Re: {Collins} 6BF5



I would also check the voltages, especially the bias. This is not a self biased tube. There should be around -11 volts on the control grid, pin 1,7. If it is more than a volt or two low I would check the main bias at C60. it should be around -65 volts. Not as big of a problem on the S3 but the S1 uses a selenium rectifier that fails with age and lowers the bias voltage. The plate voltage on this tube should be in the 125-130 volt range. If it is much higher check your line voltage. These sets were designed around 115V which is seldom the case today. Mine runs 123+ all the time.  

 
Doug
WA3DSP
www.crompton.com/hamradio


>________________________________
> From: David C. Hallam <david.hallam@xxxxxxxxxxx>
>To: Collins List <collins@xxxxxxxxxxxxx> 
>Sent: Tuesday, December 18, 2012 4:00 PM
>Subject: {Collins} 6BF5
> 
>Does the 6BF5 have a reputation for being microphonic?  My 75S-3 seems to have lost some audio output and I was puttering around the hood.  I noticed when I did the old pencil erasure tap on the 6BF5 I could hear it in the speaker.  Naturally there was no spare 6BF5 in my tube caddy, so I had to order a couple.
>
>David
>KW4DH
>
>-- Before I came here I was confused about this subject.  Having listened to your lecture I am still confused. But on a higher level.
>Enrico Fermi
>
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