RFT CM7151 reconstruction - plate voltage? Frequency response? Any info?
- Thread starter ln76d
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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?
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?
Attachments
Matt Nolan
Well-known member
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!
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!
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
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
Attachments
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.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
Menschmeier
Member
Hi, I'm revisiting this old topic because I'm currently restoring an RFT CM7151 and am dissatisfied with the original circuit diagram. I know that Andreas Grosser had developed a heavily modified circuit at the time, where the first EF12 is operated with a bias voltage from a negative heater voltage, the second EF12 compensates for the limited frequency response of the original output transformer and where the anode voltage is 210 volts and the capsule bias voltage is only 80 volts. When I spoke to Andreas back then, I didn't yet have a CM7151. Now I can no longer ask him. 
Does anyone have this circuit diagram?
Best wishes from Berlin and a happy new year!
Does anyone have this circuit diagram?
Best wishes from Berlin and a happy new year!
Menschmeier
Member
It's a pity that nobody replied. In fact, Andreas had not built the second stage as a cathode follower, as I had first assumed, but he had built in feedback from the output of the transformer in order to linearise it. I don't yet know the values of the components used. But you can certainly try it out and measure it yourself.
I am restoring my microphone using the original circuit with an external power supply, but I have revised it. Only a few changes are important here:
The two cathode electrolytic capacitors must be replaced with modern ones with 100 µF 25 V. Then you do not need to bridge them with a film capacitor due to the better ESR. The voltage divider for the capsule bias voltage must be dimensioned so that 80 volts are applied to the capsule. With the original 140 volt anode voltage, this results in an 80 kOhm resistor. Furthermore, the primary side of the transformer must be connected between the capacitor and earth and not via the anode resistor. The grid leakage resistance of the second stage must not be increased to 3.9 MOhm. The 1 MOhm is correct. The treble weakness of the microphone is due to the transformer and not to the Miller effect. Increasing the resistance is actually counterproductive.
If you want to improve the CM7151 beyond that, you have to form the bias of the first stage firmly from a negative heating voltage (voltage divider as with the Neumann CMV5/b) and not with a cathode resistor. Furthermore, a feedback loop from the output of the transformer back to the second stage must be installed. This linearises the frequency response while at the same time reducing the gain. As we have very high levels anyway due to the additional second stage, this doesn't interfere. Andreas had also converted the microphones to 210 volts anode voltage. However, this requires a recalculation of some resistors and a different power supply unit.
I am restoring my microphone using the original circuit with an external power supply, but I have revised it. Only a few changes are important here:
The two cathode electrolytic capacitors must be replaced with modern ones with 100 µF 25 V. Then you do not need to bridge them with a film capacitor due to the better ESR. The voltage divider for the capsule bias voltage must be dimensioned so that 80 volts are applied to the capsule. With the original 140 volt anode voltage, this results in an 80 kOhm resistor. Furthermore, the primary side of the transformer must be connected between the capacitor and earth and not via the anode resistor. The grid leakage resistance of the second stage must not be increased to 3.9 MOhm. The 1 MOhm is correct. The treble weakness of the microphone is due to the transformer and not to the Miller effect. Increasing the resistance is actually counterproductive.
If you want to improve the CM7151 beyond that, you have to form the bias of the first stage firmly from a negative heating voltage (voltage divider as with the Neumann CMV5/b) and not with a cathode resistor. Furthermore, a feedback loop from the output of the transformer back to the second stage must be installed. This linearises the frequency response while at the same time reducing the gain. As we have very high levels anyway due to the additional second stage, this doesn't interfere. Andreas had also converted the microphones to 210 volts anode voltage. However, this requires a recalculation of some resistors and a different power supply unit.
Attachments
Menschmeier
Member
A few words about the Miller effect:
The grid leakage resistance of the tube forms a low-pass filter together with the Miller capacitance. To ensure an upper cut-off frequency of 20 kHz, the grid leakage resistance should be selected so that the resulting cut-off frequency is above this. For an EF12 tube with 6-8 pF as pentode, the recommended maximum value according to the data sheet is 500 kΩ if the EF12 is used as an amplifier and not just as an impedance converter.
So if you want to improve the circuit, you should not increase the grid leakage resistance of the second stage to 3.9 MΩ, but you can reduce it to 500 kΩ. However, the EF12 only has 3.5 pF in the triode circuit, so the 1 MΩ is also fine here.
I now strongly doubt that the circuit published here in the forum at the time was created by Andreas. He would not have made such a mistake and he would never have installed film capacitors in parallel with the old electrolytic capacitors.
The grid leakage resistance of the tube forms a low-pass filter together with the Miller capacitance. To ensure an upper cut-off frequency of 20 kHz, the grid leakage resistance should be selected so that the resulting cut-off frequency is above this. For an EF12 tube with 6-8 pF as pentode, the recommended maximum value according to the data sheet is 500 kΩ if the EF12 is used as an amplifier and not just as an impedance converter.
So if you want to improve the circuit, you should not increase the grid leakage resistance of the second stage to 3.9 MΩ, but you can reduce it to 500 kΩ. However, the EF12 only has 3.5 pF in the triode circuit, so the 1 MΩ is also fine here.
I now strongly doubt that the circuit published here in the forum at the time was created by Andreas. He would not have made such a mistake and he would never have installed film capacitors in parallel with the old electrolytic capacitors.
Attachments
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