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In Reply to: RE: Final pics of Eico HF-89 clone posted by Wojciech on September 10, 2024 at 17:47:37:
I also refubished the original about 20 years ago. It was OK, but no great shakes. A few years ago I upgraded a Jolida amp for a customer and was surprised by how good it sounded. It has a similar circuit topology to the Eico.
My PPP KT-120 fully differential Class A transformer coupled mono-blocks require matched octets of output tubes. They're heat monsters. I have in boxes many quads of 6550s and KT-88s and thought about redoing the Eico considering how good the Jolida sounded. However there is not enough room in the stock chassis to do what I wanted to do. In the end reused the output trannies, the pin jacks used to measure the bias and the fuse holder. I also recycled the Cardas RCA jacks and binding posts, Cardas coupling caps, new bias and balance pots from the first rebuild.
IMHO the weak link in the original design was the power supply and chassis grounding scheme. New power transformer, Lundahl LL1650. Added a resistor/cap. across its secondary to critically damp the leakage inductance/winding capacitance resonance. I feel this is necessary in any amp with a cap input PS. Fast recovery HEXFRED diodes. A FET cap. mult. following the first filter cap. The input stage is fed from a CCS fed active shunt reg. The long tailed phase splitter has CCS tails. The input stage was changed from a simple cathode biased triode stage with a gain of uRl/(Rl+Rp) to a half mu stage with a gain of u/2. Simplified the feedback loop by removing the phase compensating cap and cap in the step filter as set out by Norman Crowhurst in "Optimize Your Amp for $1". - Audiocraft Magazine April 1957
Added a volume and balance control. The 100k volume control is the "faking" resistor for the balance control as set out in Morgan Jones 4th ed. Would have liked to use stepped attenuators but the budget dictated ALPs pots.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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- RE: Final pics of Eico HF-89 clone - JKT 20:08:27 09/10/24 (0)
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