[rmburrow]

Howdy folks, need some help wiring a 5703wb Raytheon tube in one of my D49’s.
I have the datasheet denoting the lead next to the red dot but its numbered: 1(red dot) 3_4_5_6

help me please

From the old Tung-Sol tube manual for the 5703...

Lead 1  (red dot on envelope adjacent to Lead 1):  Plate
Lead 2:  no connection; gap between leads 1 and 3
Lead 3:  Heater
Lead 4:  Heater
Lead 5:  Grid
Lead 6:  Cathode

Ef 6.3 v 200 ma

[TillM]

Acrylic plate or a bottom you design with Frontpanel express.
Mdf will work. So yes, you have to do it on your own 😉

[duantro]

Hello, I’ve built a second m49 with ac701. My first one I built with Oliver’s conversion and it is dead quiet, but a little light in the low end so I thought I’d try the b version here. I’ve used aml acrylic plates with poctop psu boards. I’ve got most of the voltages  in place. Psu is 116 vdc b+, heater is 4vdc. Bias is 1 .6vdc. Plate was 53vdc  and only giving 63mA. I tried to swap in a 120k plate resistor to drop the plate voltage down some. It only gave me 52vdc on the plate. The main problem is I’ve got a slight hum. I’ve swapped out the pin 7 on the binder jack in the psu to pin 3 on the psu pcb and pin 3 Binder to pin 7 on
Psu. Pin 3 and pin 7 is jumpered in the mic binder pins per m49 original schematic. I’ve only burnt the new tube in for a few minutes and will give it some more time. The hum goes away when the psu is turned of though. Any thoughts on what to try?

[TillM]

Grosser told me, as I ask him, if he has an idea to get the b-version quite, that there is a reason why Neumann make a c-version

Maybe you need another grounding scheme.
Especially the audio ground.
But I had always a little hum with the b-version.
So after a while I changed everything to c and the hum goes away.

[duantro]

Thanks Tillm, I followed your hookup chart as well as Delta's.I  still have the same slight hum. I tried grounding to case from iec side to 0v on psu pcb, then disconnecting it there and trying 0v on binder side of psu pcb, removing jumper, removing shield from mic side binder,...etc. Hum, hum ,hum and still hum! I may have to change bias.

[duantro]

I switched it over to C bias and it is dead quiet and sounds great. I am happy here.

[TillM]

A must realize, that the grounding scheme I post is the best for the c version and it makes the hum on the b version much lower.
But you still have a noticeable hum with the b version, even with the grounding.
That’s why I always recommend the c version to everyone 

[aazaa]

Never mind, guys.
I've found a good answer here: http://www.neumann.com/forums/view.php?bn=neumann_archive&key=990963033&v=f
The R3 is part of a bass roll off filter.
I suspect that the problem with the AC701 “Electronic tube” is the very hefty phase shift staring at 10 Hz (yes, I've measured it). That turns a negative feedback into a positive feedback at very low frequencies (around 4 Hz).
The size of the R3 determines the amount of bass roll off. If removed, there will be no roll off at all.

What did you found is not an answer but crap.
Neither R3 or R5 doesn't make "bass roll off".
R3 is feedback resistor, so if you want to get same level and other "artifacts" then don't mess with it.
If you want to increase low end then change C3 for higher value - that's it.

In the "Operations Instructions for Condenser microphones M49b/M50b and accessories",  Neumann document 04/1960 I read this:

"12. The frequency response of the M49b microphones shows a drop below 40 cycles. Should you desire a response substantially flat down to 20 cycles, you must remove the resistor shown in the schematic M 49b /3-S (4) diagram as resistor no. 3."

[Delta Sigma]

In the "Operations Instructions for Condenser microphones M49b/M50b and accessories",  Neumann document 04/1960 I read this:

"12. The frequency response of the M49b microphones shows a drop below 40 cycles. Should you desire a response substantially flat down to 20 cycles, you must remove the resistor shown in the schematic M 49b /3-S (4) diagram as resistor no. 3."

I don't get how a a single resistor can be a filter in NFB. C2/C3? That doesn't compute for me either. I must be missing something.

[aazaa]

I don't get how a a single resistor can be a filter in NFB. C2/C3? That doesn't compute for me either. I must be missing something.

 It would be interesting to have further explanation about the functioning of the circuit.
The circuit with R3 (5Mohm) is missing in the older M49. The German version of the document (03/1960) is comparing the M49 with the  M49b.

Betriebsanweisung für Neumann-Kondensatormikrophone M 49b/ M 50b und zubehör.
"Im Gegensatz zu dem älteren Typ M 49 hat das M49 b Mikrophon einen Frequenzgangabfall unter 40 Hz, der dazu beiträgt, unerwünschte Geräusstöringen durch Wind oder mechanische Erschütteringen zu vermindern."

translation:
"In contrast to the older type M 49, the M49 b microphone has a frequency response drop below 40 Hz, which helps to reduce unwanted noise from wind or mechanical vibrations."

[LRRec]

I don't get how a a single resistor can be a filter in NFB. C2/C3? That doesn't compute for me either. I must be missing something.

R3 and C3 form a Low Pass Filter.

[aazaa]

R3 and C3 form a Low Pass Filter.

Ok, thanks.
Very clever way of filtering the lowest bass in this high impedance NFB circuit.

[0dbfs]

The LPF presents to the backplate which modulates anti-phase. Get's a little dodgy with phase around the knee.

Cheers,
jb

[Delta Sigma]

R3 and C3 form a Low Pass Filter.

I had thought that, but I pumped the numbers into a calculator and got around 3Hz. I obviously did something wrong. Thanks!

[aazaa]

I had thought that, but I pumped the numbers into a calculator and got around 3Hz. I obviously did something wrong. Thanks!

fc=3Hz is correct. 6 dB/oct. curve. see picture.

[Spencerleehorton]

Want to replace the pot for a stepped switch to go through the patterns, is there a schematic for this please?

[Spencerleehorton]

Want to replace the pot for a stepped switch to go through the patterns, is there a schematic for this please?

Ah ok, it's a 100k lin pot, i need to make up a stepped lorlin to 9 steps.

9 x 12k ok?

[Spencerleehorton]

Or just 10 steps of 10k but was trying to use an odd number so the middle setting was cardioid!! Now I'm stumped!!

Anyone?

[doqmemory]

The R5, R9 values in the original P/S circuit are 100K, just making a voltage splitter, right? 

For 9 positions you would use 8 resistors, so the 5th position has an equal split with 4 on each side.

My understanding is that the absolute values are not that important, so 8 x 12K should work, and 8 x 20K should work, too.  I would go for 1% or better, since you want an equal split.

[Spencerleehorton]

Perfect thank you bud.

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