Converting Eico HF-81 to a simple PP EL84 (pics incl.)

[vinylkid58]

Originally Posted by ARIYAHOOR

Now I want to convert it to a simple PP EL84 amplifier, no phono/mic/fm/am/tone control/balance no nothing... just a nice PP project using Mullard tubes and OPTs with a single RCA input... an Alps pot will be used I want your opinion on the schema

The HF81 amplifier section is about the simplest you can create. PP EL84 amps with concertina splitter/driver sections don't sound bad, so why look for a different schem?

jeff

[planet10]

Originally Posted by Fenris

SY's Red Light District, the Baby Huey, El Cheapo

I consider these the 3 usual candidates. We have an El Cheapo variant with triode strapped EL84 (no feedback) that is VERY good. If you want more power, Baby Huey adds RH-style Schade feedback. I like the LTP front end.

dave

[directdriver]

Originally Posted by vinylkid58

The HF81 amplifier section is about the simplest you can create. PP EL84 amps with concertina splitter/driver sections don't sound bad, so why look for a different schem?

Agreed! Why need to reinvent the wheel?

[Soonerorlater]

No ultralinear taps on output transformers rules out Baby Huey.

[aroS3]

... why not something like Tubelabs Simple P-P with 12AT7 in front (input gain stage - concertina). Enough gain to use also gFB as George does in pentode EL84 configuration ...

[rongon]

Originally Posted by aroS3

... why not something like Tubelabs Simple P-P with 12AT7 in front (input gain stage - concertina). Enough gain to use also gFB as George does in pentode EL84 configuration ...

Maybe regulate the EL84 screen supply?

--

[ARIYAHOOR]

Eico HF-81 schema is so complicated with lots of In/out terminals... I just want to make it easy... I know transformers of Eico HF-81 are magical so I used them to build a new amp... all tubes are original (Mullard) I'll use them

[vinylkid58]

Originally Posted by ARIYAHOOR

Eico HF-81 schema is so complicated with lots of In/out terminals...

Just disconnect/bypass all the stuff you don't want.

Originally Posted by ARIYAHOOR

I just want to make it easy... I know transformers of Eico HF-81 are magical so I used them to build a new amp... all tubes are original (Mullard) I'll use them

I don't see how building a whole new amp is easier?

jeff

[ARIYAHOOR]

Originally Posted by vinylkid58

Just disconnect/bypass all the stuff you don't want.



I don't see how building a whole new amp is easier?

jeff

How to bypass tone control? balance? input selector?

[directdriver]

Originally Posted by ARIYAHOOR

How to bypass tone control? balance? input selector?

You can try this, easiest and only amp section and keeping the volume control:


If you want to include part of the line stage, which I think it's where the "magic" is and more gain. Purists will disagree but you can always compare the two.


Anyway, I don't see how other EL84 circuits can be simpler....

Another option is to simply copy the HF-86, which is essentially the pure amp version of the HF-86. The driver, however, uses the 7247/12DW7, which is half 12AX7 half 12AU7. You can see the similarity between the two.