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Vintage threads from the first ten years

EL34 bias

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11/11/1998 10:17 AM
Geoff	EL34 bias I am converting a US JMP 50 from 6550s to EL 34s. Once I have physically changed the bias circuit, what is a rough idle current I should be shooting for?

11/12/1998 5:16 AM
Liam	I normally go for about 32 ma on the JMP 50. I start at -45 volts and work down. A lot of people seem to bias much hotter than this, but it sounds pretty sweet to me and the tubes last very well.

11/12/1998 6:21 AM
R.G.	The Tube" target="_blank">http://www.eden.com/~keen">Tube Amp FAQ section on biasing has a chart of "typical" bias points based on experience.

11/12/1998 6:39 AM
Kevin Timm	I'd go low to mid 40's ma 43 -47. Kevin

11/12/1998 7:23 AM
Liam	Doesn't that make the plates glow on your one? Does on mine.

11/12/1998 1:52 PM
Derrick	I have trouble with even 27mA sometimes! Is this because Most of my old Marshalls/Hiwatts/Orange, etc. have such high plate voltages??? How can I make these amps work with tubes drawing more than 25-27mA??? -Derrick

11/13/1998 5:15 AM

Please explain, Mr. H!

Derrick,

You could lower B+, or play with the screen and grid resistors, but why? I honestly believe that people biasing English amps at 40ma must be measuring it wrong. I have similar amps, and I have biased at similar currents to you. All my old amps sound great on far lower bias currents than I ever see posted here. A thing I have found over the years is that different makes of tube will take different bias currents without plate glow (or worse). I normally avoid Chinese tube wherever possible, as they glow at very low currents. The Sovteks seem marginally better, but Svetlanas are my favourite from current manufacture. They seem very consistent, and don't normally glow until 36-40 ma. However I still don't normally bias them that high. (I'm trying 35 ma in a DR103 at the moment - I'll post if anything disastrous happens!)

Liam

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