I picked up a pair of HF-20s on eBay for $140 before the current inflation in price for that company's products. (I recently saw an HF-81 going for over $500; I paid $110 for one about five years ago.) The units are in perfect but totally original condition, so they need a thorough going-over (replacement of electrolytics, coupling caps and out-of-spec carbon comp resistors) before they can be used. I will be using them as mono amps only, with everything bypassed except for the volume control, which will be replaced with a switched attentuator for balance.
The original unit had some very impressive specs for the time, even compared to modern tube amps. The chematic shows a fairly typical Williamson circuit without any unusual design flourishes that I can see. I know people here must have had experience with this unit, and ideas on
what to upgrade. The PS lacks any inductance, so an obvious step will be to preplace the first resistor with a choke of appropriate DCR; I'd appreciate suggestions as to the minimum inductance to use here. Any other ideas would be appreciated as well. TIA.http://images.google.com/imgres?imgurl=http://www.miduho.com/jp/st/amp specs The schematic
The original unit had some very impressive specs for the time, even compared to modern tube amps. The chematic shows a fairly typical Williamson circuit without any unusual design flourishes that I can see. I know people here must have had experience with this unit, and ideas on
what to upgrade. The PS lacks any inductance, so an obvious step will be to preplace the first resistor with a choke of appropriate DCR; I'd appreciate suggestions as to the minimum inductance to use here. Any other ideas would be appreciated as well. TIA.http://images.google.com/imgres?imgurl=http://www.miduho.com/jp/st/amp specs The schematic
Excellent iron! Never had an HF-20, but I had an ST-40 however, I was never happy with it stock, because it was built from a kit and had a ground loop problem. Even after several weeks of trying to locate it, no dice. It drove me mad!
I ended up scrapping it with the exception of the iron, in which I built a super nice 6L6 integrated on a 17" X 10" chassis. Wish I still had it
Depending on the cosmetics of the amps, you may want to leave them stock, and get BIG bucks reselling them...then you can buy any parts you'd like and build something cool.
If their in crappy shape, you may want to do what I did and just save that iron...I think you would get more satisfaction building something from scratch than keeping the old chassis/connectors/knobs/terminal strips etc. And get rid of those 12AU7's!!!
I ended up scrapping it with the exception of the iron, in which I built a super nice 6L6 integrated on a 17" X 10" chassis. Wish I still had it
Depending on the cosmetics of the amps, you may want to leave them stock, and get BIG bucks reselling them...then you can buy any parts you'd like and build something cool.
If their in crappy shape, you may want to do what I did and just save that iron...I think you would get more satisfaction building something from scratch than keeping the old chassis/connectors/knobs/terminal strips etc. And get rid of those 12AU7's!!!
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