I can across the below post which may be of interest. It seems simple enough to try. I don' t know if the theory is correct or not.
https://groups.google.com/forum/#!topic ... e2z0fSGyww"I've read a number of threads on re-tubing the 7199 lately. I just want
to pass a message in regards to prolonging the life of the fairly
expensive (and getting rare) 7199s for the Dynaco ST70.
According to Mr. Matt Kamna who wrote a short artivle in the Sound
Practices #10 (vol3 #1) page 35. He stated that there was a design
error of the ST70 in which the 7199's heater to cathode was too high
(over +100 VDC, to as high as +135 VDC). This exceeds the maximum
heater to cathode voltage, resulting in "leakage as low as 100K ohms!
This leakage resistance is in parallel with the 1% matched 47K cathode
resistor, thereby lowering its value and reducing drive to one of the
EL34s. This results in a 12% loss of output power and a 25 dB increase
in second harmonic distortion!" ... "If you have to replace the tube,
you will want to protect the new one by elevating the filament to about
+75 volts Dc. This is most easily done by using a resistive divider
off the output stage B+ supply. Connect a 390K 1 watt resistor between
the B+ and filament center taps, and a 82K 1/2 watt resistor from the
filament center tap to ground. A 1uF 200 volt capacitor connected
across the 82K will reduce the divider output ripple and unwanted signal
when full power, very low frequancy notes are being reproduced."
Hope this helps and credits goes to Mr. Matt Kamna and Sound Practices.
regards...Kevin
My opinions are mine, they do not represent my employer. Be very
careful with lethal high voltages !"
I found the howto in the post below but I"m not sure what tap is Lug 3. I think it's the middle 20uf going to the grid of the triode side of the 7199 but i'm not 100% sure.
http://www.audiokarma.org/forums/showpo ... stcount=12"I would consider the voltage divider an essential mod. I don’t know whether I can describe it adequately without a diagram, but what I do is mount a terminal strip along the left side of the power transformer and at a right angle to the 7 lug terminal strip where the filament wiring is connected. Run a wire from lug 3 of the quad cap to a 1 watt 360K resistor attached to the new terminal strip. From the other end of the 360K resistor, run a 120K, 1/2 watt resistor to ground (lug 6 of the existing terminal strip). Also from this end of the 360K resistor, run two 330K, 1/2 watt resistors to the filament center taps (one to lug 5 and one to lug 7 of the existing terminal strip). When you fire up the amp, you should have 70-75 volts DC measured at lug 5 and 7 of the existing terminal strip. Also, the dual 0.1 ceramic cap remains in place. "