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Fabio Berutti
 
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Default 6J7-EF36 in phono stages

Dear RATS,

I just finished a phono preamp using a 6J7 as first amplification stages,
followed by a passive RIAA, followed by a 6F5 as a second amp stage, and
finally by a DC coupled 6J5 CF as "output" tube. It sounds great (who ever
said "my project is bull.."?) but it is noisy, even with DC filaments. OK,
it has a lot of gain, but something is definitely wrong.
I swapped different pentodes and it seems that most of that noise depends on
tubes.
If anybody is interested in using these tubes in low noise application,
here's what I found:

- the less noisy are the metal envelope ones (6J7M, Svetlana)
- a bit worse are the European painted-glass shouldered bulb type (Brit
military CVsomething, supposedly Brimar)
- the more noisy so far are the the painted-glass, big bulb, straight-sided
EF36 (US/Canada type)

I suppose I have to work a bit on the "grounding" scheme of my unit, but the
noise I found is not only 50Hz hum.
Conclusion: 6J7 is surely a great tube in other applications (see Leak
"point one" gain stage) but not in this case. In fact, it was not described
as a low noise pentode like the EF86.

Suggestion appreciated.

Ciao to all

Fabio


 
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