[R-390] 12BH7A

[Drew P.]

Ron wrote:
> 
> My R-390 has a 12BH7A in place of the 3TF7 Ballast tube. Is
> this a documented modification?
(snipped)

Add 2 jumpers to the underside of the 3TF7's socket, plug in a 12BH7, 12BY7, or 12BV7, and away you go.  The tube used can have gas, shorts, poor emission, grid emission - a large number of faults as the only requirement is that the heater be good - the tube is being used only as a resistor.  I like this mod - you can plug in a dud tube that is good for nothing else, and it will serve you - kinda like getting sumpin' for nuttin'.

If the 12BH7 is good, swap it out with that dud 12BY7A driver tube from your HW-101, and then sell the 12BH7 to the highest bidder on the 'Bay; the 12BH7 is coveted by the audio guys.

Drew