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Is 220mA too Hot?

5/23/2000 5:17 PM
Dan Z Is 220mA too Hot?
Help! I tried to blackface my 71 Bandmaster Reverb, and now I read 220mA for the bias with the Transformer shunt method as described in G. Weber's book. Yes, the tubes get an orange streak on the plate, so I haven't had it running for more than a minute of two at a time. Where did I mess up to let the tubes see that much current? Any suspect components to look at first, or should I just start over?  
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5/23/2000 5:50 PM
Peter S

220ma is a HUGE amount of current. I would double check to see that your meter is working correctly and you are measuring it correctly. With 220ma of current draw the tubes should be glowing like the core of nuclear reactor. Also your plate voltage will be way down......maybe around 320 to 350 volts. Check your plate voltage and the negative bias voltage on pin 5 of the output tubes. With a "normal" current draw on that amp your plate voltage should be around 450 and you should see something like -42 to -50 VOLTS DC on the grids(pin5) of your power tubes. If your voltages are somewhere near these, then you probably arent drawing that much current. Verify that your meter is working correctly with another one if you can. Double check to see that your are measuring current across the primary of the OT. Alternatively you could lift your power tube cathodes from ground, install 1ohm matched resistors and measure the voltage drop across it. You can then directly convert the voltage drop to cathode current which will be approximately the same as plate voltage.....about 2 ma higher with 6L6's cuz you'll be seeing the screen current too.  
5/23/2000 6:13 PM
Dan Z
Thanks for responding Peter. 220mA seemed pretty ridiculous to me, but I didn't want to leave the amp on long if that reading was right. I placed the negative lead on Pin 3 and the positive at the junction of the circuit board where the red wire out of the OT connects. (Obviously, I'm new at this.) What worried me was that turning the bias adjustment pot didn't seem to have any effect. How do I work back from the tubes to the bias pot to the power supply to find the faulty component?  
Do I measure the negative bias voltage simply by placing a lead on both power tubes' pin 5? How do I check the plate voltage?  
Dan "Stupid Novice" Z
5/23/2000 6:45 PM

"Do I measure the negative bias voltage simply by placing a lead on both power tubes' pin 5? "
No, with volt meter set up to read voltage, connect your black lead to the chassis and the red lead to pin 5.  
"How do I check the plate voltage?  
Place black lead to chassis and red lead to pin 3. Careful, lot of volts there and a slip could mean trouble.  
5/23/2000 6:30 PM
Jim S.

I think you have a bias fault. With the standby OFF, measure the voltages on pin 5 on each output tube socket. Black lead clipped to ground and red lead clipped to the pin 5 terminal. You should read a negative voltage somewhere between -42 and -50 volts. If you don't see this, then something got miswired or disconnected in the bias or phase inverter circuit, or maybe one of the wires connecting to the 1K5 control grid resistors at pin 1 got broken off. Also check those grid resistors and make sure they haven't opened up.  
5/23/2000 9:22 PM

Dan Z,  
You seem like an excellent candidate for a good tech.  
Check out your bias circuit fully, with the amp on standby, as the previous post said. Get some Fender schematics and study the bias circuits, it's really only a handful of parts. Also make sure your bias control is adjustable and not a 'bias balance' control. This could be your problem of not having the ma vary as you turn the pot. I recommend the adjustable bias control style found in Fender blackface amps. If you don't already have that style of circuit, consider putting it in. Once you're sure the bias circuit is good then progress on to the actual biasing.  
I think, until you've reached a better comfort zone in your amp knowlege, you might consider installing the 1 ohm resistors from pin 8 of the power tubes to ground and biasing your amp in that fashion, as has been previously detailed.  
This could be alot safer for you and you won't inadvertently screw up your amp!  
It takes a while to learn what parts not to put your tongue on.  
5/24/2000 12:36 AM
Dan Z
Thanks for the kind words, Bob.  
I will try all of the suggestions. I converted to a real bias adjustment pot when I blackfaced. But that work may be the problem. The steps above seem like a good way to attack the problem.  
Is there any info available on test points throughout the circuit, with instructions for a beginner like me?  
Dan Z

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