How to identify type of bias used for my tube amp

Rispoli · 17th October 2009, 01:11 PM

What are some easy identifiers, which will tell me what type of tube bias is used in my amp. (no schematic available) Each mono-bloc has a 1AT7WA input and a 6829 phase splitter/driver tube. There are 2 banks of 4 each EL-34's and there are 2 massive output transformers. They are Antony Michaelson Audio (Musical Fidelity) Chronos Monoblocs, which were designed by Antony and Paravicini of EAR fame. I believe the construction and the OT's are custom built by Manley, as it was built in the US and there are some original tubes with the Manley label.There is a feature article in Hi-Fi news from 1991 that gives a general overview, but few details. It is rated at 112 watts output. Not sure what mode of operation/ triode, pentode, ultralinear, etc. It does not appear that there is any way to adjust the bias via any pot adjusters. I replaced all the tubes with matching EH EL-34's about 3 years ago when I got the amps and they still test pretty strong. Will probably get another year or two out of them. I probably have nothing to worry about, but am curious about the operation of them. Any insights would be appreciated. By the way, they sound very, very good.

pjp · 17th October 2009, 01:35 PM

Resistor from cathode to ground ?
What exactly are you asking ?

analog_sa · 17th October 2009, 01:55 PM

Based on your desription it is most likely pentode or ultralinear in class AB with some form of automatic bias - cathode resistors or CCS.

boywonder · 17th October 2009, 07:58 PM

If you could sketch (or describe in a fair bit of detail) what Pins 5 and 8 of the EL34s are connected to, that may help to answer your question.

Rispoli · 18th October 2009, 12:49 PM

Hi and thanks, I used a mirror to get under the circuit board without disassembling and could not tell about pin5, but cathode pin 8 has a 330 ohm resister going to ground on each el34. It looks like this is cathode biased. I thought, from what I have read that this is rare for a high powered amp 112 tube watts? If this is the case, I should be good to go with any tube in the el34 family without any alterations/ adjustments, as the e34L has different voltage ratings (for example), right? When I first got the amps, they had 3 different brands of tubes that I'm sure were not matched and they still sounded good. But, I soon replaced with EH matched octects. I am looking now to try the fatboy types.

17th October 2009, 01:11 PM	#1
Rispoli diyAudio Member Join Date: May 2006	How to identify type of bias used for my tube amp What are some easy identifiers, which will tell me what type of tube bias is used in my amp. (no schematic available) Each mono-bloc has a 1AT7WA input and a 6829 phase splitter/driver tube. There are 2 banks of 4 each EL-34's and there are 2 massive output transformers. They are Antony Michaelson Audio (Musical Fidelity) Chronos Monoblocs, which were designed by Antony and Paravicini of EAR fame. I believe the construction and the OT's are custom built by Manley, as it was built in the US and there are some original tubes with the Manley label.There is a feature article in Hi-Fi news from 1991 that gives a general overview, but few details. It is rated at 112 watts output. Not sure what mode of operation/ triode, pentode, ultralinear, etc. It does not appear that there is any way to adjust the bias via any pot adjusters. I replaced all the tubes with matching EH EL-34's about 3 years ago when I got the amps and they still test pretty strong. Will probably get another year or two out of them. I probably have nothing to worry about, but am curious about the operation of them. Any insights would be appreciated. By the way, they sound very, very good.

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17th October 2009, 01:35 PM	#2
pjp diyAudio Member Join Date: Nov 2006 Location: under the rainclouds	Resistor from cathode to ground ? What exactly are you asking ?

17th October 2009, 01:55 PM	#3
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	Based on your desription it is most likely pentode or ultralinear in class AB with some form of automatic bias - cathode resistors or CCS.

17th October 2009, 07:58 PM	#4
boywonder diyAudio Member Join Date: Jan 2008 Location: So.Cal.	If you could sketch (or describe in a fair bit of detail) what Pins 5 and 8 of the EL34s are connected to, that may help to answer your question.

18th October 2009, 12:49 PM	#5
Rispoli diyAudio Member Join Date: May 2006	Hi and thanks, I used a mirror to get under the circuit board without disassembling and could not tell about pin5, but cathode pin 8 has a 330 ohm resister going to ground on each el34. It looks like this is cathode biased. I thought, from what I have read that this is rare for a high powered amp 112 tube watts? If this is the case, I should be good to go with any tube in the el34 family without any alterations/ adjustments, as the e34L has different voltage ratings (for example), right? When I first got the amps, they had 3 different brands of tubes that I'm sure were not matched and they still sounded good. But, I soon replaced with EH matched octects. I am looking now to try the fatboy types.

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