Anyone built a sela t24/25 clone?

[Purplenoise]

Hey guys, thinking about building a sela t24 “clone”. I’ll probably be using a 6s6b tube. Anyone built one? I’d be interested in tips and suggestions. Here is the schematic:

 

[kingkorg]

Correct me if i'm wrong, that seems to me like a C12/Elam cardioid only hybrid, with higher pol. voltage.

 

[Purplenoise]

Correct me if i'm wrong, that seems to me like a C12/Elam cardioid only hybrid, with slighly lower pol. voltage.

I believe they “borrowed “ elements from c12/m49/u47 to make a “frankenmic”. Neumann was suppling them with tubes, capsules and possibly psus. T24 had an m7 and T25 had a k47.
Are you familiar with sela mics? Old Swedish company…very little info on the web but I have heard they sound great.

 

[kingkorg]

I've seen few, very nice mics. I can read Swedish, this page was quite nice to read. Google Translate might work for you. Interesting to see higher voltages used with k47

https://www.filmsoundsweden.se/backspegel/sela.html
Update :
I am not sure but it seems they used both Neumann Berlin and Gefell capsules. The claim that they had more high end due to higher pol. voltage is wrong. They had less low end, which could come across as more high end. That can be seen in the graph as well.

 

[Purplenoise]

I've seen few, very nice mics. I can read Swedish, this page was quite nice to read. Google Translate might work for you. Interesting to see higher voltages used with k47

https://www.filmsoundsweden.se/backspegel/sela.html

Oh great, thank you for the link. I think it would be interesting to experiment with this since it’s a very simple circuit. Thinking of using an output transformer of 8:1 or 10:1 ratio and play around with plate resistors for the 6s6b. A member here that I really respect suggested changing the 100k plate resistors to 56k for the 6s6b

 

[Purplenoise]

I've seen few, very nice mics. I can read Swedish, this page was quite nice to read. Google Translate might work for you. Interesting to see higher voltages used with k47

https://www.filmsoundsweden.se/backspegel/sela.html
Update :
I am not sure but it seems they used both Neumann Berlin and Gefell capsules. The claim that they had more high end due to higher pol. voltage is wrong. They had less low end, which could come across as more high end. That can be seen in the graph as well.

Very useful info thank you so much. In the link there pics of the inside too..very helpful.
Are those caps ( possibly coupling and filter) electrolytics?

 

[mhelin]

Zero volts biasing, is that intentional or an error? M49 schematics show a way to (fixed) bias the Ac701k.

 

[Purplenoise]

Zero volts biasing, is that intentional or an error? M49 schematics show a way to (fixed) bias the Ac701k.

That’s the first thing I noticed too. Seems right I guess? Of course that’s with 701 so maybe with 6s6b it’s a different story.

 

[kingkorg]

Damn, missed that XD

 

[Purplenoise]

Damn, missed that XD

Loosing your edge kingkorg? Lol

 

[tomas.borgstrom]

As described on the very informative site filmsoundsweden.se the Sela (Svenska Elektronik-Apparater AB) microphones were designed to be used for recording dialogue. The Neumann and AKG microphones were too heavy to be used on a handheld boom for a long time. Besides being smaller and a lot lighter the capsule was tilted. All Sela transformers were specially made in house. The early Sela mics had the output transformer in the PSU but the T24 transformer is in the microphone. The T24 has K47 from Neumann Berlin and was more expensive than U47. The very high polarization voltage (above 90V) results in a higher output (better signal to noise ratio) and less bass, as kingkorg explained. I've read that K47 is increasingly nonlinear above 60V so you would expect a fair amount of distorsion with 92V.

I used a T24 a couple of years ago and was not impressed but perhaps it wasn't performing as intended since a lot of engineers I know praise them. I'm going to service one next month and will return with more info and the correct schematics. To be honest, since you will be using another tube, transformer, and body your "clone" will not share much with T24 besides a cranked K47.

Many years ago I had the chance to buy a PT6 for about $1500. I regret I didn't do it because it sounded really cool. It didn't have much bass but a massive nicely distorted mid push which gave a Tom Waits aura to every source. Especially great on double bass. Probably it was in need for service...I didn't do that myself back then.

 

[Purplenoise]

As described on the very informative site filmsoundsweden.se the Sela (Svenska Elektronik-Apparater AB) microphones were designed to be used for recording dialogue. The Neumann and AKG microphones were too heavy to be used on a handheld boom for a long time. Besides being smaller and a lot lighter the capsule was tilted. All Sela transformers were specially made in house. The early Sela mics had the output transformer in the PSU but the T24 transformer is in the microphone. The T24 has K47 from Neumann Berlin and was more expensive than U47. The very high polarization voltage (above 90V) results in a higher output (better signal to noise ratio) and less bass, as kingkorg explained. I've read that K47 is increasingly nonlinear above 60V so you would expect a fair amount of distorsion with 92V.

I used a T24 a couple of years ago and was not impressed but perhaps it wasn't performing as intended since a lot of engineers I know praise them. I'm going to service one next month and will return with more info and the correct schematics. To be honest, since you will be using another tube, transformer, and body your "clone" will not share much with T24 besides a cranked K47.

Many years ago I had the chance to buy a PT6 for about $1500. I regret I didn't do it because it sounded really cool. It didn't have much bass but a massive nicely distorted mid push which gave a Tom Waits aura to every source. Especially great on double bass. Probably it was in need for service...I didn't do that myself back then.

Thank you so so much, really valuable info. Yes I would be very interested in your findings after the service.
You are absolutely right..not going of course for a “100%” clone but I find the circuit is a nice base for experimentation and maybe making something that resembles those mics. It’s very unlikely that I will ever use a real one or have one for comparisons.
I think I’ll build it as close as I can to the schematic with the adaptations for the different tube and then playing around with polarising voltages to see what happens. I have a feeling that something really nice can come out of this.

 

[Purplenoise]

Until the 6s6b’s arrive I ordered an EC71/5718 to try out. Anyone have any experience with 5718 as a substitute for ac701? Thanks.

 

[doqmemory]

Until the 6s6b’s arrive I ordered an EC71/5718 to try out. Anyone have any experience with 5718 as a substitute for ac701? Thanks.

I have several that I plan to use for M49, ELA M 251 and perhaps M269c, but have not got around to it yet. I would be interested in anyone else's experience, too.

 

[Recording Engineer]

I’m in the middle of comparing 5718 and 6533 in M50 types with AMI BV11R transformers.

 

[Purplenoise]

I’m in the middle of comparing 5718 and 6533 in M50 types with AMI BV11R transformers.

That’s…perfect timing haha. Looking forward to your impressions. What do you think so far?

 

[tomas.borgstrom]

Does 5718 and 6533 sound better than 5840? I found 6s6b to sound better than 5840.

 

[Recording Engineer]

Need more time to say anything yet.

I have a pair of 19A13 type mics with 6S6BVs in them that I’d never change. I have 5 different mic types with 5840Ws. The 269C type is the only one I’d consider changing, just to open it up a bit more; as long it doesn’t loose the “cool vibe” it already has going on! I have 6947s in a pair of 49b types with AMI BV11Rs that I’d never change.

 

[Purplenoise]

Almost done…waiting on the tubes.

 

[Purplenoise]

Ok so the first tube to arrive was a 5718a. The mic sounds really really good even with the stock cheap Mxl capsule. Quiet with plenty of level and big sound…really excited about this little mic.
I changed the circuit a little after the very useful info in the responses.
300k resistor replaced by 1M to get normal 60v polarization instead of 92v and plate resistors changed to 56+56k.
Transformer is a cinemag cm-2510 8:1ratio.