milfordcubicle
New Member
- Joined
- Mar 24, 2021
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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.
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.