6F6S tubes from Ukraine, useable or not?

[DrMorbius]

Hi I have a couple of 6F6S tubes and wondered if they could be used with the Moreplay preamp with the upgrade kit? It looks like the 6F6S are very close from what I see, and wondered if anyone has tried them? Thanks for your help.
Steve

 

[Paul Birkeland]

The heater current draw is 50% higher for the 6F6, so the stock circuit is unlikely to support that without modifications. The upgrade module may be able to handle that, though I don't remember off-hand how much headroom there is on the heater winding.

The plate voltage in the stock circuit will land at around 110V, which is close enough that it should be fine. The C4S loads in the upgrade ought to land it at about the same spot, so OK there too.

-PB

 

[DrMorbius]

Thank you for the info! Now I have to find out what the upgrade module headroom is on the heater winding.

 

[Paul Birkeland]

I looked up the spec on the power transformer and there is enough headroom on the heater winding for this to work.

 

[Paul Joppa]

I looked up the spec on the power transformer and there is enough headroom on the heater winding for this to work.

In theory, the heater voltage regulator was designed to - just barely - handle EL84s, which want 0.76 amps. That has not yet been tried, but you should be good at 0.7A unless your power line voltage is too low.

 

[DrMorbius]

In theory, the heater voltage regulator was designed to - just barely - handle EL84s, which want 0.76 amps. That has not yet been tried, but you should be good at 0.7A unless your power line voltage is too low.

What would too low mean to you? The reason I ask is that I have 3 choices - 114.5 volts, 118.5 volts and 122 volts. Would the 114.5 be too low? Or best choice 118.5 or 122? Thank you for all your help!
Steve