MusicMan RD50-112 Question

Hi everyone. This is my first post and has to do with my 112 RD 50.
This past summer I had a power tube(6L6) short that burnt one of the screen resistors this amp.
I accessed a schematic diagram from Ernie Ball-http://www.ernieball.com/mmonline/te...p s/1650-rd.pdf and picked up some 470 Ohm 1W resistors. After installing them and a new fuse post (also needed) and some new SED Winged C 6L6's to my surprise the amp is louder, brighter (but really ice-picky) and somewhat distorted on the clean channel which can also be heard over on the overdrive as well.
I then tried some Sylvania STR's 6L6's instead of the Winged C's and it was more distorted. Could this be a cap issue and could all of this result from a failed power tube that resulted in a burnt screen resistor, until which, this amp sounded great?
What else needs to be addressed?
I'm not a tech, so the reainder of this repair may be outside of my abilities, but want to understand what may be needed to get this amp back making music again.
Thanks for any thoughts.

Sound like first, you need to have the bias checked. If the tubes are not red plating, you may have it severely cold, in which case it's into crossover distortion. Bias properly and then check.

Thanks Mike. I thought that the 1650 chassis was self-biasing. Please let me know if it is otherwise.
Thanks.

They're neither. The 1650RD circuit is fixed bias (as in fixed-fixed). The bias ranges from 'very cold' to 'so cold you get x-over distortion' depending on your output tubes and the condition of your cathode driver transistors. In my own specimen there is crossover distortion when the tubes are in one position (i.e. which tube goes in which socket). If I swap the tubes, the problem goes away.
Either way, you need to have the amp checked with an oscilloscope to see where the distortion is introduced. As Mike "210" Kaus suggested, the most likely culprit is crossover distortion.
Sincerely
Lars Verholt

Thanks, Lars. I'm finally getting the whole "Fixed biasing does not mean the bias is not adjustable" concept here. Given Mike's comment above, what is the optimum mV value, per tube, that is suitable for these amps?

Steve/ Lars- I read Steve's post below and just want to make sure that I understand this situation better:
"Posted on Sunday, April 28, 2002 - 09:57 pm:
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The 1650-RD chassis (110RD-50 & 112RD-50 models) appears to be a self-biasing design so all you should have to pay attention to is to use matched pairs of output tubes to keep the amp happy.
Steve"
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Therefore, there is not an adjustment pot in this amp to set the mV current per tune. The crossover distortion is being introduced somewhere else in the amp's circuit. I'm calling a tech, but as mentioned, want to understand this amp (to the degree that I can)better.
Thanks.

I should add that the 75k boot-strap resistors on the cathode driver transistors do provide a degree of self-regulation without making the tube circuit self-biased in the classic sense.
I'm sorry I don't know what the actual design idle point for the RD-50 is - my own amp is an odd-ball with a different bias circuit. Others should be able to chime in here. Ed? Mike? Steve?
Sincerely
Lars Verholtwww.captain-foldback.com

Not having done any work on an rd50, I'm a little un-armed for this one. Looking at the schematic, as Lars says, it provides a cushion of self regulating TO A POINT. The amp was set up though with the old world tubes in mind and todays new production are WILDLY out of range in both directions. Thus the need lately for a lot of people altering the control grid voltage of a LOT of amps to get them "In Range". I have put in tubes that were so cold that the bias voltage had to be decreased so much that I ran out and had to change the bias circuit as much as I could. Had to actually make a high voltage circuit once because the low voltage bias winding wasn't enough to get the cold assed tubes to warm up. My rambling point is, new tubes are crazy wide in bias range. Some hot, some cold. Each one is a test in itself. I would still suggest finding someone who could look at the output with a scope and adjust from there. OR wild ass guess, find a "hot" set of tubes that run a little wilder with a set voltage an try them. It might be hunt and peck but if it's not red plating the tubes, it;s sure not OVER driving them. Just my 2c's worth. Mike.

Oh and by the way, I want to thank Lars for being here and answering a LOT of questions that guys like him are more qualified to answer than I am. He does this a lot more than I do anymore and is really on the front lines a lot more than I am. I thank him and hope he stays with us. Mike Kaus.

AND- Ed Goforth-can't forget him!

All you guys are awesome. Much thanks and appreciation from way out here in the peanut gallery..

Fast forward to now, please...
Shortly after this posting, I took the amp and two matched sets of NOS 6L6 power tubes to a qualified local repair facility.
The work was done and I played some gigs, but the clean channel sounded very stiff and strident to me...different than my recollection of how it previously sounded. These are known to have headroom and fast-attack, but this seemed very stiff.
Note that at the time I was mainly playing a 5E3, so I just chalked my perception of the RD's sound as being stiff/trident due to being familiar with the tweed.
Shortly thereafter, the RD50 went into mothballs.
Last weekend, I noted(to my surprise) that I must have taken both a set of GE 6L6's- that weren't installed AND a set of Sylvania 6CA7's, as they were what was in the amp.
I'm thinking that the 6CA7's, while/if properly biased in this amp, may have likely lead to what I have described above.
That said, I now want to get the set of 6L6's rightfully back in and have a Weber Bias Rite that I'd like to use to reset the amp for them.
Does the procedure to re-bias the amp to the 6L6's involve using a trim pot of some sort to arrive at a desired idle value, given an observed plate voltage?
Thanks for any thoughts.
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Yes, if it's the ss driver, you will measure a voltage across the driver resistors. I haven't been in an rd50 but the pot will be on the schematic. It will be listed as tr1 or something like that. It's actually not based on a given voltage, just a desired voltage period. Mike.

RD-50 does not have a bias control. It's a fixed-fixed bias if one can say so.
-Lars Verholt

As usual, Lars is correct! Just looked at the schem an I ASSUMED(and we all know what THAT does) that the RD's had the same SS driver and bias adjust as the other SS drivers. NOT. Fixed bias voltgae that is POSITIVE, not negative. Like I said, haven't worked on one. Old Chinese proverb-SHUT YOUR MOUTH unless you're sure. I'm not sure it's Chinese but it works! Mike.

That said, can I simply put the GE 6L6's in and move forward?
I just want to be sure.
Thanks again.

Well, that depends on your electronics skill. There IS no adjustment other than altering the control grid voltage coming from the source. That being said, IT is fixed and would require changing resistors to get it to change. There is a Zenar in front to to bring the voltage down. Like I said, I have NOT worked on one though. Looking at the schematic, I would say plug them in and try. The only way of actually measuring the current would be to put 1 ohm resistors in FRONT of the cathode(it's cathode driven) and measure the voltage across that. That being said, I'd PROBABLY just plug them in and go unless you are well versed at working on HV stuff. I WILL bow to Lars on this though. He's worked on them, I have not TOUCHED an RD50. Mike.

Just plug them in and try it. It will either work (i.e. no cross-over distortion) or not (those particular tubes result in cross-over distortion). I would not alter an RD-50 to suit the tubes unless it was an emergency. You may experience a bit of cross-over distortion for a minute after initial power-on until the tubes warm up.
As previously stated on this forum - my personal pet RD-50 110 has a set of old blue label Philips 6L6 in it. If the tubes are in one position I get cross-over, if I swap them I get none (after a short warm-up).
Cheers
Lars Verholt

Thanks to both of you. I'll give it a shot.