Hi there, hoping someone can help me. I have an Audio Research CA-50 amplifier and want to check it out properly, put all new tubes in and check the biasing. I have the instructions on the output tube biasing but cannot find anything on input and driver tubes. I have seen the great VT100 input bias video and found a number of old threads that refer to the need to bias the 6922's correctly, unfortunatley the documents linked to in those threads are no longer on line. So does anyone have instructions they could share on how to set up RV1 and RV2 correctly. They may be fine on my amp but without the information as to what they should be set to I cannot tell.
Many thanks
Simon
Many thanks
Simon
Adjust each for the correct plate DC voltages for that stage.
First stage 60V, second stage 240V.
They will interact, so go back and forth until both are correct.
First stage 60V, second stage 240V.
They will interact, so go back and forth until both are correct.
The two stages are direct coupled, so there will be considerable interaction between those two adjustments.
The first adjustment will directly affect the second adjustment. But the reverse will not happen.
The first adjustment will directly affect the second adjustment. But the reverse will not happen.
Hello Rayma and friends on this thread:
I also have an AR CA-50 and am having some difficulty setting the voltages on the input driver tubes. Would really appreciate some help.
I replaced the driver tubes with Electro Harmonix 6922s which is the current tubes recommended by AR. The original tubes that came with the unit were Sovteks. I am replacing them because I'm hearing some crackling sounds when the amp is turned and I see some sparking inside the tubes. Reading some of the forum threads online, owners have said the CA-50 works best on EH 6922s as the unit is "hard on tubes."
I bought a set of 4 from a tube dealer and asked for them to be checked for balanced triode tested.
Setting RV1 for 60 volts is easy but at 60, I could not set RV2 for 240 volts... the closest being at ~280+ volts. I also noted the interaction between RV1 and RV2 and so adjusted both so that RV1 and RV2 were closer to their specified values.
I rotated the tubes and found a combination that got me a bit closer but in the end I was only able to get the values closer but both were off by more than 20% of where they should be.
The amp ran fine for a few months but then I noticed a progressively louder hum in one then both of the channels. A few weeks later the amp started crackling when turned on and exhibited the same behavior as the old tubes.
Question: Does anyone know how the original AR tubes were matched and what tolerances are required?
Does anyone know a reputable vendor that sells EH 6922s and can test/match them properly for the CA-50?
AR will sell a re-tube set of EH tubes, but they want ~$800 for driver and output tubes that they have selected.
Is there a more economical way around this?
Thanks,
Derek
I also have an AR CA-50 and am having some difficulty setting the voltages on the input driver tubes. Would really appreciate some help.
I replaced the driver tubes with Electro Harmonix 6922s which is the current tubes recommended by AR. The original tubes that came with the unit were Sovteks. I am replacing them because I'm hearing some crackling sounds when the amp is turned and I see some sparking inside the tubes. Reading some of the forum threads online, owners have said the CA-50 works best on EH 6922s as the unit is "hard on tubes."
I bought a set of 4 from a tube dealer and asked for them to be checked for balanced triode tested.
Setting RV1 for 60 volts is easy but at 60, I could not set RV2 for 240 volts... the closest being at ~280+ volts. I also noted the interaction between RV1 and RV2 and so adjusted both so that RV1 and RV2 were closer to their specified values.
I rotated the tubes and found a combination that got me a bit closer but in the end I was only able to get the values closer but both were off by more than 20% of where they should be.
The amp ran fine for a few months but then I noticed a progressively louder hum in one then both of the channels. A few weeks later the amp started crackling when turned on and exhibited the same behavior as the old tubes.
Question: Does anyone know how the original AR tubes were matched and what tolerances are required?
Does anyone know a reputable vendor that sells EH 6922s and can test/match them properly for the CA-50?
AR will sell a re-tube set of EH tubes, but they want ~$800 for driver and output tubes that they have selected.
Is there a more economical way around this?
Thanks,
Derek
Amazon has a sale.
https://www.amazon.com/Electro-Harmonix-6922-Balanced-Triodes/dp/B06W5SDH34/ref=asc_df_B06W5SDH34/
ARC buys large quantities of the tubes, burns them in, and matches them for proper operation in the circuit.
Unless you are prepared to do the same, there are no real alternatives other than a redesign of the driver stage.
Of course, you could sell the amplifier and choose something else less tempermental.
https://www.amazon.com/Electro-Harmonix-6922-Balanced-Triodes/dp/B06W5SDH34/ref=asc_df_B06W5SDH34/
ARC buys large quantities of the tubes, burns them in, and matches them for proper operation in the circuit.
Unless you are prepared to do the same, there are no real alternatives other than a redesign of the driver stage.
Of course, you could sell the amplifier and choose something else less tempermental.
Thank you for your reply rayma.
Does ARC match their tubes to some specific characteristic/parameter or is "matching" for balanced triode really a matter of tolerance?
For example ARC matches within 5% vs others that may have a wider variance?
Upscale Audio for example says "Triode balance in Platinum Graded tubes is typically 10% or tighter, but may be higher depending on available stocks. They will also measure at the tippy top for transconductance."
On the other hand if ARC is selecting for some other unpublished proprietary parameter, then I get that I should buy from them.
Thoughts?
Derek
Does ARC match their tubes to some specific characteristic/parameter or is "matching" for balanced triode really a matter of tolerance?
For example ARC matches within 5% vs others that may have a wider variance?
Upscale Audio for example says "Triode balance in Platinum Graded tubes is typically 10% or tighter, but may be higher depending on available stocks. They will also measure at the tippy top for transconductance."
On the other hand if ARC is selecting for some other unpublished proprietary parameter, then I get that I should buy from them.
Thoughts?
Derek
In this circuit, both AC and DC matching are required not only between the triode sections in the same envelope in either stage,
but also between the cascaded triode sections in the first stage and the second stage. The latter is very critical,
because the first and second stages are direct coupled. These are matched quad sections, for both AC and DC characteristics.
ARC elected to omit provisions for DC balance in both stages, so the tubes must be extensively burned in
and then closely matched. Of course, with time they will drift apart, and require replacement.
Aftermarket "matched" for AC tubes will not function properly in this circuit for very long, if at all.
Selecting tubes for maximum gm will be at the expense of the DC characteristics, which are critical in this circuit.
but also between the cascaded triode sections in the first stage and the second stage. The latter is very critical,
because the first and second stages are direct coupled. These are matched quad sections, for both AC and DC characteristics.
ARC elected to omit provisions for DC balance in both stages, so the tubes must be extensively burned in
and then closely matched. Of course, with time they will drift apart, and require replacement.
Aftermarket "matched" for AC tubes will not function properly in this circuit for very long, if at all.
Selecting tubes for maximum gm will be at the expense of the DC characteristics, which are critical in this circuit.
Thanks rayma, really appreciate the explanation.
Btw... here are the adjustment instructions from ARC for the driver stage of the CA-50:
1) Measure pins 1 + 6 of V1 + V2; set rv1 for average of 60 Vdc between triode halves.
2) Measure pins 1+6 of V3 + V4; set rv2 for average of 240 Vdc between triode halves (25v max. difference between halves).
Replace front end tubes as necessary if difference is greater than 25 Vdc.
3) Measure across R35 in each channel and adjust rv3, rv4 for 65 mV.
Btw... here are the adjustment instructions from ARC for the driver stage of the CA-50:
1) Measure pins 1 + 6 of V1 + V2; set rv1 for average of 60 Vdc between triode halves.
2) Measure pins 1+6 of V3 + V4; set rv2 for average of 240 Vdc between triode halves (25v max. difference between halves).
Replace front end tubes as necessary if difference is greater than 25 Vdc.
3) Measure across R35 in each channel and adjust rv3, rv4 for 65 mV.
The input bias sets only the average plate DC voltages (sum/2), not the individual voltages.
That's the problem with tubes not specially selected for this circuit.
A pot could be added to each stage (with the wiper connected to the current source, and the ends of the element
connected to the cathodes) for adjustment for DC balance in each stage independent of tube DC matching.
Each current source would still have its own adjustment for total current (sum of both).
That's the problem with tubes not specially selected for this circuit.
A pot could be added to each stage (with the wiper connected to the current source, and the ends of the element
connected to the cathodes) for adjustment for DC balance in each stage independent of tube DC matching.
Each current source would still have its own adjustment for total current (sum of both).
Hi there,
Thanks for all the info, I managed to get mine working within spec (just) and was going to try duplicating the current sources so as to give seprate bias for each valve. Rayma's idea of a single pot to give balnce adjustment is an interesting and more easliy implemented solution so will try that. Does anyone have any ideas how to derive a balancing solution for the output stage?
Thanks for all the info, I managed to get mine working within spec (just) and was going to try duplicating the current sources so as to give seprate bias for each valve. Rayma's idea of a single pot to give balnce adjustment is an interesting and more easliy implemented solution so will try that. Does anyone have any ideas how to derive a balancing solution for the output stage?
The other alternative would be ditch R14 entirely and use a 10k pot. Any thoughts on what might work best ?