Ef 86?????

[Keljoe228@aim.com]

What is the advantage to using a ef86 pre amp tube as opposed to the 12AX7?

[hasserl]

The EF86 is a single pentode, the 12AX7 is a dual triode. The EF86 is capable of more gain than one of the triodes in a 12AX7 (though maybe not both triodes cascaded). The EF86 has a different tone than a 12AX7 which some people prefer; not everyone, but some. They have a wider bandwidth and amplify a lot of bottom end, so smaller coupling cap values can be used than would be typical with a 12AX7 circuit. The downside is EF86's tends to be microphonic. You can get non-microphonic tubes, but you have to go thru quite a few noisy ones to find a quiet one. Once you find one though they sound great. I like to use 6AU6 pentodes, but these also tend to be microphonic. Fortunately they are cheap and plentiful.

[jrfrond]

Well, when designing my amps, I wanted to reproduce a Tweed-like tone at higher power (70W). Yeah, everyone goes the OTHER way, designing down. I was looking for something like a "super 5E3". The typical 5E3 design, ironically, did NOT work well by just using a beefier output section. Too much rolling, farting, blooming, etc. Plus, I wanted to keep the signal path as short as possible. Using a single EF86 solved the problem. It's a tighter tone, and coupled with high-wattage, it did just what I wanted it to. So basically, it's EF86 preamp, 12AX7 long-tail PI, and cathode-biased 6550's in the power section, and that's it. There's a post-PI dual MV, and a tone control (cut) after that, like the Vox BRILLIANCE control. I had to throw the 5E3 design completely out the window, and it took awhile before I tried the EF86, and I'm glad I did. Yeah, lots of gain in a single stage can present problems, but weeding out the noisy tubes is worth it.

[hasserl]

Hey John, we should compare notes sometime. I wanted something similar, but not 70W, more like 30. I built a 5E3X2, which is basically a 5E3 with 4 x 6V6 power section and two 12" speakers. Only I went with a single 15", and a 6AU6 preamp. Other changes from the 5E3 circuit were reduced coupling and bypass cap values for reduced bottom end and less fartiness; and I stayed with the cathodyne PI but gave it a fixed DC bias. The Tone control is a Vox style Cut control post PI. No MV, but I do use an attenuator when I need to cut some db's, or I cut off two of the 6V6's by lifting the cathodes.

I've considered experimenting with a LTPI in it, but it sounds so damn good as it is I hate to mess with it.

[jrfrond]

It took a lot of design changes and tweaks to arrive at my design. If yours sounds good, don't mess with it. You can get caught up in an endless tailspin of tweaking. If that's "it", then leave it alone.

The use of a pentode allows a short signal path in both cases. To me, it translates as more natural guitar sound. I like when you can "hear the wood". In conventional multi-stage designs where a lot of gain is thrown away, I don't hear that "X-factor" as much.

[mooreamps]

I just picked up an EF-806 from Lord Valve in Denver. It was untested and considered to be an engineering sample. I'm going run this up over the next couple weeks, and see what it does.

-g

[jrfrond]

I will say that EF86's are more of a hassle to pick, and also tend to develop hum after awhile. I've actually had more trouble with NOS EF86's than new stock by Sovtek, which is actually a pretty nice tube.

FYI- I add a band of heat-shrink about 1/2" wide around the center of the EF86 bulb. It seems to help these rattle-prone tubes from causing a racket in closely acoustic-coupled quarters.

[hasserl]

I've added a rubber grommet to preamp pentodes to quell vibrations. Just a standard rubber grommet like you would use to protect wires passis through a hole in a chassis. Just find one with the right ID to fit snugly around the tube. In the OD groove I've inserted awind of 1/8" solder wire, which I have from my old stained glass days for soldering the lead came. It adds a bit of heft to the device to change the resonant frequency and quell vibrations; kind of like adding lead weights to the handle bar ends on a motorcycle to quell the vibrations. I'ver tried other bits in the grommet OD, like flat metal washers, but they tend to be too heavy. I think the solder is just right.

[mooreamps]

Well, I can certainly add the band of heat shink around the tube. A couple of other things that might be in my favor. It will go into a head, instead of a combo. Also, I have the spring loaded retainers normally used for the 12ax7's. I was going to clamp it down with one of those as well.

-g

[Steve Conner]

Hi guys

I used an EF86 as the front end of the dirty channel in my recent Ninja Deluxe build. It was one of those things, I'd never have tried it except I had some old Mullard EF86s kicking around, salvaged from a batch of tube record players.

I basically stuck it in front of the standard Marshall/Fender preamp circuit, as if it was a booster pedal, with a gain knob and a bass cut switch. I thought I'd put it at the front end, since it's meant to be a low-noise tube and all. I guess it is kind of against jrfrond's philosophy of not throwing gain away, though.

It seems to work really well. Like a previous poster said, it has more gain than half a 12AX7, but not as much as a whole one, which is kind of cool. With the gain knob turned down, it just fattens the sound a bit. With gain up full it will drive the heck out of the following stages. I lent the amp for use as backline at a gig recently, and while I was watching the other bands, someone managed to activate the dirty channel and blow a Hot Rod Deluxe off the bandstand

Going on advice from other members here, I mounted all of the tubes on a subchassis that was fixed to the main chassis with rubber mounts, the kind you sometimes see reverb pans mounted with. Even though it's a combo amp, I've had no trouble with microphonics.

If they had turned out to be impossibly bad, I'd have tried all that stuff with heatshrink, grommets etc. all the way to replacing it with a 12AX7 or even a JFET.

[Robert M. Martinelli]

"boutique" vs. "commercial" amps

It must be said that, when it comes to "boutique" amps, which are built with great care and where attention is brought even to the smallest detail, and since boutique amps are built for obvious reasons in very small quantities, using EF86s can be rewarding, the harmonic content, if compared with 12AX7s is higher ( I think mainly due to lower inner capacitances ) and there' s no need to cascade both the 12AX7 triodes to get a high gain. A boutique builder can "afford" to "waste" some time selecting "good" tubes and finding the proper countermeasures, no matter what the cost is.

On the other hand, "commercial" amps are likely to be built in huge numbers, so I imagine it would be quite a hassle to sort out say, 10-20000 "good" EF86s with small microphonics, meaning you should probably buy 100000 of them ( and then what are you supposed to do with the discarded 90000 ? ). And even if you put in good tubes, microphonics is always likely to occur later, like a "time bomb", especially if the amp is gigged with....after all "rock and roll" are not just three words put together, aren' t they ?

We have a good example of this approach in the way Dick Denney dealt with this issue ( transition from the early AC30 model to the second model, yet with no top boost ), he dropped the EF86 in favor of the ECC83 to eliminate microphonic issues... Some say, being probably right, that the early "treble model" AC30s were great sounding amps, the second version probably lacked some "brilliance" but then they issued the "Top Boost" circuit to compensate for this, so I wouldn' t say a 1964 T.B. AC30 sounds any worse.
Regards
Bob

[Steve Conner]

Yeah, that's totally true. I'm not even a boutique amp builder, I just mess around with tube amps for my own personal enjoyment. So I just use whatever tubes I want, put features into them that I couldn't even think about if I had to sell them for a living, and if it breaks, well I can probably fix it in about half an hour.

I thought the same about the lower capacitances. The EF86 has less to start with, and then being a pentode, it has no Miller effect to increase the input capacitance. The capacitance of your guitar lead makes this less of an issue than you might think (even with Miller effect, the input capacitance of a 12AX7 is about 100pF, which is like 3 feet of guitar cord, so using an EF86 instead is like cutting 3ft off your cord) but it can still take your head off with the bridge pickup on a Strat

[Robert M. Martinelli]

Or a Tele!

Regards

Bob

[jrfrond]

...but it can still take your head off with the bridge pickup on a Strat

Exaxctly!!! BUT.....you can roll off some of that top end if need be. I always like to have more highs than needed in that respect, because you can always pull them out, and it makes for nice tones with Strats, Teles, and hell, even dark Les Pauls!

Steve, my philosophy of not throwing gain away is really an extension of the Tweed Deluxe thing, which I tend to prefer. Simple circuits with a short signal path. There is just SO much tone to be had, and dumping gain, to my ears, dumps tone. I'd rather shape the guitar tone with an outboard EQ or other box, and leave the amp circuit intact. Amps with tone stacks and or multiple stages, by comparison to simpler fare, always sound "constipated" to me. To be fair, part of that also has to do with NFB in many cases, ANOTHER thing I tend to prefer little of, or sometimes none.

[Robert M. Martinelli]

To be fair, part of that also has to do with NFB in many cases, ANOTHER thing I tend to prefer little of, or sometimes none.

There you are!!! I didn' t choose "voxrules!" as my nickname for nothing!!

AC30 : EL84s, AB1 class ( biased pretty hot to be fair ) and NO NFB at all!

Regards

Bob