Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bbmade on June 11, 2025, 10:38:38 am
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I've asked a similar question and was able to refer back to it.
Right now I've got a mid-60's Super Reverb (AB 763) and replaced the non-original bias capacitor with a 25uf @ 50V like the original schematic. My thought is, make it as close to original as possible, for the person who wants to buy it.
Many people (and someone previously) put a 47uf and I'm wondering what the rational is?
Also - what are you opinions on leaving it as Leo intended :m11
Plenty more work to do on this and I'm not opposed to changing it to 47uf.
Thanks all
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If you grab a handful of 50uF e-caps and measure their capacitance on a tester, it’ll be +/- 10% or so. Same with 47uF caps. In fact for the purposes of tube amps, either will do the same job (if the voltage rating is sufficient)
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I should add, tube amp design is a ballpark game. If it’s in the ballpark, it’s in the game.
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I've got a mid-60's Super Reverb (AB 763) and replaced the non-original bias capacitor with a 25uf @ 50V like the original schematic.
The 50V rating is insufficient. Raise this to at least 63V, but I prefer an even 100v. You can measure this voltage for yourself to prove that 50v is too low.
And since you asked, this is my preferred bias arrangement these days. I've taken to using the positive swing of the bias tap for heater elevation on vintage Fender style amps. I'll add a link to that thread and discussion.
Update: here's that thread. https://el34world.com/Forum/index.php?topic=30961.0
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Thank you! I will up the voltage for sure...
Thoughts on the capacitance rating? Is upping the capacitance beneficial? I'm sure I have some 47uf @100V around here.
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I find increasing the capacitance to be beneficial, also adding a second filter node helps with ripple without adding too much capacitance.
You'll see this on the marked up schematic I posted.
It's definitely possible to overdo the extra capacitance, and there are diminishing returns over about 47μF--which is likely more than sufficient on its own, honestly. If you study the evolution of the AB763 amps into later Silverface designs you'll see lots of variation. Then looking backwards from there to the Bassman/JTM arrangement you'll see lower capacitance, but two filtering nodes. Lots of ways to skin this cat.
It's been pretty standard to see 47-100μF @ 80-100V with no second node on many well maintained and serviced vintage Fender amps.
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I've got a mid-60's Super Reverb (AB 763) and replaced the non-original bias capacitor with a 25uf @ 50V like the original schematic.
The 50V rating is insufficient. Raise this to at least 63V, but I prefer an even 100v. You can measure this voltage for yourself to prove that 50v is too low.
And since you asked, this is my preferred bias arrangement these days. I've taken to using the positive swing of the bias tap for heater elevation on vintage Fender style amps. I'll add a link to that thread and discussion.
Update: here's that thread. https://el34world.com/Forum/index.php?topic=30961.0
Edit: I had a question but I figured it out. This looks like a great idea, solves the problem of not having a convenient place to elevate heaters in a fixed bias amp.