# Help with jubilee biasing too cold



## rpurdue (Nov 9, 2011)

Hey all I recently bought a canadian Marshall jubilee 2554 combo. I went to use my bias probes and install some new groove tube El34's and the bias maxed out on the DMM at 17mV. I initially thought it was the tubes and tried the old set as well as another new set and all were the same. The board looks unmodified and i don't think these ever came with 6550's. I measured the plate voltage (443) and the bias range on pin 5 is -40 to -50. I was snooping on forums and found a schematic on Drtube (which is attached). It looks like the bias supply resistors are:

R23 - 56k
R22 - 56k
R21 -220k
R47 (right of bias trim pot) - 15k
On the schematic R22 is 220k and on mine it's 56k. 



I thought that if i lowered the resistor at R23 it would make the bias higher but it didnt. In fact, multiple resistor values did nothing. I looked at my board and it looked like R22 feeds into the bias pot so i gave that a try. I added a 150k resistor and it biases perfect now. It sounds great as well (although it actually did before as well). I'm hoping this remedies the problem. Apparently the amp has went through a lot of revisions but i'm only able to find the one schematic. I've attached the schematic as well as a photo of the bias part of my jubilee. This was before i piggybacked the 150k resistor to R22 though. I also attached a schematic for the 50 watt head in which R22 looks like it feeds the bias pot. I'm in Regina and there's no one really knowledgeable about tube amps so  any help you could offer would be awesome.


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## WCGill (Mar 27, 2009)

R23 is the one you want to change if your bias trim pot won't give you the range you need. I'd say R22 is 220k as well as R21.


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## rpurdue (Nov 9, 2011)

Thanks...i tried R23 and it did nothing though


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## rpurdue (Nov 9, 2011)

When I changed R22 though it based in range. I'm lost


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## Wild Bill (May 3, 2006)

Leave the 220k resistors alone! They feed the grids of the EL34s and have nothing to do with your bias supply. Remember that when your idle current is too low, it's because the bias voltage is too high a negative voltage! As you bring that voltage less and less negative the idle current will increase.

The trim pot and the 56k resistor from its bottom end to ground work together. Reduce that 56k resistor and the bias voltage will be less negative. The range will also go up, since the trimmer becomes more of the total resistance - the total resistance is more adjustable, if you like.

However, you have to pay attention to the maximum negative voltage available. Increasing the 56k plus the trimmer value will hit an upper limit, as the load becomes so high it no longer pulls that bias voltage down. If your tubes are running too hot and you need a higher bias negative voltage then you need to reduce that 15k resistor between the two bias filter caps.

Your problem was that the bias negative voltage was too high, reducing the idle current to a point too low. Changing the 56k resistor was exactly the right thing to do. As I said, it is in series with the trimmer to become the total load on the bias supply. The lower that load, the lower the maximum negative bias voltage. You try a new value just low enough to set the upper limit and bring the trimmer into a useful range.

It seems your schematic is not true to the board. It's hard to tell from a picture but I can dimly see the connecting traces as a shadow through the pcb. I can see how R22 feeds the trimmer. I can't see where the hell R23 goes but who cares? It has nothing to do with the bias supply.

You've done the right thing! Be happy!:wave:

Wild Bill/Busen Amps


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## rpurdue (Nov 9, 2011)

Thank you so much Wild Bill! It seemed to work out ok but i was unsure if i may have screwed something up down the road. Your reassurance helps me out enormously!


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## WCGill (Mar 27, 2009)

R23 *is* the 56k bleeder resistor, according to schematic, fooled me.


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## rpurdue (Nov 9, 2011)

WCGill said:


> R23 *is* the 56k bleeder resistor, according to schematic, fooled me.


Ahhh, that makes perfect sense then as to why it did nothing to alter the bias. I know i'm terrible with schematics but i was sure on that one that R23 fed the RV1 trim pot.


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## dtsaudio (Apr 15, 2009)

Be very careful working on this amp using the schematic as reference. The markings on the schematic are really far out. To the left in the picture on the board is R29,21. On the schematic they're R21, 22. Also notice next to those resistors on the board is C14 (designating a capacitor) it's a resistor mounted in there. I'm sure there are more examples.
You did find the correct part to change however.


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## rpurdue (Nov 9, 2011)

I think I'm done fiddling thankfully. It may need a cap job at some point but ill leave that to someone more capable than myself. I am disappointed in the inaccuracy of the only schematic I could find.


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## Wild Bill (May 3, 2006)

rpurdue said:


> I think I'm done fiddling thankfully. It may need a cap job at some point but ill leave that to someone more capable than myself. I am disappointed in the inaccuracy of the only schematic I could find.


Don't give up on schematics! You just found the wrong one, is all. The connections were right but the numbering of the components was different.

I truly believe that one can't become a good tech until he's comfortable with schematics. You will know when you reach that point when someone giving you a picture really, really frustrates you!sigiifa

You see, a schematic clearly shows all connections. Pictures don't! The wires or the board traces always are covered by stuff so that you can't see where they go. Usually, they will go into some ribbon cable or a big bundle of wires all the same colour. Even if you were actually working on the amp yourself, it might take a long time trying to trace a connection, while a quick glance at a schematic will show it plain as day.

So really, most times getting a picture just pisses a tech off! LOL Spend the time learning and it will pay off, big time! Trust me!

As you learn more and become familiar with how circuits actually work you'll find that things like different numbering won't bother you. You know what should be there so any "R21" vs "R41" type discrepancies won't bother you. Instead, you will say "It needs one of those if it's gonna work, where the hell did they put it?" or "Well, if it runs from this point to that, it must do THIS!".

It's like trying to become good at jigsaw puzzles by only looking at shapes. It will not only take forever, but you will never get any faster! Instead, you learn to look at the overall picture and UNDERSTAND IT! Then the shapes become unimportant.

I still have electronic books and magazines I've been carting around since I was a kid back just before Woodstock. I got hung up on learning as much theory as I could because I am basically lazy and can get easily frustrated! I started off like most folks randomly trying this and that to make something work, spending a LONG time at "trial by terror". Then I started reading more textbooks and I discovered formula and stuff that would let me find answers in a minute or two of "back of an envelope" math. That minute or two of using theory could and did save me hours and days of using bad words!

Keep at it! It WILL pay off!

Wild Bill/Busen Amps


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## jb welder (Sep 14, 2010)

Aside from the model number, schematics often also have board numbers and revision numbers. In this case the schematic says it is for the JM1120 circuit board. You will probably find your circuit board has some other number.
Also, Marshall sometimes have variations for Canadian models. There is probably a proper Canadian schematic in a box of old Efkay files somewhere lol, maybe Erikson/Jam have a copy.


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