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Can anyone be specific about what I would need to do to the schematic to replace the EL34's with KT88's on a EICO HF-60 design ??
Tube sockets the same ???
Resitors ??? caps ??? jumper wires ??? transformer changes or requirements ???I will be using custom made transformers and a solid state power section so no problem with changes there.
By the way, I want to thank everyone who has already helped me get this FRANKENSTIEN going with all your tips and insight.
Follow Ups:
I posted quite a bit on this and core is correct as far as the driver circuit but you need to change the bias resistor to a 10k instead of the 22k in the bias circuit and you can also put in a 25k pot instead of the 10k and this will give you enough range to go from el34 to 6550s..I also changed the 3k resistors in the feedback loop to 1.5k because there was a tiny bit of distortion and this appeared to clear it up when I swept the amp..Also you will love kt66s in this amp..Its so nice when you get the right 6550 or 6L6 product in this amp and when I mention the 6550 and 6L6 I mean those tubes and all of their relatives and ancestors.
Michael
Just too make sure what are the respective resistor numbers on the schematic ?? I am using the schematic that was posted on EICO page for the hf-60.
R2 and R26
RCB
You dont have to change the pot but if you want to go back to el34s,you have to change resistors again and by having the pot you can use either tube.
MikeDo you have a hf-60 manual/parts list that I can buy off you or maybe a copy or scan of at least the parts list ????
Hope all is well
You don't need to do anything. The KT 88's will draw more heater power than the EL 34, but not a whole hell of a lot more, so you may be okay. Ordinarily, you would want to put 350VDC on the screens, as opposed to the 400VDC on an EL 34, but the HF 50/60 is an ultralinear design and will abuse either tube in exactly the same way. The differences in pin out for the two tubes make it sometimes tricky to move from a KT 88 to a n EL34, but not from an EL 34 to a KT 88. So, other than ensuring that your power transformer has plenty of heater power, building for one output tube will allow you to utilise the other. Just remember to rebias for the different types.
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