Heater Voltage too high on one tube only

Larpy · 1175

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Offline Larpy

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on: October 23, 2020, 12:11:20 PM
Here's an odd issue (for me, anyway).  While I'm deliberating over whether or not to buy some high-priced 300Bs, I got a replacement for my died-too-young Gold Lion 300B.  It's burned in 15 or so hours, so I took measurements.  Everything looks normal except for the heater voltage:  it's 5.64 VDC.  That seems alarmingly high to me.  The other tube measured 4.96 VDC.

I checked my notes and the stock EH 300Bs measured 4.96 VDC and 5.01 VDC.  So I don't think it's the DC filament supply.

Can tubes draw too much heater current and run at too high a voltage?

Larry


Offline Paul Birkeland

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Reply #1 on: October 23, 2020, 12:24:49 PM
Swap the 300Bs and recheck the filament voltages.  It would have to draw less filament current to see a higher voltage there.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Larpy

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Reply #2 on: October 24, 2020, 05:45:29 AM
The unbalanced measurements follow the tube, so I guess that means it's not the amp.

Is a tube that draws low enough current to throw off the heater voltage by over 10% defective or just part of the variability of tube manufacture?

Larry


Offline Paul Birkeland

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Reply #3 on: October 24, 2020, 06:15:18 AM
I have never seen a 300B that draws less heater current than it's supposed to (unless the filament is open and not working at all).  The tube you have in question is drawing about half of the heater current it's supposed to, I would send it back and ask for a replacement as there's very likely something wrong with it and I can't imagine it's going to sound very good with half its emission.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man