Separation of preamp and power amp

off1 · 7th November 2010, 07:23 AM

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!

korey · 7th November 2010, 09:02 AM

That would work, however, you may lose your desired gain going in at the 6AV6. You could also bypass the tone circuit and add a .01 or .1 coupling cap at pin#2 of the second 12AX7 tube.

Ian444 · 7th November 2010, 10:10 AM

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?

off1 · 04:27 PM

This is the preamp:

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

This should work, right?

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

jfitz57 · 8th November 2010, 12:06 AM

Quote:

Originally Posted by Ian444

Or just connect the line level to the AUX input and change nothing.

That's what I would do. I would wire a 1/4 inch jack to the MIC.1 input and see how that works before doing anything else. If you connect where you are asking about you loose the fuctionality of all the controls including the feedback switch!

Jim

off1 · 8th November 2010, 12:56 AM

Quote:

Originally Posted by jfitz57

...If you connect where you are asking about you loose the fuctionality of all the controls including the feedback switch!

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

jfitz57 · 8th November 2010, 05:58 AM

Quote:

Originally Posted by off1

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

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

Ian444 · 8th November 2010, 09:33 AM

+1 looks good, try it and see.

off1 · 8th November 2010, 06:37 PM

Do you guys suggest any coupling network between the input tube grid and the preamp?

Ian444 · 9th November 2010, 06:30 AM

No...just wire the wiper of the preamp's 1K output pot straight to the grid of the power amp's input 12AX7 and you should be good to go.

7th November 2010, 07:23 AM	#1
off1 diyAudio Member Join Date: Oct 2010 Location: Alpine, UT	Separation of preamp and power amp 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!

7th November 2010, 04:23 PM	#4
off1 diyAudio Member Join Date: Oct 2010 Location: Alpine, UT	This is the preamp: I think I'm starting to get it... This should work, right? Larger verion => http://www.ebodius.com/misc/tube_amp_2.gif Last edited by off1; 7th November 2010 at 04:27 PM.

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7th November 2010, 09:02 AM	#2
korey diyAudio Member Join Date: Mar 2008	That would work, however, you may lose your desired gain going in at the 6AV6. You could also bypass the tone circuit and add a .01 or .1 coupling cap at pin#2 of the second 12AX7 tube.

7th November 2010, 10:10 AM	#3
Ian444 diyAudio Member Join Date: Nov 2008 Location: Brisbane QLD	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?

8th November 2010, 09:33 AM	#8
Ian444 diyAudio Member Join Date: Nov 2008 Location: Brisbane QLD	+1 looks good, try it and see.

8th November 2010, 06:37 PM	#9
off1 diyAudio Member Join Date: Oct 2010 Location: Alpine, UT	Do you guys suggest any coupling network between the input tube grid and the preamp?

9th November 2010, 06:30 AM	#10
Ian444 diyAudio Member Join Date: Nov 2008 Location: Brisbane QLD	No...just wire the wiper of the preamp's 1K output pot straight to the grid of the power amp's input 12AX7 and you should be good to go.

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