EICO HF-30 - Schematic

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I am in the process of collecting parts to clone a pair of HF-30's. I have been able to find some schematics, but they are conflicting.

The schematic that I have from Eico shows the driver tubes as 6AV6 and 6C4, but the schematic I have from Photofact shows a single driver tube using the 7247.

I assume that during productioin of this amplifier, Eico switched to the 7247, which I am told was design manufactured for them.

What I would like to know, has anyone compared the 2 versions of this amplifier?

Any advise or opinions are welcomed!

Big

[pchw]

The 7247 is essentially the other 2 tubes in one bottle. Pretty much the same circuit from the component properties perspective.

[TheGimp]

Check out the Eico group on Yahoo. Lots of knowledge there.

[kevinkr]

I owned a pair of the HF-30 using the 6AV6/6C4 driver, and a more mediocre sounding 6BQ5 amplifier design I have not heard.. (In fairness the transformers were not that great, but the overall design is one big, cheap compromise IMNSHO..) if you are going to all the trouble of building an amplifier like this I would choose something else as the basis for the design.. I've no objection to PPP at all, just this particular design.

Perhaps Eli or SY can steer you in the direction of a design worthy of your construction effort, or they may disagree totally with my opinion.

The old Acrosound 2020 design might be worth considering if you are up for a real challenge. IMHO this is the best sounding 6BQ5 I have heard, but it might be tricky to get working at all as a PPP amp, and the compensation may be problematic as well..

[SY]

Originally Posted by kevinkr

they may disagree totally with my opinion.

Nope, I wouldn't. Eli has published an excellent design that will fit in there. So have I. And, sous entendu, the Bevois Valley design by Morgan Jones would be a top candidate for a rebuild.

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Thanks for the input. BTW I don't believe the 7247 is the 6AV6 and 6C4 in 1 tube, I believe it's 1/2 a 12AX7 and 1/2 a 12AU7.

I am looking to use some very nice 4K3 outputs (big massive iron - rated at 50W - I have 3 pairs), I don't want to use EL34's so I thought PPP EL84's would be nice (I also have a large stock of EL84's)

Where would I find the designs by SY or ELI?

Thanks

Big

[Eli Duttman]

Thanks for the input. BTW I don't believe the 7247 is the 6AV6 and 6C4 in 1 tube, I believe it's 1/2 a 12AX7 and 1/2 a 12AU7.

Your belief is incorrect. The 7247/12DW7 is an asymmetrical twin triode, where 1 section is like the triodes found in the 12AX7 and the 2nd triode is like those found in the 12AU7. The 6AV6 is triode/dual diode. The triode in the 6AV6 is like the triodes in a 12AX7. The triode in a 6C4 is like the triodes in a 12AU7.

I am looking to use some very nice 4K3 outputs (big massive iron - rated at 50W - I have 3 pairs), I don't want to use EL34's so I thought PPP EL84's would be nice (I also have a large stock of EL84's)

PPP EL84s will work with your "iron". You will need to use "fixed" bias and an individual trim pot. for each EL84.

SY's design is named "Red Light District". Search the archives for details. FWIW, my design is named "El Cheapo". I've uploaded the schematic for your consideration.

If follow the "El Cheapo" model, buffer the LTP with DC coupled ZVN0545A source followers. "Fixed" bias has a lower grid to ground resistance limit than cathode bias does. You will not get the gain you need, without help from FETs.