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Old 7th February 2012, 08:18 PM   #1
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Default 18-Watter with EF86

What do you, respected people of DIYAUDIO, think about adding a preamp stage with EF86 pentode to the existing schematic of 18watter?
Something like this:
http://www.ampbooks.com/home/tutoria...-preamp-6a.gif
to something like this:
http://www.dreamtone.org/images/18wattLite.jpg
Of course, with some modifications and calculations and ... who else knows what...

I want to have 2 preamp stages (don't ask me why) with switch selector.

Good or bad idea, the question is now?
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Old 7th February 2012, 08:30 PM   #2
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the answer to the question Why? might allow for better answers...
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Old 7th February 2012, 08:49 PM   #3
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Quote:
Originally Posted by infernal View Post
What do you, respected people of DIYAUDIO, think about adding a preamp stage with EF86 pentode to the existing schematic of 18watter?

I want to have 2 preamp stages (don't ask me why) with switch selector.

Good or bad idea, the question is now?
EF86, 2 channels, EL84 finals...you have just described a Vox AC15. Why not just clone that? Or did you want both channels to have EF86's (which doesn't make sense to me)?
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Old 7th February 2012, 10:12 PM   #4
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Yes, VOX AC15, but I love Marshall 18W-er British sound
But then, EF86 "magic" sound is for my "express feelings" days...
I'm to complicated, it seems that I want AC15-18W hybrid...
And I have 25 Siemens EF86 on "stock", what to do with them?
And EF86 microphonic sensitivity is challenge for my fresh tube amp knowledge...
(sorry for bad English)
I hope that this explains more my desires
P.S.
I will be making my own OT and PT, so this amp will be my free time killer...
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Old 7th February 2012, 10:55 PM   #5
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An AC15 and Marshall 18 have the same back end, EL84 driven by 12AX7 LTP. Just take an AC15 schematic and change a few resistor and cap values to match the Marshall 18W and you are done.
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Old 7th February 2012, 11:09 PM   #6
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@leadbelly:
If this works, you will be proclamed "God of solution finding"...
So as I understanded:
take AC15 schematic, change cap and resistor values in power stage to match 18W ones, and 12ax7 stage do the same thing as to the power stage, EF86 part leave untouched?
I'll switch cathode resistors with pots for fine biasing and, simple as that....
Tnx... will update on my progress.
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Old 8th February 2012, 12:56 AM   #7
kevinkr is offline kevinkr  United States
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Please read the comments at the top of the tube/valves forum about posting guitar amp threads in the Instruments & Amps forum.. We've tried to make it fairly obvious, it is forum policy. Obviously I'm going to move it..

FWIW there are a lot of guitar guys hanging out on I&A these days.
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Old 8th February 2012, 01:03 AM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by infernal View Post
<snip>

I'll switch cathode resistors with pots for fine biasing and, simple as that....
Tnx... will update on my progress.
I'm wondering how advisable this would be in the cathode circuit of the output stage (my assumption is this is what you meant) of a guitar amplifier. It wasn't uncommon in hifi amps but they are not subject to the vibration and handling common in guitar amplifiers. Would need to use fairly sturdy pots with good power handling and I would run them with fairly large cathode resistors in series so that if they were adjusted to maximum current this would not quite fry the output tubes..

The other possibility would be to use fixed bias with a -20V bias supply feeding locking pots allowing you to adjust the bias to each output tube independently over a wide range. (Of course they should be set to the same current.)
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Old 9th February 2012, 02:37 AM   #9
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Quote:
Originally Posted by infernal View Post
@leadbelly:
If this works, you will be proclamed "God of solution finding"...
So as I understanded:
take AC15 schematic, change cap and resistor values in power stage to match 18W ones, and 12ax7 stage do the same thing as to the power stage, EF86 part leave untouched?
I'll switch cathode resistors with pots for fine biasing and, simple as that....
Tnx... will update on my progress.

The new AC15 uses 130 ohms for the EL84's cathode as compared to the 18W 125 ohms, no difference. Both amps can use 47uF for the cap. The phase inverter is 56k, 820R, 100k for the 18W (and most people use a 47k now) and the AC15, 47k, 1.2k, 100k. So using a 47k, and 1.2k resistor with a switch to parallel a 2.7k across it will give you 830 ohms. The Vox has a 4n7 and 250k pot for the cut control, not a bad addition to the 18W. This takes care of the PI to the back of the amp.
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