served

Nuvistor Microphone questions.
« on: June 30, 2020, 05:09:47 AM »
Hello,

I have a pair of un-known made Nuvistor mics.
They have one 6C51H inside.

I have had no contact with nuvistors before. So I am looking for suggestions.
Currently the mic sounds really strange. It has a lot of high end and a lot of gain. Very agressive as they are.
Almost useless for recording.

I dont know what transformers are inside. I dont know what capsule is inside. But I would like to try to mod it a bit to use them.
« Last Edit: June 30, 2020, 05:13:07 AM by served »


Khron

Re: Nuvistor Microphone questions.
« Reply #1 on: June 30, 2020, 06:37:16 AM »
Wouldn't be outside the realm of possibility that the 22uF cathode bypass cap has lost capacitance and now only boosts the top end.

That being said, you might be better served if you could ask a moderator to perhaps move this topic to the Microphones subforum (for possibly obvious reasons?) :)
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

Gus

Re: Nuvistor Microphone questions.
« Reply #2 on: June 30, 2020, 06:40:26 AM »
Can you measure the voltages at the plate, cathode, B+ and heater? This will help figure out where on tube curves the Nuvistor is operating.

If the cathode cap dried out and is a lower uF value it could cause rise in higher frequencies. Your description does not sound like this is the issue however it is something to check.

Is it a small diameter or large diameter capsule? It could be the capsules.

Pictures should help
« Last Edit: June 30, 2020, 10:37:24 PM by Gus »

RuudNL

Re: Nuvistor Microphone questions.
« Reply #3 on: June 30, 2020, 07:30:39 AM »
I don't think it is very likely that the resistors are 20 G.ohm. Probably 2 G.ohm, which is already a pretty high value.
I have experimented with 6C51H nuvistors in the past, and the result can be very good.
The ones I used had an extremely low self noise, even better than the best low noise tube that I could find.
There is a solution for every problem!

http://www.vansteenisaudio.nl

abbey road d enfer

Re: Nuvistor Microphone questions.
« Reply #4 on: July 01, 2020, 01:00:24 PM »
Hello,

I have a pair of un-known made Nuvistor mics.
They have one 6C51H inside.

I have had no contact with nuvistors before. So I am looking for suggestions.
Currently the mic sounds really strange. It has a lot of high end and a lot of gain. Very agressive as they are.
Almost useless for recording.

I dont know what transformers are inside. I dont know what capsule is inside. But I would like to try to mod it a bit to use them.
Remote diagnosis is always on the verge of black magic. You have no idea where they come from? When manufactured?
ther are so many plausible causes, like dried caps (not likely since you mention high gain), collapsed or stiffened diaphragms, whatever... Do you have the possibility to swap in a capsule known to work well? Or conversely to install the capsules in a working mic? Is there not any dose of EQ, however extreme, that makes these mics usable?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Gus

Re: Nuvistor Microphone questions.
« Reply #5 on: July 01, 2020, 03:20:42 PM »
If the resistors are marked 206 they should be 20meg.


abbey road d enfer

Re: Nuvistor Microphone questions.
« Reply #6 on: July 01, 2020, 03:38:11 PM »
If the resistors are marked 206 they should be 20meg.
I don't read 206, I read 20G. A picture of the actual circuit would probably end the debate.  :)
Nuvistors are known to be capable of operating with very high grid resistors but I've never seen 20G resistors.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

analogguru

Re: Nuvistor Microphone questions.
« Reply #7 on: July 01, 2020, 04:25:55 PM »
My bet is that there are 200 Mohm resistors....

RuudNL

Re: Nuvistor Microphone questions.
« Reply #8 on: July 02, 2020, 04:09:05 AM »
Agreed. In the nuvistor era resistors in the G.ohm range were unusual.
100 to 200 M.ohm resistors were much more common.
There is a solution for every problem!

http://www.vansteenisaudio.nl

abbey road d enfer

Re: Nuvistor Microphone questions.
« Reply #9 on: July 02, 2020, 05:04:05 AM »
What's the point in using bandwidth for speculating? I'm waiting for more evidence from the OP to form an opinion.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


served

Re: Nuvistor Microphone questions.
« Reply #10 on: July 02, 2020, 05:17:29 AM »
Hi.

The mics are made somewhere in Russia. Into LOMO cases.

I made a picture of the resistor. See if it makes sense to you. I think it reads 20C not 20g.
Cant measure it with my DMM, its too big value.

served

Re: Nuvistor Microphone questions.
« Reply #11 on: July 02, 2020, 05:18:31 AM »
The whole think looks like this without the cover.


Gus

Re: Nuvistor Microphone questions.
« Reply #12 on: July 02, 2020, 06:29:23 AM »
19a19

I looked at two of  the 19a19s one in a light green color and a white bodied one. That was about 15 years ago and I have not found my notes.
The light green one had a glass tube and the white one had a Nuvistor. the light green one sounded good.

The white one looked like someone might have changed parts in it  similar like the one you have things like the transformer, there was hot glue holding the transformer and it and it looked like one out of a China made microphone. We changed the transformer to a cinemag it  helped a little but we did nothing else because of the glue used to hold parts.

Plate, cathode, B+ and heater voltages will help

If you search lomo 19a19 you can find different images showing the different versions. I think there might have been three versions.

A thread from the past https://groupdiy.com/index.php?topic=21724.0

link with schematic link https://groupdiy.com/index.php?topic=65218.0
Other links I found with a web search
https://www.worthpoint.com/worthopedia/lomo-19a19-19a4-rtt-mkl-mic-spare-160301707
https://www.gearslutz.com/board/high-end/563832-lomo-19a19.html
http://www.coutant.org/lomo/


 

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