering overall about the voltages in this one as measured and shown in my chart, compared to the Bill Krinard modded one as voltages are higher in this one even though I used the same filter/resistor values. If all the voltages are within a tolerable spec, I'm fine with it as the amp sounds good, but interesting to compare. On the chart this amp I've been working on is amp #2, the Krinard modded amp is amp #1.

[JMPGuitars]

I ended up changing the el84 grid stoppers to 22k. Works well. And added a 1k screen resistor for each el84. That dropped the voltage on the screens by about 5 volts, so now screens are slightly lower than plates, instead of being 3v higher.

Left the volume pot, works fine.

I am wondering overall about the voltages in this one as measured and shown in my chart, compared to the Bill Krinard modded one as voltages are higher in this one even though I used the same filter/resistor values. If all the voltages are within a tolerable spec, I'm fine with it as the amp sounds good, but interesting to compare. On the chart this amp I've been working on is amp #2, the Krinard modded amp is amp #1.

Glad you sorted that out.

Your voltages don't mean much to me since it's not a circuit I deal with, and I don't know what your targets are.

Beyond that, you should calculate your tube dissipation for the power tubes and make sure they're not crazy hot. My guess is they are hot as is, but you can calculate that and decide if you want to up the cathode resistor value. I think your schematic had a 130 ohm cathode resistor, I don't know what you actually used.

[yello]

I ended up changing the el84 grid stoppers to 22k. Works well. And added a 1k screen resistor for each el84. That dropped the voltage on the screens by about 5 volts, so now screens are slightly lower than plates, instead of being 3v higher.

Left the volume pot, works fine.

I am wondering overall about the voltages in this one as measured and shown in my chart, compared to the Bill Krinard modded one as voltages are higher in this one even though I used the same filter/resistor values. If all the voltages are within a tolerable spec, I'm fine with it as the amp sounds good, but interesting to compare. On the chart this amp I've been working on is amp #2, the Krinard modded amp is amp #1.

Glad you sorted that out.

Your voltages don't mean much to me since it's not a circuit I deal with, and I don't know what your targets are.

Beyond that, you should calculate your tube dissipation for the power tubes and make sure they're not crazy hot. My guess is they are hot as is, but you can calculate that and decide if you want to up the cathode resistor value. I think your schematic had a 130 ohm cathode resistor, I don't know what you actually used.

I used a 150 ohm cathode resistor as that is what Krinard used. I need to do the math and see where I stand.

[yello]

Looks like my two el84 power tubes are at 14+ watts each and 120%+ plate dissipation.

Stock cathode resistor was 130, but I used 150 ohm.

Looking at my voltage chart, to compare this amp to its twin (same amp circuit but different power transformer as built in different countries, Canada versus Japan) that Bill Krinard modded, the voltages produced by each PT are different. I used the same B+ cap/resistor scheme, so not surprised I'm getting different results.

What is the best approach to address my higher voltages? Zener? Higher B+ node A resistor? Higher Cathode resistor? Some combo of those?

[JMPGuitars]

Some combo of those?

Correct. Shoot between 180 to 220 for the cathode, and figure out what you want your B+ to be, then drop it there. Take some new voltages, do some more math, and play some guitar.

[yello]

Some combo of those?

Correct. Shoot between 180 to 220 for the cathode, and figure out what you want your B+ to be, then drop it there. Take some new voltages, do some more math, and play some guitar.

The math is my new frontier, learning better how to calculate the needed value of parts, especially on the B+.

[JMPGuitars]

Some combo of those?

Correct. Shoot between 180 to 220 for the cathode, and figure out what you want your B+ to be, then drop it there. Take some new voltages, do some more math, and play some guitar.

The math is my new frontier, learning better how to calculate the needed value of parts, especially on the B+.

You can cheat for your dissipation here: https://robrobinette.com/Tube_Bias_Calculator.htm

For B+, well, that's usually determined by how the circuit behaves at different values.

[yello]

Correct. Shoot between 180 to 220 for the cathode, and figure out what you want your B+ to be, then drop it there. Take some new voltages, do some more math, and play some guitar.

The math is my new frontier, learning better how to calculate the needed value of parts, especially on the B+.

You can cheat for your dissipation here: https://robrobinette.com/Tube_Bias_Calculator.htm

For B+, well, that's usually determined by how the circuit behaves at different values.

That's the exact tool I use for dissipation.

[yello]

This one is wrapped up and sounding nice. Got the bias set at a good level, and my friend and I both like how it sounds, so he is getting it back to enjoy.

Thanks for the great guidance - I learn more with every one, and each amp I work on I feel more skilled and confident, and can get things done more efficiently. Certainly makes it more fun!

On to the next one...