EF 86 pentode

[loudthud]

That Weber schematic is close to what I've been thinking about for my next project.

Compare the Weber (or Sunn) to an old Ampeg that used a 7199.

http://www.el34world.com/charts/Sche...Ampeg_B25B.pdf

The Ampeg uses a voltage divider on the screen and another voltage divider to set the operating point of the triode. I like that circuit because it would let me use a lower voltage rating on the screen cap. On my prototype, I used a 6AN8 with a .47@600V soldered right to the socket pins. I'll have to try smaller values as Alexander suggests.

I also had trouble setting the operating point of the pentode (using the Sunn circuit). Every NOS 6AN8 I tried gave a different plate voltage. I ended up just putting a trim pot in the cathode.

Have any of you guys had problems with getting a consistant pentode plate voltage between different tubes?

[Guitarist]

Man I love Ampeg's aproach. I'm seriously looking at this power amp section too. Only going SE with an el34 or kt88.

Would this work 6Bm8 (or ef86) direct coupled into 1/2 of a 12au7 giving the force feed into the power tube grid which is cathode biased and class A? I think with a hefty pentode and a 1628SEA Hammond it would keep up with 95% of the drummers out there. I called Ho Yukong today who is Vancouver BC's finest and there are ALOT of guitarists out there tuned into that rich tone. I find I never get to crank normal push-pull amps so why?

Chuck I appreciate the wisdom, thanks. I've been specing out toroids all over to try and get this sized down to fit on pedal boards or in a 1 to 2U rack space. I figure, with one channel, 6.3 volts @2A and 300-400V @350mA should do for the PT. Correct? Hard to find in one unit though and a really modest req.

Combined with 1 or 2 preamp tubes and your a 5.5K ish should be a amazing. The only shocker is the SIZE of those OT and the cost ($99 for the Hammond) and conversely a gapped toroid will likely cost the same.
Perhaps a guy should tone it back to a 6L6 for a more mini OPT. Who needs 20Hz?

All this talk on the power amp section is due to some georgeous sounding $40-$120 pedals I've heard of late. DS-1/TubeScreamer, Silver Dragon, Rat, Big Muffff and the EH English Muff'n. As well the TC Electronic Nova Reverb, Marshalls Echohead, or the forthcoming Bhrngr Reverb/Delay DR400. Talk about dream tones and no friggen effects loops or midi controllers.
I've been blessed to play for years into Ampeg VT22's and Hammond Organ reverbs but not any more. Actually playing stereo reverbed amps is an experience.

[ecc83]

EF86

Yes it is noisy and microphonic compared to a triode I suppose but I did not find this 30+yrs ago when we only used Mullards!

Still, even then for a really low noise mixer I used triodes and DC heating, 1:65 tranny's and 30R ribbon mics.

You can get a lot of the pentodes gain benefits without the noise by using T's in a cascode configuration. see if you can find: Valve Amplifiers by Morgan Jones (Newnes).

For some nice pedals look at www.blackstaramplification.com

Dave.

[rocket]

Hey Guys-
The glass octal version of the 6SJ7, the 6SJ7WGT sounds awesome too

where can I get this in Europe? - no luck at the usual sources.

[Alex R]

Did you try Langrex? They have more stocks than anyone else over here I think but you have to call them up usually.

www.langrex.co.uk/

[gypsydanger]

EF86 as opposed to twin triode

Thought I'd throw in my two cents here about the benefits of the EF86 over the traditional 12ax7 config as your first voltage amp section. First off the plate curves are that of a pentode so that's a big tonal difference right off the bat (rise time and high current gives a pretty in your face touch sensitivity). In a non-technical sense I much prefer the tone of the EF86. I've recently been experimenting alot with various EF86's (from telefunken to mullard to orange amperex to old russian military's) and AB-ing them with 12a_7's (cascaded and parallel configs, no cascodes yet) of the same pedigree. My bench buddies and I all think the EF86 'just has more.' It also is very pedal friendly given the increased headroom. This is one reason Dr. Z really likes the EF86.

Unfortunately you pretty much have to go NOS to get a good EF86. The JJ's are ok, but are definitely a massive step below even the standard russian military NOS's imho.

I highly recommend giving the EF86 a shot.

[Guitarist]

Has anyone tried a couple of these pentodes in series? They're usually in clean channels and PIs but with their forgiving inputs and overt tonal flattery they might be the ultimate crunch mothras. Would a 500K pot work well force feeding a 100K grid stopper on V2? Just how gracious are they before blocking distortion?

A slightly depressing thought that NOS sound so much better than current runs which suck hind-tit so badly.
Read a GP Award-giving review of the Genz Benz Black Pearl. Nicest inside layout I've ever seen for a PCB. No pentodes in pre. though.

[Guitarist]

Any further findings with the ef86? I notice the Seymour Duncan Mayhem, which is a very heavy pedal, used a pentode and a triode. Above, i had figured that 2 crunchy pentodes would get real gainy sounds but perhaps a triode following may do well enough if the subminiatures are any indication. I want that pedal!

How is that an ef86 shapes the waveform so immediately distorted? I thought that happened by slamming the grid?

[Steve Conner]

I like EF86s, I used one as the first stage of the dirty channel in my new build. I've just bolted it onto the front of the usual Fender style preamp circuit, with a switchable bass cut network in between.

I heard bad things about the current production, but I have three NOS Mullards that should last a while It is a combo amp, but I've shock mounted all the tubes, and so far there have been no problems with microphonics, hum or noise.

[kg]

i use the svet version as the 2nd stage of the lead channel in my preamp. it's actually not too microphonic. iirc it is LED biased.

having said that i should mention that IME microphony is a large part of a really great tube amp tone. :lol:

[mooreamps]

Ef 86

I've looked at this tube for a clean channel. I think instead I'm going to experiment using the EL-84. The EL-84 would be easier to source, and I'll be looking at ways how to midigate the hiss.

-g

[ecc83]

EF86

Hi Garry,

Using the EF86 for a "clean" amp is IMHO a contradiction in terms! Being a pentode is has much higher intrinsic distortion than a triode. To mitigate this in hi fi circuits the '86 was always used with NFB ( e.g. RIAA input stages) but if you "straighten out" the valve with NFB I think you might have just as well use my namesake anyway.

The bugbear of single ended power stages is hum! There are ways to keep it down but they are either complex, costly or power hungry or all three. If you go ahead with the project can I suggest you put in an extra octal valvebase and wire it for a 6V6? This will give you some tone options.

As neither of the above power valves will give more than about 3-4W of clean juice the amp will have to be a fairly "intimate" and quiet one, a recording amp perhaps?

Please note, these are just the ramblings of an old valvejockey. Have fun!

Dave.

[kg]

Using the EF86 for a "clean" amp is IMHO a contradiction in terms! Being a pentode is has much higher intrinsic distortion than a triode. To mitigate this in hi fi circuits the '86 was always used with NFB ( e.g. RIAA input stages) but if you "straighten out" the valve with NFB I think you might have just as well use my namesake anyway.

dave,

don't make the mistake of equating "clean" with "linear."

a guitar amp with 0% THD sounds like shit.

-ken

[kg]

I've looked at this tube for a clean channel. I think instead I'm going to experiment using the EL-84. The EL-84 would be easier to source, and I'll be looking at ways how to midigate the hiss.

-g

gary, you're going to have a tough time with the microphony, even with the relatively low gain of a clean channel.

the hiss aka noise is not much of an issue ime... but finding an sv83/el84 that doesn't rattle sure is!

[black_labb]

you refer to el84 being the same tube used as an output tube in amps such as ac30's? would it not be difficult to get the same level of gain from a pentode usually used as an output tube, or is it possible to adjust the gain to preamp tube levels with how it's used in a circuit. im curious about this as it could make finding interesting preamp pentodes easy to find an source.