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Old 11th April 2004, 12:13 AM   #1
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Default converting EF86 to take 12AX7

I have the EICO HF-35 monoblocks that use EF-86 in the pre-amp section. Gain is high, and could be noisy depending on the EF-86 used. I recently converted the EF-86 in triode mode and this has reduced gain and noise. While I can live with this definitely, I wanted to replace the EF-86 section altogether with 12AX7. I want to have the flexibility of tube rolling with 12AX7 and also, avoid having to buy the rare and expensive EF-86s.

A diagram of the HF-35 can be found here:

HF-35

I'd like to use both triodes (12AX7) strapped in parallel. Can someone share how the circuit would look like? Also, the HF-35 has a feedback circuit: Would the values change as well?

Thanks in advance.

Vd
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Old 11th April 2004, 02:47 AM   #2
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In order to get gain out of a 12AX7 approximating that obtained from a pentode wired EF86, you will have to use CCS loading.

The description you provided a link to indicates that the HF-35 uses DC coupling between the EF86 and the 6SN7 phase splitter. Monkeying with DC coupled circuits is VERY tricky business. You will have to find an operating point for the 'X7 that puts the same voltage on its plate as the EF86 has on its plate. Remember, you need higher plate voltages on triodes than you do on pentodes for a given voltage swing. Frankly, I don't think the chances for success are high.

PERHAPS you could get adequate performance out of current production Russian EF86s.
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Old 11th April 2004, 03:58 AM   #3
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Thanks Eli,

I'm also looking at Eico's HF-87 stereo circuit which uses 1/2 section of a 12AX7 into a 6SN7 phasesplitter. Similar topology as the HF-35 monos with slightly different voltages.

HF-87

Perhaps I can retrofit the HF87 circuit into the HF35 but using a parallel 12AX7. What values do I need to change on the 12AX7?

I have another EICO monoblock coming - the HF60's, and I can surely do the mod there as well.

regards,

Francis S
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Old 11th April 2004, 04:08 AM   #4
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Quote:
Originally posted by Eli Duttman
In order to get gain out of a 12AX7 approximating that obtained from a pentode wired EF86....
actually, i find the gain with the EF86 high on my system (with HF35), so i can live with lower gain. the gain on the HF87 is just about right...

thanks again.
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Old 11th April 2004, 05:04 AM   #5
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Replicate the HF87 circuitry exactly. Wire 1 monoblock to use 12AX7 triode #1 and the 2nd monoblock to use 12AX7 triode #2. When the working triodes wear out, swap the 'X7s between the 2 amps.

BTW, GENUINE NOS 7025s or current production Sovtek 12AX7LPS tubes will give you a lower residual hum level. A spiral wound hum bucking construction helps when AC is employed to energize the heaters.
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Old 11th April 2004, 05:25 AM   #6
DougL is offline DougL  United States
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I have a couple of suggestions.

Try a Russian current production EF-86 before you modify your amp. They are about $15 and appear to be excellent tubes.

The other suggestion is to investigate using A 6SL7 rather than a 12AX7. There is some NOS available, at least 1 current production (Sovtek) and have a good reputation for sound.

Cheers

Doug
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Old 11th April 2004, 09:40 AM   #7
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Quote:
Originaly posted by Eli Duttman
Replicate the HF87 circuitry exactly. Wire 1 monoblock to use 12AX7 triode #1 and the 2nd monoblock to use 12AX7 triode #2. When the working triodes wear out, swap the 'X7s between the 2 amps.
That will work. Try to get the PS voltages to 200V for the first stage (12AX7), 400 - 420V for the LTP stage (6SN7). Also keep the same feedback arrangement and the RC anti-ringing network R4 (15k), C6 (47p) because the output trannys are different for each model. The feedback resistors and the compensation caps from the OPT secondary MAY need adjusting, I don't know.
If you're still set on using a paralelled 12AX7, I've attached a schem. But IMHO I wouldn't. I don't like paralelling tubes. Plus there a lot of other good 9-pin valves out there that you could use in that position. Like 6BK7B, 6BQ7, 12AZ7 etc. They are cheap here in the US anyway (my favs). Others are the 12DT8, 6AQ8/ECC85, though a little expensive here.

Hope this helps
Wayne
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Old 11th April 2004, 09:55 AM   #8
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Talking ooops!

I must be asleep! The RC network 15k and the 47p cap should go from the plates of the 12AX7 to 145V! Also the B+ is the same as your HF-35, with about the same current draw. So you should'nt have to modify the PS.

Wayne
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Old 11th April 2004, 11:49 AM   #9
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thanks Eli, Doug , Wayne...

i'll get hold of the russian ef86s and try them out. i'll also probably experiment with changing the circuit to take the 12ax7, either as a single triode (per Eli) or as parallel (per Doug)...

thanks again...
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Old 11th April 2004, 11:57 AM   #10
SY is offline SY  United States
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A couple of observations:

Paralleling the 12AX7 sections will not get you much. The HF87 circuit as it sits has quite adequate bandwidth, and the gain increase from paralleling is pretty small.

Using a CCS (a simple bipolar one in the HF87 I have on hand) will allow you to use balanced plate resistors in the 6SN7 diff amp.

The HF87 circuit is classic and works great.
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