408a 6028 - where's that similarity to VF14?

[ln76d]

As in the subject
Any theory, facts, informations?

 

[Pip]

It's not a direct replacement by any means and to get close as IOaudio AKA Max Kircher has done you need to run two tubes with the heaters in series and the tubes in parallel. There are some who would have you also buy the 407a as a perfect replacement which is two tubes in a single envelope. They also charge a lot of money for the selected ones.  I think that the IOaudio solution works fine. A lot of the reality of the U47 and what we love about it has to do with its Capsule and Pattern Switching architecture  as well as the transformer and the single voltage power scheme. I have never built a mic using the EF80 or EF800 tubes, although its on my to do list, Oliver Archut touted their validity in this mic!

http://groupdiy.com/index.php?topic=50694.0
http://www.westernelectric.com/spec_sheets/407A.pdf
http://www.moxtone.com/mU47_U47.html
http://tab-funkenwerk.com/id64.html
http://tab-funkenwerk.com/id64.html
http://www.wagner-microphones.com/vf14_background.htm

 

[shabtek]

407a works well--cannot comment on similarity to vf14

 

[ln76d]

Thanks Pip

From what i know "telefunken" US (and saturn sound?)  is using single 408a/6028 as replacement + heater dropping resistor.
I wonder how this tube could be used instead VF14 if this is true that tele is using it.
I know MK-47 and most of related topics and i'm not building U47 clone
Am working on different circuit but by occasion i asked out of curiosity.
I tried few options of connections, voltages, plate and cathode resistors values. What i observed is that this tube have too high output impedance to work with 6:1 or 7:1 ratio transformers with similar values of voltage and plate resistors to the U47.
Currently my circuit is with 7:1 transformer, because i don't have any decent 10:1 - which would better option.
Fixed bias and much lower plate resistor - currently 83k.

 

[ln76d]

407a works well--cannot comment on similarity to vf14

Probably yes
I have few 6028 and i was thinking to use a pair for some U47 style circuit in the future (when i buy a decent body for it).
After my last work with single tube i think i will go for some metal tube replacement as EF12,13,14 etc. I am little bit disappointed with 6028

 

[Pip]

As far as I can tell Telefunken and Saturn Sound are using the 407a and strapping the two elements heater in series plates in parallel. Telefunken even hints at it when they say they are using a high end tube that was used in the Telco Industry. I don't see how you would implement a single 408a in all earnestness the drop down resistor would have to be huge, the heat and subsequent noise would be prohibitive!

When you strap two tubes in the above configuration you change the output impedance no? In parallel it drops the impedance!

 

[Pip]

407a works well--cannot comment on similarity to vf14

Probably yes
I have few 6028 and i was thinking to use a pair for some U47 style circuit in the future (when i buy a decent body for it).
After my last work with single tube i think i will go for some metal tube replacement as EF12,13,14 etc. I am little bit disappointed with 6028

Look at my earlier post and see what moxtone says about the EF13

 

[ln76d]

I'm not sure about saturn sound, but all the "U47" 408a madness started from "tele" usa.
https://www.gearslutz.com/board/attachments/moan-zone/112994d1236332957-vf14k-tube-dsc04061.jpg
They are using (or used)  standard dropping (ca 1.8k) resistor plus another resistor hidden inside tube metal housing.
Yes i know about EF13 and moxtone measurements