Replacing 12ax7 with 12at7 in Crate VC508

[inducedblues]

I recently picked up a VC 508 as a practice amp. I was reading the article on how to improve the sound and saw that it mentioned reaplcing the preamp tube with a 12at7.



I was wondering whether or not this is just a question of swapping the tubes out or whether or not it will involve more modification as the article goes on to detail.

I'm not an electronics guy, in fact I'm pretty clueless but if I can improve the sound of my new amp with a simple tube swap that can be done without any messing around with anything important in the amp, I'm for it and I can call it my little mod..along with slapping on some ampeg avt style grille cloth on there (I hate metal grills).

So any advice on this matter would be appreciated. Since this is my first real all tube amp and I am kinda ignorant of electronics in general, I don't mind if you explain it to me in laymens terms. I am a guitar player so I know woods and strings and pickups but I'm still learning about amps and their electronics

[Mini-Strat_Maine]

inducedblues said:

I recently picked up a VC 508 as a practice amp.
{snip}
So any advice on this matter would be appreciated. Since this is my first real all tube amp and I am kinda ignorant of electronics in general, I don't mind if you explain it to me in laymens terms. I am a guitar player so I know woods and strings and pickups but I'm still learning about amps and their electronics

I can't answer all the good technical stuff about biasing the tube(s) and all that, but just wanted to mention that the Fender Musicmaster Bass amp that I got a couple of months ago (and have been driving everyone here nuts with questions about since) came with a 12AT7 in place of the usual 12AX7.
I did a little reading up on it, and the change is supposed to make the MM Bass amp a little more "mellow." Having never heard it with an AX in it, I can't comment on any difference, but I can say I like it as it is now, and I'm not inclined to change it back.
If you have an AT and an AX on hand, and there's no biasing or other complicated stuff involved in swapping them around, you can do "blindfolded taste tests" on willing subjects (or unwilling ones, if you have enough duct tape ;D ) and then decide.
Whichever tube is left out after you decide could be stashed away for an emergency spare.
Yeah, there's a lot to learn about all this ("All this electronics stuff, and music theory, too!?" ), but this is a great place to learn it.

[pollyshero]

I replaced the 12AX with a 12AT in my VC508 - simply swapped them out. The difference was immediately noticeable, however it's important to note that there was no radical change in tone. The amp sounds about as it did with the 12AX, but I've got a little more control over the distortion that made the swap worth the 5 minutes it took (for me anyway).

[sumgai]

IB,

Hey, to these here forums! Stick around, you'll learn more by osmosis than you ever thought you'd want to know!

The 12AT7 is a lower-gain version of the 12AX7 (about '60' versus '100'), they require the exact same socket wiring, so no mods are needed at all. There are also three more tubes in this 'family', avoid them for their extremely low gain factors.

12AU7 = 20
12AV7 = 41
12AY7 = 45

For your purposes, the same voltage input to the AT it will be amplified less, thus 'hitting' the next stage a little less forcefully than with a AX. That may, or may not, sound "better" to your ears, and it will definitely depend on the rest of the amp's circuitry. It's all subjective to your personal tastes, so as Doug said, try them both, and keep the 'unwanted' tube as a spare.

HTH


sumgai

[ChrisK]

Can you imagine the mess we'd have if OPAMPS had fixed gains (about so much or so)?

Instrumentation my eyeball!

Keep the spare tubes. In about 30 years you won't even need that IRA. (Assuming that another slug of boomers comes along that is interested in the toys of OUR generation!)

[UnklMickey]

this seems to be "off topic day" so....

"Can you imagine the mess we'd have if OPAMPS had fixed gains (about so much or so)?"


"ackshally" Chris, they do.

(and the gains are ridiculously high)

it's just the stuff we put around them that allows us to control (reduce) the gain.


we can reduce the gain with tubes too.

sometimes it's just easier to plug one in that has the gain we want.


unk

[vjmodman]

I tried out a bunch of diffrent tubes in my valve junior and finally settled in on a GE 12AX7 made in usa of unknown year (sometime before 1980 ish I'm pretty sure)

The Sylvania 12AT7's I tested basicly sounds just like a 12AX7 only with slightly lower gain (ie you have to turn it up more for it to distort as much as a 12AX7)

But I found that the 12AT7's distortion sounds much more bright and peircing (not in a good way) and kinda "ragged". I thought the 12AX7 had much "smoother" more round distortion. I tested around 20 of each diffrent kind from diffrent manufactures from diffrent years etc etc but they all showd the same findings of the AX being much smoother.

IMHO of course. and this was on a valve junior (which is already a bright amp) so maybe the crate needs the 12AT7 to sound good.



Bottom line is go get yourself some of both and see what -you- like the best. and in case your still unsure and no ones made it clear enough, you can swap out the preamp tube all you wan't without changing anything on the amp. Just wait till they cooldown or you will have a nasty burn.

[xStonr]

Funny, with the speakers I use with my VJ, I have found it to be dark sounding. I have used the lower gain at7 tube with excellent success. To each their own.