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RE: I want to make an EICO HF81 or Rogers clone, sans preamp

Posted by SteveSchecter (A) on August 15, 2017 at 05:12:19

In Reply to: RE: I want to make an EICO HF81 or Rogers clone, sans preamp posted by DAK on August 14, 2017 at 10:08:53:

No, sorry, I didn't draw the as built schematic. I used the exact stock Eico schematic with the following changes:

1. Since I was building stereo and 12DW7 was looking scarce at the time, I used a 12AX7 and a 12AU7, such that each tube is used for both channels. I know this might make some purists blanch, but it works just fine.

2. Output transformers were salvaged from a Stromberg-Carlson basket case pentode-connected EL84 amp.

3. Power transformer was a new Hammond unit with 300v-0-300v HV and a 5v rectifier winding. B+ came out a little high. 360v as I remember as opposed to 350v for the factory amp. I have a 100ohm hum balance pot in there.

4. Because I had 5v available, I used a single 5AR4 instead of the twin 6CA4/EZ81 rectifiers.

5. Because my outputs did not have a 32ohm taps, I adjusted the feedback components and tied them to the 16ohm taps.

6. I used metal film resistors and polyprop film-and-foil coupling caps. Nothing fancy.

7. When I built the amp, I was fortunate that Ei was still in production. So I have a full set of wonderful Ei tubes in the signal chain.

Early on, I had a set of Tesla (pre-JJ) EL84. One of them gave me a light show fairly early on, so I replaced the set of EL84s with Ei. B+ is pretty high and the specified cathode resistor has the outputs running pretty hot.

I also had an early-ish Ruby (Chinese) 5AR4 in there at the beginning. It was fine for a number of years, then it gave me its own light show. Been running it with a fairly recent JJ since then. I've also tried an NOS GE and a Japanese 5AR4. I never can hear any sonic difference with rectifiers, so I just run it with the cheap JJ and keep the "mystical" rectifiers for possible trades!

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