Studio Series, tubes, and self biasing.

21fret

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well, I'm not familiar with what self biasing is all about. I always biased like others have, doing the math and setting the bias to that.
The stock power tubes that come with the Studio Series have current draw numbers on them. How necessary is that current draw number when replacing? Are we supposed to get the same number or is that what the self biasing does?

So, with self biasing, is there a way that you could bias a little hotter or colder like other Marshalls? It doesn't seem that you could do that with self biasing, but I don't know.
 

Gene Ballzz

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well, I'm not familiar with what self biasing is all about. I always biased like others have, doing the math and setting the bias to that.
The stock power tubes that come with the Studio Series have current draw numbers on them. How necessary is that current draw number when replacing? Are we supposed to get the same number or is that what the self biasing does?

So, with self biasing, is there a way that you could bias a little hotter or colder like other Marshalls? It doesn't seem that you could do that with self biasing, but I don't know.

"Self biasing" is somewhat of a misnomer. More correctly referred to as "cathode bias" instead of fixed bias to the grids. That's about as far as I can explain it and urge you to read up and study the differences!
Just Sayin'
Gene
 

PelliX

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Long story short (and it's a looong one), you can put in any output valves you want. Matched pairs for best results, but unmatched pairs won't really hurt anything. This is cathode biasing - you can plug and play, but you can't tweak. Let's not get into the nitty gritty. If you like the way the original valves sounded, try to get a pair of JJ's with the same or close spec (many retailers will do their best if you ask them). I presume they're JJ - AFAIK all Studio's had JJ's - if I'm wrong, replace JJ with the correct manufacturer, of course. If you want to experiment - well, go try anything that's compatible (another topic unto itself)!

So, with self biasing, is there a way that you could bias a little hotter or colder like other Marshalls?

Not nitpicking here, but only the 40-ish Watt and higher Marshalls are 'fixed bias' (so, adjustable - confusing, huh?). Smaller amps tend to be cathode biased. The only way to adjust the bias is by modifying the circuit - doable, but I would say not worth it, most likely.
 

Gemon

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As far as I know, the stock tube are Marshall branded, and the number on its only to identify where they were produced.
The ampli up to 20W usually are Cathode bias, and usually don't have trimmer to modify the current value.
 

PelliX

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As far as I know, the stock tube are Marshall branded, and the number on its only to identify where they were produced.
The ampli up to 20W usually are Cathode bias, and usually don't have trimmer to modify the current value.

At least on the EL34 Studio's the JJ sticker on the base was left in place. This is also where the rating OP is referring to is noted.

full


These are from my SC20h.
 

K2JLX

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I believe those are Marshall rebranded JJ EL34-II tubes, I have the same stock in my SV20h. If you can’t sleep one night reading up about fixed bias aka ‘grid bias’ vs cathode biasing may put you into a deep slumber. The tldr~ is when you swap power tubes on a fixed bias amp you need to bias it, cathode you don’t.
 

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