Heater anomaly

[LarryN]

I bought a Crate GT-50 combo for clubs gigs for $100. It seems to function fine, bit I measured the filaments and one side is 4 plus volts and the other, 1.8 volts. Is there anything i can do to correct this? Also, the highest the bias voltage will go is 31 volts, which seems a little low. (4X6V6's) I'd appreciate any help I can get. Thanks.

 

[LarryN]

It isn't humming, which I would have expected with those numbers.

 

[Malcolm Irving]

The filament voltage sounds OK to me. Each tube wants to see 6.3Vac from heater pin to heater pin, so 4+1.8 = 5.8 is 'close enough'. The voltage from each side to ground would depend on how the heater circuit is referenced to ground. If there is a 'humdinger' pot for example, it can be adjusted away from the half-way position (which would give 3.15Vac on either side) to give minimum hum.

 

[LarryN]

I just noticed the resistors at the end of the filament traces, one is 100 ohm and the other 47 ohms. I'm wondering if it was a production error or intentional. Should I assume the tube doesn't mind unequal voltages per side, as long as it's in the 6 volt ballpark? Thanks, Malcolm.

 

[LarryN]

I measured again and it's 4.2 and 1.8. That part looks OK.

 

[Malcolm Irving]

I just noticed the resistors at the end of the filament traces, one is 100 ohm and the other 47 ohms. I'm wondering if it was a production error or intentional. Should I assume the tube doesn't mind unequal voltages per side, as long as it's in the 6 volt ballpark? Thanks, Malcolm.

That's interesting. Could be a production error, or could be 'designed in' to get that 'minimum hum' sweet spot. The tube won't mind, so I think I would leave it (since you are happy with the hum level anyway).

 

[LarryN]

Cool, that's kind of what I was thinking. No second guessing engineers for me! Thanks a bunch from the colonies, Malcolm.

 

[jjohannes]

If it works, don't fix it!