Tube rating for cathode biased amps - Any concerns?

napalmdeath

napalmdeath

Well-known member
So, will a stupid hot 55 rated pair of KT77's be an issue in a cathode biased Marshall SC20H? I'd like to try them out, but don't want to cook it!😄
 
midnightlaundry said:
Those are Cathode Biased?? I assumed they were fixed like the big ones..
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Yes sir, all the Studio Series Marshalls are cathode biased. I just don't know if a pair rated that hot would cause issues, or if the circuit adapts and only allows a fixed bias point.

I know when I had the 2525H the first time I pulled it apart to check, and no bias adjustment, (this was when they first came out). Plate Voltage is low in the 250-ish range, and the bias reading was a crazy cold 45-50% dissipation. I was curious if a hotter pair would bump that to 60-70%, or stay the same.

I've biased amps for years, but know little about cathode bias, and/or what, if any, difference a hotter tube would make, or if they could possibly be too hot. I know in the case of Mesa Boogie, stock tubes read crazy low, but hotter does make a difference, which I guess answers part of my own question.
 
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napalmdeath said:
So, will a stupid hot 55 rated pair of KT77's be an issue in a cathode biased Marshall SC20H? I'd like to try them out, but don't want to cook it!😄
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I don't know exactly, never worked with SC20, but somehow I remember info, that they are biased around 50mA (per tube) and plate voltage is 260V. So the tube runs around 50% of allowed dissipation. I believe some hotter tubes should cause no problem.
 
mcstinger said:
I don't know exactly, never worked with SC20, but somehow I remember info, that they are biased around 50mA (per tube) and plate voltage is 260V. So the tube runs around 50% of allowed dissipation. I believe some hotter tubes should cause no problem.
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Guess there's one way to find out! I'll get out the Pro One and get some readings tonight. My only concern was that dissipation % would shoot way up to too hot. Tubes rated at 55 are pretty damn hot. I got them when I had another amp that was super cold with a narrow sweep. They had to be that hot to get in range, (Randall Thrasher 50). They would take EL34's or 6L6, but for some reason, EL34's would only get up to 18mA for a medium-ish rated pair, (which was odd, and obviously a design flaw somewhere). I'd put in the hot ones and get it there.
 
Hotter tubes will def run hotter and might be too hot. My experience is mostly with cathode biased EL84s but I did build a 2xEL34 CB amp years ago. In my Rocket and ZWreck clones, once I had chosen the shared cathode resistor (56ohms IIRC) it would be quite tolerant to different tube ratings but there was def a limit.

In short, way more tolerant than fixed bias but still susceptible to over dissipation. Also, with tubes that big I would monitor the transformer temps.

EDIT: I don’t know the amp but my CB amps were running near 100% dissipation. That’s why they call them ‘Class A’.
 
SpiderWars said:
Hotter tubes will def run hotter and might be too hot. My experience is mostly with cathode biased EL84s but I did build a 2xEL34 CB amp years ago. In my Rocket and ZWreck clones, once I had chosen the shared cathode resistor (56ohms IIRC) it would be quite tolerant to different tube ratings but there was def a limit.

In short, way more tolerant than fixed bias but still susceptible to over dissipation. Also, with tubes that big I would monitor the transformer temps.
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It's designed for EL34's.
 
napalmdeath said:
It's designed for EL34's.
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IIRC, it may have been the screens overdissipating before the plates did. If you can measure voltage across the screen grid resistor at idle and then divide by the resistance it will tell you the screen current. Then multiply that by the voltage to get power. Can’t remember but I think it’s 8W max, check data sheet. Plane taking off gotta go.
 
midnightlaundry said:
Those Probes are great. It won't hurt anything to plug in those Tubes to see where they're at when you're not paying them.. They might be right on. Or a quick Resistor change..

https://robrobinette.com/Tube_Bias_Calculator.htm
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It was a non-issue. The factory tubes were rated 53. Plate voltage came in at 260 and the KT77's I put in read 56mA, which is in the 60% ballpark, really. Amp sounds like an aggressive Marshall!
 
midnightlaundry said:
Those are Cathode Biased?? I assumed they were fixed like the big ones..
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Yeah me too. Never opened one up. I wonder why they did that? Do these have power scaling?

I’d love to mod one but when you can plug in a hot mod to an SC20H why bother? That’s as easy as it gets on a densely built lunchbox amp where space is a premium and everything is PCB
 
There is a whole write up on these Marshall’s over on the Marshall forum from the guy that helped design them. He says you can run any big bottle tube you want in them.
 
glpg80 said:
Yeah me too. Never opened one up. I wonder why they did that? Do these have power scaling?

I’d love to mod one but when you can plug in a hot mod to an SC20H why bother? That’s as easy as it gets on a densely built lunchbox amp where space is a premium and everything is PCB
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Yes, to keep it at 20 watts, and switchable to 5 watts.
 
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