Re: {Collins} 51S-1

Hello Steve.

Yes, the 7543 is a special low hum version of the 6AU6. I believe a 6133 is also the same tube as the 6AU6.

However, I am curious as to why you found so many tubes that showed shorts. This is not a typical failure mode in the 51S-1. In fact, I have seen 51S-1's where some of the 6EA8/6U8's showed virtually no emission at all on a transconductance tube tester, but the radio still worked. I know the two tube conditions may not be related, but shorted tubes are not as prevalent as weak ones.

If it does occur, a filament to cathode short especially in that PTO tube will cause a hum in the audio. It effectively injects the 6.3VAC filament signal into the cathode to plate current.

73, Charlie k3ICH

----- Original Message ----- From: "Steve Wender" <tobiebw2@xxxxxxxxxxxxx>
To: <collins@xxxxxxxxxxxxx>
Sent: Wednesday, November 19, 2014 11:58 PM
Subject: {Collins} 51S-1

Hi All,

I am trying to bring a 51S-1 back to life. I have tested the tubes and found many showing shorts and I replaced them. The 7543 oscillator tube is also showing a short and I replaced it with a 6AU6. How bad is that? It seems to work. Should I bother buying a 7543? They don't seem too expensive. What are your thoughts?


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