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...