American octal steel tubes in LDC microphones?

[rock soderstrom]

The most legendary tube in microphones is without doubt the German steel tube VF14.

But what about the American octal steel tubes from this era? I'm talking about the wonderful 6C5, 6J5, 1620, and 6J7. Were these also used for this job in the past?
Has anyone tried this before? The specs are not that far off from a EF12/13/14 or even VF14. The American steel tubes mentioned have, as triode, all a gain around 20 and an internal resistance around 10k or less. No idea how these tubes would behave with the extremely high grid resistors, it would depend on a test.
Sure you would have to select for microphonics (as always) but at the current prices for NOS tubes this is much more pleasant than with their German counterparts. Another advantage are the gridcaps, which would isolate the high impedance part very well from the rest.

Does anyone have any experience with this?

 

[Ricardus]

Interesting. I'm curious also.

I'm kinda baffled that no one had tried re-making VF14's, also.

 

[Murdock]

I was asking this myself lately. I think Ben Sneesby uses a 6j7 in one of his mic series.
And I believe to have read something about not so favorable internal capacitance. But I could be wrong about that.
Curious to hear your findings.

 

[emrr]

I've seen plenty of early '30's condensers that were DIY updated at some point to use steel octals once 864's/etc became hard to find.

Consider the 'official' (if you want to call it that) RCA low noise preamp tube generations were:
864 - 4 pin glass
1603 - large 5 pin glass, grid cap
1609 - large 5 pin glass
1620 - smaller steel octal, grid cap
5879 - small modern noval glass

 

[rock soderstrom]

I think Ben Sneesby uses a 6j7 in one of his mic series.

Interesting, I found this model on their website with a russian 6ZH8 (6SJ7). No gutshots, unfortunately.
https://beesneezproaudio.com/product/arabella/
What I also found was a forum entry from 2010 on gearslutz from a company called Ronin Applied Sciences about a planned microphone with a 6J7/1620.

It was clear that others had already had this idea, so I'm not surprised.

I am a big fan of steel tubes, I think they have a great sound, without getting too audiophile. I have already built a few mic preamps and headphone amplifiers based on them.

Another big advantage is that due to the lack of visible glow of the tubes, they are not particularly popular with the HIFI guys. As a result, even today you can still buy many inexpensive NOS tubes from really reputable manufacturers. This is especially true for the American steel tubes, the German ones are already more expensive and no longer available in large quantities. There are also many equivalent Soviet types to the American steel tubes. See Beesneez, they can't be that bad...

I'm kinda baffled that no one had tried re-making VF14's, also.

I think we have to wait another 2 months, then maybe there will be further information on this topic...go Hans, go!!

https://groupdiy.com/threads/vf14-tube-will-be-produced-again.80294/

 

[dmp]

I've wanted to build a U47-alternate design using a 6J7/1620 triode-wired based mic for sometime. I have a EF800 version that is my favorite mic build, that uses the 30k+100k plate resistor and the 100+29 ohm cathode combination. Would trim to find the 1620 bias current. The RCA BA-2 preamp operates the first triode strapped 1620 at 1.5mA. And used a 10:1 output transformer I think.

The capacitances of the 1620 are 7pF and 12pF for input (gk) and output (ak)
where, for comparison:
12AY7 1.3pF and 0.6pF
EF86 3.8pF and 5.1pF
EF800 3.6pF and 8.1pF

Reading a few threads on tube internal capacitances means higher capacitances create lower output via feedback
Possibly shunting high frequencies?
Thus, the higher internal capacitances may reduce performance?
Interested to try it and see.
FWIW, I have no interest in chasing a mythical original u47 sound - have never had one

 

[rock soderstrom]

Thanks for info!

FWIW, I have no interest in chasing a mythical original u47 sound - have never had one

Same here, just trying something outside the normal paths. I like the US steel tubes (I have a good collection of them too) and if it's technically possible and there's enough space in the body it could be very promising. Some steel tubes get quite warm, so maybe that could be an issue.

 

[dmp]

Also low noise. The grid on the cap is pretty sweet, aka AC701, surprising it hasn't been done in a mic unless there is a reason it sucks (i.e. high capacitances)
I bought a bunch 10 yrs ago when I built an RCA preamp, but 1620s are much harder to get these days
I really like the 6SN7 too, which is a low mu steel (edit: glass) tube, used in lots of 'classic' amplifier circuits. Lower capacitances, 3 pF and 4pF, another option. And I have a ton of them.

 

[rock soderstrom]

Yes, noise is always a challenge in this position.

The grid on the cap is pretty sweet, aka AC701, surprising it hasn't been done in a mic unless there is a reason it sucks (i.e. high capacitances)

That's what I thought too, the isolated grid on the top really makes sense here! I'm curious to see if I can get this to work.

I really like the 6SN7 too, which is a low mu steel tube, used in lots of 'classic' amplifier circuits. Lower capacitances, 3 pF and 4pF, another option. And I have a ton of them.

6SN7 is a top tube. Is it also available in the steel version? I like their (single triode) predecessors 6C5 and 6J5 very much. Absolutely recommended for those who don't have them yet. I have a lot of them and they are still very good and inexpensive to get, which unfortunately can no longer be said of the 1620. They are becoming really rare, at least on my side of the pond.

 

[emrr]

Considering all the high frequency equipment that was built with early large tubes, this shouldn't be a real problem.

 

[dmp]

For some reason I was thinking I had steel 6sn7 but I think they are all glass. Don't really care though could make a great mic.
I wouldn't buy 1620s at today's prices unless you somehow knew they were mint NOS. How would you know it would still perform as a low noise 6j7? Buy 6j7 and test for low noise. 6j7 are still cheap.

 

[dmp]

Some steel tubes get quite warm, so maybe...

1620 is 0.3 amps @6.3v (1.89 watts) same as 12ay7, only slightly higher than ef800 (.275 A). Black steel would dissipate heat out better; higher radiation than glass

 

[emrr]

I can't say I see any clear trend of available 1620's being any better than 6J7's, after years of running a bunch of channels of that type. They also seem to last forever in conservative circuits, which mics usually are. I pulled 6J7's out of Gates SA-70's I'd run nearly 20 years then selected another best new set, took measurements, the old ones were barely down on current draw, no different on noise.

 

[rock soderstrom]

I wouldn't buy 1620s at today's prices unless you somehow knew they were mint NOS. How would you know it would still perform as a low noise 6j7? Buy 6j7 and test for low noise. 6j7 are still cheap.

I agree, I did the same. I have a few 1620s but many 6J7s to select for noise. The difference was not that big in V1 position in my mic pres, though a mic is a different matter.

There are still european EF36/37/37a mostly from Mullard which are technically identical to the 6J7/1620 but in my experience built a little higher. Also very fine tubes, the EF37a is the best and most expensive. According to the data sheet, they have an anti-microphonic construction and reduced hum from the heater, although this should not make much difference with DC heaters. I'll have to measure how much longer they are than the 6j7.

 

[emrr]

I really wonder if EF37a isn't closer to 1603/77 than 6J7/1620....all interchangeable in most places with the right socket.

 

[rock soderstrom]

I really wonder if EF37a isn't closer to 1603/77 than 6J7/1620....all interchangeable in most places with the right socket.

I think they all belong to the same family in terms of pure technical data. If you ignore the socket, EF6, EF12, EF40, Ef86, EF804, EF806, but also 5879, 6SJ7 and many other types belong more or less to the same big family. Even the very early pentodes on both sides of the pond are also part of this...

 

[emrr]

I have seen 6SJ7 perform less well when dropped into 6J7 positions, higher current too.

 

[dmp]

What are ef800 and ef804 equivalent to?

 

[rock soderstrom]

I have seen 6SJ7 perform less well when dropped into 6J7 positions, higher current too.

I have never compared them directly in the same circuit. I wouldn't be surprised if there were audible and measurable differences in all the tube variations mentioned, as there is also a lot of time and material development between the individual variations, despite very similar technical data. The production methods and goals have also changed over time. There must be differences.

 

[rock soderstrom]

What are ef800 and ef804 equivalent to?

The EF804 technically belongs to the EF12, EF40, EF86 ancestral series. It is a special quality longlife tube optimised for authorities/military/telecommunication use. It is optimised for AC heating compared to the EF86 but otherwise technically very similar. It has a different pinout. The EF804s is an even further improved version, also for critical audio use (e.g. V72/V76.) and stuff like Electroencephalography.

The EF800 is a longlife EF80 from the same series of "Behördenröhren"(=authority tubes - for the former west german authorities like the Deutsche Bundespost, Bundeswehr etc.) as the EF802/EF804/EF805/EF806. Its pedigree is based on the EF14 and EF42.

The EF806 is the SQ and longlife version of the EF86 with the same pinout. Very popular in Neumann U67, unfortunately very expensive and probably quite rare now.