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Post subject: More Tube Talk
Posted: Fri Jan 21, 2011 12:55 pm
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So, it appears that the affect of putting a low gain tube in the Phase Inverter position is to reduce over all volume which allows you to crank the master volume to get more preamp gain.

For those electronically inclined what does this mean in terms of how the power flows from one stage to the next? How does this affect both preamp and power tube life or any other element of the amp?


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Post subject: Re: More Tube Talk
Posted: Fri Jan 21, 2011 2:21 pm
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No impact. The amp still produces the same amount of power. You just have to turn the volume up higher or drive the amp harder. The amount of distortion is the same, it just occurs later.

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Post subject: Re: More Tube Talk
Posted: Fri Jan 21, 2011 2:52 pm
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Actually I have discovered, with a low gain tube in PI, the distortion occurs earlier and there is more distortion at lower volumes.


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Post subject: Re: More Tube Talk
Posted: Fri Jan 21, 2011 3:23 pm
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This could be due to the electrical parameters around the PI tube, rather than the tube itself. Say the amp has a 12AX7 PI stock. 12AX7 like to run around 1-2mA idle bias for a good "swing" needed to correctly drive the power tubes. Now, if you replace the 12AX7 with a 12AT7 (which likes something around 7-10mA for a good "swing") then you maybe running the PI section in the non-linear range. Even at idle. Result would be quicker onset of distortion.

Same goes for 12AU7's (which also like around 10mA idle bias).

Which is ok, if you don't mind the sooner onset of distortion.


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Post subject: Re: More Tube Talk
Posted: Fri Jan 21, 2011 3:29 pm
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Yes, 12ax7 are stock i am using the 12au in the PI.

It does nice things in my opinion maybe not for everyone.


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Post subject: Re: More Tube Talk
Posted: Fri Jan 21, 2011 3:40 pm
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What do you mean by nonlinear?


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Post subject: Re: More Tube Talk
Posted: Fri Jan 21, 2011 4:38 pm
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BMW2002Ti wrote:
This could be due to the electrical parameters around the PI tube, rather than the tube itself. Say the amp has a 12AX7 PI stock. 12AX7 like to run around 1-2mA idle bias for a good "swing" needed to correctly drive the power tubes. Now, if you replace the 12AX7 with a 12AT7 (which likes something around 7-10mA for a good "swing") then you maybe running the PI section in the non-linear range. Even at idle. Result would be quicker onset of distortion.

Same goes for 12AU7's (which also like around 10mA idle bias).

Which is ok, if you don't mind the sooner onset of distortion.


Not sure I agree.

If I am not mistaken, preamp tubes are cathode biased. Unless you really throw an odd ball tube in there, I believe the bias level will self adjust to what is needed by the tube.

I believe that the distortion is caused by the fact that in order to achieve the same loudness level with a 12AT7 that you had with a 12AX7, you are having to turn the volume control/preamp gain/master volume up to a higher level which means that the preamp tubes are possibly being overdriven into mild distortion, plus you may be hitting the PI tube with a larger input signal than before, thereby driving it into distortion.

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Post subject: Re: More Tube Talk
Posted: Fri Jan 21, 2011 6:11 pm
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My Super Reverb( reissue) has a 12at7 in the PI location, I've read somewhere alot of guys run 12ax7's in that position as a "mod"...what does this achieve? With the tubes in their stock location, my amp breaks up too much I think.


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Post subject: Re: More Tube Talk
Posted: Fri Jan 21, 2011 6:26 pm
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randini wrote:
My Super Reverb( reissue) has a 12at7 in the PI location, I've read somewhere alot of guys run 12ax7's in that position as a "mod"...what does this achieve? With the tubes in their stock location, my amp breaks up too much I think.


Its designed that way. A 12AX7 will have more gain than a 12AT7. It may drive the power tubes harder making them break up even more.

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Post subject: Re: More Tube Talk
Posted: Fri Jan 21, 2011 6:37 pm
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....maybe I'm just not a Super person. I think the breakup is too "ratty" and thin...it has Fender/GT labeled Sovtek 5881wxt's in it currently, they were in their when I got it


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Post subject: Re: More Tube Talk
Posted: Fri Jan 21, 2011 7:02 pm
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randini wrote:
....maybe I'm just not a Super person. I think the breakup is too "ratty" and thin...it has Fender/GT labeled Sovtek 5881wxt's in it currently, they were in their when I got it


You might try re-tubing it:

https://ssl.eurotubes.com/cart/index.ph ... gory_id=36

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