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Old 22nd Dec 2019, 8:53 pm   #9
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,206
Default Re: Marconiphone 538

A couple of minor snags have cropped up. The volume will not reduce to an acceptable level. Checking the resistance between the KTZ63 AF pre-amp grid 1 ( also connected to the vol pot slider) reveals an "end float" resistance at minimum vol setting of 3.3k on a 2.9M pot. Tomorrow I will replace it with one closer to zero Ohms end-float and see what happens. Secondly there is some IF instability that can be induced by wiggling the screening can on the KTW63 IF amp valve. I have cleaned the skirt and inside of the can with Emery paper but the problem is still there. I'll try soldering an earth bonding lead onto the can. On the similar set that I have (HMV469) someone has changed both the KTW63 tetrodes for 6K7G pentodes maybe because they are more stable in this configuration? Cheers, Jerry
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