Marshall Major Blowing Power Fuse

[catalin gramada]

Cause you work at you power supply, I suggest to do a simple useful change. Broke the wire between minus rectifier pole and bias ground reference. Let the bias ground reference to run to chassis through those black wire. Run a separately wire (should be a relative thick one) from minus pole rectifier straight to minus pole first series batery cap, directly on the minus cap lug not chassis lug . like blue line in my sketch. That.s a heavy dirty noisy supply return, is better to keep it separately and not run ,contaminate ,the chassis

[cluster]

Cause you work at you power supply, I suggest to do a simple useful change. Broke the wire between minus rectifier pole and bias ground reference. Let the bias ground reference to run to chassis through those black wire. Run a separately wire (should be a relative thick one) from minus pole rectifier straight to minus pole first series batery cap, directly on the minus cap lug not chassis lug . like blue line in my sketch. That.s a heavy dirty noisy supply return, is better to keep it separately and not run ,contaminate ,the chassis


[ATTACH=CONFIG]43922[/ATTACH]

so basically leave the black bias ground to chassis but move the bridge rectifier ground to the ground lug at the filter cap?

[catalin gramada]

so basically leave the black bias ground to chassis but move the bridge rectifier ground to the ground lug at the filter cap?

Exactely, bridge rectifier negative pole straight to first batery cap negative lug (the cap lug wich will be tied further to ground chassis lug)
And the bias goes as it is to original ground conection ( you have a black wire there) . Just disconect the bridge and run a separate wire to the cap. You will keep noise supply return out of chassis. That is very important from noise consideration cause you run through chassis signal returns. It is in whole beneffit to give no chance to not contaminate it an keep as clean as posible, Those return is very noisy return containing AC ripple and have a lot of energy, Run a AWG 18 there if you can and try to keep it as short

[cluster]

Exactely, bridge rectifier minus straight to first batery cap minus lug (the cap lug wich will be tied further to ground chassis lug)
And the bias goes as it is to original ground conection ( you have a black wire there) . Just disconect the bridge and run a separate wire to the cap. You will keep noise supply return out of chassis. That is very important from noise consideration cause you run through chassis signal returns. It is in whole beneffit to give no chance to not contaminate it an keep as clean as posible

Thanks for that. I noticed that the amp had a noticeable hum at idle today. this cure some of the hum.

[catalin gramada]

No worries. clean the ground lugs when you screw in back and instal properly
We talking about 100 hz ripple noise, or 120 ...sorry

[catalin gramada]

What are those two wires tied together at first ground lug? It is a green one, suppose it is CT from bias winding and a yelow one-suppose to be a shield or what ? can you enlight me ,please ? Which one is bias center tap ? and what a heck is the other ? Cluster the heater wires, including heater CT are on the other side you said ?...

[catalin gramada]

Cluster From you pics looks like bias secondary is also made from two series windings. Green and yellow are tied together to form a center tap. Am I right ? (I.m not sure hundred percent so need to check it)
If is so,in this case you can lift green and yellow from ground lug, Keep it tied together and run a wire from that connection to the turret where bias circuit is referenced to the ground( black wire connection on the small board). You can run this wire in chassis bundled with red bias wires. Keep black wire bias reference very safe , just lift yelow/green and run it to that spot on bias board. You will separate in this way bias supply by signal path in bias circuit

[catalin gramada]

Trying to do more understandable

[cluster]

Trying to do more understandable

[ATTACH=CONFIG]43932[/ATTACH]

Thanks. Have you heard of the Larry ground scheme? Here is an image.

[catalin gramada]

I know what Larry did and he is right. There are a lot of schemes functionaly talking but be aware no schemes worth nothing if you don't understand what you did. And that means one thing, one principle : keep as much izolate signal path by supply path. Separate as much low currents path by large currents path. Stick on this principle and can develop any schemes you want in respect of complexity of your amp.
Of course in many situations inevitable mixing but good practice mean to have care to minimise the effects as much as possible

[Tom Phillips]

Caution for readers of the ground scheme illustrated in post #114 claimed to be "the Larry ground scheme." Note 6 of the posted illustration, Power Cord Ground, contains a major mistake. It indicates the black hot lead is the power cord ground. It should be the green wire which is correctly shown in the photo bolted to the chassis. However, the overlay note "6" is in the wrong location. These kind of typos are a problem for novices who build from layouts because they haven't learned to read schematics and don't yet understand the circuit principles.

[DrGonz78]

Caution for readers of the Larry ground scheme in post #114. Note 6, Power Cord Ground, contains a major mistake. It indicates the black hot lead is the power cord ground. It should be the green wire. This is correctly shown in the photo but these kind of typos are a problem for novices who build from layouts because they haven't learned to read schematics and don't yet understand the circuit principles.

That was the first thing I set my eyes on in that picture. My first thought was "Oh great some noob is going to spend hours on that one!" Hopefully not their last hour either...

[Justin Thomas]

My question is, I can't see where that black wire actually IS going... But it sure doesn't look like it's going to the center lug of the fuse, does it? Which is where it SHOULD be going FIRST, right? Do I see the Neutral fused but not the Hot?

Just asking for my own security... if I've been doing it bass-ackwards all these years, I have a few amps to fix!

Justin

[catalin gramada]

Guys take it easy. This pics is a drawing made from a user who dīnd.t understand nothing by basic circuits at the moment trying to understand what Larry Grohmann aka Novosibir on metro forum trying to explain. It is a whole valuable topic in Metro about with very valuable infos.and Thanks to Larry Grohmann for that. But Larry never did a drawing. There are hundred of almost identical pics on internet made from different novice users wich didn.t understood what they dīd. If you are really interested by subiect go to Metro and folow the related topics under Larry - novosibir- directions
For this reason I mentioned above- no schemes worth nothing if you don.t understand what you did

[DrGonz78]

My question is, I can't see where that black wire actually IS going... But it sure doesn't look like it's going to the center lug of the fuse, does it? Which is where it SHOULD be going FIRST, right? Do I see the Neutral fused but not the Hot?

Just asking for my own security... if I've been doing it bass-ackwards all these years, I have a few amps to fix!

Justin

No Justin you been doing it right. Hot "Black" wire goes to fuse and that is how it should have been done. I had one instance where there was a two-prong 1970's Roland amp power cable and the white wire was connected to the hot blade of the plug. Never encountered anything like that before or since.

Edit: Also make note that the two prong cable I mentioned had marker on it to show polarity when plugging it in. If you looked in the amp they used white wire everywhere you would normally expect to see black wire too. Of course this was the 70's...