[R-390] 6AQ5 audio mod

[Drew Papanek]

Hello all,

Reading of 6AQ5 modifications for the R-390x series leads me to relate my 
experience with a very
simple 6AQ5 mod I did a few years ago.   After hearing stories of generally 
short life with the stock
6AK6 it occurred to me that the beefier 6AQ5 might  provide longer life.

All I did was to disconnect one of the grid leads at the tube socket and 
move the other grid lead to
accommodate the 6AQ5's different basing.   I did not alter any resistor 
values.   Cathode current
with the stock cathode resistor measured about the same as for the original 
6AK6.

I contemplated lowering the cathode resistor to increase plate current and 
make use of the 6AQ5's greater power capability,  but  was concerned that 
increased plate current would lead to core  saturation of the stock R-390a 
output  transformer with attendant distortion and loss of low frequency 
response.  Hence, the stock cathode resistor (network) was retained.

Results?  Same gain, same maximum power output capability.  The audio, 
however, sounds cleaner
with less distortion than the stock R-390x setup.

The downside?  The 450 mA heater current drain adds more heat to that lower 
compartment, but I
did not notice a temperature increase using the highly scientific 
"calibrated hand" technique.

All in all, the mod works quite well, but for those wanting good sound I 
suggest using the diode load  connection with an external amp/speaker.   
Some of those el cheapo amplified computer speakers sound pretty good, 
better than regular R-390x audio.

Drew

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