Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on September 20, 2013, 06:21:16 pm

Title: bias circuit on musicman rd50-112. not getting how it works
Post by: phsyconoodler on September 20, 2013, 06:21:16 pm: I have an old RD50 112 combo that has a real noise issue and one tube is red plating.I'm looking at the schematic and I see +30v going to the control grids. ??? How is this amp biased with positive grid voltage?

I'm going to change a few components that look like someone had jury-rigged but they aren't connected to the bias supply.
I'm thinking the PI has something to do with the way it works but this is pretty weird to me having only worked with all tube bias circuits.
Any insight into how it actually works would be quite welcome!
Title: Re: bias circuit on musicman rd50-112. not getting how it works
Post by: HotBluePlates on September 20, 2013, 06:27:41 pm: Quote from: phsyconoodler on September 20, 2013, 06:21:16 pm
... I see +30v going to the control grids. ??? How is this amp biased with positive grid voltage? ...

The cathodes must be even-more positive. Then the grids will be net-negative.

I don't really follow it, but the output tube grids appear pegged to a fixed voltage while the drive signal is applied to the cathodes.
Title: Re: bias circuit on musicman rd50-112. not getting how it works
Post by: Willabe on September 20, 2013, 07:27:17 pm: Quote from: HotBluePlates on September 20, 2013, 06:27:41 pm
The cathodes must be even-more positive. Then the grids will be net-negative.
I don't really follow it, but the output tube grids appear pegged to a fixed voltage while the dive signal is applied to the cathodes.

Yes.

Here's a link to a thread that loogie started on a MM 65. He got it fixed.

The cathodes sit around +60v and are driven by the transistors. There's probable enough information in the thread for you to figure it out and fix it. I think it's biased up to class B.

I had a MM rd50-112 and it was a pretty good amp IMO. I even played out with it a few times and liked it.

http://www.el34world.com/Forum/index.php?topic=15910.0 (http://www.el34world.com/Forum/index.php?topic=15910.0)

Brad :icon_biggrin:
Title: Re: bias circuit on musicman rd50-112. not getting how it works
Post by: phsyconoodler on September 21, 2013, 12:31:29 pm: Thanks Brad!

It looks like I'll be replacing a pile of parts in this amp after all.Not a biggie at all.It's a cool little amp,perfect for small gigs or rehearsal.What little sound I did get was promising.
someone was inside trying to do some work already and it's sloppy so I'll clean it all up.
I can see why musicman amps are so hard to work on;you have to wrap your head around the SS circuitry to understand what's happening first before you are able to diagnose anything.Leo should have stuck with all tube designs,but that's history!
Cheers guys!
Title: Re: bias circuit on musicman rd50-112. not getting how it works
Post by: Willabe on September 21, 2013, 01:56:16 pm: :icon_biggrin:
Title: Re: bias circuit on musicman rd50-112. not getting how it works
Post by: Willabe on October 16, 2013, 06:33:41 pm: Any news/update on this amp?

Brad :icon_biggrin:
Title: Re: bias circuit on musicman rd50-112. not getting how it works
Post by: alerich on October 17, 2013, 12:45:26 am: Quote from: phsyconoodler on September 21, 2013, 12:31:29 pm
I can see why musicman amps are so hard to work on;you have to wrap your head around the SS circuitry to understand what's happening first before you are able to diagnose anything.Leo should have stuck with all tube designs,but that's history!

As I understand it Leo Fender didn't have anything to do at all with the Music Man amplifier line. His involvement was strictly in the guitar end.After a falling out with the folks at MM he ditched them and co-founded G&L. It makes sense. It's hard to believe that the same dude that brought us the Super Reverb would build hybrid amps.
Title: Re: bias circuit on musicman rd50-112. not getting how it works
Post by: loogie on October 17, 2013, 05:20:22 am: What worked for me was pulling the tubes and measuring voltages. If the thing's been hacked that adds to your woes because if you un-hack it then you've introduced uncertainty. But it could be the hacks that are the problem. In my limited experience this has always been the case.

Its just another circuit. If something failed you'll find it. Especially when PRR provides some guidance. I would have flamed out if hadn't pointed me in the right direction.

As I grow older I've come to learn that transistors aren't always the evil things we think they are. If that model has the same trem circuit as the one I worked on I think you might be pleasantly surprised.

I don't know much about Leo. Maybe he was a paragon of virtue or maybe he was just chasing the buck. My guess is he was some of both.
Title: Re: bias circuit on musicman rd50-112. not getting how it works
Post by: phsyconoodler on October 17, 2013, 11:07:30 am: I put it off to the side for now.It needs lots of parts to get it up and running and most are not labelled on the circuit board so it makes it tougher to replace stuff that has been changed out with the wrong parts in the first place.
Title: Re: bias circuit on musicman rd50-112. not getting how it works
Post by: Jack1962 on October 24, 2013, 08:20:55 am: Sometimes it is necessary to do what I call shotgun replacement(replace everything in a circuit) :laugh: