Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => EZ Board postings => Topic started by: Geezer on June 18, 2005, 06:36:25 am
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Hoffman Amplifiers
> Tube Amp Building - Tweaks - Repairs
> plexi bias
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jerrydyer
Junior tube assistant
Posts: 37
(4/22/04 11:56 pm)
plexi bias
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watt is the correct bias for a 50 watt and a 100 watt both els4's thanx also can you bias it while on or should I just stick to standby?
Tiny Daddy
I will work on all amps
Posts: 631
(4/23/04 8:05 am)
Re: plexi bias
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Bias current depends on the tube's plate voltage. Install 1-ohm resistors on the cathodes for measuring purposes, then adjust the current for 70% plate dissipation (17.5 or 18 Watts for EL34). This cannot be done in standby. For example if the plate voltage is 500V then 18 divided by 500 = 36mA per tube. Measure the plate voltage at the plate before and after adjusting and you may have to re-adjust.
jerrydyer
Junior tube assistant
Posts: 38
(4/23/04 10:08 am)
bias
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is it safe to touch pin 5 with a meter while amp is on ? thanx
Tiny Daddy
I will work on all amps
Posts: 632
(4/23/04 10:31 am)
Re: bias
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Pin 5 being the control grid: If you have a good high-impedance meter with fet or tube input, you may read the bias voltage. If it's a low-impedance VOM it may load the bias to a lower level, give an inaccurate reading and maybe cause the tube to over-dissipate if left connected.
Be sure the speaker is connected any time a tube amp is on.
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