JTM45 screen resistor and bias issue

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HH1978
PAT. # 2.972.923
PAT. # 2.972.923
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Location: Brussels, Belgium

JTM45 screen resistor and bias issue

Post by HH1978 » Wed May 15, 2019 9:20 am

Hello,

I changed the KT66 on my JTM45 clone, but got unexpected results, as with the bias pot set at coldest point, I have the same plate voltage reading on both tubes (460V with a NOS Mullard GZ34, 440V with a chinese GZ34), but about 10ma plate current difference (I.e. 36,5ma on tube 1 and 46,5ma on tube 2).
The KT66 ar Shuguang, supposedly matched.

I tried reversing the tubes, and got the same results on the same sockets, which means the issue is the amp and not the tubes. I checked the and components from the sockets. It appears that one of the screen resistors has drifted in value : around 225 ohms instead of 470 ohms. The socket with the incorrect value is the one on which I monitored the highest plate current.

Does that seem logical or should I check for something else causing the issue?

Many thanks

Hugues
Last edited by HH1978 on Wed May 15, 2019 10:01 am, edited 1 time in total.

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HH1978
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 507
Joined: Tue Aug 11, 2015 2:51 pm
Location: Brussels, Belgium

Re: JTM45 screen resistor and bias issue

Post by HH1978 » Wed May 15, 2019 9:57 am

here's a picture of the chassis :

Image

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BlueSparkle
PAT. # 2.972.923
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Re: JTM45 screen resistor and bias issue

Post by BlueSparkle » Fri May 17, 2019 3:20 am

Pictures aren't really helpful at this stage. unless there are some obvious scorch marks.
Take it to your amp tech if you can't trace the fault.
:o)
----
'95 JDMJMCS Blue Sparkle w/ McNelly 46/58's.
other non-offsets.

User avatar
HH1978
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 507
Joined: Tue Aug 11, 2015 2:51 pm
Location: Brussels, Belgium

Re: JTM45 screen resistor and bias issue

Post by HH1978 » Fri May 17, 2019 10:54 am

Yes, that sounds wise and that's what I'll do. The screen resistors were faulty, one crumbled into pieces, and the other had drifted badly, but replacing them made the issue worst, so there's something else going on that I can't identificate.

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