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Old 28th Jun 2020, 4:59 pm   #1
Robert Gribnau
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Default Guitar amplifier with 2 x EL86 push-pull

Hello all,

I have just finished and tested the electronic part of a guitar amplifier. All that is left to be done, is making and installing the bottom plate (MDF with an aluminum cover on the amplifier side as a screen).

Because the figures for a pair of EL86's in push-pull are a little bit better than those for a pair of EL84's in push-pull, I wanted to build an amplifier using EL86's for some time now.

One of the design problems was to keep the needed 200 V supply voltage for the screen grids of the EL86's constant (enough) while the anode supply voltage has to be 250 V. The screen current varies from 4 mA (no signal) to 26 mA (full power). Dropping the 250 V to 200 V with a resistor would cause to much voltage sag. But by using two ZZ1040 voltage stabiliser tubes i solved this problem.

The amplifier works very well. No noise at all with the volume turned down and very loud when turned up. And the ZZ1040's give off a lovely bright-purple glow which diminishes a bit when you give the guitar a good 'hit'. I can not wait to see how it will look after sundown.

Edit: Because the schematic is hard to make out, a link to the schematic on my internetsite: https://1.bp.blogspot.com/-SwqQZAEWU...600/Schema.png

Greetings,
Robert
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Last edited by Robert Gribnau; 28th Jun 2020 at 5:27 pm.
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Old 28th Jun 2020, 5:52 pm   #2
GrimJosef
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Default Re: Guitar amplifier with 2 x EL86 push-pull

It's nice to see the EL86 getting some use. They are quite plentiful and cheap compared with the EL84. It is slightly ironic that that your stabilised screen grid supply doesn't use another EL86. One of the things they were designed for was to act as the pass valve in stabilised HT supplies .

Cheers,

GJ
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Old 28th Jun 2020, 7:35 pm   #3
merlinmaxwell
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Default Re: Guitar amplifier with 2 x EL86 push-pull

I like the "If it is still glowing you are not playing loud enough" indicators!
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Old 28th Jun 2020, 8:16 pm   #4
Robert Gribnau
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Default Re: Guitar amplifier with 2 x EL86 push-pull

Thanks GJ, Merlin.

It was the combination of the available chassis and available power transformers that made me not use a pass valve for the screen grids. The maximum currents for the two high tension windings are 120 mA and 60 mA.

About the indicators: The more Deep Purple they glow, the better?

Greetings,
Robert
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Old 30th Jun 2020, 2:32 am   #5
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Default Re: Guitar amplifier with 2 x EL86 push-pull

Excellent job - well done I'm sure you get a lot of enjoyment out of using it.
I did a conversion from PP El36 Philips PA amp to guitar amp around 10 years back, my son still has it and at times uses it for recording so I guess it can't be all that bad.
I know you have your circuit "sorted out" but just as a matter of interest the power supply was a regular transformer with ct HV winding, and a bridge rectifer - ct was used for the screen supply as in the attached circuit, the 6DQ6 is also a TV valve..
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Old 30th Jun 2020, 7:08 am   #6
Robert Gribnau
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Default Re: Guitar amplifier with 2 x EL86 push-pull

Thanks.

Sounds familiar. A picture from 2007 when I was running my guitar stereo through a 2 x EL84 pp amplifier and a 2 x EL36 pp amplifier.

Greetings,
Robert
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Old 30th Jun 2020, 7:41 am   #7
SeanStevens
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Default Re: Guitar amplifier with 2 x EL86 push-pull

I wish you were my neighbour - as a guitarist I'd offer to soak test the amp for you!

Looks excellent

SEAN
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Old 30th Jun 2020, 9:54 am   #8
Robert Gribnau
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Default Re: Guitar amplifier with 2 x EL86 push-pull

Thanks.

I wonder if my other neighbours would like that idea. One home musician in the block is probably enough as it is for them. But I only play between 12:00 and 18:00 hours (i also picked up electronic drumming and keyboard) and that is going fine.

Greetings,
Robert
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