Metropoulos Forum

All voltages appear normal. Swapped out all tubes. There is a frying egg sound with occasional popping that increases with master and preamp pots. I am leaning toward filter caps as the problem but would like to get ideas first. Thanks.

I usually find dirty sockets or loose pins when those noises pop up. If you are pretty confident that they are not the issue, I would look at other suspects in the b+ line. Standby switch contacts may be failing (you can temporarily remove the switch and use a wire jumper to complete the circuit for troubleshooting), filter caps or possibly a bad plate resistor on the preamp tube. If the channel volume affects the noise level, the bad resistor would be before that control. You can pull v1 and see if the noise stops.

danman wrote: ↑

Mon Jul 15, 2019 6:19 pm

I usually find dirty sockets or loose pins when those noises pop up. If you are pretty confident that they are not the issue, I would look at other suspects in the b+ line. Standby switch contacts may be failing (you can temporarily remove the switch and use a wire jumper to complete the circuit for troubleshooting), filter caps or possibly a bad plate resistor on the preamp tube. If the channel volume affects the noise level, the bad resistor would be before that control. You can pull v1 and see if the noise stops.

Standby switch, plate resistor on V1, tube sockets all eliminated as source of noise. I pulled V1 and the noise remained. I swapped in both mains filters and noise remains. I am starting to doubt filter caps as the source. The choke is new but I will check it. Also my current limiter glows pretty good when I take it off standby, not sure if this matters it is 100 watt beast. Nothing smells or seems to run hot as I probe with a heat sensor. I will order a whole set of filter caps but I don’t think it’s the problem.

The treble cap in the tone stack can cause those types of noises and many people have reported problems with silver mica in the past few years. It may also be beneficial to reflow the solder joints at the preamp tube sockets as there may be a bad joint that is causing the noise.

There are also treble peaker caps (470pf) in the preamp that could be going bad and introducing that noise. If the pre-volume knob affects the noise volume, the issue should be in the circuitry before that volume pot.

danman wrote: ↑

Wed Jul 17, 2019 5:58 pm

The treble cap in the tone stack can cause those types of noises and many people have reported problems with silver mica in the past few years. It may also be beneficial to reflow the solder joints at the preamp tube sockets as there may be a bad joint that is causing the noise.

Thank you for your input. The board and components are all new but I will check those areas. I pulled V2 and noise remains. I will post again after I have changed the filter caps. There is an excess current draw on the output side or when high voltage standby is on. Like you said we look at the B+ line. If the AC Voltage is normal including the bias and filament and is rectified properly we can assume the PT is fine. Next we have the choke, this is also new but will check it. If all thi sounds checks out we may look at the Output transformer. It is acting like something is shorting to ground. There is also a huge chance I am overlooking something misplaced somewhere. Stay tuned.

Sorry...I didn't see your post above until today. Your assumptions about the PT would be correct. You mentioned that the lightbulb limiter bulb glows when you take the amp off standby. What wattage bulb are you using in the limiter? A 60 watt bulb will glow moderately in my 50 watt amp but it only glows brightly for a second or two as the filter caps charge up.

Do not overlook those silver mica caps because they are new though. Several builders have reported gremlins in their new builds caused by bad mica caps. Many folks have switched over to ceramic in their Marshall style builds to avoid the issues. If you do decide on mica, be sure they are quality caps such as Cornell Dublier. The bulk of the problems with them seem to be isolated to the black, unbranded ones used in many kits.