Mesa DC-3 bias too hot

[rockon1]

The EL84's in my fixed bias Mesa DC-3 are slightly redplating. Checked the grid voltage at pin 2 and its -10.65. Found a schematic for the amp and it shows the voltage at -11 vdc at pin 2 of the EL84's. Both seem pretty low for an EL84 as far as I can google. It seems -14 would be a better level. Thinking of piggybacking a 100K on the 33k in the bias circuit to bring it down to around 24K ? Any suggestions as to the value or if this is even a good idea? Heres a link to the schematic Ive found.

Thanks Bob

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These might help. Sorry best res I could get.Thats a 33K at E where it states -12VDC. It shows -11 at pin 2 in the second pic. bob

[rockon1]

Hello?

[Jazz P Bass]

Try the 100K in parallel.
See what you get.

[rockon1]

Thanks.Heres a picture of the guts. Im OK with an iron and pcb. That said its like pulling teeth for me to figure out stuff! lol! Hunting for the resistor now! Its got to be close to one of the 220uf caps. There is a 33k(Or,Or,red,red, brown) resistor Ive pointed to in this pic. Im thinking thats the resistor? Underneath the blue one in front of it I think it says -12 so maybe Im close? Bob

[Jazz P Bass]

That looks like the one.
Measure the voltage on it.
Resistance should read 2 x 220K to the grid pins. (amp off)

[rockon1]

That looks like the one.
Measure the voltage on it.
Resistance should read 2 x 220K to the grid pins. (amp off)

I Paralled a 100K with that 33K resistor and the voltage at pin 2 remained virtually the same. -10.6 . Im lost. Bob

[Pedro Vecino]

This bias circuit is very rigid. Look at the 2M2 resistors between power tube grids and earth.
Raising its value (by adding a 1M5 resistor on each one, for example) you can adjust it.
It´s a laborious process but it works.

[Jazz P Bass]

Thanks for the tip.
Nomads are the same configuration.
( As Pedro advised, no they are not)

[rockon1]

This bias circuit is very rigid. Look at the 2M2 resistors between power tube grids and earth.
Raising its value (by adding a 1M5 resistor on each one, for example) you can adjust it.
It´s a laborious process but it works.

Yes,thank you for this info! This may be my next try. Bob

[Pedro Vecino]

Thanks for the tip.
Nomads are the same configuration.

Nomad (the three models) has flexible bias circuits. No problem with them.
Regards

[Jazz P Bass]

I stand corrected on the Nomad bias supply.
Thanks Pedro.

[GainFreak]

Check out the Dynawatt thread and the bias circuit I used. It's the same power amp:

http://music-electronics-forum.com/t22622/

[rockon1]

Check out the Dynawatt thread and the bias circuit I used. It's the same power amp:

http://music-electronics-forum.com/t22622/

I couldnt figure out what exactly you did from that thread to modify the DC-3 bias. Theres got to be a simple way of doing this. Bob

[GainFreak]

I couldnt figure out what exactly you did from that thread to modify the DC-3 bias. Theres got to be a simple way of doing this. Bob

I used the adjustable bias circuit below. Whatever you do the voltage on those EL84 is going to be high and the amp will periodically "eat" them.
I had a Peavey Classic 50 with +400V and I was changing tubes every couple of months but finally I removed the original board and replaced it with a new one with 2xEL34 but that's another story.

[rockon1]

OK so I replaced the 33K highlighted here in this picture with a 50K pot. Got -15 going in and at the pots lowest setting Ive got -14.5 going out to the two 330k resistors. Still minimal change at pin 2 at -10.75 now from -10.65. Wondering if I could simply lower the values of the 330K resistors or should I perhaps find the 2.2M resistors as previously suggested and mess with them? Heres the link to the schematic. Also Im going to have to recheck my voltage coming out of my pot. Cant figure out why if I have -14.5 at E (two 330k resistors in schematic) I dont have more - voltage at pin 2.

http://www.schematics.ca/modules/PDd...pdf&title=Mesa

TIA Bob