GZ34 arcing

Nope. That’s a capacitor input filter (CLC, although you’ve only drawn the first CL).

There’s only one 55 uF cap though?

Reading in the direction that the power flows (left to right - mains transformer then rectifier then 55uF C then L) it’s CL. It’s possible that there’s another C off to the right, which would make it CLC. But the 29k resistor across the C would then be odd. With a CLC filter there’s sometimes a bleed resistor there to make sure the C discharges after the amp’s switched off, so the service engineer doesn’t get a belt when he opens it up. But it would be at least 10 times bigger than 29k.

29k would be a plausible value for an LC filter because these only work properly if there is at least a minimum current draw, which might be 10mA or so. The resistor’s there to make sure that there’s always some current draw even if the 300Bs are cold (they’re directly heated, but there’s a brief period after switch-on before they conduct) or if they’ve failed or if they’ve been removed. Is it possible that your circuit shows the C before the L, but in reality it’s after the L ? You wouldn’t be the first to get the location of that C wrong (voice of experience).

actually looks like there is another 41uF cap after the choke which is mounted on the top plate rather than inside.

The big film cap (55uF) is connected to the input of the choke, I measured the DCR from the output of the choke back to the 5V tap and it is 130 Ohms

DCR from the choke input back to 5V tap is zero (1 Ohm)

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Ok, sounds like a CLC filter.

Might be worth checking what the series resistance of the mains transformer’s HT winding is & if each ‘arm’ is less than 100 Ohms, add the resistance needed to bring them up to that.

you may lose a little HT but given your speakers it shouldn’t matter & you won’t have the amp eating GZ34’s.

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each arm/leg (Rt) is 121 Ohms

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Whenever there is a pause in this thread, I check BBC Local News to make sure that emergency services haven’t been called to a house fire on the South Coast.


It may be a duff GZ34. What were you using before?
You could go to the 400-0-400 winding and add a little more resistance. (Provided the caps will stay comfortable)

I like 400v across a 300B @ 80mA into a 5K primary (Cathode bias, 1Kohm) But I tend to use EH rather than spendy 300Bs.

I’m not sure your amp’s power supply would do that.

Check insurance. Turn amp on and have a glorious nap. If your beard is singed upon awakening the house is probably on fire. Curse Japan Man & irresponsible forum suggestions, claim money. If it’s not put another record on.


Put some TJ mesh plate 300B in and the rectifier went full batshit crazy, lot’s of arcing and then red plated.

Not an expert but I think the TJ Mesh aren’t quiet rated the same as other 300b’s. Was recommended not to use the ones I have in my BP 300b by Kevin.

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No, was quite expecting a firework show so was ready to switch off.

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He was not wrong

They are not equivalent to 300B

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Can you not video the fireworks for us?

Just as long as there’s some nice light jazz playing in the background so we can properly evaluate the sonic properties of the system.

Well there’s something very wrong with it.

NOS Mullard GZ34 lasted about 15 mins then red plated and amp died

Gahhh, time to deploy operation return?