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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Separation of preamp and power amp

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off1

Member
Joined 2010
ofirmx.com
Please take a look at the following schems:

http://www.ebodius.com/misc/tube_amp_1.gif



I am modding a nice 35 W tube PA amp to use with my guitar.
My idea is to replace the built in preamp with one that is more suited for the guitar pickups. I marked with a couple of yellow dots what I think are the connections between the preamp and the power amp. Am I right? In other words, if I remove everything before the yellow dots, I will be left with the power amp untouched and ready to receive a line-level signal?

If I'm wrong, could you please help me accomplish my goal?

Thank you all in advance!
 
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Ian444

Member
Joined 2008
The 6AV6 is a cathodyne phase splitter, it has no voltage gain. Since the 6L6's are biased at -38V you will need at least +/-38V voltage swing from the preceding stage, which you won't get with a line level output, line level is usually around +/-1V. The lower right hand 12AX7 is the gain stage for the 6AV6 so you would do better to feed the line level into the 1MEG Master volume pot which is the input for that half 12AX7. The anode resistor looks to be 68K which might be a bit low. Or do what Korey said - you could feed your line level into pin 2 of the upper right hand 12AX7 and bypass the tone controls. Or just connect the line level to the AUX input and change nothing. There's many ways to do it. Are you thinking of building a specific preamp into the amplifier? Or running an external line level into it?
 
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off1

Member
Joined 2010
ofirmx.com
This is the preamp:

pre.gif




I think I'm starting to get it...

An externally hosted image should be here but it was not working when we last tested it.



This should work, right?

Larger verion => http://www.ebodius.com/misc/tube_amp_2.gif
 
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off1

Member
Joined 2010
ofirmx.com
jfitz57 said:
...If you connect where you are asking about you loose the fuctionality of all the controls including the feedback switch!
Click to expand...

Thank you, but that's something I actually want to. The preamp I'm going to use has a better tone control (3-way instead of 2), and I don't mind about the anti-feedback switch... the amp gives me a very guitar-ish feedback and I love it.

In fact, by removing the current preamo I'll free 5 pot-holders that I can now use for the gain, preamp volume, bass, treble and mid controls :D
 
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jfitz57

Member
Joined 2010
off1 said:
Thank you, but that's something I actually want to. The preamp I'm going to use has a better tone control (3-way instead of 2), and I don't mind about the anti-feedback switch... the amp gives me a very guitar-ish feedback and I love it.

In fact, by removing the current preamo I'll free 5 pot-holders that I can now use for the gain, preamp volume, bass, treble and mid controls :D
Click to expand...

For some reason I can't see your preamp circuit, but I can see the other amp circuit and it looks like you know what you are doing. Go for it.

Jim
 
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