Should Pentode/Triode switch affect the &nb

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bluesbear
I only work on Fender's
Posts: 518
(5/26/05 3:36 pm)
Reply  Should Pentode/Triode switch affect the bias... Found it!!
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 Here's one I've never checked before; maybe they've all been this way.
If I bias the amp on pentode and switch to triode (using one ohm cathode resistors), the bias current jumps from 25ma to 60ma. If I bias it to 25ma on triode and switch to pentode, it drops to 3 or 4ma. Is this how it's supposed to work? If so, how do the tubes survive? If this is normal, which setting should I use to bias?
Thanks,
Dave

Edited by: bluesbear at: 5/27/05 5:59 pm
 
OldHouseScott
Hey get your own solder
Posts: 223
(5/26/05 3:55 pm)
Reply  Re: Should the Pentode/Triode switch affect the bias?
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 Are you using screen resistors and if so what value? Also if so, are you keeping them in circuit when switching to triode mode? Also, are you using a choke, or a resistor between the plate and screen taps on the PS?

With no or low valued screen resistors, and with no choke, I can imagine a vast difference in screen current between pentode and triode mode. This is a case where the self-regulating nature of cathode-biasing would help greatly.

OldHouseScott
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bluesbear
I only work on Fender's
Posts: 519
(5/26/05 5:02 pm)
Reply  Re: Should the Pentode/Triode switch affect the bias?
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 Hi Scott,
That's why I'm confused. I'm using 1K 5w sandbox resistors and they are staying in the circuit when switched. I'm using a choke.
Thanks!
Dave
 
bluesbear
I only work on Fender's
Posts: 521
(5/27/05 6:14 pm)
Reply  Confusion is the problem
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 I traced back from the screen resistors and took a closer look at the filter caps. I haven't a clue how I screwed it up so badly or why the amp worked as well as it did. This is a 5F6a in a Princeton chassis. I had already changed the can style cap to 40-20-20-20 so I thought I was ready to go... except that I had everything except what went to the first terminal wired to the wrong spot! I even had the choke wired across the 10k resistor... and IT was in the wrong place. I corrected it all and everything is fine. The bias jumps from -25 to -26 when switched now.
Scott, I don't know when or how I would have found this if I hadn't been double checking your thoughts. Your assistance has been vital!
Thanks!!!!!
Dave
 
 
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