Klangfilm 063a help with a PSU

[duantro]

I picked up a Klangfilm 063a that I need to build a PSU for. Most of these had an m7 type capsule and an AC701 tube. Mine has a 6AU6 and a smaller Schoeps looking capsule with small holes around the back similar to m221b capsules. I've reached out to Schoeps as they made most of these, and they kindly confirmed it probably is very similar to Schoeps a CM51schematic. I've tried to trace out the few differences on this schematic in red. Can someone help me verify and suggest any PSU changes, or should I make it identical? Would something like Dany B's m49 PSU boards work here with a 250VAC plate transformer? I saw on another Schematic for CM51 that the Heater is -5.7 so his boards might not work. Maybe could cut the heater ground trace and swap the choke to the negative path, I'll have to look those over again.

 

[duantro]

I have wired up a PSU after tracing the pinouts. The mic works, but sounds really thin. Maybe I need to try a bigger output cap? Can anyone tell my what the 100k that goes from the plate to the transformer is doing here? It seems to be bypassing the output capacitor.

 

[Khron]

That 100k is indeed weird there - unless the output transformer is gapped, that'll put the plate voltage DC straight through the primary, which is... bad.

And yes, 22nF versus 1uF WILL make a very audible difference (the "thin" sound you describe).

 

[duantro]

That 100k is indeed weird there - unless the output transformer is gapped, that'll put the plate voltage DC straight through the primary, which is... bad.

And yes, 22nF versus 1uF WILL make a very audible difference (the "thin" sound you describe).

Thanks for the reply Khron. I’ll double check the connections on that 100k and make sure it’s for sure how I originally traced it. I’ll also try a bigger cap there and report back

 

[duantro]

I've measured 1.6 mA across the 50k plate resistor, so seems about right. It has plenty of gain. I do measure 71vdc on the transformer. I tried a 1uf and even a 3uf cap, and don't hear any low end improvement. I disconnected the 100k that goes from the plate to the output transformer and it still sounds thin. So thinking maybe the capsule is suspect, I tried a k47 style capsule and it is still thin. Something is up with this circuit. This Attachment is the cm51 schematic that Schoeps had generously sent me thinking it might be similar. I'm not sure the difference between the T43 and T44 transformer that mine has. Should I maybe follow this schematic more and take the end of the 100k that was on the transformer and ground it?

Edit: I tried taking the 100k end to ground and It sounds the same, with slightly less gain. What are the optimum plate voltages with the 6au6? I guess I could try to tack on 150k on the plate resistor and 100k on the plate to ground side Like this second schematic. not sure what else to check.

 

[Murdock]

Do you mean you measure 71v at the transformer leg? There shouldn’t be any DC voltage. That’s why the 3uf cap is there. I hope your transformer isn’t shot because of that...

 

[duantro]

Yes, It looks factory wired this way-see first schematic as I have traced it out as. Quite baffling.

 

[duantro]

Ok so I have implemented this scheme- see attachment. I’ve included bias/voltages in blue. I’m a bit confused with the negative 6v heater in respect to bias voltage. It has good gain, but is still thin. I tried another k67 style capsule and it is thin as well. Swapped out another 6AU6, and same results. Is there anything in this circuit that is obviously cutting bass that anyone can see? Maybe I'll try another transformer?

 

[Murdock]

If you swapped capsule and tube I would suspect the transformer... do you have an LCR meter?
Or even a multimeter where you can at least measure the windings to see if one winding is open.

 

[Tubetec]

The thing about that mic is all the electronics are quite exposed to dust and dirt ingress ,maybe theres leakage in the grid circuit .
A clean with q-tips and isopropyl might be a good plan .
The negative heater line isnt difficult to understand , the 150k/50k resistance derives -1.5v for the grid , the 10uF 3v cap provides some extra reduction in noise .
The tube socket is made from phenolic , thats probably not the best material for insulation resistance , again careful cleaning might improve matters .
The additional 1.5M and 1M resistances ,why did you add them ? it drops your capsule bias voltage to 2/5ths of the 150v supply , that may not be appropriate for this capsule and might explain why it doesnt sound right .
Either or both coupling caps ,in the grid and anode might have become leaky over the last 60-70 years , likewise any of the other components might have suffered drift that may effect opperation .

 

[Murdock]

He changed the capsule and the coupling cap which did not help. I would think the divider is there to bring down the polarization voltage for the other capsules he tested which also didn‘t help.
But you’re maybe right about the two mega ohm resistors. If they drift downwards they can cause high pass filtering.

 

[duantro]

Thanks for the replies! I don’t have a lcr meter. And good point about electronics being exposed, I’ll clean everything up. I received the mic that had factory components as per the first schematic. The red items in the first schematic are factory parts that didn’t match the schematic from schoeps. I have been trying to decipher what they were thinking with it. Perhaps it was for television and wanted a low cut? So the 1.5m and 1m to ground resistors were there originally. Should I bypass these and see if it helps? I replaced the anode coupling cap. I’ll try to replace the grid coupling cap and see if that helps.

 

[Murdock]

Do you have a Multimeter and can measure the dc resistance of the transformer?

 

[duantro]

Yes I’ll check transformer resistance. I swapped out the 10uf grid cap and no change.

 

[Tubetec]

Ahh right , so the mic was as it left the factory ,
Its fair to assume the capsule is meant for around 60v polarisation in that case .
The original 0.022 uf anode coupling cap will give very little bass , Ive seen two other versions of the CM51 ,one with 3uf and another with 0.5uf but with quite low voltage rating(75v) to keep the size of the component down , with the tube out of circuit the ht would easily go beyond the caps rated voltage ,so best never to do that . 3uf at 200v might not fit in there . Ive generally found a cap rated at around double the anode voltage to be satisfactory , but many will fit a higher voltage part if they can ,just to be on the safe side.

https://medias.audiofanzine.com/files/schoeps-cm-51-9f-475000.pdf

Theres a link to the service document for a version of the CM51 , in French though ,
It does mention the use of trichlorethane to clean grease ,oil or other contamination but thats not used much these days , as I said a q-tip moistened with isopropyl alcohol does the job , put the mic in a warm place after and allows to dry before applying power.

 

[duantro]

Good info! I’ve scrubbed everything that is reachable with isopropyl. Yeah I could only find a 1uf 250v anode coupling cap that would fit in the space. On that French link is it stating that the 100hz is -6db when using the 50r windings? I measured 63r at these arrow drawn points in this photo. Perhaps it is wired up as 50r?

 

[Murdock]

Could be but I don’t think so. Try to measure between ground and the point where the transformer primary and the 1uf coupling cap meet.

Edit: you’re right, the manual in French states -6dB at 100Hz when wired for 50 ohm output impedance.
But as you measure 63 ohm I doubt it is wired that way. Then it would be more like 5-10 ohms I would think.

 

[duantro]

I de soldered the primary /coupling cap side of the transformer and measured an open circuit. It does look like the tube has singed one wire here also. Wires are pretty brittle.

 

[Murdock]

So you measure an open circuit between the desoldered primary winding and ground?
And you’re sure the other side of the primary winding is still connected to ground, right?
If so, than it looks like the transformer is shot...
luckily there are people like Moby who recreates transformers like these. I think his T14 should perfectly fit.

 

[Tubetec]

It appears to have the secondaries wired in series for 200 ohms as it is .
DC resistance of the coils will usually measure much lower than the nominal impedence ,
If two coils in series measures 63 ohms , so each half secondary is around 30 ohms , two 30 ohm coils in parrallel will measure around 15 ohms.
If you see an open circuit across the primary , yep its toasted , the good news is there are a few choices of modern made transformers with ratios in the right ball park , Igor at UTMindustry does a very cost effective range of tube mic transformers , Moby also has a few options which come highly reccomended by users here , there are a few USA made transformers also , Tab/funkenwerk AMI , kinda pricey .