Amp Bias, hotter or colder?

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NippleWrestler
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Re: Amp Bias, hotter or colder?

Post by NippleWrestler »

You need a multimeter, a crocodile clip, and a mini screwdriver.

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Re: Amp Bias, hotter or colder?

Post by tubeswell »

The hotter you bias the output toobs in a push-pull amp, the less maximum output power you will have as the amp get louder.

If both toobs are biased to operate in Class A (where both toobs are conducting all the time throughout the signal cycle), there is no point within the signal cycle at which either toob will be able to go into the 'extra power' zone that Class B delivers.

Running a toob at 60-80% of its possible dissipation limit means that you're reserving the additional 20-40% of the Class-B part of the signal cycle (where each on-toob goes into over-dissipation) for signals above the idle power. It's what lets you have a bigger output power than Class A. It's also what lets you increase the B+ some to get that bigger power without overheating the toobs.

Tone-wise its hard to say what the difference is where you're talking about an output toob, because as the amp gets louder the finesse around what the human ear can detect in terms of tonal differences gets reduced as your eardrums get hammered more. The effect of hotness or coldness on bias is more detectable where you're talking about a pre-amp toob.

If your aim is to have a 20W amp you can hear well in a pub, then bias the output toobs at about 60-70%, so the amp puts out more power.
He who dies with the most toobs... wins

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griff
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Re: Amp Bias, hotter or colder?

Post by griff »

tubeswell wrote:The hotter you bias the output toobs in a push-pull amp, the less maximum output power you will have as the amp get louder.

If both toobs are biased to operate in Class A (where both toobs are conducting all the time throughout the signal cycle), there is no point within the signal cycle at which either valve will be able to go into the 'extra power' zone that Class B delivers.

Running a valve at 60-80% of its possible dissipation limit means that you're reserving the additional 20-40% of the Class-B part of the signal cycle (where each on-toob goes into over-dissipation) for signals above the idle power. It's what lets you have a bigger output power than Class A. It's also what lets you increase the B+ some to get that bigger power without overheating the toobs.

Tone-wise its hard to say what the difference is where you're talking about an output valve, because as the amp gets louder the finesse around what the human ear can detect in terms of tonal differences gets reduced as your eardrums get hammered more. The effect of hotness or coldness on bias is more detectable where you're talking about a pre-amp valve.

If your aim is to have a 20W amp you can hear well in a pub, then bias the output toobs at about 60-70%, so the amp puts out more power.
Cool man, that explains a lot.

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Re: Amp Bias, hotter or colder?

Post by chur »

griff7628 wrote:
chur wrote:
griff7628 wrote:Thank god I know a tech who can do this shit!
I have my JCA22H in getting new power valves currently, one started arcing and randomly glowing bright red.. highly annoying and distracting mid solo with volume cutout.. oddly, other band members didn't say a thing, clearly they weren't listening to my awesomeness :wtf:
hahaha
Biased too?
Yep, I think it's compulsory.
No one ever died of hard work.. but why take the risk..

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