substituting 5703 for ac701

[mfilter]

I've been rebuilding a couple Hungarian 221b amplifiers and now they work but there's a lot of hiss, which I think is due to bad ac701's. Has anyone tried a 5703 as a direct substitution?

AC701

5703

Thanks!

Michael

 

[dale116dot7]

I haven't tried it as a substitute, but I have built mics based on one of: 5702, 5703, 5744, and 5840. You will need to adjust the heater voltage up to 6.3V for it to work properly. These tubes are hissy when underheated (the ac701 is a 4V filament). You may need to go through about 5 or 10 tubes to find one that will work properly. It will work, but the performance isn't going to be exact - but a working 5703 would be better than a non-working anything.

 

[mfilter]

Sorry, meant to say direct substitution except for the heater voltage.

Will the performance really be worse, and why? Is there something really magical about the ac701? It seems like finding a decent ac701 isnt going to be easy even if I shell out the money....

 

[mcs]

The AC701 was made by Telefunken - so it's automatically magical and better than everything else :wink:

Best regards,

Mikkel C. Simonsen

 

[gyraf]

The AC701 is one of the few tubes that were actually designed for microphones. The only other that I know of that was specifically designed for mics is the VF14.

So yes, there is something magical about it.

JAkob E.

 

[Brent Casey]

All of the 5703's I've tried are too noisy for microphones.

Brent Casey