New X-101-B owner

Except for a startup crackling in the Left channel which I've tracked down to a worn out socket on the X-101-B( checked & tested with 4 different sets of tubes including 2 T-S's, 1 set of EH7591's, and a quad of Westie's) I love them. I've got quad in the Executive Model 950 (1963), a Sansui 1000A, in addition to the X-101-B. They had the most impact on cooling factors in the Sansui where the construction of the chassis basically encases the tubes on 3 sides with transformers, block off plates, Power resistors(which were moved after the 1st quad), and Tuner Front end. Lousy layout for tube cooling. I've managed to get temps down into the low 130's now instead of the original 160-170 region on the Sansui. Due to the smaller envelope they leave more room between tubes in the FISHER's which help with cooling on the 800c's. They DO tend to run a bit hotter for a set bias setting on fixed bias. Cathode bias I haven't checked yet although I did set mine to 43V and they've been running for over a year and without problems. If you do get a quad, I highly recommend Jim McShane as he tests them to higher levels than the factory or anyone else I've seen.
 
I have been running the amp using my phone as the source through the Aux input for ~2.5 hrs over 2 nights. The cathode current is still high for V5. I assume this was run long enough for it to pass all of its gas?

V4 - 439V, 42.85V, 41mA = 16.24W
V5 - 436.6V, 43.18V, 60mA = 23.60W
V9 - 440.1V, 43.07V, 40mA = 15.88W
V10 - 439.6V, 42.95V, 39mA = 15.47W

Is this what red plating looks like?

IMG_20180328_203852.jpg
 
Not quite. You'll see the outer metal casing (plate) start to glow cherry red. An area somewhat smaller than a dime shouldn't hurt it. But if you see the whole or most of the plate glowing, YANK THE PLUG! That tube is at least 95% of the time, TOAST!

With regard to the other question you asked about and wasn't answered. Grid Return resistors and output coupling caps. On cathode biased units, the 7591A/7591/7868/6GM5 all have a max resistance of 1 meg ohm. (1,000,000 ohms) vs. 300K for fixed bias. No change is necessary for cathode bias units . On fixed bias units, FISHER exceeded the max resistance rating of the grid by 30K (330Kohms) so it's reduced to approx 220K and the output caps are changed to maintain the R/C timing. Ideal cap for maintaining the timing is now a .068uf cap with the 220K resistors. Remember that this change is necessary only in FIXED BIAS.
 
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That's what I figured in regards to the cathode bias not needing parts changed.

My V5 has probably run it's race, correct? Even if it's not quite red plating, it's only a matter of time with the numbers I posted, right?
 
If the current drift has stabilized, but at a higher current draw than the others, then the gas has all been collected by the getter now and the tube just needs more negative grid voltage to bring its current draw down to a typical operating level than the other tubes in the set do. However, the bright orange color from the center of the tube is a glowing screen grid, which does not bode well for the tube. That is due to either physical damage or poor manufacturing tolerances. Either way, if the glow does not go away when the tube is correctly biased, I would strongly consider replacing the entire set with a new matched quad.

Dave
 
Since I don't have an individual bias setup in this, it looks like the best bet at this point is a new matched quad.
 
Got my McShane matched quad Tung Sols and here are my numbers (@121V wall):

V4 - 437.8V, 42.01V, 43mA = 17.13W
V5 - 437.7V, 41.94V, 43mA = 17.01W
V9 - 437.5V, 42.08V, 45mA = 17.83W
V10 - 437.2V, 42.00V, 48mA = 19.04W

Should I worry about V10? Or do I have to let these "burn in" for a while?
 
It's good. Within 3 ma total between the pair is great. Just keep an eye on that pair. If they start climbing, look for loose connections between the tubes and sockets 1st. Cathode biased FISHERS run their tubes hard. It's the nature of the beast. IIRC Dave explained why in a previous post on this thread or another one where the unit under test was cathode biased. What are your heater voltages on the DC String (look at the schematic and ID them in the bias circuit. Should be no higher than 11v per. or 44V total on pin 5 cathode output tube). Any higher and you need to back off the bias voltage a little. I use 43V (10.75v per tube avg) as I've gotten used to having a grandkid before these heat up :D.
 
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