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Old 26th Feb 2021, 5:48 pm   #1
poppydog
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Default Philips B2G25U

I have a Philips B2G25U that is causing some concern.

Firstly it has a pcb but I think it’s an extremely low hour radio as the pcb is almost immaculate so I don’t think there is any leakage of any kind. It has a rather unusual 800r speaker, so I need to make sure this isn’t going to be damaged.

It was initially powered up on the lamplimiter after the usual checks and it worked albeit a bit quiet and distorted, the limiter bulb (100W) stayed a bit too bright for my liking.

Going around with the meter checking resistors I found that r17 was OC, well I think its OC as I couldn’t get anything out of the multimeter or even the Chinese tester. In fact there were quite a few resistors that were out of spec by quite a bit, these were all replaced as was the grid coupling cap. When power was reapplied, I found that I had pos voltage on pin 3 grid 1 of the UCL82 and it was climbing steadily, voltages on pin 6 anode and pin 7 grid 2 were dropping like a stone and r20 was red hot.

I purchased 2 NOS UCL82 and tried them but I still have around 69mv on grid 1, should I not have a bit of a negative voltage on that pin?

The rest of the voltages on the UCL82 are -
Pin2 11.80v Pin6 143v Pin7 179v pin9 89.2v
They all seem a tad high but is that of concern?

Also cathode resistor r20 which i replaced with a 2w WW, as the original was miles out of spec, is still quite hot. On the sheet i have its rated at 1/2 (half) watt??

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Old 26th Feb 2021, 11:35 pm   #2
Herald1360
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Default Re: Philips B2G25U

Even on the basis of the cathode volts shown a 220R resistor is going to dissipate 0.6W so a half watt resistor is underrated.



Have you checked the actual cathode voltage for the UCL82? There could be other problems causing it to be high such as a leaky grid coupling capacitor or a dying valve.
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Old 27th Feb 2021, 12:26 am   #3
G8UWM-MildMartin
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Default Re: Philips B2G25U

All voltages look perfectly OK now.
Grid 1 should be negative with respect to the cathode, which is the same as it being very nearly 0V (69mV) with respect to chassis and the cathode positive (11.80V) w.r.t. chassis, giving -11.73V grid bias, all very close to the values on the circuit diagram and well within normal tolerances.
R20 is dissipating 630mW. (It would be 600mW at the 11.5V given on the circuit.)
A half-watt one would be over-run, even without allowing any tolerance for the resistor and for valve characteristic and mains voltage variations, so your choice of a 2-W one is good.
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Old 28th Feb 2021, 7:16 pm   #4
poppydog
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Default Re: Philips B2G25U

Thanks for the replies, its good be reassured that those voltages are ok.

The other areas of concern is that the heater voltage on the UCL82 is at 54.8Vac but the rest of the valves heater voltages are fine. I don’t want to run it and destroy the heater on a new valve.
On the UCH81, I have 107.8v on pin8 the anode of the triode section of the valve. The 15k resistor to pin 8 was replaced as the old one was miles out of spec, so I could do with some advice regarding that as well.

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