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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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In Reply to: Eico HF20 posted by Winefood on October 20, 2003 at 09:06:09:
Too bad about the MC225's. The HF 20 was a conservative 20 W/ch integrated amp running 5881's. The preamp section was so-so. The power amp design was basic Mullard circuit and fairly well done, I thought.The Chicago output transformer is way oversized for 20 W. I eventually redid mine to get 50 Watts with EL-34's with that transformer. The power transformer is marginal to undersized and runs very hot. I also blew a filter cap on mine, crappy quality and not conservative enough rating.
I ended up with a mono power amp of 48 Watts from 40 to 18 kHz and a few watts less at 30 and 20k Hz. The circuit used was the Eico voltage amplifier and phase splitter driving the EL-34's in a cathode bias circuit borrowed from Dyna. I had to rig up a bias voltage circuit and added some capacitance as I recall to kill parasitic oscillations. Using the stock circuit with EL-34's or 6l6GC's and a huskier power transformer and filter circuit you can easily get 35 Watts.
Although the modified HF-20 didn't have quite as much low frequency power as a Dyna MK II, I used them as a stereo pair for quite a while and they were indistinguishable, sound wise.
What was done to the HF-20's to turn them into mono blocks? If nothing was done to the power supply and just the preamp was stripped out, I'd avoid them. They can be built into better amps, but to do that you need an oscilloscope, a good DMM, and function generator at the minimum. Maybe the best comment is that I would not do it again if I had the chance.
Jerry
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Follow Ups
- Re: Eico HF20 - Bold Eagle 10/20/0311:18:21 10/20/03 (1)
- Re: Eico HF20 - Richard CWY 02:22:29 10/22/03 (0)
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