bias mod for the mesa boogie subway rocket

[JC@]

Hi, I can't find a pair of tubes that will "fit" one of these amps that I have on the bench. All the tubes I have draw excessive current.

I would like to ask if there is an article or something on the web about the best way to make the bias adjustable, or which components should I change.

Thank you very much.

[stokes]

On the schematic I have it shows a 33k resistor in the bias supply,use a 50k cermet pot in place of this resistor and you will be good to go.If you have trouble finding this component look for a small electrolytic cap that is reverse oriented,that is the + end connected to ground.At -connection of this cap you will see another 15k resistor,this feeds the cathode of the PI,check the voltage here before you change to the adjustable cermet pot and be sure when you adjust the bias voltage this voltage remains the same,it shouldnt change but just check to be sure.

[JC@]

It's being a little more complicated but not much,
the 50K pot doesn't give enough range (still too little bias) so I had to replace the "main" 120K resistor with a lower value one, and now I have to find the correct value for the resistor that will go in place of the 15K one so it keeps the 3 volts supply for the phase inverter which is now a bit too low (2.3 volts)

thank you stokes

[stokes]

Yeah,sometimes you gotta tweak it some.But I think instead of changing the 120k,maybe if you put a 10k or so in series before the pot will get you closer and not change the volts at the 15k.Also,I should have pointed out before not to use a standard volume type pot for this.You are putting dc volts on the pot and that type will eventually burn the trace,the cermet pot can take the dcv's better.

[unparalleled]

Mesa Rocket 44 bias problem

I have tacked onto this old thread because I have a related problem.
This amp has just had PT rewound ( the OPT has also been rewound earlier ) and the EL84s are drawing excess current.
The PI cathodes are connected via a resistor to the bias supply and this arrangement draws current from the bias supply.
When the 12AX7 is inserted the available bias to the output tubes drops from -20v to - 3.3v
causing the EL84s to cook.
A design which draws current from the bias supply seems to me to be asking for trouble!
Has anyone successfully remedied this problem?

[Enzo]

Well, I think the amount of current through that PI tube is normally just a very few milliamps. Not enough to load it down. Does it do this with ANY 12AX7? or just the one?

Hard to say if anyone else has had this failure.


Have you the schematic, and can post it here?

[unparalleled]

Well, I think the amount of current through that PI tube is normally just a very few milliamps. Not enough to load it down. Does it do this with ANY 12AX7? or just the one?

Hard to say if anyone else has had this failure.


Have you the schematic, and can post it here?

Does it with any tube.
I am thinking of modifying the PI cathode circuit and bias supply to something more standard.
The bias cap takes a painfully long time to charge up.

[billyz]

I did it even simpler. I just parallel another 33k resistor with the bias feed resistor. Gives a little more voltage to the tube grids. Did not seem to negatively affect the bias of the PI. It sounds good and no more red plates. Still not adjustable, but acceptable. I first tried an adjustable pot but it really did not change the sound much. So I thought the fixed resistor would be more reliable in the circuit.