Measured voltages & bias with 4 el84s, came out as 15.4 watts/ valve, 128 %
Pulled 2 el84s, so now I guess it's an ac15, and changed the cathode bias scheme to accommodate - it's now two pairs, each biased with a 120r/100uf pair.
I've switched from a 5AR4 to a 5V4G to stop the B+ rising too much, it's gone up by 15v (thought I think 3 of those were the wall voltage going up in the time it took me to re-wire the cathode bias scheme) which puts the 2 el84s at 16.9 watts/ valve, 140%. And that feels a bit reckless.
Is there another rectifier that'll drop in and sag things a little more? 5Y3? I've also got a voltage bucking transformer that'd knock 12v off the mains, but I wouldn't mind a bit more sag anyway....
As to the "why" of it... I want to put a celestion blue in, it's a 1x12 combo...
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16W is a bit reckless, but that's what Vox always biased them at, and they do seem to hold up OK.
"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
There's a few fair priced NOS ones on eBay, I see there are different glass shapes and also read that the modern production might not drop as much voltage. I'll pick one at random and slightly lament that the Brimar 5v4g that has been waiting in my drawer for the last 6 years is going to go back in the box.
I'm also reading that the old ac15s connected the OT centre tap after the choke, which would drop a few more volts, but have my doubts about the ac30 choke holding up even with only 2 el84s...
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Unlike someone I knew who had read about the pulling two valves to reduce output power trick, and then couldn’t understand why they only lasted two gigs. I’m actually surprised it was that long - probably only because sadly, they were old Mullards.
"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
Might do the same mod to my 2x12. Does it affect the sound, going from a shared cathode resistor to two pairs? I imagine going to 4 - one per tube - would change the power amp response since each side of the push/pull pair would be free to bias shift independently under load, but I'm struggling to work out where the difference might come between the traditional shared resistor vs 2 resistor/cap combos.
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The really big difference between shared and individual resistors is that for individuals, the AC current all has to go via the cathode caps. With a shared resistor, there is (theoretically) no AC signal across the resistor since the currents in each valve are equal and opposite... assuming perfect matching, which it never is - which I think is also why paired vs single resistors sounds different with four valves.
I worked on an AC30 once which some well-meaning hi-fi engineer had converted to separate resistors, with individual trimmers to bias each valve. It sounded quite odd, and not like an AC30. I removed the (sadly, very neatly made) little circuit board he'd built, fitted the old crude 50-ohm resistor and cap, and it sounded great again.
"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
On the other hand, It sounds fine and I've put it all back together now. So I'll probably just leave it be with a shared 60r.
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