Quick bias question. 6CA7 and EL34's.

Amp is a Randall RM100 so I can bias each individual tube. I had all Winged C 34's in the RM100 biased up each one between 30mV and 31mV. I just dropped in a set of 6CA7's to mix with the 34's and am really liking the results after day 1. I now have all of them at just over 31mV.

According to the manual, I am in range on the cooler side. Based on some numbers that someone on the MTS board posted years ago I am on the warmer side. Any thoughts on whether I can go a little hotter or should I stay where I am at?

Per the Randall manual:

6L6/5881 28mV to 35mV
EL34/6CA7 30mV to 38mV
E34Ls 35mV to 45mV
6550 35mV to 45mV

I think I'd bump them up to around 34-35. Heck, give it a shot and see if you like it better. You can always bump 'em back down. I think you are safe though.

bias range is calculated by the b+ of the given amp.

1. for an amp with a B+ of 480dcv, 70%MPD would be 36mA
2. for an amp with B+ of 400dcv, 70%MPD would be 43mA

in example #1 45mA would be way hot.
in example #2 35mA would be way cold.

so its not as simple as someone saying "bias them at 35mA" unless they knew that that particular amp had a B+ of around 500dcv.

You need to find the B+ as part of the equation when biasing and then do the math.

Mark

rockstah":ljqv80ba said:

bias range is calculated by the b+ of the given amp.

1. for an amp with a B+ of 480dcv, 70%MPD would be 36mA
2. for an amp with B+ of 400dcv, 70%MPD would be 43mA

in example #1 45mA would be way hot.
in example #2 35mA would be way cold.

so its not as simple as someone saying "bias them at 35mA" unless they knew that that particular amp had a B+ of around 500dcv.

You need to find the B+ as part of the equation when biasing and then do the math.

Mark

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Thank you. I believe the RM's are at 500dcv. I will have to dig a little deeper to make sure though.

fek":2t1ydp0s said:

I now have all of them at just over 31mV.

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That's very cold.

6CA7, EL34, KT77, E34L should all be minimum ~35mA – you're not going to be completely out of crossover distortion (for any tube power amplifier) at a setting of <=31mA.

Maybe not the best method....

I just use my ear .... if it is in the recommended range and sounds good is what I do.
With some amps the differences are much easier to hear. If you run the amp low dB's in general most people seem to like a higher bias setting. If you crank the dB's most people will like a lower bias setting.

If set too low you will get crossover distortion (as mentioned) ~ sounds kind of nasty/harsh. Some people like a little. Too hot and the midrange will be exaggerated and you are going to have a shorter tube life.
Bottom line is not everyone has the same taste or is going to use the amp the same way (dB's,etc).

Depending on the amp / taste / application.... EL34/6CA7 I have seen set from 25 to 40ma....

Thank you for all of the replies. Builders and techs, good thought provoking wisdom to go on.

King, you could be right on. The manual says 30 - 38. A tech on the MTS board said he measured the plate voltage on these from 500 to 525 so I used 515 to come up with about 34mA. The heat gets turned up tonight!

I have all 4 tubes at about 33.5 mA and all seems well. Great tone and feel so I will leave them right where they are at. Thanks again for the help.

fek":2b5ltpme said:

A tech on the MTS board said he measured the plate voltage on these from 500 to 525 so I used 515 to come up with about 34mA. The heat gets turned up tonight!

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WOW! That's some serious PV. My Herbert reads out ~480VDC. I'm very much a big advocate of the 25W JJ-E34L power tube (beautiful tone and Matador balls; don’t like JJ preamp tubes at all – for my tone taste). I run them "each" in my Herbert at ~36mA which is ~70% dissipation for each (25W/480VDC = 0.052 * 0.7 = .036 or 36mA).

This gives me plenty of power tube life expectancy. The JJ-E34L power tube is a good deal more resilient than any other EL34 type power tube I can find.

Wow Rockstah spam necrobump from the other side! We hear you

Yeah, that was strange... Dam