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Dirty-South Blues Harp forum: wail on! > Double trouble tube swap
Double trouble tube swap
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128 posts
Nov 10, 2021
7:06 AM
I recently bought a HarpGear Double Trouble secondhand. To get more "crunch", I swapped in a 5751 for the 12ay7 preamp tube. Could that cause any damage to the amp?
398 posts
Nov 10, 2021
8:14 AM
No damage.
Just FYI, I looked at some old notes -- I have played the DT with an LW Octave pedal in front; that pedal adds a little crunch (and some feedback resistance). I was happy with that. Ah, looks like I had some reverb later in chain.

Last Edited by TetonJohn on Nov 10, 2021 8:18 AM
3661 posts
Nov 10, 2021
9:01 AM
Preamp tubes do NOT need to have the amp rebiased at all, but with power and rectifier tubes that are different than what was originally in the amp, absolutely yes! BTW, a 5751 is a higher gain tube than a 12ay7 is and in terms of gain, it's in between that of a 12ax7 and a 12ay7.
Barbeque Bob Maglinte
Boston, MA
CD available at http://www.cdbaby.com/cd/bbmaglinte
990 posts
Nov 10, 2021
12:38 PM
I ran a nos Jan 12AY7 in mine, sounded great with no effects.
Real fat, but too dark. I swapped out the Webers for Jensen RI P8R's.
That's all it took!

Since crunch comes from pushing a speaker/amp to break up, you may or may not get that? I didn't get crunch, just a real fat thick tone.

Last Edited by Harpaholic on Nov 13, 2021 9:42 AM
129 posts
Nov 10, 2021
2:08 PM
@barbequebob- Yes,I know about the gain factor; that's why I made the swap. But my question was would swapping in a preamp tube with a higher gain factor cause damage. The amp has a solid state rectifier which of course remains the same and the power tubes haven't been changed.
7012 posts
Nov 10, 2021
11:34 PM
The internal resistance of a swapped preamp tube can sometimes be a problem, but very unlikely in the case of 12AY7 to 5751. They are pretty close. It’s more of a concern when you go from 12AX/AY/5751 to 12AT/AU. The AT and AU tubes have a much higher internal resistance which draws more current and can stress the plate resistors. Some amps eg the Bassman RI, Hot Rod Deluxe/Deville have plate resistors which are already a little marginal in the circuit so it doesn’t hurt to beef them up to 1/2watt types if you plan on swapping in these tubes. But for many amps it’s no drama.
The 12AX7 and 5751 are very swappable though as they are virtually the same apart from the amplification factor. The 12AY7 is reasonably close too and also pretty much always a safe swap.
130 posts
Nov 11, 2021
2:05 PM
@SuperBee - Thanks. That was the kind of information I was looking for.

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