RFT CM7151 reconstruction - plate voltage? Frequency response? Any info?

[ln76d]

Slowly i'll be reconstructing my VEB Funkwerk Leipzig CM7151.
Any info, tips and others are highly appreciated

Does anyone knows what's the DC plate voltage value?

 

[ln76d]

In attachment is simplified schematic - without PSU and filament connections.

Anyone knows why frequency response is limited to 10Khz?
It's not transformer by itself, because it's pretty linear in 20Hz-20kHz range.
Connection of transformer one side to the plate potential (not ground) or is there anything else which can cause LPF?
Does anyone see anything suspicious on schematic?

 

[Matt Nolan]

It might be the Miller Capacitance of the second Pentode working with the 1 Meg. 15pF is all it would need to be. Try a larger resistance there.

You know that mic produces pretty much line level output.

I think the plate voltage is around 137V. If I remember correctly, the capsule is run at something insane like 105V. So, you really want to change that polarisation divider to bring it down some. At most 85V, preferably less.

There's a nice Andreas Grosser alternative circuit for this which I believe he has made public. It uses the two tubes. Though, you might also want to keep it original.

I had one of these a while ago. It really made me chuckle how much hum there was. The mains transformer is right there in the base of the mic. Something else you might want to consider is moving that out to an external box so you don't have mains wires going into your microphone. Safety first and all that. The mains transformer is in a mu-metal can with ventilation holes. So they went to the bother of trying to reduce the hum that it induces in nearby wires by putting it in a magnetic shielding can. Then, they ran the audio output cable *through* the shielding can via a couple of the vent holes. Hilarious!

 

[ln76d]

Hey Matt!

Thanks

I have schematic from Andreas Grosser, even newer version than this online.
It's really interesting circuit, but first i want to make something close to the original with some mods.
PSU of course will be in the outside box and filament voltage will be regulated.
After you replied i looked on Andreas schematic, and he also put higher grid resistor on the second stage, so probably you are right, that the main problem is the Miller capacitance.
Replacing 30k/100k divider with 100k/100k polarisation voltage should be less than 70V.  It is a good value for the start, then i will empirically choose the best value for my ears

For now i want to make something like in attached schematic.
Any comments appreciated

 

[ln76d]

Warning pictures!

That how it looks when some ***** wrong connected mains voltage to the signal output pins!

 

[ln76d]

And other ***** bought it "as is" previously not asking about the pictures of the inside...

These are the residues of the burnt output transformer.

Currently cleaning the mess.

 

[TLRT]

Hey Matt!

Thanks

I have schematic from Andreas Grosser, even newer version than this online.
It's really interesting circuit, but first i want to make something close to the original with some mods.
PSU of course will be in the outside box and filament voltage will be regulated.
After you replied i looked on Andreas schematic, and he also put higher grid resistor on the second stage, so probably you are right, that the main problem is the Miller capacitance.
Replacing 30k/100k divider with 100k/100k polarisation voltage should be less than 70V. It is a good value for the start, then i will empirically choose the best value for my ears

For now i want to make something like in attached schematic.
Any comments appreciated

FYA,This schematic is not correct it won't work.First tube needs external bias,(from heater) the second amplifier tube the cathode bias is ok.