Help w/ Marshall Major bias

[Jonas]

Hi, I am working on a Marshall Major build with (2) KT-120's.  I am having trouble figuring out the bias circuit. I did some research and it sounds like the original Marshall Major's had a center tap on the bias winding.  Some schematics floating around on the internet are difficult to read, and may have a few errors on them.  I do not have a center tap on my bias winding. 

I am looking for someone to recommend / share a schematic for a bias circuit compatible with a Marshall Major.   

Any help would be appreciated, and Thanks!!!

Jonas 

[SILVERGUN]

What power transformer are you using?

Please post any specs you have.

Are you building an exact Major but just subbing in KT120s?..what schematic are you working off of?

[Jonas]

Hi Silvergun.  Thanks for chiming in.  This is not an exact copy of Marshall Major, but I am using a mercury magnetics MP100-67 power transformer.  The PT secondary spec's are  197-0-197; 6.3V @ 7A; 90V bias (two wire) no center tap.

The amp uses a voltage doubler at the first filter stage exactly like the Marshall Major schematic shows.  I'm having issues with turn on due to the high voltages, and I cannot seam to find a bias point that works well with the KT-120's.  How do you trouble shoot a bias supply?  Can I pull the output tubes to trouble shoot the bias supply? What is a good range of negative voltage to check for? -50 to -100?

Thanks

Jonas

[DummyLoad]

Hi Silvergun.  Thanks for chiming in.  This is not an exact copy of Marshall Major, but I am using a mercury magnetics MP100-67 power transformer.  The PT secondary spec's are  197-0-197; 6.3V @ 7A; 90V bias (two wire) no center tap.

The amp uses a voltage doubler at the first filter stage exactly like the Marshall Major schematic shows.  I'm having issues with turn on due to the high voltages, and I cannot seam to find a bias point that works well with the KT-120's.  How do you trouble shoot a bias supply?  Can I pull the output tubes to trouble shoot the bias supply? What is a good range of negative voltage to check for? -50 to -100?

Thanks

Jonas

use a FWB for the neg. bias supply. alternatively use a 1/2 wave and ground one side of the bias secondary.


see attached for schema.


--pete

[SILVERGUN]

How do you trouble shoot a bias supply?  Can I pull the output tubes to trouble shoot the bias supply? What is a good range of negative voltage to check for? -50 to -100?

Yes, you can work on the bias supply without the tubes installed, in fact it is a good practice to make sure you have the adequate bias voltage before installing the tubes to begin.

The appropriate range depends on what your actual B+ voltage is.

[SILVERGUN]

If you wind up with a FWB and you want to use a pot for adjustment:

[2deaf]

So the B+ might wind up around 520V and you might want a bias voltage of -65V with KT120's.  If so, the attached could be a good starting point for a bias circuit. 

[Jonas]

Thanks everyone! I will give it a try over the next few days (hopefully......) and re-post