Marshall must be confident...

[AdmiralB]

...of the quality of Shuguang KT66s. I've looked in two of the new Vintage Modern amps (I bought the 100W) and they bias them to 85% dissipation.

The 50W is bad enough - about 445V into a 6.7K primary - but the 100W is a killer, 530V on the plates into a 4K primary. The power transformer might be the largest I've ever seen in a 100W amp.

[Steve Conner]

Yes, the evidence so far from looking at forums has been that the 100W one eats tubes for breakfast. The plate and screen voltages are viciously high for NOS EL34s, let alone those Chinese KT66s. I saw the Jimi Hendrix reissue version self-destruct at the Marshall Roadshow in Glasgow. I don't know what made it die, but I'd guess it was the tubes.

If I owned one, I would convert it to 6550s, since the rumour is that Hendrix's tech had to do that to stop them dying on him.

[AdmiralB]

Does anyone know if the Vintage Modern uses the same PT as the Hendrix?

[Steve Conner]

No, the Hendrix uses transformers made by Drake to the original spec, according to Marshall. One stand-up and one drop-through. The Vintage Modern has two drop-through trannies, and one (appears to be the PT) has the cheesy welded stack I'm more used to seeing in microwave ovens than $2000 guitar amps

Marshall have close-up shots of the chassis on their site:

http://www.marshallamps.com/product....pageType=SEEIT
http://www.marshallamps.com/product....pageType=SEEIT

[Ray Ivers]

Steve,

So the Vintage Modern's OT is in the front-right corner of the amp, next to the input jack? Kewl.

Ray

[AdmiralB]

It is, but it's isolated - it's got a metal 'cage' around it inside the chassis.

[AdmiralB]

one (appears to be the PT) has the cheesy welded stack I'm more used to seeing in microwave ovens than $2000 guitar amps

The 100W PT does have the weld, as does the PT on the JVM four-channel. I don't think the 50W PT is welded, though.

I think the problem with reliability is less due to the voltages involved as it is the very high stock bias setting and the extremely high OT primary. Flyback has to be severe.

I'm running the VM with the impedance set one 'notch' high, so the four KT66s see a 2K load. It should be happier, and it sounds better too.

[Steve Conner]

Oh, that's interesting. I don't think of it in terms of "Flyback", rather I believe that a too high impedance makes the screen grid draw excessive current and melt itself. But either way I agree with you that the too-high impedance would be stressful for the tubes.

[Smitty]

...I believe that a too high impedance makes the screen grid draw excessive current and melt itself...

Why would higher impedance cause the screen to draw more current?

[Enzo]

Offhand I'd guess that the lower the impedance of the screen compared to the plate becomes, the more current the screen will conduct. The higher the plate impedance, the more attractive that screen looks to the electrons.

[Steve Conner]

What Enzo says. If the plate isn't loaded heavily enough, its voltage dips way down, and it doesn't have enough of a positive voltage to attract electrons to it. So the electrons coming from the cathode that were meant to go to the plate just turn back and hit the screen. Screen current goes through the roof. You can see the effect if you look at the characteristic curves on the tube datasheet.

Or for a practical demo, try cranking an amp with EL34s into a load that's mismatched one tap too high, and watch through the holes in the tube plates. You can see the screen wires light up like lamp filaments as the power stage reaches the point of clipping. Then try again with the correct load and see how much cooler they run. Use care when doing this experiment, or you'll trash your tubes and maybe other things too. BTW, now you know what those holes are for

[Smitty]

Offhand I'd guess that the lower the impedance of the screen compared to the plate becomes, the more current the screen will conduct. The higher the plate impedance, the more attractive that screen looks to the electrons.

That would have been my guess, too.

[AdmiralB]

the cheesy welded stack I'm more used to seeing in microwave ovens than $2000 guitar amps

A friend brought over an original Matchless Chieftain last night. Great-sounding amp, not something I'd like to work on though.

It has a welded PT too....