Triode Mode with Fixed Bias?

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eddiejg
Senior tube assistant
Posts: 242
(3/7/04 8:13 pm)
 Triode Mode with Fixed Bias?
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 Does it work?
I've only set up cathode biased amps with triode mode so far.
If it works, will I have to adjust bias when switching between triode & pentode?
Thanks.
-Ed
 
GroundhogKen
Forum Moderator
Posts: 2311
(3/7/04 8:47 pm)
 Re: Triode Mode with Fixed Bias?
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 No problem and no need to rebias unless you have a big dropping resistor ( instead of a choke ) between the center tap and the screen supply.

Ken
 
eddiejg
Senior tube assistant
Posts: 243
(3/7/04 10:00 pm)
 Triode Mode with Fixed Bias?
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 Ken-
Thanks.
The dropping resistor is 2.7K.
That's probably OK, right?
-Ed
 
GroundhogKen
Forum Moderator
Posts: 2312
(3/7/04 11:36 pm)
 Re: Triode Mode with Fixed Bias?
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 In pentode mode if the screen voltage is somewhat less than the plate voltage, then the plate current will increase when you connect the screens to the plate for triode mode.

As long as the tubes are not biased really hot in pentode mode, they should be OK in triode mode.

Measure the bias in both modes and then you'll know what going on each time you flip the switch.


Ken
 
eddiejg
Senior tube assistant
Posts: 244
(3/8/04 11:44 am)
 Triode Mode with Fixed Bias?
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 Ken-
So, just find a happy medium so it's not too hot or cold in either mode.
Thanks.
I figured that it should work with fixed bias, but the Torres book has a section on triode mode & he says set the amp up in cathode bias.
The book has a lot of good info (I keep going back to it), but it is poorly written.
-Ed
 
eddiejg
Senior tube assistant
Posts: 246
(3/12/04 11:20 am)
 Triode Mode with Fixed Bias?
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 What value resistor should I use for the screen grids in this setup?
470?
1K?
Thanks.
-Ed
 
GroundhogKen
Forum Moderator
Posts: 2316
(3/12/04 12:30 pm)
 Re: Triode Mode with Fixed Bias?
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 Typically,

470 ohms for beam tetrodes like 6v6, 6L6, 5881, 6550,

1K ohms for pentodes like EL34 although 470 ohms is probably good enough for EL84.

Ken
 
eddiejg
Senior tube assistant
Posts: 247
(3/12/04 3:00 pm)
 Re: Triode Mode with Fixed Bias?
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 Ken-
Thanks for the thorough answer, especially since I didn't specify that I'm suing EL84's.
I've used 1K for 6V6's in the past.
Is voltage / current a factor when choosing the value?
-Ed
 
 
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