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I’ve been hearing an awfull lot about the EF 86 preamp pentode. Does anyone know what the big hoopla is about it? What’s the advantage over the famous 12AX,Y,U7 tubes? Is it tone, gain, distortion characteristics????
Norm
Norm
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What I know is, as preamp pentodes they have loads more gain than the triode types and thus offer the possibility of a less complex signal path (whatever that's worth) with overdrive-friendly levels of gain. Dr Z and VHT use them. However with the exception of some 60s NOS types they have a strong tendency to noise and especially microphonics. Quite expensive new manufacture 'premium' versions are currently marketed by JJ and EH.
Many 60s Brit amps use them and I see quite a lot of them. Vox AC15, Bird, some Selmers spring to mind. Attempted replacements of bad EF86-es with the new types mentioned above has given me a few headaches as even the premium new types have sometimes proved impossibly microphonic. I'd be pleased to hear that these were bad batch/teething troubles and have been ironed out in current production. Any news? -
Thanks, I am about to build a stripped down version of an AC30 (the Hoffman layout) using a Weber 6M36 chassis kit and VOX trannys. I was thinking that I might want to substitute a 12AX7 with the EF86. At first, I thought that I would put in the 1st preamp stage (like the old AC15) but I saw the schematic from the new Amp kit from Weber (100 watter) that uses the EF86 in the 2nd stage w/ boost switch (kind of like DR Z). I think I'll keep to the 12AX7 for now, maybe Ill modify it later.
Thanks for your help
NormComment
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Good decision. The EF86 is wired quite differently from the ECC83 and has very different characteristics, in addition to the worry about the modern types being unreliable/noisy.Comment
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Hey Guys-
Pentodes as preamp tubes offer an incredible variety of options to tune responses, from cathode bypass caps, to different screen resistors, screen bypass caps, plate resistor values, their ability to still be wired as a triode...I love using their high gain characteristics to drive a tone stack AND a reverb circuit using split-loads off the plate circuit. That way you don't load down the circuit like Fender does with the 2M2/10 pf thing. Just remix the reverb return at the PI. Gives you a parallel reverb circuit that doesn't affect the lead tone one bit!!
I assume you're here to learn stuff- take the plunge and get a couple premium EF86's or try the 6136 version of the 6AU6A. 6136 is a lo-micro tube (and cheap) though 7-pin. But you will learn a TON if you take notes, measure, and listen. Plus using a pentode in a simple 5E3-ish amp gives you fat nearly D*mble-esque tone & sustain, highly addictive! You'll wonder why you wasted so much time with triodes :-)
Pentodes produce very wide frequency response with much smaller coupling cap and cathode bypass cap values. From the plate, start with .0022uF/630v and a cathode bypass of only 4 or 5uF/25v. You'll be amazed how much bass that can produce. Just a suggestion for a starting point. Don't go too big on screen bypass caps, max value is around .1uF/400V no matter the pentode. Start with .047 or .01uF/400V and then go up or down and hear what the changes do. Some guys wire in trimmer pots to adjust screen values...Also just change one variable at a time or you will make yourself crazy pretty quick. Don't ask how I know that. :-)
The new EF86's I get from JJ & new EH production have been very good. Plus there's loads of NOS USA 7-pin 6AU6A variants & similar sharp-cutoff pentodes that work well for audio. The glass octal version of the 6SJ7, the 6SJ7WGT sounds awesome too (but avoid the metal ones).You must use tube-dampers & suspend the socket to cope with microphonics with most pentodes because hey, a gain of 200 means EVERYTHING'S louder! But for using in a head they are fine.
Anyway, I highly recommend trying some out- you'll learn a lot in the process. Have fun!
Cheers,
Alexander
Retrodyne AmplificationComment
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WOW, Thanks; that’s a great explanation of the positives of the EF86. Although there are some negatives I see that there are advantages over the triode. Now I understand why I’ve been hearing a lot about these things lately. I've got plenty of room in the chassis for an extra 9 or 7-pin tube and I'll even leave some room on board
Thanks again
NormybabyComment
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Its a nice "clear" sounding gain stage with a wide range and good even control at least in my homebrewed matchless clubman.
I found the Pentodes may be noisey it this application. I think they used them in the old ac30's in the begining but were noisey then.
The EH ef86 I bought is much Less microphonic than the old 6sh7 pentode but it is electrically very similar.
I built a adapter and plugged one into the octal of the 6sh7 situation and it works very well.
Could probably do the same with an amp which used a 6sj7 too
I think the only problem may be noise with older tubes. the 6sh7's were terrible as were some other pentodes I used as were probably the old ef86's.
Heres to the usefullness of the ef86. Saved me a lot of noisey headaches.
Might be interesting to stick one in an old fender V front super or something...NAH!Comment
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The 6SH7 was not designed for audio so the chances of finding a quiet one are slim. I bought a box of 50 of the Svetlana EF86/6267 tubes a few years ago and overall they are less noisey than my NOS versions (Amperex& Mullard). I find the 6SJ7 has a slightly "richer" sound than the EF86. Pentodes require a lot more tweaking to get what you want out of them. By adjusting the screen voltage you can often get the microphonics under control while still maintaining sufficient gain. I have built quite a few pentode front ends at this point.Comment
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How about using the EF86 in the 2nd stage(post tone stack) with a gain switch (see attched picture). I'm not sure but it may be similar to the set up that DR Z uses.
Thanks
http://i133.photobucket.com/albums/q...s100_schem.jpgComment
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That circuit is very similar to the Dynaco HiFi amps and the Sunn amps where you have a pentode driving the cathodyne splitter. The earlier amps used a triode/pentode combo tube like the 7199, 6AN8, or even a 6U8. Same idea, a proven circuit.Comment
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I suspect the use of an EF86 after the tone stack is largely due to noise issues, since the gain at that point is significantly lower than if used as first stage. So a good way to use up vast quantities of sub-par already-bought Svet EF86's and yet still impart some pentode punch to the proceedings.
Just a gut feeling of course. :-)
That Weber schematic does indeed show the worlds most expensive 7199 replacement! And I bet their claims of very loud and clean power are just that. I don't know about your local clubs, but 'round here you can barely crank 30W amps so that topology garners little interest from my perspective!
I like the front-end pentodes primarily for replacing the need for conventional tonestacks; set the bass response to 'adequate' and then tune the rest of the circuit for screamin' lead tones via the purest and simplest inverter/power section you can build. The upside in juicy, organic & dynamic tones vs. mere adjustability of so-so ones makes the choice clear to me!
"the older I get the fewer knobs I need" indeed! LOL
Cheers,
Alexander
confirmed minimalist
Retrodyne AmplificationComment
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That circuit is about a third the cost of a 7199 and the worlds cheapest replacement. The Sovtek 7199 is a repinned 6BL8 and will not work in all circuits.
I think Weber was trying to emulate the Sunn/Dynaco circuit to a large degree. In fact, my Dynaco Stereo 70 has an aftermarket board in it that uses two EF86s and a 12AU7 that sub for the 7199s. Same circuit with discreet tubes. This far back in the circuit microphonics is not usually a problem. But I agree the biggest impact is as the first stage.Comment
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Thanks to all for your comments and advise. I’ve learned a lot. I’m kind new at this amp building and repairing thing. I recently purchased my 1st kit, 6A14HP from Weber, and I was very pleased with it. I decided to install other switches, jacks and tubes of higher quality components as recommended on this forum. When I received the kit, nothing was missing and everything went together easily and worked perfectly. The cabinet was excellent quality and the speaker is awesome. I kind of like that Weber and a few other kit suppliers are offering something different. I’m kind of board with the fender tweed and blackface kits/boutique amps that are out there. Don’t get me wrong, I love that stuff but once and awhile you like to learn new things; keep things interesting. It’s fun discussing, agreeing/disagreeing with other members on all kinds of ideas. After all that’s what it’s all about; that’s why we love coming back to this forum
Thanks
NormybabyComment
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Alexander,
Minimalism is trick, for sure. No pcb req. either. What would you recommend for a 30 wattish power amp that is dead simple but still has a wide frequwncy response?
That amp above with the '86 into twinned 12au7s and loadin' out cathode biased KT88s looks MEAN. Could be ideal with a pedal board too. Funny nobody makes a clean tube preamp pedal. Maybe you should!Comment
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Wide bandwidth yet simple 30 watts? A lot depends on your preferred distortion flavor, but any pentode-preamp (EF86, 5879, 6136, 6SJ7WGT etc) into a 6SL7 or 12DW7 split-load PI driving a pair of cathode biased EL34'S at 400 or so Vp into 5-6.6k sure sounds good to me. I'm just sick of LTPI's so don't even ask me about them
. I prefer burned-in power tubes but mismatched by at least 10ma. Try a pair of Eminence 10" Ramrods at 4 or 16 ohms and prepare to have your Celestion perceptions changed. Eminence is making GREAT speakers these days, screw Chinese Celestion- (and Vox too for that matter) using slave labor and throwing honest Brits out of work. Don't forget, you vote with your money first and foremost!
Don't forget that pentode preamps don't need typical 12AX7 value caps!!! plate circuit try .0022 and bypass the cathodes with only 4-5 uF/25v to start! Also don't forget about split loads off the plate to tame the feed to the PI. Use a 1-5.6k grid stopper on the input of the PI if you're going to run it hard. Blocking distortion is not your friend.
Also use a .1uf input cap from the input jack to the pentode grid since incoming DC ain't no good either. Keep screen bypasses under 1uF, usually .1uf or under gives you excellent response. And instead of copying Matchless or Gibson GA circuits, pull out the data sheets and really study what combinations of values at given Vp's give you 'X' voltage gain, pick a set and develop from there. Much more fun that way!
Cheers,
Alexander
Retrodyne Amplification
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