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
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
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.
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.
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
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
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.
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.
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
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
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
Attachments
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.
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.
Hi,
That should be possible as most I've seen and heard often outperform NOS EF86s.
If you want a super EF86 and are willing to put some major green ones on the table go for the EF806S, E80F, EF804 and better still the EF804S.
The best NOS EF86s I know of are the mesh plate Philips, Mullard and daughter companies, top of the bill is the unobtainable GEC Z729.
No need to dabble with TFKs unless you really know how to identify them correctly.
Most were made for them by RFT in Eastern-Germany, not bad but there's better.
BTW, the EF86 production has been halted for quite some years already, even in the USSR.
As SY points out, you don't boost the amplification factor by putting both sections of a twin (are they really?) triode in //.
Cheers,
That should be possible as most I've seen and heard often outperform NOS EF86s.
If you want a super EF86 and are willing to put some major green ones on the table go for the EF806S, E80F, EF804 and better still the EF804S.
The best NOS EF86s I know of are the mesh plate Philips, Mullard and daughter companies, top of the bill is the unobtainable GEC Z729.
No need to dabble with TFKs unless you really know how to identify them correctly.
Most were made for them by RFT in Eastern-Germany, not bad but there's better.
BTW, the EF86 production has been halted for quite some years already, even in the USSR.
As SY points out, you don't boost the amplification factor by putting both sections of a twin (are they really?) triode in //.
Cheers,
Hi,
I think it was the other way around., Sovtek's just a brand name.
Anyway, yes. Those are the ones.
Let them burn in for a day or two before passing on judgement.
EF86 HISTORY.
Enjoy,
I think it was the other way around., Sovtek's just a brand name.
Anyway, yes. Those are the ones.
Let them burn in for a day or two before passing on judgement.
EF86 HISTORY.
Enjoy,
Hi vintage_dog
I would stick with an EF86. One of the best AF pentodes. You said you wanted to do some tube rolling with some AX7's and wanted less gain. If I were to use that family of tubes I'd try a NOS 5751. Slightly less gain (mu 70) and a little more linear than a AX7. I think the only advantage of // AX7's is the lowish B+ of 145V, if my thinking is right! A 5751 is supposed to have better balance between sections and higher heater current.
@Frank
Just out of curiosity one valve I'd like to try in a new PP EL34 amp I'm thinking of building (mono blocks) is a 5965A, since I have about 20 of them. Good computer tube by the way! HeHe! Thing is about 60% of them are a little microphonic. What's your opinion on them. I also have two NOS 6EJ7's mesh plate that my little brother missed when He and a friend shot up my stash of tubes (about 200 or so, most NOS in box from the 50's and 60's) with a Daisy pump BB rifle in my parents basement back in 1982
Cheers
Wayne
I would stick with an EF86. One of the best AF pentodes. You said you wanted to do some tube rolling with some AX7's and wanted less gain. If I were to use that family of tubes I'd try a NOS 5751. Slightly less gain (mu 70) and a little more linear than a AX7. I think the only advantage of // AX7's is the lowish B+ of 145V, if my thinking is right! A 5751 is supposed to have better balance between sections and higher heater current.
@Frank
Just out of curiosity one valve I'd like to try in a new PP EL34 amp I'm thinking of building (mono blocks) is a 5965A, since I have about 20 of them. Good computer tube by the way! HeHe! Thing is about 60% of them are a little microphonic. What's your opinion on them. I also have two NOS 6EJ7's mesh plate that my little brother missed when He and a friend shot up my stash of tubes (about 200 or so, most NOS in box from the 50's and 60's) with a Daisy pump BB rifle in my parents basement back in 1982
Cheers
Wayne
The 5965 is a GOOD tube. It's a very close relative of the 12AV7. The whole 12AV7 family are excellent audio tubes. Damp the suckers with "O" rings if they are microphonic. Mu for the 12AV7 family is in the low 40s.
FWIW, the 12AV7 was developed in response to complaints about the 12AT7 in audio service.
FWIW, the 12AV7 was developed in response to complaints about the 12AT7 in audio service.
Hi,
Most of those computer types tend to be microphonic but they're fine when not asked to amplify such as in CF service.
If you want to build a preamp with them then I'd recommend some countermeasures against microphony.
Here's where Bill Perkins accesories such as the Iso-sockets and Tubedampers can be put to excellent use.
As for the way they sound, I can't really comment as I've only seen them as drivers in VTL amps and such where they're mostly used either as phase splitters or CFs.
I can't see any reason why they'd all sound bad so give'm a shot. Ouch!!
The 6EJ7 is the equivalent of the EF184,I think and can be used en lieu of the EF86. I think Julius Futterman used one of those as the input tube of the H3 OTL.
Errrr....Your brother, is he still alive??
Cheers,
Most of those computer types tend to be microphonic but they're fine when not asked to amplify such as in CF service.
If you want to build a preamp with them then I'd recommend some countermeasures against microphony.
Here's where Bill Perkins accesories such as the Iso-sockets and Tubedampers can be put to excellent use.
As for the way they sound, I can't really comment as I've only seen them as drivers in VTL amps and such where they're mostly used either as phase splitters or CFs.
I can't see any reason why they'd all sound bad so give'm a shot.
Ouch!!The 6EJ7 is the equivalent of the EF184,I think and can be used en lieu of the EF86. I think Julius Futterman used one of those as the input tube of the H3 OTL.
Errrr....Your brother, is he still alive??
Cheers,
5965 marked as 12AT7
A long while back I ordered some 12AT7's and two of them were 5965's marked as 12AT7 in white paint, go firgure! You could see 5965 with a pattern of dots under the number etched on the glass. They are branded as International Service Master. They were definitly from different manufactuers and dates, 'cause they all had different plate structures. All of them became very noisey after a few hours of operation except the remarked 5965's.
Yes that's how I plan on using them. Currently I'm using them as CF's in my preamp. A homebrew Marantz/VTL rippoff! And yes I know loads of NFB. But it'll walk all over your average sand preamp.
I showed him some prices on some NOS tubes like the 7591's recently and he couldn't believe it! I have one left out of a matched quad. He was doing some high speed photography with a strobe light I bought him. He rigged the strobe light to fire with the sound of the BB rifle. I have to say cool pictures tho. Errrr maybe not so cool.
Wayne
A long while back I ordered some 12AT7's and two of them were 5965's marked as 12AT7 in white paint, go firgure! You could see 5965 with a pattern of dots under the number etched on the glass. They are branded as International Service Master. They were definitly from different manufactuers and dates, 'cause they all had different plate structures. All of them became very noisey after a few hours of operation except the remarked 5965's.
Yes that's how I plan on using them. Currently I'm using them as CF's in my preamp. A homebrew Marantz/VTL rippoff! And yes I know loads of NFB. But it'll walk all over your average sand preamp.
I showed him some prices on some NOS tubes like the 7591's recently and he couldn't believe it! I have one left out of a matched quad. He was doing some high speed photography with a strobe light I bought him. He rigged the strobe light to fire with the sound of the BB rifle. I have to say cool pictures tho. Errrr maybe not so cool.
Wayne
Hi,
Made by GE telling from the dots.
Way back in the early nineties 12AT7s were growing really scarce and that's why you saw the silliest of rebrand jobs.
Count yourself lucky..The samples I received from one of the biggest dealers/manufacturers in the world varied from 7044 to 6463 to 59xx whatever they had plenty of with the same pinout as a 12AT7...
Let me tell you that they received a very angry letter from me and they lost a customer that will never do business with them ever.
Watch out for rebranded 5963s too...
Cheers,
Made by GE telling from the dots.
Way back in the early nineties 12AT7s were growing really scarce and that's why you saw the silliest of rebrand jobs.
Count yourself lucky..The samples I received from one of the biggest dealers/manufacturers in the world varied from 7044 to 6463 to 59xx whatever they had plenty of with the same pinout as a 12AT7...
Let me tell you that they received a very angry letter from me and they lost a customer that will never do business with them ever.
Watch out for rebranded 5963s too...
Cheers,
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