I’ve tried a number of different preamp tubes in this amp since building it and wanted to share my results. I’ve used a raytheon NOS 12au7 and a Phillips JAN NOS 12at7 in addition to several kinds of 12ax7. Alot of what I read online says that these are bad substitutes for 12ax7s – I see alot of “dead tone,” “tone killing,” “its like turning your guitar volume pot down” etc. These are not useful descriptions because they don’t explain what these tubes do to the sound or volume of the amp..
I find both of these tubes very useful for me in this amp.
I find that the 12au7 sounds alot like a 12ax7 whereas the 12at7 seems to emphasize lows more. I wouldn’t call it darker, just like there’s more oomph to the lows.
The 12at7 gives about a 30-50% volume reduction vs 12ax7 while the 12au7 gives about a 70-80% volume reduction.
The 12au7 will not get you breakup at all. the 12at7 probably only starts to breakup about 9 or more on the volume. These won’t give you breakup at lower volumes if that’s what you seek.
The amp sounds quite different with these tubes than with a 12ax7 with the guitar volume rolled down, particularly a stock fender guitar which sounds lousy with the volume rolled down to me due to the treble loss.
I think both tubes sound great, and the 12au7 in particular allows for a great clean sounding amp at VERY low volumes in my opinion.
I am mostly using the two stroke with an RCA NOS 6k6gt and most of the time want a clean tone that is of a useable volume for inside the home practice. The 12au7 is great for this.
It’s great to have an amp that I can run a 12ax7/EL34 through a 1×12 cab and hear it up and down the block; then put 12au7/6k6 into the 8″ speaker combo cabinet I have the amp in amp turn it up to 6 and barely hear it on the other side of the house. And have the tone be great in both cases.
I’m about to get a simple mic/usb interface/protools setup going and then i’ll start recording sound clips and posting them online – I’ll let this forum know when I do.
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