KT88s - screen grid resistance and bias

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KT88s - screen grid resistance and bias

Postby sitka_spruce on Mon Dec 15, 2014 8:31 pm

I was thinking about going with some KT88s in my 50W Marshall. It was originally made for coin base 5881s so there are 470R resistors in place. With the greater TC of the 88s I figured I might have to increase this value, but looking up on Marshall Major schematics it seems these hade a mere 250R. But obviously back then the 1959 using EL34s often had no screen grid resistors in place. Then in the data sheet the KT can supposedly take 800V on the plates and the g2. So what would you suggest?

Also I would like to know how you bias a KT88 seeing that 70% of 35W is 24.5W which is very close the the amp's clean rating. But then I'm aware that the amp obviously is unable to put out more than it can bring to the table - which basically means this is a moot point, right?

Thanx in advance,
Jonas
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Re: KT88s - screen grid resistance and bias

Postby Gooch on Mon Dec 15, 2014 9:29 pm

sitka_spruce wrote:I was thinking about going with some KT88s in my 50W Marshall. It was originally made for coin base 5881s so there are 470R resistors in place. With the greater TC of the 88s I figured I might have to increase this value, but looking up on Marshall Major schematics it seems these hade a mere 250R. But obviously back then the 1959 using EL34s often had no screen grid resistors in place. Then in the data sheet the KT can supposedly take 800V on the plates and the g2. So what would you suggest?


Earlier Marshall Majors had the 250-Ohm screen resistors, but they had 1-kOhm resistors in place from at least 1971 until the end of their production. But you also have to realize that they had a B+ voltage upwards of 600V. In your 50W, you should be fine with the 470's, although moving up to 1K would be fine. Personally, I wouldn't bother changing them. The KT88's will be under light stress in that amp, and their screen grids are much more robust than the EL34s.
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Re: KT88s - screen grid resistance and bias

Postby sitka_spruce on Tue Dec 16, 2014 1:31 am

Gooch wrote:
sitka_spruce wrote:I was thinking about going with some KT88s in my 50W Marshall. It was originally made for coin base 5881s so there are 470R resistors in place. With the greater TC of the 88s I figured I might have to increase this value, but looking up on Marshall Major schematics it seems these hade a mere 250R. But obviously back then the 1959 using EL34s often had no screen grid resistors in place. Then in the data sheet the KT can supposedly take 800V on the plates and the g2. So what would you suggest?


Earlier Marshall Majors had the 250-Ohm screen resistors, but they had 1-kOhm resistors in place from at least 1971 until the end of their production. But you also have to realize that they had a B+ voltage upwards of 600V. In your 50W, you should be fine with the 470's, although moving up to 1K would be fine. Personally, I wouldn't bother changing them. The KT88's will be under light stress in that amp, and their screen grids are much more robust than the EL34s.

Thanx mate! You're right about the B+: this amp measures around 420-430V on the plates with the 6L6GCs, KT66s and EL34s I had in there.
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Re: KT88s - screen grid resistance and bias

Postby PeterS on Tue Dec 16, 2014 2:50 am

Also I would like to know how you bias a KT88 seeing that 70% of 35W is 24.5W which is very close the the amp's clean rating.


I would try biasing it like that but then check to see how hot the PT and OT are getting. If you imagine the extreme case that you had tubes that could dissipate say 100 watts, you'd be biasing at such a large current that the power supply would overheat/sag/hum. At some point you are biased in class A and there's no point going any hotter.
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Re: KT88s - screen grid resistance and bias

Postby sitka_spruce on Tue Dec 16, 2014 3:51 am

PeterS wrote:
Also I would like to know how you bias a KT88 seeing that 70% of 35W is 24.5W which is very close the the amp's clean rating.


I would try biasing it like that but then check to see how hot the PT and OT are getting. If you imagine the extreme case that you had tubes that could dissipate say 100 watts, you'd be biasing at such a large current that the power supply would overheat/sag/hum. At some point you are biased in class A and there's no point going any hotter.

Thanx man! I reckon the strain would be an additional 14-16% compared to the KT66s that are in there now biased at 21W. At that level putting my hand onto the PT I can feel some heat is generated but probably a mere 30-35*C as I usually am pretty sensitive to heat.

Reading up on the KT88 so with the stock 220k bias feed resistors the valve should be seen as a 35W one, and the B+ available would never push it effectively beyond that either. Should these be an issue though I figure I could just piggy-back resistors of equal value for 110k.
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Correction

Postby sitka_spruce on Sun Dec 28, 2014 7:59 pm

I just realised that the transformers for the 9100 are really rated at 60W in power and output - one of the reasons they sound a bit stiff perhaps; may have to replace the trannies on the EL34 side. So that should cut me some slack with the 88s I figure, especially as they're going to be run clean/ cleanish.
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