Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on October 30, 2022, 03:08:31 pm
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Hi guys - Concerning an Ampeg B-15-NF would you consider a static bias setting with Sovtek 6L6WXT power tubes right about 73% of 30W acceptable?
PV - 493 VDC
PI - 45 ma
Or should I up the 100K bias resistor to get the static dissipation down to around 60% ? If so what value would you suggest.. ?
https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_b15nf_schematic_4_65.pdf
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Or should I up the 100K bias resistor to get the static dissipation down to around 60% ? If so what value would you suggest.. ?
You need to decrease that 100K in order to decrease plate current. Or you could increase the 47K for the same results.
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by adding 1 Ohmer's you can monitor real-time results, I was pretty shocked at how much current hard pickin likes.
even at 60-some % Average current was getting close to the warning zone.
but these are guitar amps, so usually tone wins over engineer, hands down :laugh:
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Oh yeah sorry decrease thanks.. And it's a bass player that will play it. Old guy like us so not going to play that loud. I was thinking more around 60% or even less would be better for the amp.
Thanks guys.
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...shocked at how much current hard pickin likes....
That extra on-demand power goes to the load, not the tube. And is ultimately determined by supply voltage and load impedance, not bias.
If 63% is the coolest you can go with THIS set of tubes, find a way to get less bias current (more bias voltage), so when you replace tubes and get a "hot" set you can dial it down.
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Thanks PRR I replaced the 47K with a 56K and the amp quiescent current is at 50% and I feel a whole lot better about it. It also sounds pretty good to me.
Shooter I have a Hoffman bias checker I made eons ago .. used it and trying to keep the amp as stock as possible.