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Old April 21st, 2012, 07:39 PM   #1 (permalink)
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EF86 Preamp

I know the old Vox's used these, and Weber has a mod kit for making a really simple EF86 preamp.

I've just always wondered, how do these tubes sound? As I understand it, they have so much power that they usually don't need a phase inversion. This seems like a pretty beefy sounding preamp. Does anybody have any experience with EF86's? How do you like them?

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Old April 21st, 2012, 08:10 PM   #2 (permalink)
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Quote:
Originally Posted by itsa68
I know the old Vox's used these, and Weber has a mod kit for making a really simple EF86 preamp.

I've just always wondered, how do these tubes sound? As I understand it, they have so much power that they usually don't need a phase inversion. This seems like a pretty beefy sounding preamp. Does anybody have any experience with EF86's? How do you like them?
I think Vox only used them for a few years in the AC15?? Since then the likes of Matchless, Dr. Z and Bad Cat have used them extensively.

Yes, it is bad ass preamp tube. But it comes with a price. They are extremely finicky tubes. Something like 8/10 new productions are microphonic and unusable out of the box.

I have Dr. Z Prescription ES a VERY powerful amp. When I first got it it was so noisy. I spent a day at a local store going through all the tubes trying to isolate the noise. I never found an EF86 that solved the problem that day. I had to resort to buying expensive NOS tubes, but it cured the problem!!! My amp is sort of the exception because it's a 45 watt combo. That's a ton of power. If it were a head/cab, and less prone to rattle - I'm sure the tube wouldn't be as problematic.

It's a trade off. You get beautiful, harmonically rich tones at lower levels from the preamp section but will spend a lot of time finding the right tube. I also have one in a Dr Z z28, it's less of a problem likely due to the lower wattage.

Hope that helps.
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Old April 21st, 2012, 08:34 PM   #3 (permalink)
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Quote:
Originally Posted by itsa68 View Post
I know the old Vox's used these, and Weber has a mod kit for making a really simple EF86 preamp.

I've just always wondered, how do these tubes sound? As I understand it, they have so much power that they usually don't need a phase inversion. This seems like a pretty beefy sounding preamp. Does anybody have any experience with EF86's? How do you like them?
You may not have a good understanding of tube amps. If an amp has a push-pull output it pretty much needs a phase inverter to operate whether there is a EF86 type of tube up front. The EF86 is a pentode and it has a lot more gain than a triode which is normally used in a preamp. But throw two triodes in series and the gain is much more than a single pentode.

What the pentode has is rich harmonics when it is overdriven. There are other pentodes that can be used in the preamp, some sound better than others though. The tubes can become microphonic as you have a lot of gain and the vibrations from the speaker shapes the guts of the tube. One way around this is to feed the pentode a stronger signal than the guitar pickups and reduce the gain of the pentode. As far as the sound

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Old April 21st, 2012, 08:38 PM   #4 (permalink)
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I love small signal pentodes, but they have problems that are seldom effectively addressed or used to their full tonal potential.

The major problem is all the gain, they can provide a lot and designers seemed to get seduced by it, ignoring that with high single stage gain comes the likelihood of microphonics. There is a solution that does not compromise their sound, and that is to control the gain to reasonable levels. Keep the gain below 100 and microphonics is no more a problem then it is with a 12AX7. But that means that you need more gain stages. Not a real problem really, and it gives you the flexibility to use more of the pentodes tonal range.

Pentodes are usually used as the input stage because the high gain gets the signal out of the noise floor fast, but if we are controlling the gain to high triode levels, there is no reason to use the pentode as the input. Placing it later in the chain gives you the chance to overdrive it and get the cool tone it yields being overdriven, without having to use a pedal.

You can also use it as a compressor by putting the screen bypass cap on a pot.

There are lots of cool reasons to try small signal pentodes, but if you use them traditionally you have the traditional problems.
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Old April 21st, 2012, 09:40 PM   #5 (permalink)
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I am already thinking of a Bassman with a 6U8A or the like with a squish in place of the 12AX7 tone control/cathode follower position.
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Old April 22nd, 2012, 12:52 AM   #6 (permalink)
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I have a Z28 style head built by local amp tech Robert Hinson, it uses the EF86 preamp tube.
When I got it came with a new EH EF86, which sounded good, BUT ... I got some older stock EF86 tubes off eBay , and what a difference!
I've got several different brands , none of them are microphonic in this amp, and there is something special about the sound of a EF86 preamp tube.
OD pedals work really well into this circuit too.

I've head that the main problems with the EF86 as far as going microphonic is using them in combo amps, or in a real high gain circuit, neither of which I have.
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Old April 22nd, 2012, 01:12 AM   #7 (permalink)
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I think they are all slightly microphonic by design. Some circuits and configurations accentuate it more than others.

You're exactly right. When it comes to EF86's NOS is the only way to go. For several reasons. Although I have heard Darios are nice.
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