The preamp needs 18 volts. Can you supply external 18 volts to the preamp and see if the hum disappears?
The 0 volts ground wire connection to the preamp may cause this problem if there are two grounds made. Power amp or power supply or speaker grounded to chassis along with input sockets. 

Thanks for the info.
Checked a few sites.
Seems like a bicycle tool, made of zinc or aluminium alloy.
Does not feel strong.
Regards.

Has anyone seen this box spanner?
Saw in a Japanese YouTube video
Many sizes in one tool.
Any idea where to get one?

Thank you in advance

Found KMD GV 100S schematic.
May be useful to you.

The reverb level is at the output of the first amplifier after the reverb output.
Pull out the reverb tank inputs and see if the hum exists.
If no, the drive to the tank is the culprit.
If yes, pull out the output from reverb tank and see if the hum exists.
If yes, the first amp after the tank output need to be checked.
If no, see if there are any other transformer nearby.
The tank will make hum if kept nearer to other transformers.

Regards.

FET to mute power amp during powerup???

Did you try to connect the preamp output to another amp?
Or from another audio source to the power amp input?

Regards.

The picture you posted shows common cathode (connected to the -ve).
The other two are for red or green (or yellow if you power both) goes to the +ve
The limiting resistor may be 1 if wired on the cathode or 2 nos. if wired from anodes.
regards.

Quote from: saturated on September 07, 2023, 02:58:08 PMyes sir just looked it up....

there is a schematic for the inside...transistors...diodes...zeners...resistors...
nm

but thats cool i had wondered if some transistors are simply called voltage regulators....like the one we put in my Peavey Audition Chorus because they have three legs and look like transistors.
so the answer is
NO THEY ARE NOT

Your reply is funny.

Disconnect the SMPS and power the amp with regular transformer power supply.
The problem could be elsewhere.
Regards.

Have you heard the Yamaha G100 amps? The preamp and speakers are part of the sound.

A comparison of the 3 models...
https://www.youtube.com/watch?v=cY8VNDOZpts

Regards

These line filters live for many years.
Probably, the ac supply voltage exceeded more than the rating of some component for too long.

Regards.

The diodes may be producing some RF induced hum. Try connecting 0.01 uF capacitors across each of the diodes to see if it reduces the hum.
Regards.

An interview with Bob Galien by sweetwater where he explains some designs of his amlifiers.
(30 minutes)
For those interested.

Regards.

https://www.youtube.com/watch?v=aT4PwrW9Rdk

One schematic with 3886 output with +/- 31 volts supply has 130 ohms in the PSU circuit.
The other schematic with 2050 output with +/- 22 volts supply has 10 ohms in PSU circuit
No mention about the output power in both the schematics.

Regards