Marshall 9005 stereo power amp question.

[Captain chunkulus]

So I was able to get this thing up and running, fid a full recap job, New tubes, new output Transformer and now it works. However the power transformer is an after-market one so they tied both standbys together on one standby switch. Which, doesn't bother me I'm using both sides anyways. Here's where the problem comes in. Even before I did anything to the amplifier, when you engaged the standby there's a loud buzz hum for like maybe 2 or 3 seconds and then it settles down and makes no noise at all its dead quiet. Then I did a recap job. I used in 100/100uf jj caps instead of the 50/50uf that were originally in the amplifier. The other capacitors I just changed ouy for the same values using Illinois brand. It biases up just fine but that buzzing hum sound when you Engage The standby switch it's just strange. Never experienced it before. So what I've been doing is leaving the standby on and turning the amp on and just letting it warm up naturally. That way I don't get that noise. I remember reading something that Merlin said about standby switches actually not being even necessary. Just wanted to pick your guys brains about this issue if you've experienced it if it's a real issue or if it's fine. Thanks in advance for any help.

[pdf64]

Perhaps the bias supply isn’t charging up whilst in standby? That would be very bad, can you check that?
ie applying HT to warmed up valves, whilst then having to wait for the bias supply to come on line.
There’s no technical need or benefit to standby, it’s best seen as a user convenience feature.
But not if it’s slowly wrecking the output valves.

[Captain chunkulus]

It appears that the bias voltage is not coming up with the standby off. So when you engage The Stand by the readings overload my meter at the DC voltage setting. Then it settles back down to round 56 negative volts. But it's reading about 35 milliamps per tube at that setting. If the standby is engaged from the very beginning when I power the amp on it jumps to overload and then drops right back down to 55 negative volts. I also don't get the buzzing sound that I get when I just engage standby switch.

[Latole]

schematic here ;

https://schematicheaven.net/marshallamps/rackmount_poweramp_9005.pdf

[sluckey]

You could probably remedy that issue by moving the standby switch(s). I would put the switch between the HV fuse and point X. Of course pdf will have a different solution.

[Captain chunkulus]

So replace the fuse with the standby switch? Am I thinking right on that?

[pdf64]

The buzzing sound would seem to be the output valves screaming in agony.
So don’t use standby, at least till you get it sorted.
But make sure to get it sorted, as a self destruct switch is not something that should be a user option.

You could probably remedy that issue by moving the standby switch(s). I would put the switch between the HV fuse and point X. Of course pdf will have a different solution.

On the contrary, I’m pleased to say we are of one mind in this matter
As it’s solid state rectified, it ought to be pretty difficult to get it this wrong, though to be fair, in JMP era 50watters, Marshall managed to the same thing.

So replace the fuse with the standby switch? Am I thinking right on that?

No, put the standby before the fuse.

What value HT fuse have you got in there? As 4 unbiased output valves, even momentarily, should cause it to blow.

[Captain chunkulus]

Appears to be an 800ma fuse.

[shooter]

WAG;
lift one leg of C17 and test

[Captain chunkulus]

On both sides of the power up? And then power it up and use the standby?

[shooter]

whatever condition makes it hummm for a few seconds then go to normal

[Captain chunkulus]

No difference. 

[Captain chunkulus]

So, I'm going to move the standby switch likes Slucky suggested. Question is, do I run a separate wire from each capacitor on both sides to the standby switch?

[pdf64]

I don’t understand the question.

[shooter]

this is how I understand Sluckey

[sluckey]

So, I'm going to move the standby switch likes Slucky suggested. Question is, do I run a separate wire from each capacitor on both sides to the standby switch?

Don't you still have two standby switches even though it's been modified to only use one switch? If so, disconnect the wires on the single switch and solder all wires together. Now wire one channel switch as I stated above and wire the other channel's switch the same. This puts you back to having two functioning standby switches and the bias circuit will remain active no matter which position the switch is ni.