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aletheian 12th October 2005 05:01 AM

So what are some good 7-pin, hi mu, low noise/microphonic tubes??
I want to mess with some 7 pin tubes, but I don't have any experience with them. So what are some good 7-pin, hi mu, low noise/microphonic tubes? I looked up hundreds of data sheets, figuring I'd start by looking at the A.F. or special quality ones first, but I found very little info on usage... other than the VHF specific ones.

Yvesm 12th October 2005 05:23 AM

Re: So what are some good 7-pin, hi mu, low noise/microphonic tubes??

Originally posted by aletheian
. . . . other than the VHF specific ones.
No, except the 4HA5 / 6HA5, a remote cut off VHF one !

However, interesting tube with CCS load ;)


aletheian 12th October 2005 05:32 AM

Re: Re: So what are some good 7-pin, hi mu, low noise/microphonic tubes??

Originally posted by Yvesm

No, except the 4HA5 / 6HA5, a remote cut off VHF one !

However, interesting tube with CCS load ;)


Very cool. wish he had some performance data up there.

6ha5, I had considered that one mu over 70, Gm over 14... petty good numbers.

I also looked at the 6AN4, specifically designed for grounded grid, cathode drive, so I thought it would make a nice top element in a srpp/cascode setup.

EC8010 12th October 2005 07:09 AM

I've never measured a PC900 that met specification for gm...

ray_moth 12th October 2005 08:52 AM

Why would you want to mess around with a remote cutoff pentode? It's non-linear. The 6AU6/EF94 is a miniature 7-pin sharp cutoff pentode that was popular as a microphone amplifier at one time. There are many versions of this tube and some are more microphonic than others. I have a couple of RCA ones that are good but my GE ones are not so good.

SY 12th October 2005 11:38 AM

I've never used a 6HA5 that wasn't microphonic.

Tubelab_com 12th October 2005 02:02 PM

The 6HA5 is a triode. I never heard of a remote cutoff triode, but I suppose it is possible, but I see no mention of the words "remote cutoff" on the GE data sheet. I tried this tube for driver use with a CCS IC for a load, and a mosfet follower for a buffer (PowerDrive) and found that it worked quite well. The 6HQ5 is similar. I did not try them for low level use, therefore I didn't pay much attention to microphonics.

The 6AU6 is a pentode, and in pentode configuration I have found a very wide distribution in performance. Some are noisy and microphonic, while some are dead quiet. The characteristics of the noise changes from tube to tube. The distortion and gain varies quite a bit from tube to tube, and with the operating conditions. In triode mode the distribution of performance is better, but the tubes that were noisy in pentode mode are still noisy in triode mode. The ocean roar is now a mild hiss. If you have a lot of these you could grade out the good ones. I have at least 1000!

The related 6BA6 IS a remote cutoff pentode, and is generally useless for audio, except for some unusual fuzz type distortion in guitar amps.

For an output tube the 6AQ5 absolutely rocks! Triode or pentode, this one sounds good, and takes quite a bit of abuse, although I did manage to melt the glass on one at 400 volts! It was still cranking out sound when it sucked air! Consider it a baby 6V6, at least that is what the tube manual says. Found in early Gibson guitar amps. There are some similar 7 pin output tubes 6CU5, 6FX5 and others.

For the common stuff 6AV6 is half of a 12AX7, 6AT6 is the same with lower gain. These work as well as their 12A*7 counter parts.

The military type 9002 is an excellent sounding little tube, although the Mu is only 25.

The 6J6 is a 7 pin dual triode. Both sections share a common cathode. You can parallel both sections. I haven't tried this. You can use the tube as a differential amplifier with a CCS in the common cathode pin. This works well, although again some of these are niosy.

The 6AB4 is also a good sounding hi Mu triode, but I only had 3 to try, so I didn't spend much time with it.

The 6AK5 pentode is a good little amplifier, in triode or pentode mode. The tube to tube variation is much better than the 6AU6. I have about a million of these.

The 6AS6 is a pentode with interesting characteristics. G1 and G3 can be used as independent control grids. I have tried this as a vacuum tube VCA, with promising results, but more experimenting is needed.

There are several other VHF type tubes that I have collected, but haven't had the time to try out.

Yvesm 12th October 2005 02:48 PM

I admit "remote cut off" is somewhat incorrect, say "variable Gm"

Probably the stated Gm value is only attainable at highest plate current.

Fortunatly, from 10 units (PC900, labelled Philips) I've found 5 that was not seriously microphonic, they just produce a short and damped "ding" when gently knocked with a pencil.
Otherwise, thermal noise is, as expected, vy low.

AFAIR, the RIAA preamp showed around 1% THD for an output level of 7v pp into 20K, not so good, not so bad.

This tube is obviusly not the better choice for the second stage :cannotbe:


nafunga 12th October 2005 05:20 PM

Don't forget the 8532 (Darling Amp) and the EC91 - both very useable.

aletheian 12th October 2005 05:46 PM

Great stuff guys! I think I'll buy them all and try a bunch out since they are wicked cheap at a buck or two a piece. Thanks for steering me away from the 6AB6. I was going to pick up a few of those.

Hey tubelab, if you want to unload some of those 6AU6 and 6AK5 's. Let me know how much you want for a handfull of good ones... hit me up at alex<at>

So overall, are partially sheilded triodes decent sounding? Which works better, to ground the sheild or connect it to cathode?

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