If the tubes are still warm they can conduct quickly on turn on. Never turn the amp off and on without letting at least 10 minutes pass. Take out the input tubes and try to turn on the amp slightly warm. If it still pops you know it’s not them. Do the same with the driver 300b. If it still pops it’s somewhere in the output circuitry. Remember to not do the off and on quickly! Give it at least ten minutes to not fry anything. This will point you to the area to look at.
Don't attempt to swap the output transformers, you are not technically qualified to attempt this.
There is no simple way to locate the problem if you have already swapped tubes.
It could be anything from a poor connection (switch, socket, solder, etc.) to a bad part.
There is extremely dangerous high voltage in this amplifier. Only a qualified technician should proceed.
There is no simple way to locate the problem if you have already swapped tubes.
It could be anything from a poor connection (switch, socket, solder, etc.) to a bad part.
There is extremely dangerous high voltage in this amplifier. Only a qualified technician should proceed.
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Yes, have tried swapping the tubes, same result. I believe it is not the tubes.
Immediately after the power switch.
Thanks!!! I will try the process and get back. BTW, it will still pop after say 30 minutes. Have taken note that it will not after about 2-3 hrs.
Yes, I am well aware of the danger. Definitely will not attempt to fix it myself if it will be a more than changing a resistor or capacitor job. Thank you for the kind warning.
It may be related to the bias metering drive circuit that is tied back to the 8 ohm speaker tap, it seems... Try selecting a different feedback level and switching the meter in and out. See if either has some affect. Probably not. The meter drive is powered on without delay and may be sending a voltage pulse to the OPT. If that is the source, it could be a faulty diode or transistor that's stays warm long enough to malfunction until cold. Or leaky cap, or.... ??? Good Luck. It may need to go in for factory service if you don't have a real tech you can trust.
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My apology, I should have stated I am running off the 4ohm tap with my 4ohm speakers as well as on the 3rd NFB at all time when this happened. Nevertheless, greatly appreciated for your input. I have also done some more testings, please see my reply below.
OK, update on my testings based on @mdpaudio 's suggestion.
Input tubes (on)
300B (on)
OPT (on)
Input tubes (off)
300B (on)
OPT (on)
Input tubes (off)
300B (off)
OPT (on)
- no pop
So it seems something to do with the 300B section. Any suggestions where should I look? Again, thanks a lot for your help.
Input tubes (on)
300B (on)
OPT (on)
- pop on left channel if turned back on after 10-30 minutes
- will not after 2-3 hrs
Input tubes (off)
300B (on)
OPT (on)
- same as above
- same as above
Input tubes (off)
300B (off)
OPT (on)
- no pop
So it seems something to do with the 300B section. Any suggestions where should I look? Again, thanks a lot for your help.
The speaker tap you use won't matter because the meter circuit is internally connected to the 8 ohm tap to get the output readings, if what I see on the schematic pointing to the 8 R is correct.
I’m glad my method of deduction helped! Make sure are the tube socket connections are tight. Use something like a dental pick to snug them up a little, not too much. Put the tube back in and test. If no help look at all the solder joints in the 300B area to make sure they’re shiny. If that doesn’t help look at the coupling caps for leakage, not goop coming out but dc voltage getting through. Tap on all the resistors to make sure they’re not cracked, micro cracks can cause pops on heat up. Let us know what happens and don’t kill youself!
First, I would really like to say thank you all again for the help. As you know I am still an amateur but also wanted to learn more about tube amp as I play it. I find tube amp is really a piece of art...😁
Hopefully, I would be able to learn enough to build a simple 300B SET amp one day.
Today, I have noticed something I didnt pay attention to before until @mdpaudio mentioned goop. Could this be the resistor (highlighted in green) that causes the pop? Is that leakage? Looking at the schematic, it is R115 25W 10KΩ aluminum chassis mount resistor. If so, what brand would you recommend to get for the replacement?
Hopefully, I would be able to learn enough to build a simple 300B SET amp one day.
Today, I have noticed something I didnt pay attention to before until @mdpaudio mentioned goop. Could this be the resistor (highlighted in green) that causes the pop? Is that leakage? Looking at the schematic, it is R115 25W 10KΩ aluminum chassis mount resistor. If so, what brand would you recommend to get for the replacement?
Resistors don't leak, that's thermal goop for better heat transfer into the chassis
Yes, have already filtered out the possibilities on tube issue, including using another pair of 300B with same result.