Filament voltages very high

Joneberr

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My 5F1 clone measures super high filament voltages and i'm not entirely sure why, but i suspect a bad power tube (JJ 6V6).
Without any tubes in I measured 3VAC at each lug of the pilot light.
With rectifier in but no other tube I also measured 3VAC.
But when I put in the 6V6 and measured they slowly started increasing to as high as 50VAC.
The filament CT is DC elevated at the 6V6 cathode.
Any suggestions to what could be wrong are appreciated.
Thanks!
 

andrewRneumann

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If you are measuring voltage reference to chassis, you’ll get weird results when DC at the 6V6 cathode is added to the AC on the winding.

Try measuring with the black probe on the heater CT.

Or measure across the whole heater winding and confirm it is 6.3Vac (or in your case maybe only 6.0Vac).
 

King Fan

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Try measuring with the black probe on the heater CT.
Or measure across the whole heater winding and confirm it is 6.3Vac (or in your case maybe only 6.0Vac).

Andy, you're ahead of me on this, or I'm coffee deficient. Would it be enough to measure heater VAC pin-to-pin instead of pin-to-chassis (say on the lamp and sockets)?
 

andrewRneumann

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Andy, you're ahead of me on this, or I'm coffee deficient. Would it be enough to measure heater VAC pin-to-pin instead of pin-to-chassis (say on the lamp and sockets)?

I’m known to suggest something bone headed—and it has nothing to do with coffee! Yeah winding to winding voltage could be measured on the socket pins or the lamp lugs.
 

Joneberr

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If you are measuring voltage reference to chassis, you’ll get weird results when DC at the 6V6 cathode is added to the AC on the winding.

Try measuring with the black probe on the heater CT.

Or measure across the whole heater winding and confirm it is 6.3Vac (or in your case maybe only 6.0Vac).
You are absolutely correct! Measuring from pin to pin between the filament wires I actually measure 6.3VAC.
I'm trying to learn as much about tube amps as I possibly can, so if you could give me a brief explanation of how this works or give me a reference to read, that would be highly appreciated!
First of all, how come there is AC on the 6V6 cathode? I understand that I would see different results if the heaters were DC instead of AC. But when the amp is in idle, shouldn't the 6V6 cathode only be showing DC? Or am I seeing leakage from another winding through the PT?
And where does the number ~+47VAC come from? It's obvious the increase of VAC on the heater winding relative to ground has a direct correlation to the 6V6 warming up.
And also how is it possible to measure heater voltage from pin to pin? I assume it's because the two filament windings would be out of phase, thus having a differing potential of ~6.3VAC, but I want to double check.
Perhaps I'm just overthinking all of this, but I am very interested in learning the theory behind these weird measurements.
Thanks!
 

Jon Snell

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There is only one heater winding and if your meter does not differenciate between AC and DC voltage, (with all due respect most lower end meters don't) it will add the DC level to the AC voltage, giving a false reading.
A genuine Fluke meter doesn't. Yes there are plenty of Chinesium knockoffs out there in fleabay/amazon land.
If you measure as you describe as 'out of phase', you will get zero volts as this is a single phase transformer.
Yes you are over thinking a little.
Read lots of books from the old days and you will soon pick up what you need to be able to fault find and change the way your amplifier works, to a recommended or your taste. Most people call it 'modding'.
Elevating the heaters is only useful in reducing hum with low grade valves or if you have a phase splitter with a cathode follower that has its cathode above the heater cathode limit of that valve. ECC83 is 100volts etc.

This is a good place to learn theory and put things into practice; https://vdocuments.mx/mullard-circuits-for-audio-amplifiers.html
 




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