Preamp Tube Tidbit

[skoora]

Was picking up an amp from a local tech and we got to talking about using AT7’s when you want to tame the gain in a high gain amp and he told me that it would be better to use an AY7 as it has the same impedance as an AX7 and would mess with the EQ response a lot less. An AT7 has a different impedance than an AX7 which would explain when I’ve tried them before I didn’t like that they reduced top end as well as gain. This may be old news to some but it was new news to my old ass.

 

[eclecticsynergy]

Hadn't heard of these and now I'm intrigued. What's the gain factor of a 12AW7?
Must be fairly rare, it doesn't seem to be listed on any readily available tube charts...

 

[VonBonfire]

I had had heard of the 12AT7 swap and I tried it once for the heck of it and thought it was lousy. I found a good production preamp tube I like so I just stick with that for gigs.

 

[skoora]

Hadn't heard of these and now I'm intrigued. What's the gain factor of a 12AW7?
Must be fairly rare, it doesn't seem to be listed on any readily available tube charts...

Crap, my mistake on memory. He must had said 12dw7 and I remembered wrong. It was the W that was stuck in my head. I edited my OP to correct it.

 

[skoora]

I’m actually going to reach back out to him to double check exactly what he said. He may also have mentioned a 5751 so not sure if that’s the same.

 

[skoora]

OK, he did say 5751 or 12AY7. Edited OP.

 

[griff10672]

OK, he did say 5751 or 12AY7. Edited OP.

I think Dave Friedman has mentioned using a 12AY7 in the first position on certain amps in several podcasts

 

[SpiderWars]

I knew it but still think that a 12AX7 sounds better in the PI in some old Fender amps that originally used a 12AT7 there. However, imo the PI circuit they used was actually better for a 12AX7 anyway.

But I would never use a 12AT7 to reduce gain. A 12AY7 maybe but it’s a big reduction in gain. A 12DW7 is one triode like a 12AX7 but the other is a 12AU7 (even less gain than a 12AY7). They can good for FX loops as long as the 12AU7 triode is used for the Send.

The biggest difference between a Tweed Bassman and a JTM45 is the first preamp tube…yet same circuit. 12AY7 in a Bassman.

 

[70 Mach 1]

I never was happy with swapping V1 lower gain tubes in any of my amps

Always ended up going back to the 12AX7s

 

[eclecticsynergy]

5751 is a bit closer to a 12AX7's gain factorthan the 12AT7 is, and as your friend noted it's MUCH closer in terms of resistance.
There's a big discrepancy between the 12AX and the 12AT.
Makes sense that the AT would alter the behavior of a circuit far more than the 5751.
5751 is a much better option for reducing gain a bit without screwing up the tone.

Here's a preamp tube reference chart:

 

[Aynirar27]

I used to run a 5751 in the V1 of my Uberschall. Still had more gain than I'd never need, but it did seem to tighten it up a little bit.

 

[dcburn]

I have a 12AY7 in a drawer.
Purchased to try lowering gain, maybe different tone.
Tried in all my amps, tube is in the drawer...

 

[mcstinger]

Just for fun and imagination, simulation of different tubes in a somehow standard stage.

And here with 500p caps across anode resistors.