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Bias 64 Super Reverb

Messages
60
I tried biasing but only got from -11 to -23 mA when turning the bias trim pot.

I first want to make sure I'm not doing something stupid - I am very careful doing this and respect amperes, believe me.

Shunt method:
Connect multi-meter (red) to the center tap of the output tranny. Connect black to pin 3 of one of the power tubes (Svet 6L6 Winged-C).

That's it but I can't get -30 to -35 on the meter (only from -11 to -23).

1. The center tap of the OT is red and is connected to the board. There is also a black wire from the OT connected to about the same spot with a short little jumper. Is that ok? The reason I'm asking is because the OT is not original - it is Schumacker 4ohm from 1968 - probably from a bassman.

2. Should the meter be set to V or mA? I am setting for 200 in order to get the reading.

If I am doing this right, it's possible that the resistor, the electrolytic, the diode or the trim pot is shot in the bias feed supply, right? If so, how can I know for sure which is fried? I guess the electrolytic should be replaced anyway as it is original 1964 (I've replaced all the caps under the pan, a few years ago).

Anyway, thanks for your help on the bias questions.
 

Blue Strat

Member
Messages
30,190
Assuming that everything you're doing is ok, which it appears to be, the specific pair of tubes you're using may be weak or have low conduction. JJ's and Sovtek 5881s are famous for low idle current.

To get them into the 30s, you'll have to decrease the value of the fixed resistor attached to the bias pot by one or two standard values.
 
Messages
60
Thanks for all the answers everyone. William, you don't come across as condescending at all - you are absolutely right. I've been reading from lots of sources and still feel I may be doing something wrong. Ironing out the details is great advice, especially for something like this. I tried setting to 200 mA and got those low readings.

I have 3 sets of power tubes: Svets, Sovtek 5881 and Phillips 6L6WGB. Blue Strat mentioned that could be the issue. I haven't tried the Phillips yet. I guess I should.

The rectifier is a GE 5AR4 that is suspicious. I have a spare Ruby 5AR4 which I think is Chinese. I'll swap out the tubes and try again. If still no go, I may change the cap in the bias supply. If that doesn't do it - does anyone know a good tech in or around NYC?

Thanks again for the info!
 

Blue Strat

Member
Messages
30,190
Try at least one other pair of power tubes...that's the most obvious first step.

A failing bias supply cap will give you too high of a bias current reading, not too low.
 




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