12av7's and use in microphones

Any  thoughts?

I had a few lying round and since  12**7 are usually interchangeable I put a pair in my el chepo china mics  and I am  doing more testing but so far I am liking it.  Outside of power requirements any reason to avoid this?

Works ..  mostly ...  the higher heater current requirement notwithstanding.

But as always, tailoring the operating conditions for the tube choice generally improves things.

That said, I quite like av7 subs ...  a little higher in the thd, a little lower in the hum-noise margin ...  but can sound quite good imho.

For a reasonable price, one can roll some nice nos ... it's fun to try ax7/at7/av7 and even 6dj8 types (with some re-wiring!)

Probably not worth the effort for new builds  .... I think the modern 5751, AY7  work better nearly all the time than any nos tube I've yet measured ... as far as simple and dry performance goes.

I'm using them in my 4 chan NYDave One Bottle pream box ....  have some nice old rca, ge,  valco and national.

On measurement, I get about 3 out of 5 good for each brand  - the best are the valco   

pucho812 said:

Any  thoughts?

I had a few lying round and since  12**7 are usually interchangeable .........

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A lot of people seem to think this but it is not really true. The characteristics of the tubes in this range are very different and random substitution will only lead to tubes operating at unsatisfactory conditions. if satisfactory operation is achieved it will be purely by chance. Also sorts of erroneous folklore is built up by being rolling tubes in this way.

Cheers

Ian

ruffrecords said:

A lot of people seem to think this but it is not really true. The characteristics of the tubes in this range are very different and random substitution will only lead to tubes operating at unsatisfactory conditions. if satisfactory operation is achieved it will be purely by chance. Also sorts of erroneous folklore is built up by being rolling tubes in this way.

Cheers

Ian

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I read r get asked about this way of changing tubes too. It is very often encouraged by tube sellers, without mentioning it is not only about correct pinout. Building a few high quality pieces correctly would give good reference sound, where wrong tube type ruins it completely.
I switched to new production tubes a few years ago and often compare them to various good NOS like what Tesla made for Telefunken. EH works very well, sounds almost the same. The only difference seems longer heating time before tube starts to sound good, even when it isn't new anymore.
Ian, you like Sovtek 12AX7WC for mic input, how many from 10 do you have to discard from V1 because of noise or other problems?

My3gger said:

Ian, you like Sovtek 12AX7WC for mic input, how many from 10 do you have to discard from V1 because of noise or other problems?

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Probably 20%. My Lindos test set has a speaker output you can use when measuring noise. I often keep it turned up as I turn tubes on and off. You can clearly hear the tube metalwork heating up and in a likely microphonic tube you can clearly hear the grid resonating as it heats up. Once this stage is over you hear the shot noise and as the tube heats up this reduces to a minimum. You can then tap the tube to see how microphonic i t is. Really bad tubes are obvious because they act a microphones and you get howl round. Lastly it is often instructive to listen after you power the tube off. The HT tens to stay up long enough for the space charge to disappear and you can hear the shot noise increase. Using this test set it is very easy to weed out noisy ans microphonic tubes.

Cheers

Ian

ruffrecords said:

A lot of people seem to think this but it is not really true. The characteristics of the tubes in this range are very different and random substitution will only lead to tubes operating at unsatisfactory conditions. if satisfactory operation is achieved it will be purely by chance. Also sorts of erroneous folklore is built up by being rolling tubes in this way.

Cheers

Ian

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this is why I ask the experts  8)

ruffrecords said:

Probably 20%. My Lindos test set has a speaker output you can use when measuring noise. I often keep it turned up as I turn tubes on and off. You can clearly hear the tube metalwork heating up and in a likely microphonic tube you can clearly hear the grid resonating as it heats up. Once this stage is over you hear the shot noise and as the tube heats up this reduces to a minimum. You can then tap the tube to see how microphonic i t is. Really bad tubes are obvious because they act a microphones and you get howl round. Lastly it is often instructive to listen after you power the tube off. The HT tens to stay up long enough for the space charge to disappear and you can hear the shot noise increase. Using this test set it is very easy to weed out noisy ans microphonic tubes.

Cheers

Ian

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None of the tube manuals publish data for tubes operating at grid impedance of 100 meg ohms or above, typically encountered in condenser microphones.  Commercial condenser microphones successfully used tubes such as the 6AB4 (EC92), 6AU6, 5840, 6267 (EF86), 6072, 7586, and maybe a handful more.  I never liked the 12AX7.  That tube could be microphonic in its cardboard box.    High mu double triode with gain of 100 per section.  Noisy as well.  Better choice to try is the 12AY7 (6072).  Medium mu triode.  RCA used these tubes for mic preamps in some of their broadcast consoles.  Some of the old Westrex gear used them, also.  Like Ian says, don't roll over tubes... the mic circuit should be optimized for whatever tube you settle for.  That means some measurements and probable component changes.

The av7 has a gain rating of 41 and is close to an au7.  Yes measurements need to be made and I am certain components need to be swapped out.  Should I post a schematic?

pucho812 said:

The av7 has a gain rating of 41 and is close to an au7.  Yes measurements need to be made and I am certain components need to be swapped out.  Should I post a schematic?

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No, no, no ,no, no. A 12AU7 is nothing like a a 12AV7. The AU7 has a mu of only 17 compared to the 41 of the AV7.

Cheers

Ian

Oliver Archut supplied his C12 kit with 12AV7, so I figure it can’t suck that bad.

The 12av7 that Oliver had sent me with the c12 kit was by far the noisiest, most terrible tube I have ever heard in a microphone. But perhaps he had sent me a bad one from the junk pile by mistake.  :-\

Nikos

ruffrecords said:

No, no, no ,no, no. A 12AU7 is nothing like a a 12AV7. The AU7 has a mu of only 17 compared to the 41 of the AV7.

Cheers

Ian

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Sorry Y and U are next to each other on the keyboard. It was a typo

oliver's schematic....

pucho812 said:

Sorry Y and U are next to each other on the keyboard. It was a typo

Click to expand...

LOL, no problem, easy to do.

Cheers

Ian

I just looked at the Oliver schematic, things I see right away

The biasing used needs a very low noise heater supply
IMO there are missing noise bypass caps

Beside microphone circuits to replace Chinese designs, Oliver also redesigned power supplies. Some early mic circuits from him were not as high performance like original, he wrote about it on his website.
As Gus wrote, check both circuits for low noise.
Tube types on this C12 schematic seem like suggestions, not all will sound very similar to original. Even known mic tubes like 6072 need to be selected for various conditions like very high value grid leak resistors. I tested many EF86/EF806 and still haven't found one from new production having low noise as some NOS EF806. E80F often works better in mics that use EF806s.

Yes all true. 

a good reason to try different tubes is that there are some sleepers out there that can be had on the cheap.

like 12SN7 instead of 6SN7 etc.