Converting Eico HF-81 to a simple PP EL84 (pics incl.) - Page 2 - diyAudio
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Old 14th June 2012, 05:45 PM   #11
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Originally Posted by ARIYAHOOR View Post
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
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Old 14th June 2012, 05:48 PM   #12
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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.

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Old 14th June 2012, 06:45 PM   #13
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Originally Posted by vinylkid58 View Post
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?
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Old 14th June 2012, 06:59 PM   #14
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No ultralinear taps on output transformers rules out Baby Huey.
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Old 14th June 2012, 07:40 PM   #15
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... 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 ...
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Old 14th June 2012, 08:22 PM   #16
rongon is offline rongon  United States
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Originally Posted by aroS3 View Post
... 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?

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Old 14th June 2012, 08:35 PM   #17
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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
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Old 14th June 2012, 09:03 PM   #18
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Originally Posted by ARIYAHOOR View Post
Eico HF-81 schema is so complicated with lots of In/out terminals...
Just disconnect/bypass all the stuff you don't want.

Quote:
Originally Posted by ARIYAHOOR View Post
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
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Old 14th June 2012, 09:13 PM   #19
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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?
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Old 14th June 2012, 10:09 PM   #20
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Originally Posted by ARIYAHOOR View Post
How to bypass tone control? balance? input selector?
You can try this, easiest and only amp section and keeping the volume control:
Click the image to open in full size.

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.
Click the image to open in full size.

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.
Click the image to open in full size.

Last edited by directdriver; 14th June 2012 at 10:28 PM.
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