Bias won't go any lower...

[Seegs]

Just opened up a 64 AA864 Bassman head that I just got and was checking bias and B+ etc.

The amp has been serviced and all electrolytics have been replaced...

Winged C were in and the B+ measured about 425...bias was about 45mvs

Swapped in a pair of NOS Phillips 7581As and the B+ measured about 407...I biased them to about 40 & 41mvs...

I couldn't go any lower and after running for awhile one of the tubes drifted about 15mv higher than the other...

Why couldn't I get the bias lower and is this an indication of a tube problem?

Chow,
Seegs

 

[VaughnC]

There should be a bias range resistor in the circuit. Adjust accordingly.

 

[zenas]

Sounds more like a problem in the bias supply. All that does is set negitive voltage and that in turn sets the bias of the tubes.
(if the coffee is working right) You need more negitive voltage to your bias pot. Normaly a resister change will do that. Unless you just don't have enough negitive voltage then it gets complicated.

This might be a job for your tech.

 

[slider313]

If one tube drifted, and not the other, then it's a tube issue.

Put the Winged C's back in and monitor the current.

 

[Diablo1]

Some amps simply don't have a bias voltage range that is right for all the tube choices, modern production and NOS. If it were my amp, I'd modify the bias circuit to increase the bias voltage range, giving increased flexibility in your choice of tubes,

 

[drbob1]

There might be a problem with other components in the bias circuit, too, like the diodes… Worth just replacing things and making sure the values are right. It's not a "mojo" part of the circuit so no one should get bent out of shape!

 

[slider313]

Yes, it could be a component in the circuit but, if one tube drifted up 15ma, I suspect the tube. FWIW [email protected] is less than 50% mpd for a set of 7581A's. You really wouldn't want them any lower.

 

[Seegs]

Thx for all the replies...some really good info. here...

The amp has recently been serviced and the diodes and caps in the bias circuit have been replaced...pretty sure that is functioning as it should...if I understand this correctly....I didn't know you could set the range that the bias pot adjusts at?

By my calculations 7581A's at 70% dissipation would bias at about 50ma so I think the range available is ok for these tubes...they were within 1-2ma when one of the tubes drifted out...the tone and punch was amazing even after the power tube drifted out...

I was just checking out the range available and will go back to the winged Cs as well as checking out another pair of the 7581A's to see what they do...

Chow,
Seegs

 

[VaughnC]

There should be a fixed value resistor attached to the bias pot...and adjusting its value will put the bias pot in the range needed for the tubes installed. Back in the day, tube specs were tighter so they were pretty much "plug & play", but not today.

 

[pdf64]

FWIW [email protected] is less than 50% mpd for a set of 7581A's. You really wouldn't want them any lower

My view is that the likely capability of the power supply should be considered when devising a suitable bias voltage / static current draw.

eg as a more extreme case, for a vintage 5F6A we should assume that the power supply was specified to accommodate 5881 (in the absence the actual power transformer B+ rating).
Subbing in power tubes that have a higher plate dissipation rating will not increase the power transformer's ability to supply B+ current. So even if 7581A were fitted, bias voltage should still be adjusted to obtain a current draw that would be suitable for 5881, in order to avoid stressing the B+ winding.
Pete