Cold Clipper EF86?

[Mr. Lime]

Most of the amps with EF86 preamp pentode use this tube as input gain stage.
Because those tubes tend to be microphonic when set up at maximum gain, it's sometimes advertised to half the available gain and/or drive it with a 12ax7 triode previously.

Did anyone try to set an EF86 up as a cold clipper?
Something like high cathode resistor and no bypyass cap just for distortion?

I always read that the EF86 clips more beautiful than an 12ax7 when driven hard, so how to get the best out of it?

[roberto]

You want to get pentode-like overdrive in a preamp?

[Andy Le Blanc]

That a good question, I haven't seen that application, You might be able to get away it by dressing the pentode as a triode, simplify the circuit and still use the tube type.

Ef86 pin out has separate pins for g1, g2 and g3. There are alternate ways to treat the pentode as a medium mu triode or a high mu triode, but I'm not sure of the practically.

In a triode, the large cathode resistance pushes the operating point up the load line resulting in asymmetrical clipping.

Its often presented in early explanations of resistance coupled circuits as what not to do.

I bet you could do it, but I'd be cautious, with an eye on the screens and dissipation limits, for the operating point the notion results in.

Worst case you won't like the result after the effort, but hey, give it a go.

[Mr. Lime]

Thanks for the replies!

Well, I‘m working on a 2 channel amp using the LTPI as channel mixer like Vox does. 5e3 kind of drive channel and EF86 clean channel. I plan do add a fx loop for each channel, so I can feed one channel into the other and vice versa.
Matchless uses only one unbypassed triode to drive the EF86 set up for maximum gain.

I consider to lower the gain for the EF86 when it‘s boosted by the other channel.
Pretty much gain and I‘m afraid I only got oscillation then.

Generally I would prefer the pentode mode over triode but I‘m open for ideas to keep the distortion under control.

Maybe I should ask for the best condition of an EF86 to be driven hard from two fully bypassed gain stages?
I basically want lots of distortion with very little gain. Preferd switchable for single channel operation..

Merlin‘s EF86 with a gain of ~100 could be a start..

[ChopSauce]

I always read that the EF86 clips more beautiful than an 12ax7 when driven hard...

I read a few things that suggest that but I'm not so sure. I believe it's all a matter of the number of gain stages.

I build two variants of the vintage Vox AC4. One with the Vox settings and another "à la Blencowe": with a triode in front and milder (around 70 gain, approx.) settings.

Using a 12AX7 in front allows for setting the EF86 with less gain for improved reliability, maybe. You need to tame the signal from the triode if you don't want to push the EF86 too hard into distortion. In between these two tubes you can insert a gain pot for overdrive tones at bedroom level. This is not something the vintage Vox AC4 is capable of (without pedals).

It is not bad but it requires you to play with two pots, which is not as convenient as a footswitchable overdrive.

In this respect, maybe it is more interesting to use it (actually a 5879, but it may translate) as an overdrive gain stage "à la" tweed overdrive special - see: https://el34world.com/Forum/index.php?topic=9636.0 - but I'm not even sure...

So, afterwards, I tend towards the belief that using a pre-amp pentode might not be worth the extra burden in the layout and I look forward to reading more experienced builders / worthy guitarists about that.

NB using an EF86 as a triode is something M. Dumble did on one famous of his amps. I didn't read that the client was overpleased with the outcome...

[TUBEDUDE]

I use a 5879 in the hot channel of most of my builds. Not a cold clipper, that's not needed.

20200910_174441.jpg

Just a pentode driven to greatness.

[Mr. Lime]

In this respect, maybe it is more interesting to use it (actually a 5879, but it may translate) as an overdrive gain stage "à la" tweed overdrive special - see: https://el34world.com/Forum/index.php?topic=9636.0 - but I'm not even sure...

The Tweed OD Special has some similarities with my amp, see the attached schematic.
Sadly there are no sound samples to hear the Tweed OD Special..

Amp Conversion K19_EF86.PNG

It is not bad but it requires you to play with two pots, which is not as convenient as a footswitchable overdrive.

Channel switching doesn't have to be stage ready, I got other amps for that.A looper would offer lots of combinations:

1) EF86 channel parallel to the other channel.
2) each channel on it's own
3) EF86 in series with other channel
4) EF86 in series boosted by the other channel

Sure, adjustments on the controls have to be done when switched.

I use a 5879 in the hot channel of most of my builds. Not a cold clipper, that's not needed.
Just a pentode driven to greatness.

Interesting, thanks for sharing your circuit.
I see others seem to pad the signal down before cascading with the EF86..

[TUBEDUDE]

Oh the hot channels are all front panel or footswitchable. Stage validated.
The 5879 pentode was manufactured for audio. You will find some variability in screen current, so play with the Rs values to optimize each particular tube.

[ChopSauce]

Sadly there are no sound samples to hear the Tweed OD Special...

Found

sample-Tweed Overdrive Special-jeffss+bluesystory.mp3

on my hard disk...

Not sure which forum member should be credited for that, though...

[Colossal]

Found sample-Tweed Overdrive Special-jeffss+bluesystory.mp3 on my hard disk...

Not sure which forum member should be credited for that, though...

Pretty sure that was 10thTx

[Mr. Lime]

Thanks for the sample!

That's quite a mild overdrive, I'm curious if it's capable for hard rock?

[Bergheim]

Slightly off topic, but I just did a 12AU7 cascode in v2. Kind of the same thing you're after, only much cheaper and more reliable tubes to do the job.
The regular triode preamp overdrive never really impressed me, and I didn't want to use a EF86 with all its drawbacks either, so after reading about the cascode in Merlins book I figured I should give that one a try.
The upper valve "screen" is grid leak biased (in stead of fixed bias by a voltage divider off the B+) to get compression when it overdrives. It has a more apparent attack before compressing than a DC CF, and it's got a rawer/raunchier sound than triodes. I guess cascaded triodes are better for smooth lead tones
My preamp topology is 12ax7 input stage, james tonestack, 12ax7 "recovery" stage with 820R unbypassed cathode, then a 39k resistor to ground after the coupling cap (to drop the gain and increase harmonic distortion of the recovery stage) and finally the 12au7 cascode. There is loads of distortion and compression on tap, and with a humbucker guitar I can almost reach fuzz territory.
I'll see if I can get a soundclip of it in near future.

[Mr. Lime]

I once tried a cascode with a LND150 replacing the lower (6N2P) triode. I haven't played with it long, just wanted to test the circuit but it sounded alright to me.

Does the cascode basically turn the phase twice?

I hope to find some time to experiment a bit!
Already found out I don't like the screen "resonance" control..

[Bergheim]

Does the cascode basically turn the phase twice?

A cascode is an inverting circuit so it only flips the signal once.