Jtm45 screen resistors

[Marcomel79]

What value for KT66? 470R with 1K flying resistor or 1K? Found contrasting opinions after a google search...

 

[lespaul339]

470 2W is what I used in my build.

 

[Marcomel79]

470 2W is what I used in my build.

Thanks man, did you use the 1K dropping resistor?

 

[Marcomel79]

@neikeel could i use your expertise on this?

 

[neikeel]

In a JTM45 I tend to use 470R 5w grey radial RS or Welwyns and often do not use the 1k flying resistor as I don’t typically notice much difference either way. On a resto I will use both.
George at Metro did some experimentation years ago and thought that there is a little less compression without, but I confess I don’t hear/feel it.

 

[Pete Farrington]

For me, the 1k shared feeding 470R individual screen grid resistor arrangement is part of the JTM45 thing. It creates local negative feedback that kinda keeps it smooth, even overdriven / presence full. It’s not a massive effect, but on a AB switch it’s noticeable.

 

[Marcomel79]

Thank you @Pete Farrington and @neikeel , im starting to plan a JTM45 build and want to slowly get the correct parts for it.
The Marstran schematic shows the choke before the OT ct, i take thats wrong...

 

[neikeel]

Thank you @Pete Farrington and @neikeel , im starting to plan a JTM45 build and want to slowly get the correct parts for it.
The Marstran schematic shows the choke before the OT ct, i take thats wrong...

Yes it is incorrect. Usual Marshall arrangement with first cap and choke to CT then screens, choke and screen filter to second node.

 

[Marcomel79]

Yes it is incorrect. Usual Marshall arrangement with first cap and choke to CT then screens, choke and screen filter to second node.

Ok! Are the 3W green Welwyn ok to use with KT66?

 

[neikeel]

You mean 2.5w? I only see those, 5w and 7w (bit big).

 

[lespaul339]

Thanks man, did you use the 1K dropping resistor?

Yes I did. Sorry for late response.

 

[Marcomel79]

You mean 2.5w? I only see those, 5w and 7w (bit big).

To be honest im buying some 1K 3W Welwyn resistors from ebay and asked the seller for two 470R 3W and he said hed check. Havent gotten an answer yet... whats the lowest safe wattage i can use for the screens?

 

[Pete Farrington]

It depends on what arrangement used and how hard you overdrive it.
I used 1W 470R with 7W 1k. 3W 1k overheated.

 

[Marcomel79]

It depends on what arrangement used and how hard you overdrive it.
I used 1W 470R with 7W 1k. 3W 1k overheated.

So if i were to use a 2W 1K to two 5W 470R, would that be ok?
The Marstran schematic shows this...🤷🏽‍♂️
http://www.marstran.com/JTM45.gif

 

[Pete Farrington]

a 2W 10K

Please check that value.

 

[Marcomel79]

Please check that value

Oooops i meant 1K😅