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Old 8th December 2004, 05:01 AM   #1
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Default bias voltage for 6BQ5 plate

I purchased an SRS biastool to help in my (clueless) efforts to rebuild/repair/get working some of the tube amps that I have been collecting. The most recent is an EICO Classic Series receiver that uses 6BQ5s in push pull as outputs. The ones that came with the receiver look fried and toasted, and test bad.

I figured that something must have gone wrong so I used my new tool and found that the voltage was about -315 on a cheapo digital VOM. I checked the data for the tube and saw that the max voltage for the tube was supposed to be 250.

I then checked the voltage on another of my amps that seems to work fine and sounds good to boot. It is a stromberg carlson 433. To my surprise, the voltage with the biastool is even higher--around 325-330.

Recognizing that I don't know what I am doing, does anyone know if the bias that I am finding is unusually high?

Thanks for any input or observations.
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Old 8th December 2004, 09:44 AM   #2
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The SRS bias tool should measure the current flowing through the tube. Two versions were available: one allows you to read current in mA directly on your multimeter, the other (for meters without an accurate current range) allows you to read current on the mV scale of the meter.
Whatever version you have, a reading of hundreds of volts would suggest either a faulty unit or incorrect connections to the multimeter (more likely)

The voltages you are quoting look like plate voltages, and in a cathode biased amplifier it is normal for these to be higher than the maximum rating for the tube due to the voltage drop across the cathode resistor.
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Old 8th December 2004, 11:45 AM   #3
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If you are measuring the plate voltage, the max for a 6BQ5 is 300V, 250V for some EL84's. That's plate to cathode voltage, i.e. plate voltage minus cathode voltage.
The idle current for a typical 6BQ5 PP cathode biased @ 300V plate and screen grid (measured with respect to ground) should be around 36mA for each tube (17W output).
The Eico HF-86 power amp using two EL84/6BQ5's in PP pentode config:
Plate=340V, cathodes sitting on 165R 5W resistors; 10V=30mA for each tube (2x 14W output).
Here's a link to the Eico Home page. Sorry no "tubed" recievers, but lots of tubed amps!

http://users.rcn.com/fiddler.interport/eico.htm

Wayne
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Old 14th December 2004, 06:03 PM   #4
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Thanks to both of you for being so thoughtful in posting replies. I will try to use each of your comments and investigate further.
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