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EF86 guitar preamp circuit
EF86 guitar preamp circuit
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Old 22nd May 2018, 02:48 PM   #1
FWAU112358 is offline FWAU112358
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Default EF86 guitar preamp circuit

Hi,

I'm looking for a simple EF86 guitar preamp circuit and (if not included) references to what a good power supply to use might be.

Any help greatly appreciated!! Thanks!!
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Old 22nd May 2018, 03:31 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Vox AC15 circa 1960 used an EF86. Take a look at that circuit. The HT should be about 200volts with the valve set to about 90volts on the anode and 70volts on the screen grid.

AC15 schematic
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File Type: pdf vox-ac15-amplifier--1960-schematic.pdf (224.8 KB, 28 views)
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Old 22nd May 2018, 06:40 PM   #3
Thomasha is offline Thomasha
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The valvewizard page explains a pentode preamp based on the EF86, and how to get away with it. It can be quite microphonic in the vox design.
The Valve Wizard -Small Signal Pentode

The spitfire also uses this tube in the preamp.

Last edited by Thomasha; 22nd May 2018 at 06:42 PM.
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Old 22nd May 2018, 06:50 PM   #4
Gnobuddy is offline Gnobuddy  Canada
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The input pentode circuitry for the VOX AC15 and other similar guitar amps was lifted straight from the back of a tube catalog. The trouble is, the schematic in the catalog was designed to extract the maximum possible voltage gain from the EF86, for use in microphone preamps, tape recorder playback preamps, record player magnetic cartridge preamps, et cetera.

Unfortunately, when this very high gain stage was used at the input to a guitar amp, it usually caused major problems with microphonics. The guitar amp would howl and screech, and even if you found an EF86 that didn't howl, it would usually "go bad" after a short time, and start howling and screeching (because the mica insulators holding the electrodes inside the bottle loosen up from vibration, eventually allowing the electrodes to move enough to cause the howling feedback.)

The problem was so bad that all the pentode-input guitar amps disappeared from the market within a few years.

You might have better luck if you modify the schematic to lower the voltage gain of the EF86 stage. Merlin Blencowe (the Valve Wizard) has a very helpful section on his website showing how to do this, and he even has a complete schematic there: The Valve Wizard -Small Signal Pentode

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Old 22nd May 2018, 06:50 PM   #5
Gnobuddy is offline Gnobuddy  Canada
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Looks like Tomasha posted while I was typing up my post.


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Old 22nd May 2018, 07:48 PM   #6
Bigun is offline Bigun  Canada
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EF86 guitar preamp circuit
I’m building an EF86 pre st present- figure it will be OK in a low power amp such as VOX AC4, but maybe not - will find out !
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Old 22nd May 2018, 08:13 PM   #7
Printer2 is offline Printer2  Canada
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Lot of widow-makers with 6AU6's been built with it being a higher gain tube. But when you have a big enough cabinet and a puny 8" there isn't that much concern. Through in another couple of gain stages and a pair of 12AB5's into a efficient 12", don't stop playing as the 6AU6 will go on singing without you.
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Old 22nd May 2018, 09:11 PM   #8
Gnobuddy is offline Gnobuddy  Canada
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I built a guitar preamp with a small signal pentode too, but I put the pentode at the output end of the preamp, not the input. Signal levels are larger here, so microphonic effects are further down in the noise. There is less gain in the signal chain after this point, so once again, less chance of microphony problems.

I also designed the pentode stage to have relatively low gain - IIRC it ended up at around x70 (about 37 dB), instead of the x200 or so (around 46 dB) in the old VOX amps. That's roughly 10 dB more breathing room before microphonics become a problem.

I haven't had any microphony problems in a couple of years of using the amp. Fingers crossed...

-Gnobuddy
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Old 23rd May 2018, 11:44 AM   #9
DrFireBottle is offline DrFireBottle  Montenegro
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This looks cool...

But it looks kinda faulty to me...

220k resistor on the anode of EF86 and drops it down from around 320 and something volts down to 80?


22.gif
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Old 23rd May 2018, 11:50 AM   #10
Bigun is offline Bigun  Canada
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EF86 guitar preamp circuit
those voltages look about right to me

see also VOX 4.... if you leave off the vibrator circuit and use a solid state rectifier it's a simple two-tube amp.
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File Type: jpg VOX AC4.jpg (127.8 KB, 55 views)
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