M49 build - changing tube type

[rstreber]

A while back, I did the TAB-Funkenwerk TLM49-M49 conversion project (read about it here:http://tab-funkenwerk.com/id84.html.)  I based it around an EC1000 tube with a Thiersch PVC M7 and TAB transformer and PSU.  The mic sounds great and I've mostly been able to keep it in daily use. However, I've been struggling with tube reliability, as the EC1000s generally run quietly for anywhere  from a week to 2 months but seem to always go noisy before too long. The noise is that typical, intermittent "wind" noise.  For example, the tube that's currently in there might be totally quiet for an hour or more but eventually it always acts up. 

When I started the project, I managed to get a hold of 8 NOS EC1000s (plus a bunch of slightly used ones),  but I've now gone through four of them and feel that I need to make a change since I can't find any more NOS examples.  It seems like the conventional path would be to switch to a 5840, and I'm hoping to get some pointers here on what to look out for in converting the circuit to this tube type.

My knowledge is pretty limited... with clear guidance I was able to build a nicely working microphone, but I don't feel confident making major changes to the circuit without some advice.  If anyone has any suggestions as to what to look out for in making this change, I'd greatly appreciate the input. I apologize in advance for what will probably be stupid questions along the way!

FYI, my mic follows the TAB-Funkenwerk AC701-based schematic exactly (http://tab-funkenwerk.com/id89.html), the only difference being the 6.3v heater supply for the EC1000.  The major appeal to me about the EC1000 was its similarity to the AC701.

Since the 5840 is a pentode, should I be triode-strapping it?  Any other voltages that need to be adjusted in this circuit?  Oliver had mentioned to me a while back some rough changes to be made, but I'd love to get a clearer idea of the proper way to do this.

Thanks in advance for your thoughts!

Ryan

 

[gyraf]

Yup, the 5840 should be triodificed - look at other 5840 mics for reference.

Very few sub-miniature tubes lasts very long in microphone head amps- other than the obvious AC701.

Jakob E.

 

[dmp]

I've built a few mics using the 5840, using the circuit here:
http://www.groupdiy.com/index.php?topic=47258.0
It would be an easy adaption from what you have.
(this circuit has a blocking cap before the grid, but the capsule polarizing from the AMI schematic can be used alternatively)

 

[salomonander]

why not pop a n ac701 in there and be done with it. i highly doubt that any other tube will give you a correct m49 sound. all mic that i have that use the ac701 share a similar characteristic that i'm sure comes from this tube. its a very lovely sound. they last forever. all my mics still use their old ac701s without a problem. most of these tubes are about 50 years old and still going strong.
well worth the investement i think.

 

[rstreber]

Thanks all for the suggestions and for that schematic, dmp.  Regarding AC701, I considered it.  Honestly, it's partly an issue of budget (I don't want to use a tube type that I can't have at least three of on hand, and that's just not possible for me right now) and also partly an issue of me not trusting my build with such a valuable and scarce tube!  The EC1000 was sounding quite nice, just doesn't seem to pass the reliability test.

I used to own an SM69 with its two AC701s, and it was indeed a lovely sounding mic.  Maybe I'll revisit the idea, but for now as long as I have a bunch of 5840s and more can be gotten cheaply, this seems like the easy route.

I'm going to spend some more time looking over dmp's schemo, but it looks like I can basically just drop a 5840 in without really changing the rest of the circuit, is that correct?

 

[dmp]

I've used 750R for the cathode resistor, as in the schematic I linked to, but in this thread

http://www.groupdiy.com/index.php?topic=44667.0

0dbfs used a 2.2k || 10uf without a problem. Also - read the whole thread - he is doing something that pertains to your question.
The bv11a is a 10:1 which is OK for a 5840 as well. I don't know why AMI lists different transformers for the 701 vs 5840.

 

[Kamel]

I've got good result with 5719 subminiature tube, a military tube especially done for low level signal with low distorsion
check data

http://www.mif.pg.gda.pl/homepages/frank/sheets/127/5/5719.pdf

 

[rstreber]

Thanks for the suggestion on that 5719 - I may give one of those a try and see how it goes. As it is, I have a bunch of 5840s, and the 5719s look plentiful, so it should be easy enough to try some different options and see what works best. 

I have a Flea 49 clone that claims to use a triode (non-AC701)... anyone know if they're using a 5719 or something else?  I haven't opened mine up in a while, but i seem to remember the tube not having any identifying markings on it. Whatever is in it sounds nice and has been working well for at least a year and a half of daily use.

 

[bockaudio]

Yup, the 5840 should be triodificed - look at other 5840 mics for reference.

Very few sub-miniature tubes lasts very long in microphone head amps- other than the obvious AC701.

Jakob E.

The 5840 being the exception. At Soundelux we made hundreds of mics with 5840s and if there is one thing I can say about it, it's reliable.
No where near the AC701, but better than other subminis.
The 5840 has an internal strap so it never gets run true pentode anyway.
The output impedance is a little different than the AC701, but a xfmr matched to an AC701 will at least function at full bandwith with a 5840 installed

 

[Steph.B]

Thanks for the suggestion on that 5719 - I may give one of those a try and see how it goes. As it is, I have a bunch of 5840s, and the 5719s look plentiful, so it should be easy enough to try some different options and see what works best. 

I have a Flea 49 clone that claims to use a triode (non-AC701)... anyone know if they're using a 5719 or something else?  I haven't opened mine up in a while, but i seem to remember the tube not having any identifying markings on it. Whatever is in it sounds nice and has been working well for at least a year and a half of daily use.

Hi,

I opened a FLEA 49 once, the tube looked close to a 5719... but on a second thought I'll say it was a russian 6S6B.

 

[rstreber]

Interesting about the 6S6b in the Flea, I'll have to take another look inside mine to see if it has that same base. 

Meanwhile, I dropped a JAN 5840W inside my "M49" and so far results are positive.  There's a little intermittent low level fluttering noise, but I'm going to let it burn in over night and give it a listen in the morning to see if it settles down.  It sounds good and the general noise floor is comparable to the flea (with slightly higher output) so that's a good start.  I didn't make any changes to the circuit so far, just pulled the old tube out and installed this one as-is (with grid 2 tied to plate.)

I also have some 5719s on the way here, so I'll drop one of those in at some point if I have a chance.

Thanks for all the help so far!

 

[rstreber]

I had a little downtime yesterday and tried swapping out the 5840 with a 5719.  The 5719 sounded really honky and bass-light, also very high output.  Would this be because of a mismatch between it and the xformer (AMI BV11r)?  Also, I had been using a 2.2k cathode resistor as per the AMI schematic, but I swapped it out for a 1.5k when I installed the 5719. 

In any case, I put the 5840 back in (but left the 1.5k cathode resistor in place.)  For now, it's sounding pretty good with the 5840 and the noise level is comparable to my Flea 49, so I'll leave it alone for the time being.  But I'd be curious to hear thoughts on the 5719 sounding so funky in this setup.  When I have more time, I'm going to install a trim pot in parallel with the cathode resistor to try out some different values and see what works best.  But at least for the moment I have a great-sounding and relatively noise-free microphone.

 

[pasarski]

The 5719 sounded really honky and bass-light, also very high output.  Would this be because of a mismatch between it and the xformer (AMI BV11r)? 

Yes, looking at the plate curves, I believe that's the case. If I'm interpreting it right, it would need a very high ratio transformer and preferably quite hot bias, near 2mA, to work properly in a circuit like M49's.

 

[Kamel]

you will have better result by polarize the grid ala C12 with the cathode directly tied to the ground

 

[bockaudio]

you will have better result by polarize the grid ala C12 with the cathode directly tied to the ground

Much more complicated power supply.

 

[micaddict]

Yup, the 5840 should be triodificed - look at other 5840 mics for reference.

Very few sub-miniature tubes lasts very long in microphone head amps- other than the obvious AC701.

Jakob E.

The 5840 being the exception. At Soundelux we made hundreds of mics with 5840s and if there is one thing I can say about it, it's reliable.
No where near the AC701, but better than other subminis.
The 5840 has an internal strap so it never gets run true pentode anyway.
The output impedance is a little different than the AC701, but a xfmr matched to an AC701 will at least function at full bandwith with a 5840 installed

It's proven pretty reliable in Brauner microphones, too.

 

[rstreber]

I changed a few things in my mic - swapped out the 25uf electrolytic for a 10uf, replaced a 1.5k cathode resistor with a 5k / 5k trim pot in parallel, and added the 600pf cap from plate to ground for some HF attenuation.  Now it's sounding nice, still some low level noise.  I'm not sure if in doing all that work I may have damaged the tube with too much heat. I also initially fired it up with what turned out to be a bad solder joint on the 5k trim pot causing an intermittent connection - any chance that could cause permanent damage to the tube?

In any case, I'm going to try another 5840 to see if I can get it any quieter, but hopefully this will be it for now.

Question:  Do any of you have opinions on the necessity of coiling the tube leads?  It seems like newer mics with subminis don't do this, whereas I've only ever seen AC701s installed that way, and it's how I've been installing tubes in this mic.  If I just keep it straight, use heat shrink and a heat sink while soldering, are there any other reasons to not keep the leads straight?

PS: I have a pair of VM1s that have been going strong for nearly 7 years now, but they have EF86s in them.  I'm guessing the newer versions with EF732s are just as reliable though.

 

[micaddict]

I'm going to try another 5840 to see if I can get it any quieter

The Jan Philips 5840  that Brauner uses is very quiet. Of course the rest of the build plays a part, too. And a percentage of the tubes no doubt will be rejected. But to give an idea, self noise in the Brauner Valvet X (pure cardiod) is below 9dB A. 
The tranny in those is a Lundahl BTW.

 

[rstreber]

Continuing to have problems with this mic, and I'm seeing that the plate voltage is low.  The original schematic specs 43v for an AC701, and I'm getting around 37v with the 5840 in. Any thoughts on whether 43v is an ok target or if I should be shooting for something else?  Also, is something like 37v low enough to be causing noise problems?

 

[PRR]

> damaged the tube with too much heat.

What did you use, an oxyacetylene torch?

In the factory, every part of the tube is run up to red heat. After being outgassed and sealed, you generally do not want to go to red-heat again (except the heater-cathode).... but any lesser temperature is fine.

Coiled leads allow multiple disconnects even if some lead is lost in the process. They also avoid strain on the lead-glass seal. Some slack is important. But the coiling is maybe a vestige of radiosonde and hearing-aid customs, not needed in a pampered studio mike.

43V-37V does not sound like a big deal to me. If you are curious, increase the cathode resistor, or decrease the plate resistor. And be sure of your heater voltage at-the-tube.

> intermittent "wind" noise

I'm no cap-mike technician, but that seems to me more likely a flaw in the capsule (dust, grime, humidity) or the capsule insulation system than in the tube.