Noval tube sockets with or without shielding when using 6.3V AC heater?

[rock soderstrom]

Hi guys, is there much difference between tube sockets with or without shielding when using 6.3V AC heater in terms of SNR?

I usually heat PCB based tube circuits with DC voltage but this time I will do it with AC voltage and with separate wiring.

I only have the standard ceramic sockets without metal shielding at home ATM and therefore the question arises: is the difference to the shielded sockets realy that big?

Has anyone compared the two? What are your experiences?

My current project is a Fender Black Face type bass preamp aka Alembic F2B, based on 12AX7 valves. The tubes are mounted horizontally on upright PCBs in a 1HU case, the AC heating voltage is supplied from the component side.

 

[musipol]

(Disclaimer)Back in the late 70's/early 80's. I did a lot of repair and mod work on Fenders so my memory may be a little foggy.

I have not measured the difference. In casual listening tests, I really couldn't hear a significant increase in hum and noise after removing the shields on blackface and silverface Fenders(twin rev, tremolux, deluxe rev, bandmaster, bassman). I have observed lower hum(60hz) after rewiring (retwisting and rerouting the heater leads) on some particularly poorly wired silverface amps. Of course none of these were used as bass amps but the circuitry is very similar as what you describe above, especially on non reverb versions.

 

[RuudNL]

In very sensitive circuits (microphone ampliers) the shielding is essential to reduce hum.

 

[rock soderstrom]

Thanks musipol and Ruud for your input! I oscillate between your positions, on the one hand I believe that it also goes well without shielding, as you can see for example with the V76 (mic pre with AC heater and unshielded tubes) on the other hand there is certainly a reason why this type of sockets have been installed for decades in sensitive places en masse.

Fortunately I was able to find two of these sockets in an old build, so my AC paranoia was calmed down.

Let's see how quiet I can get this preamp...

 

[Khron]

In guitar amps it might be arguable that, since they deal with higher source impedances, they may need more shielding than in a mic preamp..?

 

[Tubetec]

Id agree that generally the earlier stages of tube gear benefit most from tube sheilds .
Ive also seen amps when tested without sheilds that oscillated , so they can also help reduce capacitive coupling with neighbouring tubes or other components .

 

[gyraf]

I always thought the shields were mainly for RF interference protection - you tend to see them always in radio/tv, more or less rarely in audio (besides e.g. around bias osc. in tape recorders)

/Jakob E.