Checking bias in a combo amp?

[talltxguy]

I got a bias kit from The Tube Depot but am unable to check the bias of my Roadsters' tubes because there's just not enough room. Is there any way to check the bias on my 2X12 combo?

 

[domct203]

I got a bias kit from The Tube Depot but am unable to check the bias of my Roadsters' tubes because there's just not enough room. Is there any way to check the bias on my 2X12 combo?

Remove the chassis.

Weber used to make a 90 degree adaptor but they have discontinued their Bias tools since Ted Weber passed away. Maybe you can find one used?

Dom

 

[talltxguy]

Thanks Dom.

Will removing the chassis affect my warranty if I have somebody who knows what he's doing remove it?

 

[domct203]

Just make sure you put it back in properly and you will be fine. There are not any tamper resistent/evident fasteners to worry about. Also, while the chassis is removed you will see the date your amp was made and the initials of who built/tested it hand written inside.

BTW- there are DEADLY voltages inside the amp, even when unplugged (stored in the power supply filter caps). Don't go poking around in there unless you absolutly know what you are doing.

Dom

 

[talltxguy]

Thanks for the info, Dom.

I am definitely not qualified to be poking around in that amp. I've got a few different techs I trust and will be visiting them.

 

[talltxguy]

I talked to a Mesa tech, who walked me through the rather easy procedure. I unplugged the power cord as well as one of the reverb wires. The white reverb wire, however, looked like a permanent attachment to the chassis; so I left it alone. I pulled the chassis out just far enough so that I could fit my bias tool & output tube into the slots and got the readings I needed.

 

[mansfieldguitars]

I am in the process of making a bias probe that will cure your issues. This came about as a request from another Roadster Combo owner and I agreed to build it. I just have to test it and source some good wire for it and I will make them available to all. I will also post a pic of the prototype within a few days.

Thanks
John

 

[talltxguy]

Thanks John @ Mansfield Guitars. The only reason I have not purchased your adjustable bias pot for the Roadster is because I've got some years left on the warranty. I've heard great things about your adjustable bias pot for several Mesa models and would like to thank you for your Mesa dedication! 8)

And thanks for addressing the problem we combo owners have in checking our bias. I hate to have to pull out the chassis because I have no business being in there to begin with. Looking forward to seeing your combo bias probes!

 

[mansfieldguitars]

I have completed the bias probe for use with combo amps or really any amp for that matter and wanted to share a few pics. You can also check the plate voltage with this probe as well which is something you cannot do with the conventional probes. I am going to make these available so anyone interested can contact me by email. With wire being expensive as it is they will be priced at $59.95. I have ordered all the components and will be ready to ship in approx 2 weeks. THey can be ordered by contacting me by email until I get a chance to get them up on my website.

 

[talltxguy]

Very nice work. And the plate voltage is an added bonus that will help when figuring plate dissipation. Thanks!