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Gain calculating in 12ax7 guitar preamp
Gain calculating in 12ax7 guitar preamp
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Old 8th November 2009, 07:55 AM   #1
McBarry is offline McBarry  Australia
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Join Date: Nov 2009
Location: Canberra, ACT
Default Gain calculating in 12ax7 guitar preamp

Hi folks,
long story short,
crackle/hum in my guitar amp (80's Kitty Hawk M1), a few caps faulty around V1 and V2, relay was suspect also.
Couldn't resolve crackle until DC found on the board between track..
Cleaned thoroughly, left amp in sun today, fixed. Beautifully quiet now.
Here's the schematic (mine is later revision but is mostly same as this: http://www.schematicheaven.com/newamps/kittyhawk_m1.pdf

But I think clean channel is quieter than it used to be with guitar plugged in ie; less volume.

I'd like to calculate the gain from V1 and V2 - how to do??

Also, posed with the same scenario, how would other readers proceed to determine if ch1 is markedly less gain than ch2 and then track cause..?

Thx for any thoughts/opinions..
Kind regards,David
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Old 8th November 2009, 09:36 AM   #2
Arnulf is offline Arnulf  Europe
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Join Date: Feb 2009
The theory

Note however that unless you have measured your components' (passives and tubes) parameters your calculation will only yield an (over)estimate.

In reality you would take a function generator and an oscilloscope or AC voltmeter (a "true RMS" one), set up the generator to output a certain level of sinewave (say 1 Vpp @ 1 KHz) and measure the AC voltage at the output of each channel, with all control settings maxed. If one channel shows substantially lower voltage reading, it has lower gain. The difference should be huge (at least 30%) in order for you to be able to even notice it, let alone it be "markedly less" as human perception of loudness isn't proportional like the characteristic of a voltmeter is. Repeat the measurement at different frequencies.
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Old 8th November 2009, 03:26 PM   #3
Ezequiel (kel) is offline Ezequiel (kel)  Brazil
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Location: Brasil
the friend above is correct, but for you and other forum friends do the calculations and that i put down a useful link.

I strongly suggest that you make a revision in the pre. resistors, caps and even old tube sockets ok?

Tube Amplifier Circuits and Equations
Vacuum Tube Amplifier Circuits and Equations
sorry if wrong, my English is google translator
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Old 18th November 2009, 01:41 AM   #4
McBarry is offline McBarry  Australia
diyAudio Member
Join Date: Nov 2009
Location: Canberra, ACT
Found it!
Signal tracing the AC signal through both channels showed some interesting gain figures.. but nothing conclusive.
Then...on the schematic the clean ch. master vol to it's 100n cap is actually an under-board link. 110vac from pot became 2vac to cap... dry joint. Soldered, sorted, extremely happy. Both channels very similar in utput

But I need help from learned folks to "rectify" the lead ch sound.
It's very "agressive", not "smooth" like my Boogie Mk1. The o'drive pot gives a lot of distortion from 1 onwards, and really has no middle ground ie: it's either on, or really on..if that makes sense.
How can I lower the distortion early on, but retain the blistering sound when it's dialed over say 7??
I'm trying to get some pics of the signal at various overdrive levels on my cro and hopefully folks with heaps more experience than me will be able to offer some insight/ideas..
Found this post from 10yrs ago which might help - any comments anyone?
AMPAGE Archive: Re: This is really a very cool amp, anyone?

Thanks for any help - all very appreciated,
Cheers, Dave
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Old 18th November 2009, 11:52 PM   #5
jjman is offline jjman  United States
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Join Date: Jan 2009
The OD pot may be linear whereas a log type may be appropriate.
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