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Advice/schematics wanted for HF-81 opt project
Advice/schematics wanted for HF-81 opt project
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Old 19th October 2012, 02:36 AM   #1
jmillerdoc is offline jmillerdoc  United States
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Default Advice/schematics wanted for HF-81 opt project

A couple of years ago I cloned the K-502 with a pair of HF-81 Eico opts and although it sounded better than the original I still found it lacking. Since then I came across another pair of these fine opts and am now considering building another similar amp from scratch.

Just want some ideas for this next project...basically built around these opts, everything else as far as design is up for grabs.

I was thinking of creating my own design altogether using a D3a in triode to drive a pairof power tubes using Bruce Heran's SIPP configuration (OddBlock, OddWatt, podwatt, etc self inverting push pull) using an LM317HV as the current sink for the SIPP circuit. I haven't even put pencil to paper to design this, just an idea.

Could use some suggestions for an excellent sounding design using these trannies....open to all ideas, even off the wall stuff.

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Old 20th October 2012, 01:42 AM   #2
rongon is offline rongon  United States
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Location: Across the river from Rip's big old tree...
Any specs on those OPTs? Do you know what the primary impedance is (plate-to-plate)?

Were you planning on running EL84's? Pentode or triode wired?

I've never heard one of those OddWatt amps. I would think that they wouldn't balance very well, being that the output tubes are used like a long-tailed pair with a single-ended input. Any imbalance in the output stage would not only increase distortion from the tubes, but also in the output transformer.

If you only need about 5 watts per side, you could triode-wire EL84's. With a grid bias of only -12V or so, and a mu of almost 20 (in triode), those should be very easy to drive. A long-tailed pair 6DJ8 or 12AT7 should drive that well, no? Check out Eli Duttman's El Cheapo for the general idea. He uses triode wired 6AQ5 (a 7-pin mini version of 6V6) but you could adapt it to triode-wired EL84's.

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Old 20th October 2012, 02:36 AM   #3
rmyauck is offline rmyauck  Canada
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Poinz's Musical machine. Similar circuit to EL Cheapo, but triode wired. He mentions you could use EL84's instead of 6V6's which would suit the primary impedance of those OPT's better.

6v6 music machine build help!


If you want no SS and a simple circuit similar to the HF-81 try George's tubelab simple PP EL84 amp. Can be Triode wired which may suit those small OPT's better.

Another similar circuit is the tubecad EL84 PP one.


Dave Gillespie's EFB mod would be a great addition for output tube life and improved sonics also.

Post #5 has his paper on the Tronola site:

diytube.com :: View topic - Improved SCA-35/ST-35 Performance

It can be found over at diytube under the ST-35 form and he has developed a pentode version with a thread over at AK.

Teaser Pics -- Fisher SA-100 clone in progress - AudioKarma.org Home Audio Stereo Discussion Forums

Hope that helps!


Last edited by rmyauck; 20th October 2012 at 02:46 AM.
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Old 21st October 2012, 10:30 PM   #4
jmillerdoc is offline jmillerdoc  United States
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Rongon, the Oddwatt SIPP sounds great and measures very well in the distortion area. Bruce has several build pages over at DIY AUDIO PROJECTS - Do-It-Yourself Hi-Fi for Audiophiles on the various models with distortion measurements, take a look at them and see what you think. I built a pair (from scratch, not his kit) of the large KT88's and they are very good.

I appreciate all the input.
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