Taming the EF86

[Garthhog]

I built an amplifier for a customer this summer, and it turned out way better than expected. It was based on a Matchless DC-30. I used the EF86 preamp channel (no 2nd 12AX7 channel), added my tube based effects loop, and changed output transformers to encourage earlier breakup of the EL84s. The result was spectacular... a bright, chimey single-channel amplifier that can bark like a pitbull when you need it, all available at the twist of your guitar's volume knob.

I was particularly enamored with the tone coming from the EF86, and I want to create a couple of amp designs around this preamp tube. Now, the EF86 is notorious for being rabidly microphonic in most cases. I didn't have this issue because the amp I built was a head, not a combo. I want to incorporate it into a combo, though... so I am looking at ways to mechanically isolate the EF86. Before I dive head first into some over-engineered mechanical design, I wanted to know if there are some simpler, well established solutions. So, for those that have played with the EF86, what are some effective vibration isolation solutions for this tube that you've used and are willing to share? Tube rings? Rubber gasket under the tube socket?

Ryan B.

[M Fowler]

There is a TAG post on this very subject and people posted their solutions to vibration using various shock proof methods rubber o'rings under the mounting screws and some used very expensive mounts.

I cannot find that post yet.

Mark

[joeboo88]

on youtube - this guy talks about the ef86 - and has like the tube socket floating on rubber gaskets.
[youtube]
https://www.youtube.com/watch?v=z7yHtoxkdb8[/youtube]

I was helping my father-in -law at his shop, he restores jukeboxes, and I saw a chassis that was destined for the garbage, so I grabbed it and it also
had a tube socket floating on gaskets, ( it was a triangle piece of metal with 3 rubber grommets)... this jukebox was from the 40's - I don't know what tube this was for but I guess a jukebox is like a combo
sorry I'm rambling

[Garthhog]

I was thinking that a rubber gasket for the mounting screws on both sides of the chassis would work. Should I be worried about the durometer of the rubber, or would that be over-thinking?

[teemuk]

For the trouble's worth, I would mount all preamp tubes, if not all tubes, on a separate metal plate, which is then fixed to the chassis with aforementioned rubber gaskets and bolts. Chassis, of course, needs all neccessary holes to acces tube sockets underneath.

Bit off topic, but in my opinion you get more pentode characteristics out of the pentode tube when its not used as the first gain stage like it commonly is. The input signal level is way too low to overdrive the stage and the tiny bit of harmonic distortion produced by the pentode gain stage is likely quite inaudible. Additionally, dealing with such low input signal levels makes microphonic issues even worse. In later locations of the signal path the pentode gain stage can be overdriven with higher input signal amplitude and access the different ovedrive characteristics of pentode (or beam tetrode) versus triode tube. Less gain is needed, because you don't have to worry about signal to noise ratio issues, and decreasing gain decreases magnitude of micropohnic side effects. Not to mention, ratio of signal amplitude and amplitude of microphonics is likely much greater in such stages.

[Garthhog]

@teemuk

To a certain extent I agree, though my ear was hearing a warmer (harmonically) clean sound. One of my design ideas is to cascade a 12AX7 into an EF86. I would like to get true pentode even harmonic overdrive entirely within the preamp. That would allow for a lot of options in power amp design.

[MakerDP]

Consider a 5879 instead of the EF86. It was designed for low-microphonics and audio preamp applications.

[Kagliostro]

Here in Italy you can still find NOS Geloso anti shock rubber socket at HamFest

similar to this 7 pin socket



my friend used on his Standel 25L15 this method





you can use rubber fairlead as rubber units

K

[brewdude]

For the trouble's worth, I would mount all preamp tubes, if not all tubes, on a separate metal plate, which is then fixed to the chassis with aforementioned rubber gaskets and bolts. Chassis, of course, needs all neccessary holes to acces tube sockets underneath.

Bit off topic, but in my opinion you get more pentode characteristics out of the pentode tube when its not used as the first gain stage like it commonly is. The input signal level is way too low to overdrive the stage and the tiny bit of harmonic distortion produced by the pentode gain stage is likely quite inaudible. Additionally, dealing with such low input signal levels makes microphonic issues even worse. In later locations of the signal path the pentode gain stage can be overdriven with higher input signal amplitude and access the different ovedrive characteristics of pentode (or beam tetrode) versus triode tube. Less gain is needed, because you don't have to worry about signal to noise ratio issues, and decreasing gain decreases magnitude of micropohnic side effects. Not to mention, ratio of signal amplitude and amplitude of microphonics is likely much greater in such stages.

At the risk of derailing this thread...
I have built a couple amps that utilize a 12AX7 wired with dissimilarly biased parallel triodes feeding a treble/bass tone stack into a 6SJ7 pentode. On one amp I installed a VVR to adjust the screen voltage of the pentode. The effect is subtle yet effective at adjusting the amount of overdrive/compression.

[strelok]

One thing I might add. If you use rubber grommets for the mounting holes, use nylock nuts and don't tighten them down too much. If you get it too tight it wont provide as much isolation.

You could also try a cascode stage using a 12AT7. In principle its similar to a pentode though idk how it would sound in a guitar amp. Its commonly used in Hi-Fi tube amps where lots of gain is needed without the micro-phonic issues of a pentode. Supposedly they don't sound that great when they start to breakup but if its the input stage that likely won't be a problem. Just another idea.

[Garthhog]

For the trouble's worth, I would mount all preamp tubes, if not all tubes, on a separate metal plate, which is then fixed to the chassis with aforementioned rubber gaskets and bolts. Chassis, of course, needs all neccessary holes to acces tube sockets underneath.

Bit off topic, but in my opinion you get more pentode characteristics out of the pentode tube when its not used as the first gain stage like it commonly is. The input signal level is way too low to overdrive the stage and the tiny bit of harmonic distortion produced by the pentode gain stage is likely quite inaudible. Additionally, dealing with such low input signal levels makes microphonic issues even worse. In later locations of the signal path the pentode gain stage can be overdriven with higher input signal amplitude and access the different ovedrive characteristics of pentode (or beam tetrode) versus triode tube. Less gain is needed, because you don't have to worry about signal to noise ratio issues, and decreasing gain decreases magnitude of micropohnic side effects. Not to mention, ratio of signal amplitude and amplitude of microphonics is likely much greater in such stages.

At the risk of derailing this thread...
I have built a couple amps that utilize a 12AX7 wired with dissimilarly biased parallel triodes feeding a treble/bass tone stack into a 6SJ7 pentode. On one amp I installed a VVR to adjust the screen voltage of the pentode. The effect is subtle yet effective at adjusting the amount of overdrive/compression.

That just gave me another idea

[M Fowler]

I wonder how many of us really have problems with EF86 tubes?

I know I have never had any problems with my amps using EF86 tubes in regular tube sockets.

[pompeiisneaks]

I've just built a Vox AC30/4 and I have some occasionaly microphonics, it's a head unit, and I'm sitting it on top of the speaker (A 15ohm 50W Celestion Gold) and I do get some weird ringing that I'm confident is a mild microphonic character. I've thought of buying a few silicone rings for it or something similar to stop vibrations.

I've read about them and it seems that it's really just a crapshoot on the tube itself. Some are rock solid for years and then finally go microphonic. Some are horribly microphonic to start. Some, like mine, are mildly microphonic. Mines a NOS Winged C I bought on Ebay for not too high of a price and tonally I love the amp, but I just am annoyed at that occasional sound. If I go in the trem input I never hear it.

~Phil

[M Fowler]

My EF86 tubes use two red silicone tube rings per tube.

[Kagliostro]

On one amp I installed a VVR to adjust the screen voltage of the pentode. The effect is subtle yet effective at adjusting the amount of overdrive/compression.

That just gave me another idea

Yesss, very interesting, for me too is an intriguing idea

Did you set the VVR at a particular value and forget it or you use the VVR in a dynamic way rotating it when you need ?

--

BTW, one other way to tame microphonics is to adopt a lower value Plate Resistor, this will tame gain and microphonic effect

K