Heathkit AA-100, pretty major problems

[saabracer23]

That can definitely happen, readily.

One of the 7199s or the output tubes, may have more of what's called "filament sensitivity" than the other. That means, that it takes more heater voltage, for the "sensitive" tube to come up to full conduction.

That can be a sign of a "tired" or defective tube- in fact, if the heater voltage cannot be turned down to 90% of normal heater voltage, without the tube starting to drop in output- that's a sign that the tube doesn't have much life left.

Regards,
Gordon.

Good to know! I hope that’s not the case. The 7591s are new and the the 7199s are nos. They all measure very strong, hope they’re not defective.

Dan

 

[gadget73]

I wouldn't worry too much about it if the heater supply was below about 5.5v with the DBT in the loop. Power tubes especially can get cranky when run overly low.

 

[GordonW]

I wouldn't worry too much about it if the heater supply was below about 5.5v with the DBT in the loop. Power tubes especially can get cranky when run overly low.

Yes, 5.5V is about 13% down. It wouldn't be unusual for some tubes to start to drop off at that low of a heater voltage.

Regards,
Gordon.