[Drake] T-4XB gain drops as warms up Why?

[k9jmlinux at gmail.com]

I have an R-4B/T-4XB combo. The gain varies during the first half hour of  
operation. For example, on CW, to make full power when it is cold, the gain  
knob will need to be at about 10 o'clock. As it warms up, the power drops,  
eventually to about half power. Re-peaking the RF TUNE or the final tuning  
indicates that neither has changed. To get to full power again requires  
increasing the "GAIN" to perhaps the one o'clock position.

I have check and swapped out tubes. No luck. I haven't dung in to find out  
what stage is changing. The time constant is long enough that it will not  
be a quick debug. Anyone experienced this and have any suggestions.

Appreciated
73 de
Jim K9JM