Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Yosser on January 08, 2026, 05:40:30 am
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This amplifier has not been used for about 8 years so I am still checking it out before replacing anything and testing
The fixed bias resistor has been replaced by two resistors in series ( as circled in the photo) which total 153k rather than 180k as in the schematic.
This leaves no room for adjustment and also maybe the resistor should be rated at 5watt.
As it is a printed circuit board it might be easier to bypass this part of the circuit and snip out the two incorrect resistors as in the photograph
Would it be okay to leave the existing diode in place on the board as it would be a pain to remove?
(Btw I will be replacing the big psu caps)
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That bias pot is very dangerous. It will allow the bias voltage to be adjusted to zero. This can quickly kill the EL34s.
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That bias pot is very dangerous. It will allow the bias voltage to be adjusted to zero. This can quickly kill the EL34s.
BTW there is a 47k resistor in parallel with the 47uf capacitor on the pcb (see attached schematic on the first post) but
is it better like this like this with the 47k in series with a 22k trim pot:
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... is it better like this like this with the 47k in series with a 22k trim pot:
Much better
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... is it better like this like this with the 47k in series with a 22k trim pot:
Much better
What type/ rating should the trim pot be?
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Hi Yosser
See attached Bias Sch ideas. No great requirement for trimpot but I personally like Cermet (eg Bourns 3xxx series, 0.5W).
See attached link to Sluckey's site for great ideas esp the Scrapbook (thanks Sluckey for your kindness)
https://sluckeyamps.com/index.htm (https://sluckeyamps.com/index.htm)
Regards
Mirek