Verify My Mini Bias Board for a 6G3

[syscokid]

I'm still working on my bastardized 6G3 with no tremolo. I need to add a mini fixed-bias board featuring a trimmer for adjustability. Will the following illustration work properly?

Screenshot 2025-06-06 073505.jpg

Stock schematic:

6G3_Schematic.jpg

[sluckey]

Layout will work. I suggest using a filter cap rated for 150V. You may need to increase the value of the 100K resistor to get the desired bias adjustment range.

[syscokid]

Layout will work. I suggest using a filter cap rated for 150V. You may need to increase the value of the 100K resistor to get the desired bias adjustment range.

Thanks, sluckey. How about a (F&T) 33u/160v?

[sluckey]

Yes. Work fine, last long time.

[Stevem]

If you have not ordered any parts yet then you might like this set up with a bias trimmer and a balance, and it also acts like a fail safe in that if the wiper on the trimmer ever goes open then the full bias gets applied too keep the tubes well below a burning up state.

[syscokid]

Big bummer... Red plate city!! Not reading any proper negative voltage at the grid bias resistors, except for hundredths of a volt. Quadrupled checked the new wiring with the new parts. Maybe I cooked the trimmer... Is that possible? It still reads resistance at about 56K, and at its halfway point its reading 28K.

[xtian]

Big bummer... Red plate city!! Not reading any proper negative voltage at the grid bias resistors, except for hundredths of a volt. Quadrupled checked the new wiring with the new parts. Maybe I cooked the trimmer... Is that possible? It still reads resistance at about 56K, and at its halfway point its reading 28K.

Check the orientation of your diode and caps!

And of course, check bias voltage BEFORE installing tubes!

[xtian]

If you have not ordered any parts yet then you might like this set up with a bias trimmer and a balance, and it also acts like a fail safe in that if the wiper on the trimmer ever goes open then the full bias gets applied too keep the tubes well below a burning up state.

Steve, note in the diagram that the wiper is connected to -ve bias, so if wiper fails, still full -ve voltage on tube grids. Safe.

[syscokid]

Big bummer... Red plate city!! Not reading any proper negative voltage at the grid bias resistors, except for hundredths of a volt. Quadrupled checked the new wiring with the new parts. Maybe I cooked the trimmer... Is that possible? It still reads resistance at about 56K, and at its halfway point its reading 28K.

Check the orientation of your diode and caps!

And of course, check bias voltage BEFORE installing tubes!

Orientation is exactly like my illustration in the first post. I'm using UF4007's with the white bands towards the HT supply. Filter cap's positive is grounded.

I removed the 50k trimmer and replaced it with a 25k trimmer and I'm still getting the same results.

I'm assuming it doesn't matter which of the two HT leads I tap into, right? The HT leads are each connected to a UF4007, which are connected to pins 4 and 6.

[syscokid]

And of course, check bias voltage BEFORE installing tubes!

Yes... No bias voltage at the pair of 220K grid bias resistors. I originally set up this amp with cathode bias power. Sounded fine, worked fine. I just couldn't leave well enough alone!

[syscokid]

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IMG_1902.JPEG

IMG_1901.JPEG

[Stevem]

If you lift the pot side of that diode what do you read for a negative voltage?

If you do have negative voltage then is it getting over to the intensity pot?

Do not pop the output tubes in until you see a good level of negative voltage on pin 5 of each tube.

[syscokid]

If you lift the pot side of that diode what do you read for a negative voltage?

If you do have negative voltage then is it getting over to the intensity pot?

Do not pop the output tubes in until you see a good level of negative voltage on pin 5 of each tube.

No negative voltage. I'm reading .8 VDC and .4 VAC.

Amp is built with no tremolo.

Thanks, I won't install any tubes.

Taking a dining out break with the wifey. I'll check back later.

[martin manning]

Looks like your HV input lead is coming from the DC side of a safety silicon diode. It needs to be connected directly to the HV AC.

[sluckey]

Looks like your HV input lead is coming from the DC side of a safety silicon diode. It needs to be connected directly to the HV AC.

BINGO!