Non ADA Gear > Amps

Marshall 9100 bias adjustment

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vmaxvmax:
Hi,

I am about to set the bias on my power amp.

Anyone have any experience regarding the balance trim pot adjustment next to the bias pot on Marshall 9100 dual mono bloc amps?

Looking at the schematic it seems to balance the PI output. I don't have an oscilloscope so, perhaps set for minimum hum by ear?

Any suggestions are welcome!

Cheers in advance...

Aelfad:
The method which can be used with most of amps, no matter what brand, is called "Standby Switch" method and involves using a multimeter. Check out this guide http://www.carvinmuseum.com/pdf/Ka-Boom_v1a.pdf  and apply the tips accordingly. Also, there are a few videos about biasing Marshalls, like this one https://youtu.be/Me0v2jgPShQ.

MarshallJMP:
Best way is to use a scope.

Aelfad:
True, but a good oscilloscope is pretty expensive. Would this thing from AliExpress make a cut, though?
http://tinyurl.com/y269ac28

Edit: I've found some reviews on YouTube of this (DSO138) oscilloscope, and it seems it's good enough for a layman like myself.

BTW, I've never used a scope. Is using one more difficult than using a multimeter? And would it be useful for current values for home electronics?

MarshallJMP:
Not sure if that can handle the voltages in a tube amp. But normally you can find old CRT scopes pretty cheap.
On youtube are plenty of videos on how to use a scope. But yes it's more difficult to use.

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