tony777
Well-known member
:roll:
II C+ shots
- Thread starter tony777
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Looks to me like somebody changed the bias supply resistor on the C+60.If you look on the left side just below the 220uf caps the resistor on the lower edge of the board to the right of the red/blue wire there is what looks like a 22k resistor, red-red-orange stripes the leads have a gob of solder on them.FYI if you replace this resistor with a 50k cermet pot you will have an adjustable bias supply.A very simple but very useful mod.With that pot there you can use any tubes you want.Also if this resistor was changed as it appears to have been,your amp wont match Mesa's bias scheme anymore and when you try to use Mesa or GT tubes you may not fall into the right bias range.If you are able to do a simple soldering job it is well worth it,it takes longer to open the amp up than to actually solder in the cermet pot.I have the same amp and did this years ago.You wont be sorry.
Also,those caps appear to be stock,probably a good idea to change them too.If they are still stock I assume the amp is from '83 or '84.I know there are a lot of different opinions on the life of caps but many techs will agree that 10 years is about it,personally,I like to change them after 6 to 8 years even if they seem to be good.I have always noticed an improvement tonewise with fresh caps.When caps get that old they tend to fail suddenly when they do go south,which can cause other problems.Just my 2 cents.
As for the difference in tone with the same tubes the power tubes probably have a different bias in each amp,another good reason to put the 50k cermet pot in there.As long as you now know which resistor to change,why not do both amps,I havent seen the price on those pots as I got 25 of them years ago for about $3.00 but I am sure they cant be more than a buck or 2 each.
As for the difference in tone with the same tubes the power tubes probably have a different bias in each amp,another good reason to put the 50k cermet pot in there.As long as you now know which resistor to change,why not do both amps,I havent seen the price on those pots as I got 25 of them years ago for about $3.00 but I am sure they cant be more than a buck or 2 each.
tony777
Well-known member
Yes.That resistor determines where the power tubes are biased.The 60w appears to have been re-biased.So if as you say they both have the same power tubes,the 60w is probably biased closer to the tubes "sweet spot",so to speak.If you were to replace that resistor with a 50k cermet pot you can set the bias to accomodate any power tubes you want to use and find that "sweet spot".When you use the Mesa tubes you buy the ones that "match" your amps bias point.That "match" is usually within a certain range.As an example if you have 400 volts on the plate that range could be between 25 and 45ma.These numbers are not neccessarilly exact but I am using this as an example.The "sweet spot" or best sounding bias point might be 35ma,without an adjustable bias mod you wont get that.What sounds good to you isnt what sounds best to someone else.Apparently your SC is not biased to what your ears thinks is the best sounding,but your C+60 is.You know what I mean?I believe with that mod you would be able to get your SC to sound closer to your 60w.There are other differences in the amps so they may never be able to sound exactly alike,but generally,and from the way most people feel the SC should be able to "give it up" better seeing how it has the Graphic EQ option.Mine doesnt have the EQ and I think it is great,but there are many on this board who feel the 60w without the EQ is terrible.Of course tone is a very subjective issue but there is no doubt with the adjustable bias mod you can dial in a better sound.If you have basic soldering skills it is easy and fairly cheap to do.If you are not so inclined a tech should be able to do it fairly inexpensively,and then bias it to your liking.You have two great amps there and this simple mod would make them even better.
tony777
Well-known member
:roll:
Boogiebabies
Well-known member
tony777
Well-known member
:roll:
Yes there are two resistors in parallel.One is the 22k and the other should be 68k.The 22k is the one you would change to reset the bias or use a pot to make it adjustable.The same resistor would be subbed on the SC.If you go to www.elexp.com they have the pots for this.The best choice in my opinion would be the multiturn pot W/Knob- 50k should do it.The multiturn is better because the single turn is harder to fine tune.I glue them to the circuit board with the knob pointed upward to make access to the adjustment easy.
It is possible both resistors were changed.Stock is usually a 68k and one under 50k that is selected to set the bias.I have never seen them "piggybacked" as Boogiebabies described, just side by side as you see on your SC,that is why i assumed this was changed in your amp.It also looks like the solder is a little sloppy-also what makes it look like it was changed.Cant make out the colors on the other resistor too clearly but stock I believe it should be 68k.Maybe Boogiebabies knows otherwise?
tony777
Well-known member
:wink:
No,the current the plates draw must be measured.When you have it set at a safe draw you can "tweak" a bit by ear but you must then measure again to make sure you are in a safe zone.All of this is done working inside a live amp with dangerous voltages,if you arent sure of what you are doing it is better left to someone who is qualified.It is something you can learn to do but you have to be careful and make sure you know where all the high voltage is present in an amp so you dont accidentally get yourself cooked...or worse.
Boogiebabies
Well-known member
Oh,export, didnt think of that.Well that sure changes things,dont it.That 6.8k was throwing me a curve.I have worked on about a dozen boogies and never saw anything so sloppy looking,and have never looked inside an export model at all.Was trying to see what the SC picture he provided had going on but couldnt make sense of the resistors in the bias supply but the export angle sure puts a different spin on things.BoogieB you seem to know Mike B. so if you say it looks stock to you then I stand corrected.
tony777
Well-known member
So, can I measure the bias voltage myself?
And where du I put the connections?
And yes, both amps are export models, living in europe, this is what we get. Though I'm not sorry;-)
regards
Tony
And where du I put the connections?
And yes, both amps are export models, living in europe, this is what we get. Though I'm not sorry;-)
regards
Tony
The bias voltage can be measured using a DC voltmeter by connecting the black lead (common) to ground and the red lead to pin 5 of your power tube.This is the negative voltage that determines the current the tube is drawing.The current can be measured a few different ways.I like to use the "transformer shunt" method.You must have a meter that will measure miliamps.The black lead is connected to the plate of one tube,the red lead is connected to the center tap of the OT.If it is a 2 tube power out put the reading is what your tube is drawing,if it is a 4 tube output you must divide this number by 2 to see what one tube is drawing.This will only work if you have a closely matched quad of tubes.Please be aware that you are working in a "live" amp and the voltages in there can be fatal.If you are not experienced at working on amps then it should be left to someone who is.You can get seriously hurt or dead if you touch something you shouldnt.
tony777
Well-known member
Ok! cool,
though I have to spend 14 days on an offshore oilrig in waiting to get home to my beloved amps, and check it out.
Thank you both
Tony
though I have to spend 14 days on an offshore oilrig in waiting to get home to my beloved amps, and check it out.
Thank you both
Tony
tony777
Well-known member
Hey!
Bias voltge on the simul class are at the inner sockets -40.4Volts
and the outer sockets they are -30.0Volts.
The mark IIC manual says it should be -46, and -37 for export simulclass models.
The 60w model has bias voltage at -40.1.
The manual doesn't say anything for the export model, but the bias voltage should be -47.
So what does that tell me?
kind regards
Tony
Bias voltge on the simul class are at the inner sockets -40.4Volts
and the outer sockets they are -30.0Volts.
The mark IIC manual says it should be -46, and -37 for export simulclass models.
The 60w model has bias voltage at -40.1.
The manual doesn't say anything for the export model, but the bias voltage should be -47.
So what does that tell me?
kind regards
Tony
