I have just brought a non-working AC30 from 1974 back to life, and everything is now working fine except the vibrato and tremolo. I'm going to do some more work on it over the weekend, so hopefully I'll be able to work it out. Strangely you can hear the tremolo effect after the power switch is turned to off, as the sound fades. So the oscillator is definitely working, it's just not being applied properly.
But enough of that for the moment. My immediate query relates to the cathode bias resistor. Most of the schematics indicate a 47R or 50R resistor here, but none state the wattage. Because the HT voltage is higher in this circuit than in the valve rectified ones, I have fitted a 56R 7-watt resistor and am reading the requisite 10V across the resistor. It sounds incredibly good - very hot, but amazingly rich. So I think it's okay as far as the biasing goes, but am wondering if 7 watts is enough. Can anyone tell me?
Comments
P=IV, so ~2W
I think you're OK. I'm sure if I got it wrong, @ICBM will correct me.
EDIT Alternatively you could write P = V.V/R = 100/56 still giving you ~2W
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I would go for a 10W resistor for the cathode. There's no disadvantage to using the biggest one that will physically fit. You can fine-tune the value by using the original 50-ohm and adding another smaller one in series, too - there should be a spare tag right next to it on the strip. That's how I did my one, ending up with 65 ohms (15-ohm 3W added).
Are you sure the HT is higher though? Mine isn't. With the 22-ohm resistor in the power supply it's the same as a valve-rectifier one (320V) at idle - it just doesn't drop as much at full output.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
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Bandcamp
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Seriously: If you value it, take/fetch it yourself
Bandcamp
Spotify, Apple et al
Are these actually two separate effects or one and the same thing? I know the control says vib/trem but I thought one knob was for 'rate' and the other tremolo depth - or did the latter actually move it between two different effects? I ought to know as I had an early 70's AC30 myself, but (senior moment mode strikes again! >:D< ) I'm damned if I can remember!
Seriously: If you value it, take/fetch it yourself
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Seriously: If you value it, take/fetch it yourself
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein