The AM Forum

I am considering replacing the V-70D triodes in the RF section of the GK with either 811A's or 572B's ...( change out the filament transformer and should be plug N play)  the plate voltage on these are at max levels I think for the 811A's but I have seen data stating that they can take up to 2000 vdc pretty well... any input on this would be helpful, new to some of this stuff.. thanks  Tim K8WBL   

Thanks, I have not found them cheaply, in fact just the opposite, thought that the 811A's and especially the 572B's might be cheaper and more durable, certainly easier to find...the 5514 modulators, were replaced in the GK500 with 811A.  I still have good 5514's in my GK400.

73, Tim

Yup, either should work FB OM.
Dentron put 2200VDC on 4 811As in the 160-10L amp.  Meant for ssb they really sang for their supper but did well. Later the Clipperton (re-designed 160-10L) used 4 572Bs. Low B+ (cw) was 1800VDC High was 2750VDC.

Thanks, would I then be better at running 811A's a little hot (will have blower on them) rather than the 572B's signficantly below there rated voltage?  I was wondering if the 572B may not run as linearly at that low voltage.  TK

V-70D's or 812-A's whould be easier to drive and modulate more linearly than hi mu triodes like the 572-B.  I have always used the higher mu triodes for class-B service and used low mu tubes for class C service.

Commonly used combinations are 811-A's modulating 812-A's, 203-A's modulating 211's, etc.  But if you can get the 572B's to modulate linearly that would be a good replacement.  You will have to change out the grid leak, and I suspect the grid drive will be much higher.

Low mu tubes run lower grid current with higher bias voltage, while higher mu tubes run more grid current at lower bias.

In general, low mu triodes are best for plate modulated class C service.

Thanks very much Don,

73, Tim  K8WBL