Single Channel Dyna/VTA 70 - Can I Remove A Driver Tube?

[knotscott]

I have a pair of Dyna/VTA 70 "twins" that I use as single channel mono blocks. Can I safely remove the driver tube from the side that has no output tubes, or will that cause an issue with the rest of the circuit?

 

[rayma]

That will be ok, since there are both input and output coupling capacitors for that stage.
The only effect will be a slight rise in the power supply DC voltage for the remaining small tubes.

 

[knotscott]

Thanks rayma! No sense putting hundreds of hours on what could be good spare tubes.

 

[rayma]

Yes, good point.
Bear in mind that the B+ will rise somewhat from spec, due to only one pair of output tubes.

Here is the schematic I found. If you have a newer, different one, post it here.

 

[knotscott]

Understood, but was hoping the increase in B+ would be insignificant.

I do have a more recent schematic of the VTA 70 pcb.

 

[rayma]

I would guess the B+ would rise by around 20V without one pair of output tubes.
As long as the bias is readjusted after removing the tubes, it should be ok.

 

[knotscott]

Thanks again. Your info is much appreciated.

 

[6A3sUMMER]

The original ST70 used 50mA cathode current (plate + screen) for each EL34 (200mA total).

The KT88 (shown in Post # 1?) would have to be adjusted to a more negative fixed bias voltage, or the current per KT88 would be more than 50mA.
Perhaps that is OK, since there is only one channel operating.
More current into the 2 KT88s, will tend to bring the B+ back down closer to its original voltage (with 4 EL34s).
One question is: What sounds better, KT88s at 50mA each and higher B+; or KT88s at 70mA each and slightly lower B+?

. . . Just thinking out loud.

Those were not KT66 tubes, were they?