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Post subject: HRD - Phase invertor question
Posted: Tue Mar 15, 2011 3:15 pm
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Joined: Fri Jan 14, 2011 6:40 pm
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Hey guys,

I was wondering if you guys change the 12AX7 tube thats used as your phase inverter. (The 12AX7 tube on its own between the pair of 2 preamps and 2 power tubes.)

I'm not too techie with amp stuff, but can anyone tell me what it does?

Thanks!


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Post subject: Re: HRD - Phase invertor question
Posted: Tue Mar 15, 2011 8:10 pm
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Joined: Tue Jun 29, 2010 1:20 pm
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Hi jchoi5,

Using different gains/brands of tubes in the PI position, seems to affect the breakup point of the amp, especially different gain tubes, without really changing the tone. The PI tube isn't a preamp tube, so it has little, if any, affect on tone. The PI simply uses the single (1) signal that the amp produces, and converts it to 2 signals that are 180 degrees out of phase to each other, one for each side of the output, one signal for each of the 2 output tubes for push-pull. The PI position is usually where you want a balanced tube, if you care about that. Personally, I just make sure that the output tubes are a matched pair, and that the balance between them is very close, and I haven't found it necessary to specify a balanced PI tube. Some feel that a balanced PI tube is important, and it certainly wouldn't hurt.

If by HRD you mean Hot Rod Deluxe (HRDlx), the output transformer primary is imbalanced anyway (by design), so a matched set of output tubes is about the closest to a balanced output that you can get, and a balanced PI would be a waste of money IMO, but wouldn't hurt anything if one is used. You may get a desireable change in the breakup point of both the clean and drive channels by trying different gain factor tubes in the PI position. For example, instead of a 12AX7, try a lower gain tube such as a 5751 or 12AT7 in the PI spot to change the level where breakup occurs.

I have tried a few different gains and types including the lower gain 12AT7, and I seem to prefer the higher gain tubes in the PI position. I happen to have a GT 12AX7-C in my HRDlx PI position now, simply because I put the same tube in all 3 positions and it sounds great. For my BJr, I use 12AX7WC in all 3 positions, it also sounds great. And 1 spare for each amp will work in any of the 3 positions, so no need for a special tube for the PI. :)

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