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I draw: fixed vs cathode

5/1/2006 11:14 AM
drewl I draw: fixed vs cathode
so which would draw more current from a power trans, fixed bias or cathode bias?  
I'm rebuilding an old Masco into something like a champ, as the output tube started to warm up and draw more current, the output then started dropping off.  
i'm thinking the power tranny can't keep up.
5/1/2006 1:53 PM
Wild Bill

For similar operating voltages, they draw the same!  
Once again, the tube doesn't care HOW you bias it! It only cares about the voltage between plate and cathode and how much negative is the grid with respect to the cathode.  
If the Masco worked before, then why can't the power tranny keep up now? Perhaps there is something else wrong that is causing that tube to draw too much current...  
---Wild Bill
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5/1/2006 6:41 PM

Possibly the tube is failing, or the amp is going into oscillation that you can't hear, sapping the power supply.  
Method of bias indeed doesn't matter. But if you are changing from a class AB push pull to a single ended class A design, then the idle current will go up because class A runs at full current all the time.
5/2/2006 5:57 AM
I had asked for ideas on this sucker a while back, but noone had any insight.  
It's a single ended 6L6 masco that used a choke with back biasing to develop a neg. bias voltage.  
the filter caps were bad, which killed the rectifier.  
after replacing the bad components, the amp sort of worked but had very low power, and did oscillate all over the place.  
I tried making it Cathode bias to see if that would help, it did not.  
Since i needed to order a champ OT for another amp, I got a second one to try and get this amp up and running.  
well, it now has a bit more output, but not what it should.  
I'd probably be better off just gutting it, and starting over.
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