Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fazeka on September 11, 2023, 12:40:43 am
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Hi all,
I have a '72 Ampeg B25 I am refurbing.
I want to put in a trim pot to allow for adjustable bias.
I know I can just put a trim pot in (say, 50k with a ~22k resistor in series) but I want to have a failsafe to prevent damage to the amp/7027s if the trim pot's wiper fails.
How do I go about doing this?
Schematic (https://ampeg.com/support/files/Schematics/B%20Series/B-25B/B-25B%20Schematics%201.pdf) of amp in question.
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> put in a trim pot
Where?
(hint: pick the right place, an open wiper makes max neg voltage.)
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If you want to add a Trimmer as to be able to allow adjustable Bias
and you want to be safe avoiding problems due to wiper disconnection
arrange a similar circuit and add the trimmer in a similar way
(https://i.imgur.com/TarfxaN.jpg)
(https://i.imgur.com/6FbuhaP.jpg)
On this link you can see example on a B-15 N
https://www.talkbass.com/wiki/technical-amplifier/ (https://www.talkbass.com/wiki/technical-amplifier/)
Franco
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There are many circuits.
1- Most important : Use a good pot, not a trimmer 2 cents pot
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Bad pots I see too often;
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I know I can just put a trim pot in (say, 50k with a ~22k resistor in series) but I want to have a failsafe to prevent damage to the amp/7027s if the trim pot's wiper fails.
Just replace R38 with your pot/series resistor. If the pot wiper opens the circuit will put out max negative voltage, automatically protecting your tubes.
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Connected as rehostat I suppose, correct ?
Franco
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Connected as rehostat I suppose, correct ?
Yes. I like to connect the wiper to one end of the pot. This way if the wiper opens the pot resistance goes to it's max value (50K in this example) and the bias voltage will simply increase as if you had turned the pot to the coolest setting. Probably still usable. If you don't connect the wiper to the to the end terminal, the pot goes to an open circuit and the bias voltage will increase to a much higher value, probably enough to bias the tubes way to cold.
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Bad pots I see too often;
Not sure why you say the Amphenol Piher trim pot is a bad choice to use in a bias circuit. it has some pretty decent ratings and Doug has similar pots for insert into his turret boards as a bias trimmer.
It surely is satisfactory. https://www.mouser.com/ProductDetail/Amphenol-Piher/PT15WH06-103A2020-7CR?qs=F72wFm1HmI8JArhI5GqnPQ%3D%3D
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I've never had to replace a bias pot.
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I've never had to replace a bias pot.
Is a dirty wiper connection possible?
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I've never had to replace a bias pot.
Is a dirty wiper connection possible?
Anything is possible. But if you wire it as suggested no harm will come. It's FAILSAFE!
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That way you can avoid the use of a safe resistor connected between wiper and one pin of the pot
Franco
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That way you can avoid the use of a safe resistor connected between wiper and one pin of the pot
Exactly. But if you look at the Fender style bias pot, you'll see the pot is wired as a three terminal voltage divider pot. If the wiper fails you will lose all bias voltage and harm the output tubes. The Fender circuit could benefit from an additional failsafe resistor.
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Different approach with same result but I consider more elegant the rehostat connection, the added safe resistor seems something added to put a patch
K