Author Topic: Effect of tubes? (Read 3256 times)

MartinS · « **on:** November 09, 2007, 01:30:43 PM »

My tube zipper has two 12AX7 tubes. I want to buy a couple of spares, but as the 12AX7s seem rather "hot" I was wondering what the impact might be of trying 12AU7s or 12AY7s. What's the difference?

I'm more than a little ignorant on this, so any advice is greatly appreciated.

Elliot · « **Reply #1 on:** November 09, 2007, 02:47:07 PM »

In order of gain (differences in gain are an estimate):

7025*
12AX7
5751 - Has 75% of the gain of a 12AX7
12AT7 - Has 60% of the gain of a 12AX7
12AY7 - Has 50% of the gain of a 12AX7
12AV7 - Has 40% of the gain of a 12AX7
12AU7 - Has 25% of the gain of a 12AX7

Alot of valve powered pedal users find that a 12AX7 is too much and in two valve pedals find 12AY7s to be good. There are lots of macho types out there who insist on 12AX7s as being the only tube going, but cutting the gain in a pedal can sweeten things up a bit.

* by this I mean modern 12AX7s rated as 7025 which mean hi-gain, low noise examples of 12AX7s, not the original 7025s.

MartinS · « **Reply #2 on:** November 09, 2007, 02:53:23 PM »

Thanks for that. Yeah, I almost put that I find the pedal too hot, too saturated, and that the tone of the guitar's rather swamped. I think I'll perhaps try some of the lower gain alternatives.

Cheers.

MartinS · « **Reply #3 on:** November 09, 2007, 02:57:39 PM »

Just as an afterthought: is it possible to mix and match? Say, one 12AX7 and one 12AY7, or isn't there much point in that?

TwilightOdyssey · « **Reply #4 on:** November 09, 2007, 02:59:26 PM »

That's a very nice little chart!!

Davey · « **Reply #5 on:** November 09, 2007, 03:23:14 PM »

Quote from: TwilightOdyssey

That's a very nice little chart!!

very much so...

and before WFD chimes in, let me ask:
What's above the 7025 in terms of gain :D

Elliot · « **Reply #6 on:** November 09, 2007, 05:50:52 PM »

Mixing and matching is possible and may give you the sound you are looking for - Its worth experimenting with different combos on EH 2 tube, tube pedals - and swapping the direction - I have an English Muff'n and find a 12AU7 into a 12AX7 is a good combo.

the_bleeding · « **Reply #7 on:** November 12, 2007, 03:59:51 PM »

i recommend some JAN phillips 5751's. Those are some sweeeeeeeet sounding tubes. And they say 75% gain of a 12ax7, but that makes them sound like they're still pretty high gain... which they definately aren't. They're clean as hell without sacrificing the volume.

jpfamps · « **Reply #8 on:** November 12, 2007, 04:57:48 PM »

Two points.

Firstly, all these valves are twin triodes. In most applications the two halves will be cascaded. So a 5751 will have .75 x .75 = 0.56 the maximum potential gain of a 12AX7. This is quite a substantial reduction.

Secondly, the gain is not solely determined by the valve, but also by other circuit components. You can easily design a circuit, where say a 12AT7 provides the same amount of gain as a 12AX7.

FELINEGUITARS · « **Reply #9 on:** November 13, 2007, 12:57:12 AM »

Very good point on the cascading
Think I need me some of those in the Fender Prosonic to tamethe lead channel a bit - I like a lot of gain but this amp is a bit too much and will be interesting to reign her in a bit

Same goes for one of my ADA MP1s that I want to use as a clean/low gain sounds preamp

Elliot · « **Reply #10 on:** November 13, 2007, 08:31:10 AM »

5751 in V1 of Fender amps are really a must, especially the Hot Rod series - they allow the More Drive to be used, also works wonders in a Blues Junior.

MartinS · « **Reply #11 on:** November 13, 2007, 09:30:57 AM »

I've done a bit of experimenting, and a 12AX7 into a 12AY7 sounds pretty nice to me, although I've yet to try it with the band. It's just a bit less OTT than two 12AX7s, but still has attitude. Two 12AY7s sounded a bit weak.

jpfamps · « **Reply #12 on:** November 13, 2007, 12:03:57 PM »

In a pedal like the Hot Tube which runs about 250 V on the plates most valve substitutions should be safe. However, in an amplifer where there is a larger plate voltage you should be careful about using lower gain valves.

Lower gain valves generally draw more current than higher gain valves. This can cause the plate load resistors to exceed their dissipation rating. Many manufacturer use 1/2 W carbon film resistors in this position. I recently had a Fender HRD to repair that had 12AU7s in all the preamp positions. ALL the plate load resistors in the amp were dissipating more than 0.5 W. My guess is that this is not good for long term reliability. Furthermore changing the plate load resistors on some PCB based amps (eg all modern Peavey amps) is a total pain.

You will almost certainly be fine with 5751s which don't draw that much more current than a 12AX7, although I haven't been too impressed with the Sovtek version of this valve so if you want to experiment with 5751s I would advise going the slightly more expensive NOS route.

Unless you can measure the dissipation in your plate load resistors I would avoid really low gain valves such as 12AU7s. Incidentally, failure of the plate load resistors in the first stage of the first Fender Bassman re-issues was very common when a 12AY7 (the valve used in the original Tweed Bassman. The RI came with a 12AX7) was used in the first stage.

hunter · « **Reply #13 on:** November 13, 2007, 02:49:40 PM »

Quote from: FELINEGUITARS

Very good point on the cascading
Think I need me some of those in the Fender Prosonic to tamethe lead channel a bit - I like a lot of gain but this amp is a bit too much and will be interesting to reign her in a bit

Same goes for one of my ADA MP1s that I want to use as a clean/low gain sounds preamp

Prosonic? How many amps/preamps and stuff do you actually have now?

Feline, you're a little amp whore!

:lol:

Twinfan · « **Reply #14 on:** November 13, 2007, 02:57:16 PM »

Isn't that the pot calling the kettle black??????? ;)

Author Topic: Effect of tubes? (Read 3256 times)

MartinS

MartinS

MartinS

TwilightOdyssey

MartinS