Gold Lion KT77, JCM900 4500 DR: Maxed Out Bias Pot

milfordcubicle

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Previously had some Ruby EL34BSTR tubes in my 50W JCM900 Dual Reverb (which worked well and sounded good) but wanted to see what these KT77s sounded like. Rubies were biased at -38.5ma or so and were surprisingly well balanced with about a 0.4ma difference between the two tubes. Put in the GL KT77s ( sold new as a matched set; test numbers on boxes confirm that), checked my probe, turned the bias pot to its max setting and measured plate current on both tubes as -35.6ma on one and -31.77ma on the other. For a "matched" set based on transconductance, I believe, that is a considerable difference. Probably not noticeable and still relatively balanced for this type of application, but that was a bit disappointing.

Anyway, I wanted to get the plate current higher, maybe to about -40ma or so (these tubes are rated for 32w, so I have heard this is OK), but the pot is maxed out. I had this problem before, but much worse, so I swapped the bias pot resistor (R30 in earlier JCM900s; R28 I think in the later ones) from a 56k to 47k. Am I going to have to swap out that resistor again for something with even less resistance? If so, what would be a good bet? My next smallest resistor is a 20k unless I piggyback. Otherwise, what else can I do, or should I just leave it where it is? Idle current is calculated at about 60% on one tube and 65% on the other with a plate voltage of 451v on both.
 

2L man

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To get bias hotter tube grid voltage need to get closer the tube cathode voltage.

On most common bias circuit you need to reduce resistor value between potentiometer and ground OR increase resistor value between pot and bias voltage supply. No need to change the potentiometer if its adjustment sensitivity is OK.

Sometimes potentiometer is installed to ground and there is a resistor between it and grid output and then you reduce its value.
 
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pedecamp

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Previously had some Ruby EL34BSTR tubes in my 50W JCM900 Dual Reverb (which worked well and sounded good) but wanted to see what these KT77s sounded like. Rubies were biased at -38.5ma or so and were surprisingly well balanced with about a 0.4ma difference between the two tubes. Put in the GL KT77s ( sold new as a matched set; test numbers on boxes confirm that), checked my probe, turned the bias pot to its max setting and measured plate current on both tubes as -35.6ma on one and -31.77ma on the other. For a "matched" set based on transconductance, I believe, that is a considerable difference. Probably not noticeable and still relatively balanced for this type of application, but that was a bit disappointing.

Anyway, I wanted to get the plate current higher, maybe to about -40ma or so (these tubes are rated for 32w, so I have heard this is OK), but the pot is maxed out. I had this problem before, but much worse, so I swapped the bias pot resistor (R30 in earlier JCM900s; R28 I think in the later ones) from a 56k to 47k. Am I going to have to swap out that resistor again for something with even less resistance? If so, what would be a good bet? My next smallest resistor is a 20k unless I piggyback. Otherwise, what else can I do, or should I just leave it where it is? Idle current is calculated at about 60% on one tube and 65% on the other with a plate voltage of 451v on both.
I'd send the tubes back for another set.
 

Pete Farrington

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these tubes are rated for 32w
No, their design centre anode dissipation limit will be about 22W, ie they aren’t as beefy as EL34, whose design centre limit is 25W.
https://www.thetubestore.com/lib/thetubestore/genalex-kt77.pdf
32W is the absolute max anode limit of KT77, their design max is 25W.
The design centre limit is about 16% lower than the design max.
There are 3 valve dissipation rating systems; absolute max, design max and design centre. Any valve type will have a rating under each of those systems, but only one of those ratings is usually published. For receiving valves, the absolute max data was rarely released. However GEC chose to muddy the water / cause confusion, making their valves seem beefier than reality, by publishing it.
Here’s the rating system definitions from RCA RC-20
https://m.imgur.com/F1kDImI
Philips valve brands / types stayed with design centre.
My view is that a design centre rating is what should be used for the ‘idle at 70%’ bias setting guideline.
If only a design max figure is available, then idle at 60% of that.

Note that the New Sensor era Gold Lion KT77 info linked to above is just a crudely edited version of a GEC publication from the 70s https://tubedata.altanatubes.com.br/sheets/086/k/KT77.pdf
So it’s credibility seems questionable.
 
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milfordcubicle

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No, their design centre anode dissipation limit will be about 22W, ie they aren’t as beefy as EL34, whose design centre limit is 25W.

32W is the absolute max anode limit of KT77, their design max is 25W.
The design centre limit is about 16% lower than the design max.
There are 3 valve dissipation rating systems; absolute max, design max and design centre. Any valve type will have a rating under each of those systems, but only one of those ratings is usually published. For receiving valves, the absolute max data was rarely released. However GEC chose to muddy the water / cause confusion, making their valves seem beefier than reality, by publishing it.
Here’s the rating system definitions from RCA RC-20

Philips valve brands / types stayed with design centre.
My view is that a design centre rating is what should be used for the ‘idle at 70%’ bias setting guideline.
If only a design max figure is available, then idle at 60% of that.

Note that the New Sensor era Gold Lion KT77 info linked to above is just a crudely edited version of a GEC publication from the 70s
So it’s credibility seems questionable.

Thanks for this. The spec sheet shows 25w as the design max for the KT77 plate dissipation, and if I use the 22w figure my idle current is around 65-70%. I would still like a little room above the current settings for further adjustment, so that the pot isn't maxed out at least.
 

milfordcubicle

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To get bias hotter tube grid voltage need to get closer the tube cathode voltage.

On most common bias circuit you need to reduce resistor value between potentiometer and ground OR increase resistor value between pot and bias voltage supply. No need to change the potentiometer if its adjustment sensitivity is OK.

Sometimes potentiometer is installed to ground and there is a resistor between it and grid output and then you reduce its value.

https://imgur.com/gallery/M3cUUSu Here is the schematic for the bias circuit. The circled 56k resistor was changed to a 47k. PR1 is the 22k ohm trim pot. What might you suggest changing to allow for a slightly hotter bias?
 

Gutch220

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I had to do the same thing with a 4100. I just piggy-backed a resistor on top of another, I forget which one, to lower resistance to gain more range on the bias pot. This way it's not a permanent mod, just a piggy-backed resistor.
Just use standard resistance formulas for parallel resistors to determine what you want to have, and which resistor you'll piggyback.

Note:I'm not sure of any other changes you'll need to make since you're switching tube types. I wasn't. I was just using EL34's before and after. Not sure why the bias pot wasn't suitable. The stock tubes & circuit were for EL34's.
 

milfordcubicle

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Well, 220k plus 47k came out to 38.5k. Soldered in the new resistor pair and biased tubes at -36ma and -39ma. I can live with that. I think the pot has enough range in it that I can swap in my EL34s with just a bias adjustment.

Amp sounds quite good with these tubes; definitely more bottom end and still good clarity. Haven't cranked it yet, but I will. Thanks for the tips!
 

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