Difference in sound between a AC701k and 6072A in a ELA M251?

[rock soderstrom]

It was a while ago since I experimented with it. I tried a lot of tubes but got tired of it. It quickly gets quite expensive since there are so many rejects due to noise. Western electric sound good and have ok noise levels (but the best are still often more noisy than accepted if you were to sell a microphone). LM Ericsson were almost all to noisy but I remember finding one with the best noise levels. I tried other brands as well but can't remember in detail.

Thanks for feedback.

You have to use a resistor before the heater to get 20V. I think in some I didn't need any at all.

Thats the part I do not understand. Are you still talking about 408 type tubes? Does it really "differ between the brands"? AFAIK, only the 407a is able to work without dropping resistor.

You can also add a resistor to increase the cathode resistance if needed. Start with inserting trimpots in both places and adjust to taste.

Variable resistors is always a good idea during testing.

Once again, if you don't have an original U47 that you don't want to modify. Aim for another circuit. The wired dropping resistor gets even hotter and the match with the BV08 is not optimal.

I am totally on your side, normally I would not go for the original U47 route but next time I will try the 407a/b under (almost) original conditions. This would open up the door for all the VF14 (fet based) substitutes as well and maybe I will find a real VF14 in the near future?? I'm still hoping to find one in an old, dusty box at some point during my regular tube hunts. Hunter's dreams...

 

[terry setter]

Didn't Oliver use a dual 407A topology in one of his best-known U47 designs?

 

[rock soderstrom]

Didn't Oliver use a dual 407A topology in one of his best-known U47 designs?

Edit: I got 407a and 408 mixed up, I just got up, in the linked thread member shabtek says he thinks he was the first with a 407a solution.

Now old post...

Wasn't it rather Max Kircher/IOaudio who popularised this variant? It is of course also possible that Oliver Archnut built such microphones, I don't know all his designs but in my memory it was Max who also offered the dual 408a version as a kit.

There were some heated discussions about this topic, make your self a picture;

https://groupdiy.com/threads/ioaudi...configuration-in-u47-clone.57798/#post-736279

 

[rock soderstrom]

Didn't Oliver use a dual 407A topology in one of his best-known U47 designs?

I've always associated the EF80/EF800 variant with Oliver, but in the end I don't know for sure...

 

[gyraf]

Rock, do I understand correctly that you have a real u47, just no vf14 for it? In that case I may be able to help..

 

[Buttercup1]

Actually that's a really good reminder to make time to experiment. It's fun and I forget how much I enjoy tweaking stuff as time gets short and I just need to finish things so end up following safe, known 'recipies'.

 

[rock soderstrom]

Rock, do I understand correctly that you have a real u47

No, unfortunately not. I wish I had one, at the moment I only have a Neumann M7 capsule (and a Dany M7) around which I would like to build a more original U47 in terms of circuitry. Just out of (I know, stupid) curiosity and to find a VF14 one day that I can plug into it (#dream mode off)

 

[TLRT]

Andreas Grosser used small submini tubes for replacing vf14, I have a dead one, I think it is not a good idea to close a small tube into a box without ventillation.

 

[Accelerator]

@terry setter​

Didn't Oliver use a dual 407A topology in one of his best-known U47 designs?

Nope, not Oliver…

Danny’s used and use two 6028 - 408A “Pentode(s) tube wired as triodes” en parallèle in his PCB Board D-47 TubeMicrophone as Max Kircher/IOaudio’s used two 6028 - 408A “Pentode(s) tube wired as triodes” en parallèle in his Point-to-Point MK-47 Microphone…

Till this time –as I know…- nobody has used or use the 407A Double Triode tube in any microphone…

 

[Biula]

I used the search engine last night to search all posts on groupdiy for 407a mics info. Most of them refer to a rather negative statement by Oliver Archut, the only one who has had real positive experiences is member @shabtek. Maybe he can contribute something to this topic? He uses the 407a together with a Dale M7 in a DIY U47 style build and seems to be very happy with it.

The only reason to use the 407a in a U47 is try to use the 105v monosupply to somehow feed a tube filament, as this was the case with the original Telefunken VF14m. If you are trying to build a clone, go for a dedicated filament supply with a better suited tube: 407a is awful...(just follow Archut path, he used to know what was talking about this topic).

 

[Zeltec Service]

A small addendum: the 5654 family tubes feel more comfortable with a 10:1 transformer than with the BV08, which only has a 6.5:1. I don't know if TELEFUNKEN USA takes this into account in their microphones. I would do it...

That's why I find the 407a/b tube interesting for this application, as it could probably cope better with the original BV08 transformer and has the 40V heater as well. At least theoretically, I haven't tried it yet.

Jakob is right, the question remains how the 407a/b for instance will cope with the high impedance situation...

I work at Sear Sound NYC, we have original 47's and 13CW4 47's. One of the reasons I thought the 407 was a better answer was that the 13CW4 mics were often chosen for vocals. Talk about unsuitable. If you try the 407A I'd like to know the filament voltage you observe at the tube. VF14s usually are run low, 36 to 40 volts. the 407A is 60 ma current versus 50 for the 14. I would expect a similar drop on the 407A. an increase in noise might happen if the filament needs to run at 40 volts. Also there is a center tap on the filaments that is floating when running at 40 volts. A cap to ground would make it the shield between the sections it was meant to be. VF14 was a post war audio output tube for 5 tube radios. It's closest American purposed tube is actually the 50L6 octal tube. A low plate resistance is desirable to drive a transformer. It would help to double up the triodes in a 407A.

 

[rock soderstrom]

I work at Sear Sound NYC, we have original 47's and 13CW4 47's. One of the reasons I thought the 407 was a better answer was that the 13CW4 mics were often chosen for vocals. Talk about unsuitable. If you try the 407A I'd like to know the filament voltage you observe at the tube. VF14s usually are run low, 36 to 40 volts. the 407A is 60 ma current versus 50 for the 14. I would expect a similar drop on the 407A. an increase in noise might happen if the filament needs to run at 40 volts. Also there is a center tap on the filaments that is floating when running at 40 volts. A cap to ground would make it the shield between the sections it was meant to be. VF14 was a post war audio output tube for 5 tube radios. It's closest American purposed tube is actually the 50L6 octal tube. A low plate resistance is desirable to drive a transformer. It would help to double up the triodes in a 407A.

Thanks for good info!

 

[rock soderstrom]

Andreas Grosser used small submini tubes for replacing vf14, I have a dead one, I think it is not a good idea to close a small tube into a box without ventillation.

I agree with you, that is thermally problematic. If I were to make an adaptation, I would make it free-standing, similar to this adapter, but of course smaller in height...

 

[tomas.borgstrom]

Thats the part I do not understand. Are you still talking about 408 type tubes? Does it really "differ between the brands"? AFAIK, only the 407a is able to work without dropping resistor.

If it draws more current you will get a greater drop over the wirewound resistor.

I recall that the Philips miniwatt TS62 didn't need an extra resistor. Some wrote that these were the best but I didn't like the sound. To hifi-sparky for a U47. I think their noise levels are ok.

 

[lampas]

Andreas Grosser used small submini tubes for replacing vf14, I have a dead one, I think it is not a good idea to close a small tube into a box without ventillation.

Was that russian triode? 6n16

 

[TLRT]

Was that russian triode? 6n16

No, something USA tube with 20/25v heater,he removed all markings but I have many tube catalogs so it would be easy to find(honestly I had no time yet to research that)