Re: Valve failures
Interesting fault:
1. There is a continuous buzz
2. The UL84 cathode voltage is high
3. There is a lot of AC on the UL84 anode
4. The HT voltage is about right
5. The buzz is worse when FM is selected
6. The UL84 cathode bias resistor is 340 (270)
A couple of observations:
1. Even taking into account the 340 ohms, the UL84 cathode current is slightly higher than expected.
2. The FM selection removes the cathode bypass which reduces the gain but the buzz is greater.
A couple more questions:
1. What is the UL84 grid voltage?
2. Does the buzz vary with volume?
3. Do you have a spare UL84 or UABC80 you could try?
4. Try a 0.1uF from UL84 grid to chassis, what happens to the buzz?
5. Try a 0.1uF from UABC80 grid to chassis, what happens to the buzz?
Its always a bit of a challenge when the DC conditions are about right but you still have a fault.
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