6FQ7/6CG7 as regulator tube

[vetmed]

Like others I have noticed that the original schematic showed a 6fq7/6cg7 where the 12bh7 now resides. I have also heard that one could use the 6fq7 instead of the 12bh7 with appropriate changes to the socket wiring. From my calculations the tube would be run pretty hard, around 90% of maximum rated value. 6fq7/6cg7 tubes are pretty cheap, 12bh7 less so. Besides,  I think we should save them for the crack users that like them! So my question is what is the consequence to the amp if the regulator tube fails from being run so hard? If it just stops working and you put in another I can live with that. If failure has catastrophic consequences then it would be a false economy to say the least. Thanks


Robert Lees

[Grainger49]

Robert,

I guess that running at 90% of maximum is why PJ went to another tube.  I know that Bottlehead has used the 6FQ7 for a regulator in other experiments so if you got 'em smoke 'em. 

If the 6FQ7 regulates it shouldn't harm the circuit, just the 6FQ7.  As the 6FQ7 fails it will also fail to regulate.  But I suspect that means the voltage will drop.  At least I hope it will drop rather than rise.

[ALEXZ]

It's the shunt regulator. If tube in the regulator is not functioning, the voltage will rise.  The same thing is happening during powering-on, first voltage rises to 275v and after shunt tube get heated drops to 225v.

[Grainger49]

That sounds like it should be monitored periodically; even if you stay with the supplied tubes.

[ALEXZ]

Yes, it is good thing to do.   Also if reg. tube fails there will be enough hum to know.