Low end oscillation in M49c

[tomas.borgstrom]

Hello, I hope that someone can help understand the theory behind this.

Some years ago I experimented with increasing the cathode capacitor, C9, in an M49 circuit. When I reached 100uF the microphone started to oscillate in the low end. I just lowered it and everything was fine.

Yesterday I finished two almost identical M49c clones. The only difference is that one has a Moby BV11 10:1 and the other one has a AMI BV11 Classic 10:1. Both have 47uF between cathode and ground. When I connect the high pass filter with R2, C2 and C3 the microphone with the AMI transformer starts to oscillate in the low end but the one with the Moby transformer works fine. If I lower the cathode capacitor, C9, to 22uF in the microphone with AMI transformer the oscillation disappears.

Can anyone explain this?

Best Regards

Tomas

 

[Purplenoise]

Hello, I hope that someone can help understand the theory behind this.

Some years ago I experimented with increasing the cathode capacitor, C9, in an M49 circuit. When I reached 100uF the microphone started to oscillate in the low end. I just lowered it and everything was fine.

Yesterday I finished two almost identical M49c clones. The only difference is that one has a Moby BV11 10:1 and the other one has a AMI BV11 Classic 10:1. Both have 47uF between cathode and ground. When I connect the high pass filter with R2, C2 and C3 the microphone with the AMI transformer starts to oscillate in the low end but the one with the Moby transformer works fine. If I lower the cathode capacitor, C9, to 22uF in the microphone with AMI transformer the oscillation disappears.

Can anyone explain this?

Best Regards

Tomas

Hey Tomas, which tube are you using. If you are using the phaedrus fet replacement it’s a known issue. You would have to leave out the low cut filter…disconnect r5 I believe (5M)

 

[RuudNL]

Correct. At the time, I was the first person who reported this issue to Phaedrus.
Link
They even published a 'service sheet' about this problem.

 

[tomas.borgstrom]

Hey Tomas, which tube are you using. If you are using the phaedrus fet replacement it’s a known issue. You would have to leave out the low cut filter…disconnect r5 I believe (5M)

I use 6s6b, I used AC701 when I got oscillation with 100uF.

 

[Purplenoise]

I use 6s6b, I used AC701 when I got oscillation with 100uF.

Hmmm..strange. But when you disconnect the filter it stops?

 

[RuudNL]

The cathode decoupling capacitor was never meant to be 100µF (In the M49C it was 25µF).
If you remove the high-pass filter, you will get plenty of LF even with 25 (22) µF.
Here are measurements on the M49 amplifier with and without the high-pass filter.

 

[Vac11]

Built maybe 10 pcs of M49C microphone. Most with 6s6b-v inside. I had same problem in one microphone with 6s6b-v (and with 6S31B-ER substitute in same mic problem with oscilation too). It was150Hz oscilation, doesnt matter filter configuration (with or without too), doesnt matter different PSU. Didnt fix it yet, so Im very courious what cause this artifakt.

 

[RuudNL]

Strange, I have built a lot of M49C microphones with a 6S6B-V and never had an oscillation problem, even with the filter active!
A frequency of 150 Hz has no relation with the time comstants of the filter.
Is it still oscillating if you disconnect the output transformer?

 

[Vac11]

Are you thinking output BV11 transformer? Didnt try it yet.
Thanks for the tip.

 

[RuudNL]

No guarantee of course, but I would probe the plate connection without the transformer connected.
Maybe disconnecting the transformer could change something.
After all it is a pretty simple circuit.

 

[tomas.borgstrom]

The cathode decoupling capacitor was never meant to be 100µF (In the M49C it was 25µF).
If you remove the high-pass filter, you will get plenty of LF even with 25 (22) µF.
Here are measurements on the M49 amplifier with and without the high-pass filter.

I'm aware of that, but as I said I was experimenting to see if I could hear any differences lowering the phase shift. Some microphones have a lot higher values, for example Sony 800G 330uF (I know it's 6AU6). It seems like it's either the transformer or the tube that's the reason why some circuits oscillate but it would be great if someone could explain it from an electronic perspective.

 

[tomas.borgstrom]

Hmmm..strange. But when you disconnect the filter it stops?

Yes, and also when I lower value of the cathode capacitor

 

[RuudNL]

For the +HT the M49C relies on decoupling in the power supply.
Did you ever try to add local decoupling? (1µF or more.)

 

[Papa Tango Charly]

Correct. At the time, I was the first person who reported this issue to Phaedrus.
Link
They even published a 'service sheet' about this problem.

Hello Ruud
Can i use your schematic to build My M49C?
Could you tel me what tranformer i should use to build my M49C.with
I have:
Tbone 700 11.6.2 transformer
3u audio 7:1
Do you have any other idea. (BV18 or BV11, from AA)
Do you have a good schematic for the building of a M49C with a 6S6B-V
Buy the way i did you Tbone mode with a 6S6B it sound really good thank you
Best
François

 

[Papa Tango Charly]

Correct. At the time, I was the first person who reported this issue to Phaedrus.
Link
They even published a 'service sheet' about this problem.

Hello Ruud
I would like to build a M49C with a 6S6B-V triode
I would like to know what schematic i should use to build my M49C
I have.the Olivier Arhcut and the one you have posted.
Have you got a schematic for the 6S6B-V triode.

I would like to know what Transformer i should use
I have a
Transformer Tbone 11.6.2
3U 7.1
3U 6.5.1

There Is the BV11 and the BV18 from AA that seems interesting.
I have finished your Tbone 700 with the 6S6B-V triode i have put a C12 capsule its sounds great Thank you so much!
Best François

 

[RuudNL]

I used a BV11 transformer.
The Advanced Audio transformer looks like it has a small core, not a 'real' BV11.
https://advancedaudio-europe.com/webshop/parts_diy/bv11_transformer/

 

[tomas.borgstrom]

I would like to know what Transformer i should use

My recommendation is Moby BV11 10:1 ratio, great transformer. You can use exactly the same component values as in Olivers schematic, but with the Moby transformer 2K6 for R3 resulted in the least amount of distorsion. You can experiment with increasing C5 to 47uF.

 

[Papa Tango Charly]

Hello Thank You all!
I have seen on a Phaedrus Schematic that there evokes an LF instability (around 3Hz).an they advise 1M in R3

"The problem was eventually traced to the positive feedback at LF which is
incorporated in this circuitry via resistor R3 (as marked up in the diagram below).
Output voltage from the amplifier is transported by this resistor to the low pass
filter R3/C3"

I will certainly use a simple BV11 from Mic&Mod because it fits well in the GTR2 (Tbone Sct 700 body)
I will also use one of there PSU because they are well built.

I don't really really know how R3 will react with there BV11. What is the role of C5 in the sound signal.
Best regards