KT66 Tubes for Marshall JTM 45/100 100JH

[1959er]

Hi Folks,

I've just got a Marshall JTM 45/100 (100JH ) head and it comes with Gold Lion KT66.

I've read an earlier post on this forum

http://forum.metroamp.com/viewtopic.php?f=13&t=36539

that's related to this topic, but doesn't answer my question. Are the modern Gold Lion KT66's adequate for a 100JH head? If not , what do you suggest? Are there better NOS tubes or that's overkill?

Thanks

[Roe]

gold lions may work but are certainly not guaranteed to work at these voltages. datasheets spec 400v at screens and 500v at anode/plate. the 100JH runs 530-540v on both, something that may be too much for the poor gold lions. Original GECs are better, of course. even the shuguangs may work

[1959er]

Thank you for this information . I was clueless about the operational voltage requirements. I was also clueless about how expensive GEC KT66 are; I thought Mullard EL34's were expensive... this is a completely new level: 1,100-1,200 for NOS tubes is totally crazy.

I looked into the ShuGuang KT66: 400V screen voltage and 400-500 V plate voltage.
http://analogmetric.com/download/Shugua ... imited.pdf (page 23)

I looked into the reissue Tung Sol KT66 reissues: 450V screen voltage and 500 V plate voltage.
http://www.tungsol.com/tungsol/specs/kt66-tung-sol.pdf

I looked into the JJ KT66: 500V screen voltage and 500 V plate voltage? The symbols are confusing; I could be wrong.
http://www.jj-electronic.com/pdf/KT66.pdf

I was unable to find a datasheet on the Valve Arts KT66, but that's the tube in JD Simo's demo of a Metropoulos Plexi Replica JTM45/100:
https://www.youtube.com/watch?v=lbC8pWwjIEY

I'm assuming the voltages would be very similar and that's the Metropoulos tube choice for this kind of amp.

Question:

- I'm guessing it's safe to keep the gold lion reissues in, without risk of damaging the amp, while they last. I guess the worst that could happen is a blown mains fuse and no sound when you turn it on, which is time to take the amp to the tech. The mismatch in the plate and screen voltages only seem to affect the lifetime of the tube itself, right? I worry of damaging other components.

Thank you

[Roe]

valve arts and shuguangs are made at the same factory. the former is selected from the latter.

the gold lions may last years or minutes. a blown tube may take out the screen resistor and fuze, occassionally also the tube socket and the output transformer (or mains transformer) but this is rare.

what kind of voltages are you running? new gold lions take 60-80 hours before the bias really settles in

[1959er]

I don't know the voltages; I've just bought it and it's being shipped. I can ask the seller, but he probably doesn't know.

I guess I should take it for a tech check before even using it. Maybe there is a small change he can make so the amp can take current production tubes without risk of damage. Do you happen to have something in mind? I can suggest to him.

Thank you so much

[Roe]

To be reasonably safe you can either use kt88s or install a extra screen resistor ala the jtm45 flying resistor (try 220R or 470R instead of the 1k used in the jtm45)

[1959er]

Thank you. I really appreciate the suggestion and all your help. Saved my amp, man.