Marshall 9100 bias adjustment

[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.

[vmaxvmax]

Thanks all for your answers...

I have no problems biasing my amps. Done it many times - I use a bias probe.

The Marshall 9100 has a 'balance' trimpot on each channel. It is for setting the PI balance. Trouble is, ideally you need an oscilloscope to balance the onset of distortion at the top and bottom of the sine wave.

I was just wondering if anyone has done this and found another way...

One side of the phase inverter has a 100k resister on its output and the other has a 47k resister and 100k trimpot in series. I figure that setting the trim halfway would set the basic balance of the PI but Marshalls tend to be set slightly offset for tonal complexity... I.e. 100k and 82k resistors from the PI.

I have tried to set it by ear at 'household' volume but makes little difference. Maybe not worth worrying about!