12au7 in Princeton Reverb

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JoshBernstein
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12au7 in Princeton Reverb

Post by JoshBernstein »

Hi again all,
My parts are starting to come in for my Princeton Reverb, and I was curious if i could sub all the 12ax7's with 12au7's? Im looking to keep the volume on this thing as low as possible. Im sure that subbing v1 would be fine. Im wondering what the effect would be on the reverb return and the trem circuit?

Another big reason i wanted to do this was the major price difference between NOS 12ax7's and 12au7's. I can get a RCA, Philips, Sylvania etc. 12au7's for way cheaper than their 12ax7's.

What would the sonic differences be? Would there any issues with the reverb or trem when using these?

Thanks a bunch!
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xtian
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Re: 12au7 in Princeton Reverb

Post by xtian »

12AX7 has a mu (amplification factor) of 100. 12AU7 is only 17. But more than that, the AU is generally used as a high-current driver, not as an amplification stage. You'll find AUs by the hundreds in old organs and tone generators and such. But it's not a great tube for tube amps, except as reverb driver or such.

You will have better results with 12AT7 (mu = 60) or AY (mu = 40) as lower gain alternatives to AX.

That's why the AUs are cheap!
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Re: 12au7 in Princeton Reverb

Post by pompeiisneaks »

And I'd add this isn't going to make the amp much 'quieter'. The loudness of an amp is pushed by the output stage, the amount of tone and drive you get is what the preamp stage gives you. Therefore you face other problems, like potentially needing to adjust the preamp tubes bias point etc, since its a slightly different tube. The amp was designed with the 12AX7 in mind, but you'll never know how it sounds until you try it. Just make sure you understand there's more to loudness than the preamp tube used etc, and that the changing of the 12A*7 tubes isn't just as simple as 'do it'. Yes they're pin compatible, and won't kill anything, but the tonality is going to be quite different as well.

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JoshBernstein
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Re: 12au7 in Princeton Reverb

Post by JoshBernstein »

well that sounds like a no go then!! thank you both for your advice! Its appreciated!
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tubeswell
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Re: 12au7 in Princeton Reverb

Post by tubeswell »

As a reverb driver tube (in a Blackface/Silverface-type reverb driver circuit), a 12AU7 is a fantastic choice to sub for a 12AT7 if you want less overwhelmingly deep reverb. It is a lower plate resistance than a 12AT7, but much better bandwidth, and passes high tube current (like a 12AT7), and with the right amount of bias, can comfortably run into the same sort of transformer reflected load as a 12AT7) and it has lower gain than a 12AT7. The wider bandwidth means it can give a nice 'drip' to the reverb, but the lower amplification factor, means it it will not overpower the reverb.

For best results, set up the bias to be as centre-biased as possible with the particular 12AU7 plate idle voltage and reflected load that is provided by your reverb transformer. (Some RTs are designed to reflect ~22k, some ~25k, some ~15k). Note that where you are running both triodes in parallel, a 12AU7 maximum recommended plate dissipation is 5.5W. So say you have 400V at your reverb driver plate on your typical AA1164, you ideally want Zout=Va/(Pa/Va) = 400/(5.5x400) = 29,090R, so you would shoot for an RT of that ilk, or if you didn't have that, then run the 12AU7 bias a bit cooler (i.e. with a higher cathode resistor value - say, use 2k7 if you have a 25k reflected load or 3k3 if you have a 15k reflected load - YMMV)
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Last edited by tubeswell on Thu Mar 30, 2017 2:42 pm, edited 1 time in total.
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Re: 12au7 in Princeton Reverb

Post by Stevem »

You also can not get 12AU7s with low noise wound filiments for gain stage usage like you can AX7s!

Your far better off setting a AX7 voltage wise and resistor wise for making less gain.
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Re: 12au7 in Princeton Reverb

Post by Firestorm »

There's not all that much gain in Blackface circuits. The tone stack throws away a lot of first stage gain, the tone recovery stage puts it back and the reverb mix stage has its input set for is set for very little net amplification. One of the best places to control output volume in Fender Reverb amps is at this reverb mix stage. Compare the AB763 circuits with the Princeton Reverb. The AB763 models have a 220K to ground paralleling the 470K/100K reverb level pot. This slightly reduces the signal into the mix stage, which works with the LTP PI, which has gain. The Princeton split load PI has no gain so a signal boost was in order. None of these amps was intended to overdrive the PI in "normal" use (and you absolutely don't want to overdrive the split load PI).

If you insert a 250K-to-500K rheostat-wired pot in place of the 220K, you have a master volume of sorts that lets you attenuate signal to taste. You'll overdrive nothing, so you won't have the slight edge a Princeton gets when cranked, but that's the trade off.
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