ef86 alternatives Pentodes

[Ripstich]

In reference to an earlier thread Dartanion's EF86
And after the thread EF86 build use Mullard EF85 instead ?

There was some mention of alternative tubes to the ef86/6267 which sparked my interest in a search for an alternative replacement tube. Since do build 18 lites with the ef86 Akai m6-8 series amplifier platform. (They make nice conversions)

The tube that interested me the most was the 6u8, but my EBay find was a wrong listing and I recieved 20 of these 6eu8 tubes instead.
Medium-Mu Triode
Sharpe-cutoff Pentode
The tube data is actually quite good, but I could not find any charts to plot out the curves. If any one has charts on this tube or a close alternative it would be appreciated. Or if anyone has played around with these things are they a wasted effort? These transconductance levels are pretty high; Triode8500/Pentode6400 an amplification factor of 40 and they will take up to 330v on the plates.

I will more than likely take a shot at a stand alone 6eu8 preamp set up with a pot VVR controller as a power supply, and then plug it into the V2 input/driver 12DA7 and keep you posted. I’ve got a feeling that this might be like an EF86 on steroids, and probably micro-phonic.

[Ripstich]

I haven't found the charts and it's been an all nighter looking at these $.25 valves 6EU8. But this should work by cascode method of dissimilar valves even though they are in the same bottle, triode feeding into the pentode. I will be needing to work out the load lines, and I am sure I will be encountering some compression and phasing issues, but it just might be worth the trouble.

This will be great if it works a half dollar replacement for an EF86. What a concept.

[Snakeman]

Paradoxically, I can't find anything on 12DA7 tubes.

[zaphod_phil]

Try a 6BR7 instead of an EF86, for low microphonics, and sweet tones. The EF83 is also low-microphonic, and pin-compatible with an EF86. Or if you're into octal tubes, try a 6SJ7 or 12SJ7. Everyone who's used one of these loves the tone they get. Not sure about the 6EU8, but the with a 6U8/ECF82, you can just strap the triode side to the pentode, as a cathode follower. This trick has been used successfully in AC15 type amps, so should also work for 18Ws.

[Alexo]

6CM8's a good sounding triode/pentode, the triode side's very close to a 12AX7 in spec, if not identical, the pentode side's pretty different from an EF86, but plate curves are available, and you can end up with a good sound and a lower output Z than an '86.

Better buy a few though because they can get insanely microphonic. But they're very cheap!!

[Merlinb]

I haven't found the charts and it's been an all nighter looking at these $.25 valves 6EU8.

TDSL :Why look anywhere else for data?? Lots of datasheets- the last link has curves: http://tdsl.duncanamps.com/show.php?des=6EU8

[Ripstich]

Thanks Merlin

Wow, I was so close last night! I had looked at all most every thing on Duncan’s site, and I had been to Frank Philipse site, and there it was the only thing that I didn't look at. I had assumed that the PDF was going to be the same as the gif file, so I didn't look.

And after looking at the curves, it pretty easy to understand why they would be inherently micro-phonic. I am hoping that if I can tame the pentodes grids and reduce some of this high Transconductance (Gm) that I might be able to harvest some good second order harmonics stuff. Just printed out the charts and will be reaching for the slide rule next. I’ve never had the batteries crap out on one of these yet.

Finished bending a chassis for the stand alone preamp testing unit this afternoon. Will be installing 7, 8, 9 pin sockets and some quick connect fixtures, have a dedicated PT that will be combined with an adjustable VVR. Hope to get this going soon, but I have too many irons in the fire at this time. Got’a go.

I’ll keep ya’ll post with some results.

[Merlinb]

Thanks Merlin

And after looking at the curves, it pretty easy to understand why they would be inherently microphonic.

Unfortunately the high gain of so many pentode circuits is the root of nearly all complaints about microphonics. Considering that an ECC83 gives more gain than we usually need, I often wonder why people are still building pentode stages with gains of 100+.

[Cathode_Ray]

5879 works well. Sound very similar to my EF86. I have a EL84 SE with 2 channels. One is 12AX7 and other is 5879. It's brighter and louder so I used a divider to tone it down a bit.

[jaysg]

Considering that an ECC83 gives more gain than we usually need, I often wonder why people are still building pentode stages with gains of 100+.

Because it sounds a bit different and it's an alternative to the original Normal channel.

[zaphod_phil]

I think Merlin was asking why EF86 (or other pentode) preamp stages often get built with such a high stage gain, which emphasize microphonic problems, rather than why do people build EF86 preamp stages in the first place. In other places I've seen merlin suggest building the pentode stages so that they have less gain. In a simple 18W classic or Lite amp, however, where you only have a single-stage preamp, IMO the extra gain may be useful to have.

[jaysg]

oh...well...nevermind. I suppose you could use an el-84 for lower gain. Dumble is the only builder I'm aware of who uses the ef-86 in the triode configuration.

[katopan]

Melinb - I've seen you mention microphonics and gain a few times in this forum. You've sparked my interest and would be grateful for a quick (?) explaination of why the EF86 has microphonic problems and how gain and anything else influences that. In my 18W I enjoy the extra gain of my EF86 channel (without any resistive dividers) but being an Electro Harmonix valve it suffers not only microphonics but also a bit of hum. My amp is a head unit so the microphonics don't upset things and I'm happy with they way it sounds, but funnily enough I have had instances where it wasn't microphonic at all. But then next time I'd play it was again. Same circuit and valve, nothing changed other than removing the chassis for tweaking in other areas of the amp. Wondered if it had anything to do with how well the valve was seated in the socket, or temperature, or I don't know what else.

[zaphod_phil]

oh...well...nevermind. I suppose you could use an el-84 for lower gain. Dumble is the only builder I'm aware of who uses the ef-86 in the triode configuration.

In his well-known Valve Wizard web site, merlinb has a worked example of how to design an EF86 gain stage - http://www.freewebs.com/valvewizard/pentode.html In the example, he uses a 56k load resistor, which means stage gain will be considerably less than a typical EF86 preamp stage, with a 100k or 220k load resistor. I suspect this is what Merlin was kind of suggesting between the lines.... And of course with lower stage gain, tiny vibrations of internal electrodes are less likely to be amplified by an amount we can hear. However, IMO running an EF86 in triode mode would be a waste of a perfectly good pentode.

On the subject of microphonics, I was over at Trinity Amps not long ago, and Stephen was showing me one of his amp heads that has a Vox-style EF86 preamp. He told me he uses NOS Bugle Boy EF86s and doesn't bother shock mounting them - and he's never had any microphonics issues.

There's also a bunch of useful stuff in this thread about using EF86s, for anyone who's interested - http://www.18watt.com/modules.php?name= ... c&p=178148