How to bias an Ampeg Gemini VI gs15r

[davohilts]

Hi,
I've been rebuilding an Ampeg GS15r and have it up and running but have put in a pair of 7591A power tubes and need to bias the amp. The amp has a bias pot and I'm wondering how I should go about biasing the amp. I have 476 Volts on the plates of both V1 and V2 and when I check the mV level I get .3 mV which seems real low.
Does anyone know the what the correct values are for this amp?
I'll attach a schematic.
Thanks,
Dave

[acorkos]

max plate dissipation for a 7591A is 19 watts

Tube Database Search


at 476 volts on the plates, you should bias between 20mA (50% of max) and 29mA (70%)

[davohilts]

the strange thing is when I measure the mA between the center tap of the ot and the plates on V1 or V2 I get .04 mA. The amp plays great and sounds very good but I'm really stumped as to why I get such a strange reading.

[g1]

Your first post you said mV and now you said mA. You are measuring DC current through OT with meter probes connected to mA jacks of meter?

[davohilts]

yes, sorry i'm measuring mA with the meter set to read mA.

[g1]

Can you verify the mA function or try another meter? One of my meters has a fuse for the mA setting. If that DCmA reading is good, I wouldn't think the amp would sound good, especially on clean settings.

[davohilts]

i'm going to open up the meter and see if i've go a blown fuse, seems like a good idea.
I put the amp back into the cabinet and will play it for a couple of weeks, them pull it out and go through the steps again. Since it sounds so good it must be pretty close to a correct bias but it bugs me that i couldn't get it to bias properly.

[g1]

You could also measure resistance of the 2 halves of the OT primary, measure the voltage drops across the 2 halves, and get out the calculator. Volts divided by ohms equals amps.