You meant solder a diode between pin 3 and 1 or 4 of the 300B? Which diode should I get? Please advise. Thank you!!!
OK it does not explain the pop with a fresh set of 300B. But its normal to add across between grid and cathode so the cathode cannot be more negative than the grid. Its normally done with something like a E88CC not a 300B. Can you see any of the valves or anything on the underside flash with the pop with the lights off.
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From the valve wizard:
A diode must be added between grid and cathode to prevent arcing between the two electrodes before the heaters have fully warmed up. Once the valve is conducting normally this diode will be reverse-biased and completely off, playing no further part is circuit operation.
So to confirm, I will solder a diode, IN4002, in the right direction between these two points shown in the schematic....for both 300B. Right?
BTW, I have just tested the power down and up again, there is no spark or flash in the chassis and all tubes light up normally.
I can see a problem C103 when the valves are cold is exposed to more then 400V. Adding the diode will make this worse. C103 should be a 630V component. It may be C103 is the source of the pop.
Actually, I have upgraded C102 n C103 to Jupiter Copper Foil 600V 0.1uf as well as sockets recently...nevertheless, the pop was there before and after. I thought the upgrade will fix the pop issue but didnt.
Is that ok? Do I still proceed with the diode?
OK so you don't exceed 600V the diode and 47k 1/4W in series with the diode. Draw it on the schematic if you want a check.
Sorry, I lost you there....the diode and 47k 1/4W in series with the diode??? What do you mean? Do I still proceed with the addition of that diode, IN4002?
Fully understood...Thank you, greatly appreciated!!! I will work on this and get back to report.
Yes, will do that definitely. Have just ordered the diode and resistor suggested by @baudouin0. Will report back.
Again, greatly appreciated for your help....cheers!!!
1. Locate the relay J2 and read the part #.
If the relay is socketed. You are in luck. Swap the relays (good channel <-> pop channel).
If not, find out the footprint of the relay from datasheet
2. Make sure the amp is completely discharged. That is +B1 and +B2 and other high voltages are all at 0 volt.
You must use a multimeter that is at least 1000V capable to measure the voltages of +B1 and +B2.
Most multimeters are only ~600V capable. You have to build a voltage divider to scale down the voltage for measurements.
3. Measure the resistances of both gangs of J2 (see diagram). Compare the good channel with the "pop channel"
The relay should be normally open (high impedance).
If you want to try live soldering a couple of wire across the lowest bleed resistors R59 or R62 and tape them to your multimeter cables. There should be delay of a few seconds before the HT appears. If C27 is leaky then this may have changed. Tantalum cap is better here.
When does the pop appear immediately after turning back on or a delay? Is the channel otherwise the same audio performance as the right? What mods did you make to the amp?
When does the pop appear immediately after turning back on or a delay? Is the channel otherwise the same audio performance as the right? What mods did you make to the amp?