Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jonas on April 15, 2018, 07:15:45 pm

Title: Help w/ Marshall Major bias
Post by: Jonas on April 15, 2018, 07:15:45 pm

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 

Title: Re: Help w/ Marshall Major bias
Post by: SILVERGUN on April 15, 2018, 08:26:30 pm

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?

Title: Re: Help w/ Marshall Major bias
Post by: Jonas on April 15, 2018, 09:28:33 pm

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

Title: Re: Help w/ Marshall Major bias
Post by: DummyLoad on April 15, 2018, 10:23:25 pm

Quote from: Jonas on April 15, 2018, 09:28:33 pm

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

Title: Re: Help w/ Marshall Major bias
Post by: SILVERGUN on April 16, 2018, 07:47:38 am

Quote from: Jonas on April 15, 2018, 09:28:33 pm

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.

Title: Re: Help w/ Marshall Major bias
Post by: SILVERGUN on April 16, 2018, 08:31:37 am

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

Title: Re: Help w/ Marshall Major bias
Post by: 2deaf on April 16, 2018, 02:01:04 pm

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. 

Title: Re: Help w/ Marshall Major bias
Post by: Jonas on April 16, 2018, 03:01:31 pm

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