Anyone built a sela t24/25 clone?

[mhelin]

The grid voltage must arise from the capsule leakage. Next week I will post some pictures, audio and basic data.

This circuit is grid leak biased, meaning that the grid collects electrons from the heated cathode and because of the large Rg maintains the grid somewhat negative is relation to the grounded cathode. The input signal cannot be too large or the bias point will become too positive and distort the signal. Some ECC83/12AX7 datasheets have a table which define the conditions for zero ohms cathode resistor (typically the Rg is 10Mohms and Ig 0.5mA or so). Also there is variation between tubes, not all work (depends on grid structure I guess).

Good reading on grid leak bias: Grid Leak Bias or Contact Potential Bias. – tubes

 

[tomas.borgstrom]

This circuit is grid leak biased, meaning that the grid collects electrons from the heated cathode and because of the large Rg maintains the grid somewhat negative is relation to the grounded cathode. The input signal cannot be too large or the bias point will become too positive and distort the signal. Some ECC83/12AX7 datasheets have a table which define the conditions for zero ohms cathode resistor (typically the Rg is 10Mohms and Ig 0.5mA or so). Also there is variation between tubes, not all work (depends on grid structure I guess).

Good reading on grid leak bias: Grid Leak Bias or Contact Potential Bias. – tubes

Thank you soo much for this explanation!

 

[Purplenoise]

I believe the only thing C4 will do is to lower the output. Neumann selected that capacitor so the output would be the same in all microphones. It's not included in the early schematics. Does anyone experience that it affects the sound?

I played around with it in my ptp 49 clone. Without it the sound was a little hairier and it cleaned up as the values went up. I ended up with 3.3pf. I guess it depends on how the input drives the circuit. Possibly “loud” capsules ( low capacitance) might need it more.
Also styrenes sounded a little different than ceramics but this could be controversial..just my impression

 

[Purplenoise]

I finally got some time to explore the Sela T25. The build quality is good. It's very light and as small as a microphone with a K47, a tube and a transfomer can be, 114 x 41 mm. It's a two layer mesh, outer about 2.5mm, inner very dense. See the attached schematics for voltages. I don't have a voltmeter that can measure at impedances as high as 1G (I've planned to buy or build for several years). The transformer is 1:6.5 custom made by Sela. The photos are with stock components. Before the recordings I replaced the electrolytic capacitors with film. Since the T25 have the same capsule and tube as M49 (with K47) I recorded both with the guitar on a stand and the capsules at the exact same spot. It doesn't really work to record both at the same time since the distance between the capsules results in a substantial difference in sound. You have to tilt the Sela microphone quite much for the capsule to point forward.

Listen for yourself. Both sound really good but to me one sounds better. T25 has a really high output, about 10dB more than M49, and it is really quiet. When I lowered the capsule voltage to 60V the output dropped about 4dB. I expected larger differences between the two voltages. The M49 sound is often referred to as "velvety". Whatever that means I don't like that description, I think it has too much spark. Perhaps it's more correct if it is an M49b with M7. The recorded microphone is an M49c.

They sound quite similar. M49c has more air and t25 seems to have more sensitivity and “drive”. Very interesting…maybe the t25 sounds closer to the earlier m49b.
Very informative, thank you so much again.

 

[Recording Engineer]

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

Ok, ended-up with the 5718 in a pair of 50b-types with Beesneez capsules and bodies, that Chris at Barbaric built for me. Decided the 6533s are way too noisy for the circuit and way too microphonic.

The mics sound absolutely amazing! A thing I’ve noticed though is that warm-up time is the longest I’ve regularly experienced with a tube mic. Each time, it takes a good 30 minutes before they get quiet. It’ll be interesting to see if that time reduces the more I use them. Haven’t had that be a thing with my 3-dozen of other tube mics.