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Discussion in 'Glowing Bottle Tube Amp Forum' started by motwang, Nov 23, 2015.
What brand should I try and why? I hear that it smooths out volume as you turn up.
Do you have a specific tube you're planning to replace with it?
I would only recommend a 12AT7 in the phase inverter position (V3), but I don't know how that would affect the play of your volume knob. If you want to smooth out the volume, try a 5751 (which is really just a 12AX7 with gain similar to a 12AT7) or a 12AY7 in V1.
If you really want to try a 12AT7, JAN Phillips are available NOS for barely more $$$ than new production versions.
I used a 12au7 for a few years in the first position in my blues deluxe . Made the amp totally controllable, but it certainly dropped its output.. i ordered the 12at7 but while i was waiting for it to arrive i put the 12ax7 back in.. i may just leave it at that, the amp is back to its original specs and I'm loving it. . But after the next gig i may give the 12at7 a run..
12AT7 and AU7 aren't exactly circuit compatible with the AX,AY or 5751. They are pin compatible, but the tubes operating points are different. In the PI position, the tubes are run operated a bit differently, and you can substitute an AT in there, but PI don't function like gain stage, so the difference in gain factor doesn't have the same effect as swapping it into a cascaded gain stage.
So snag a AY7 if you are ordering up tubes. You might not lose much volume, and you might just lose some preamp distortion. You'll definitely gain a larger dial sweep prior to onset of preamp distortion.
The 12AT7 will bias a little cold in that position, which could add some harshness. It's kind of a try and see experiment.
The 5751 might be a better choice. Same gain factor as a 12AT7, but it will bias up the same as the 12AX7. Next step down in gain would be a 12AY7, which is what Leo's boys used for the Tweed amps.
Brands? For 5751, I would probably go NOS or ANOS JAN, there haven't been a lot of choices in those in modern production, because there wasn't a lot of demand.
12AT7 in V2 makes the overdrive much more useable. We recommend it as a replacement. JJ's seem to work very well there.
The different tubes will change the amps dynamics a little, but won't fix the problem.
The real issue is the volume pot not the choice of tubes. I don't recommend mods very often but, Fender should have done it right and used an audio pot, 1 meg or 500k I think would work. You can do the mod yourself pretty easily the pot shouldn't cost more than a few bucks and is a well worth improvement. I think you can find one that fits right in the board at www.tubesandmore.com
In my Hot Rod DeVille 4x10, I place a 12AY7 in V1 (EHX Brand sounds best to me, I've also used JJ that was nice). To my ears, it makes the clean channel WORLDS better!! I only play thru channel 1 actually. In Channel 2 (V2) you can try a 5751. In V3 try your 12AT7 (NOS RCA or Sylvania in mine now). This seems to relax the Presence & some say the Reverb too. This setup will tame the beast. Also run a volume box in your effects loop. A tiny passive volume knob allows you to CRANK the amp, while keeping the volume at bedroom levels when needed. You can pick one of these up for $20.
When I owned a Hot Rod Deluxe I put a 12AY7 in V1 to drop the first stage gain a bit and give me more control over clean volume.
12AT7s and 12AU7s are better suited for the phase inverter tube as they were not designed as an audio preamp tube and can deal with the extra current in the PI position. But they do work as a sub for other 12A_7 tubes and have been used in that way by many musicians.
Back in the day we heard that the ATs and AUs were less musical sounding than other A_ tubes but given the state of new tube quality nowadays I don't think that would be a significant factor
Wasn't that one of the big changes in the Hod Rod III series, much more balanced volume taper without the jump from nothing to LOUD.
I've heard this a lot, but the HRD schematic shows 250K Audio pots (15/85 taper) for both Volume and Drive controls. Only the Master Volume is Linear*, and if you're using the Clean channel the MV isn't even in-circuit. I think the real problem is that a 40+W non-MV amp is just really crazy loud for non-stage use.
Lower gain preamp tubes can help with the volume taper issue, but the HRD's circuit makes this complicated because changing any one preamp tube will effectively change two triode gain stages in series. Plus both "channels" will be affected.
* Counter most of the Internet speculation about the linear MV being a marketing ploy, I always assumed they used a linear control in that position because low settings on master volumes usually sound pretty bad.
The George Benson signature model uses a 12AT7 as a preamp tube. Maybe there is something to it.
Pretty much every tube I've tried has sounded pretty good in my DRRI's preamp. Including 12AT7.
Yeah, a lot of people have tried it and liked it. I wondered about that GB model, and what we don't know is if Fender biased the 12AT7 differently or did something to modify the tone coming out of those stages. Seems a little silly to market a product where the only difference is a preamp tube and a speaker, but who knows....
I certainly don't want to discourage anyone from trying a 12AT7 in V1, it just seems like the 5751 is a better bet to lower the gain there while preserving the 12AX7 tone.
I'm in the same mindset of NOT placing a 12AT7 in V1 or V2 positions. I do not see it as a musical, or tonally pleasing, preamp tube. A 5751, 12AU7, or 12AY7 as an alternative with lower gain, sure, but not a 12AT7. It will work, but...
I don't think it's silly, it's a way to market it so you get repeat sales of the same amp with only a few external, modular, changes. I would speculate the circuit boards themselves are the exact same.
Good point. There's a reason I'm not in Marketing.
Agreed -- I can't imagine they would make a different board. But if they wanted to make an effort they could have tweaked a few component values. Maybe. Probably not.
It's common practice to list the PI tube among the preamp tubes. My money says (since the GB amp links to the same schematic as the other Hot Rod Deluxe/DeVille amps) that GB just wanted a traditional 12AT7 in the PI spot, not at the input stage.
Nah. According to this article it's V1. (The official marketing blurb says "up front".) That article also points out that Fender is charging you an additional $200 for all that extra awesomeness.
Interesting, good thing I didn't bet any real money on that one! Still, a really weird choice on their part.
Only real 12at7 I would consider for tube slot designed for a 12ax7 is a Mullard cv-4024. It is lone voice in 12at7, warmer and richer by far than any other
It's not gain range of 12at7 that doesnt fitin here, it's the current draw. Most sound brittle/strident, sure you get the knob roll off, but it sounds off. A old Mullard 7024 or Sylvania 12dw7 sound much better in this use. It will give you about same gain feel as a AT but sounds much better.
For those preamp slots made for 12ax7, go 5751, 12dw7/7024 or 12ay7/6072, high>low gain to find your sound/feel on knob. It will sound much better.