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Old 29th December 2007, 07:08 PM   #1
bigwill is offline bigwill  United Kingdom
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Default UL to triode mode conversion on cathode bias amp-different cathode resistors needed?

Quick question - I have a Radford STA15 and I'd like to try it in triode mode. It currently has KT77s in it and biases up fine. Am I right in guessing ultra linear output stages essentially act as triodes at around DC? (The screen goes to B+ through the OPT through a resistance much like connecting the plate and screen together via a resistor)

Will disconnecting the ultra linear tap force me to change the cathode resistance to retain the correct bias?

Amp schematic

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Old 29th December 2007, 07:39 PM   #2
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The schematic link did not work for me.

You should not have to change the cathode resistance(s) to switch from UL to triode mode. The voltage at the screens should not change from one configuration to the other and the quiescent current through the valve should be the same either way.

Best regards.
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Old 29th December 2007, 07:39 PM   #3
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Your link didn't work for me.
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Old 29th December 2007, 07:40 PM   #4
Svein_B is offline Svein_B  Norway
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I believe that should be quite OK.

A as reference, Allen Wrights PP-1C is designed from the start for triode operation and it also uses 470 ohm cathode resistors for EL34 at around 380V.


PS. I had no problem with the link
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Old 29th December 2007, 07:48 PM   #5
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Well it wouldn't work directly for me either. Sometimes you have to go through the home page protocol first.

"It is hard to imagine a more stupid or more dangerous way of making decisions than by putting those decisions in the hands of people who pay no price for being wrong." ~Thomas Sowell
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Old 29th December 2007, 08:02 PM   #6
bigwill is offline bigwill  United Kingdom
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Thanks very much everyone, I will perform the mod and see how it sounds! I find the amp to be slightly solid statey - it has a large amount of negative feedback which may contribute to this
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Old 30th December 2007, 12:37 AM   #7
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Hi, be careful with all that feedback.
Triode connecting the outputs will change miller cap. and Rp, this will move around 3dB points and potentially cause instability.
I'm not allowed to do magic, union rules...
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