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Old 7th October 2007, 04:25 PM   #1
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Join Date: Nov 2006
Smile Weekend Fun with Tubes - Guitar Preamp

Hi friends,
Great fun modeling new guitar preamps
at weekend, so this is a new design based
in a lot of things, comments welcome:


I will put the tonestack curves next.

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Old 7th October 2007, 04:47 PM   #2
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So, the tonestack switch effect (in theory):

The intention is a mid base move from 500Hz
to something about 800Hz-1KHz, and get some
metal tones with scooped mids.

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Old 7th October 2007, 05:42 PM   #3
Merlinb is offline Merlinb  United Kingdom
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Location: Lancashire
Looking good! Some comment (ignore at will!)

Your input grid stopper is pretty large, seems a shame to throw away some gain right at the input. I'd go with about 22k or so, after the grid leak.
Also, why have yet another divider right after the first stage? I can't see the signal being that big by that point...

Your high/low gain switch has a problem. In the high gain setting, V2b is DC coupled to V2a anode, in the normal way. But in the low gain setting it isn't. You'll end up with a REALLY cold biased clipper in that setting, which I can't see being good at all. I suggest you either arrange for it to be DC coupled in both settings, or change the V2b cathode follower to an AC coupled version.

I like your tone stack switching though!
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Old 7th October 2007, 06:54 PM   #4
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Join Date: Nov 2006
Hi Merlin,
Thanks a lot for your comments,
I really want to mod this initial project
and find errors with help of experienced
people like you.

So, about your comments:
Changed the first stopper to after leak
resistor, but value is a standard radio
frequency blocker, but I will test 22K and
see (hear).

The second voltage divider was there
to take care of a wild gain (I was worried
about it), but you made me think... the standard
divider in this point, imagine it without the pot,
would be a fixed divider of 470K/470K...so, the
pot alone (1M) will handle it and add some freedom
to user. Finally... Divider removed.

Mods applied:

About the high/low gain switch... you made me
confused... Used a analog solution in my ax84
Lead preamp and it works fine... Take a look at
this mod in the third page of this schematic.


So, if you can tell me more details about this switch
will be fine, remember that I'm not a professional in
any way.

Thanks a lot
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Old 8th October 2007, 01:36 PM   #5
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Join Date: Nov 2006
(1) put a coupling cap on the plate from V2A

(2) the input grid stopper should be mounted right on the tube. Not sure about the 22k resistor though. For a resistance of 22k with 12ax7 millar capacitance of about 150pF this would give an RC cutoff value of 48kHz. 68k, which is common in guitar amps will give a more reasonable 15khz cutoff. This is much more suitable to eliminating frequencies you don't want in your amp. There will be very little signal loss with a 68k resistor.

(3) you will have a lot of gain in this amp. Try keeping the voltage dividers, then adjust to taste.

If you like, post it on the forum at hoffman amps, there are quite a few designers there specifically for guitar amps.

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Old 10th October 2007, 10:49 AM   #6
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Join Date: Nov 2006
Thanks for your reply.
I think I will need to try it alone,
no replies at ax84 or hoffman amps.
Don't know if my bad english doesn't
allow people to understand what I'm
trying to do.

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