UL to triode mode conversion on cathode bias amp-different cathode resistors needed?

bigwill · 29th December 2007, 07:08 PM

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

http://www.freeinfosociety.com/elect...dfordsta15.gif

tubewade · 29th December 2007, 07:39 PM

bigwill,

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.

Burnedfingers · 29th December 2007, 07:39 PM

Your link didn't work for me.

Svein_B · 29th December 2007, 07:40 PM

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.
http://www.vacuumstate.com/schematics.htm

SveinB.

PS. I had no problem with the link

HollowState · 29th December 2007, 07:48 PM

Well it wouldn't work directly for me either. Sometimes you have to go through the home page protocol first.

http://www.freeinfosociety.com/elect...page.php?cat=1

bigwill · 29th December 2007, 08:02 PM

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

hemgjord · 30th December 2007, 12:37 AM

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.

29th December 2007, 07:08 PM	#1
bigwill diyAudio Member Join Date: Dec 2004 Location: UK	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 http://www.freeinfosociety.com/elect...dfordsta15.gif

29th December 2007, 07:48 PM	#5
HollowState diyAudio Member Join Date: Aug 2006 Location: Taxland, New Jersey	Well it wouldn't work directly for me either. Sometimes you have to go through the home page protocol first. http://www.freeinfosociety.com/elect...page.php?cat=1 __________________ "At the heart of quantum mechanics is a rule that sometimes governs politicians or CEOs - as long as no one is watching, anything goes." ~ Lawrence Krauss

30th December 2007, 12:37 AM	#7
hemgjord diyAudio Member Join Date: Apr 2007	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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29th December 2007, 07:39 PM	#2
tubewade diyAudio Member Join Date: Jun 2007	bigwill, 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.

29th December 2007, 07:39 PM	#3
Burnedfingers diyAudio Member Join Date: Nov 2002 Location: Iowa	Your link didn't work for me.

29th December 2007, 07:40 PM	#4
Svein_B diyAudio Member Join Date: Oct 2005	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. http://www.vacuumstate.com/schematics.htm SveinB. PS. I had no problem with the link

29th December 2007, 08:02 PM	#6
bigwill diyAudio Member Join Date: Dec 2004 Location: UK	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