12AV7

[ajkimba]

I have a sleeve of NOS RCA 12AV7s. If I remember right these tubes can be used in place of a 12AX7 but with a lower gain factor simular to a 12AT7. Any one know anything about them?

[CurtissRobin]

Per the RCA book:
12AX7 mu is 100
12AT7 mu is 60
12AV7 mu is 17

Very low gain by comparison.

KennyO

[jamesrr]

Approx gain factors: -

AX7 - 100
5751 - 70
AT7 - 60
AY7 - 45
AV7 - 41
AU7 - 19

I was more interested in if it was a no-brainer decision to use AV7s where AY7s are normally used, as the AV7s are dirt cheap compared to AY7s.

regards,

jr

[Phil_S]

It isn't only the gain factor that determines performance. It is rare, as I understand it, to actually milk a preamp tube for all the gain that is has potential for. How it's biased and other circuit design considerations that I don't really comprehend come into play. This is the reason, for example, that you really don't want to use a 5751 or 12AX7 for a Fender PI that's built around a 12AT7. (Sorry about the F word here.)

I acquired a pair of nice looking black plate Canada made Marconi 12AV7's several years ago in the hope that they would tame a problem I had, but that proved not to be the case.

It occurs to me that it might be a potential sub for 12AU7 or 12AT7, but I think it's very low plate resistance will make is a poor sub for 12AX7, 5751 or 12AY7.

The bottom line? Plug them in and see what your ears tell you. I think then you will know why mine are in their boxes! I think you will find the 12AV7 will suck the life out of an 18W, and in general, you will find out why this tube has not caught on as a guitar amp tube.

[CurtissRobin]

It isn't only the gain factor that determines performance.

Absolutely, but the gain (mu) tells a lot about a tube, particularly in guitar amps where the importance of gain outweighs everything except, probably, reliability and noise.

It is rare, as I understand it, to actually milk a preamp tube for all the gain that is has potential for.

Probably true but I'd rather use a tube that's giving me the sound I want while working in the middle of its range, not at the limit of its capability. Being an engineer, I know I'm not alone in that sentiment.

KennyO

[kleuck]

Approx gain factors: -

AX7 - 100
5751 - 70
AT7 - 60
AY7 - 45
AV7 - 41
AU7 - 19

I was more interested in if it was a no-brainer decision to use AV7s where AY7s are normally used, as the AV7s are dirt cheap compared to AY7s.

regards,

jr

You can use them where 12AY7 (or 12AX7 most of the time) are used, i did.
Little less gain, and harsher sounding in my amp, i'm back to the 12AY7.
By the way, i use NOS GE12AY7, but the EH ones ares really really good, NOS do not provide a huge improvement in that particular case (slight one).

By the way, µ is NOT the gain of the tube.
In most amps, 12AT7 have a little MORE gain than 12AX7, but with a different sound and lower plate impedance.
Most of the time 12AX7 gain is about 55-60, 12AT7 : 60-65, 12AY7 : 40-45, 12AU7 : 12-14

[zaphod_phil]

In most amps, 12AT7 have a little MORE gain than 12AX7.... Most of the time 12AX7 gain is about 55-60, 12AT7 : 60-65....

In terms of actual stage gain, I believe this is only true when the preamp stage is driving a low impedance load at the output, such as a TMB tone stack. Due to the low internal anode impedance, the stage gain is less affected by the low-impedance load. At least that's the theory. However, a few days ago I was testing a Hiwatt-like amp I had built, with a 12AT7 PI (91k/82k load resistors), driving power tubes with 100k grid reference resistors. When I swapped out the 12AT7 for a 12AX7, to my surprise, the amp got noticeably louder with a clearer sound. Based on the theory, I was really expecting the 12AT7 to drive the power valves harder than the 12AX7.

The other thing to watch out for with 12AT7s, is that they have kind of unpleasant, unmusical muddiness when overdriven, which I discovered when trying to use them as preamp tubes in a higher gain preamp. Someone else I know also experimented with 12AT7s in his preamp and found the same. After that I came across a post of some lab tests with 12AT7s which showed that they generate a lot of odd-order harmonics when overdriven, especially 9th. This explained what we had been hearing. "Valve Wizard" merlinb also later published test results showing that most 12AT7s draw very high grid current compared to other common members of the 12A_7 family, which could also make them more prone to blocking distortion. So now I generally avoid using 12AT7s, except where I know they are unlikely to be overdriven.

[kleuck]

In most amps, 12AT7 have a little MORE gain than 12AX7.... Most of the time 12AX7 gain is about 55-60, 12AT7 : 60-65....

In terms of actual stage gain, I believe this is only true when the preamp stage is driving a low impedance load at the output, such as a TMB tone stack. Due to the low internal anode impedance, the stage gain is less affected by the low-impedance load.

Not really, try it : http://amps.zugster.net/tools/triode-calculator
In V1 position in my amp, a 12AT7 has actually more gain than a 12AX7, in my "Pi" too, but it's a cathodyne so...

[dotfret]

It should be remembered that the specification sheet gain is a nominal figure. Many NOS valves have a higher gain than the "spec value".

I once found a paper on the web relating to some tests that Philips made on production batches - effectively the work that determined how to select the WA, WB, WC low noise JAN 12AX7 types from production batches. It showed that the mean gain was often as high as 114-118 on standard production, but that valves with a gain close to the nominal value (90-105) had improved S/N ratio over the high gain valves in the batch.

Anyone finds that paper - please send me a message.

[zaphod_phil]

Not really, try it : http://amps.zugster.net/tools/triode-calculator
In V1 position in my amp, a 12AT7 has actually more gain than a 12AX7

Only if the input of the 2nd stage presents a fairly low impedance. That is the one place an AT7 wins over an AX7 in stage gain. The thing to be careful of is an AT7 clipping, as then they sound quite unmusical, presumably due to high-order odd harmonic content, such as strong 9th harmonics. There are more details about that on AX84.

It should be remembered that the specification sheet gain is a nominal figure. Many NOS valves have a higher gain than the "spec value".

That's another good point.

[jimipage33]

maybe it would be a great tube to use in a cascode pre?

[dotfret]

Frankly, the cascode pre-amp concept is so fraught with problems that you only want to go there to win a BIG bet.

Mr Laney did it a long time ago for a small bet - he did not go back.

There is one valve which was really designed to attack the cascode pre-amp idea, the Telefunken ECC8100.

I have a pair of these rocking horse droppings hanging around in my environment, waiting for me to take all my courage in both hands, and boldly go ...

I suspect that they will still be waiting ten years from now - LOL it is hard work, and where do you get more valves?

I have spent too much of my life being the metaphorical body double of "Gonzo the Great" to think about cascodes - I'm through with grief and bleeding.

This is 18 watt, and 18 watt works! You want to talk about cascodes, go over to Wattkins.com and look for soulmates.

Back to reality - it has to be said that there are occasions when the ecc81 in a 18W trem circuit oscillates badly, and make you pull all your hair out - put a 12AZ7 in the hole instead, tell the customer it is an upgrade, and go back to watching the river. WTH, if it works, don't fix it, as AJK would say.

[marshman]

I've a slew of those little gems, as well, and my research led to a few guys that had commented that they were designed as switching doo-dads in TVs and such, no consideration whatsoever given to their audio qualities. As such, they tended to be very noisy in audio circuits.

I didn't 'save' any of those references, but after the third one, I gave up and put 'em back in a big ol' box of "Tubes I'll likely never find a use for."

[Alexo]

Frankly, the cascode pre-amp concept is so fraught with problems that you only want to go there to win a BIG bet.

Mr Laney did it a long time ago for a small bet - he did not go back.

There is one valve which was really designed to attack the cascode pre-amp idea, the Telefunken ECC8100.

I have a pair of these rocking horse droppings hanging around in my environment, waiting for me to take all my courage in both hands, and boldly go ...

I suspect that they will still be waiting ten years from now - LOL it is hard work, and where do you get more valves?

I have spent too much of my life being the metaphorical body double of "Gonzo the Great" to think about cascodes - I'm through with grief and bleeding.

This is 18 watt, and 18 watt works! You want to talk about cascodes, go over to Wattkins.com and look for soulmates.

What's so kooky about cascode preamps? They've worked and sounded great for me (in 18 watt amps), higher gain than a triode, or paralleled triodes, lower output impedance, chimey sound.... I suppose they're not for everybody, but problematic, they ain't, at least in my experience. I've always used 6DJ8's - which were also specifically designed for cascode preamps, fwiw, scored about 2 dozen out of an old Oscilloscope.