Mesa Boogie Road King - strange fault - help? - diyAudio
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Old 13th March 2013, 12:37 PM   #1
elros is offline elros  Norway
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Default Mesa Boogie Road King - strange fault - help?

Hi,

I have a Mesa Boogie Road King guitar amp here, originally for tube swap, customer expected that would fix the problem. It didn't.

When I turn it up loud, it plays cleanly when I play soft, but when I play harder it will distort (expected) and the volume drops quite drastically (not expected).

What could be wrong?

All the tubes are new (original Mesa Boogie tubes, it was the same fault with the old tubes). This is an advanced amp with three pair of output tubes which can be engaged in several configurations, also one can choose between tube and solid state rectifier. I've tried all combinations and they all show this same fault.
Now, I don't have the schematic for this amp, but I have found for the new version (Road King 2) and it is much the same.

I have measured the B+ to 400VDC in standby, 365VDC when amp is switched on, and around 300VDC when I play hard and the volume drops. This is less than the 460VDC written on the schematic.
I also looked at the B+ with the oscilloscope, it doesn't show much ripple. Also the PSU filter caps look OK.

So, any tips would be greatly appreciated. And if someone have the schematic available, a copy of it would be very helpful.

Also, if someone knows of a better forum to ask this, please let me know.

Regards,
Hallgeir
Norway
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Old 13th March 2013, 01:25 PM   #2
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when I had problem with the mesa like this (50/50 amp) it was caused by the negative feedback having more amplitude than the input.

first time it happened. I changed the splitter tubes. It went away for a while.

then the it came back after 100 hrs or so, so I recapped the FB network and changed my presence control. also I changed my cathode bypass capacitors.

the problem hasn't came back in 5 years.
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Old 13th March 2013, 01:36 PM   #3
DF96 is offline DF96  England
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I would hazard a guess that the Instruments and Amps forum on this website might be more appropriate. Or maybe we are meant to ignore the subtitle on Tubes/Valves?
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Old 13th March 2013, 02:00 PM   #4
elros is offline elros  Norway
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I'll have to check the cathode bypass caps, thanks!

Yes I was afraid that there might be a more appropriate forum for this question, and that I was unaware of it.
Feel free to move thread, mr moderator who might read this.
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Old 13th March 2013, 02:05 PM   #5
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maybe he was looking for someone who ran into this problem on any amp.
cause this could happen to any amp that uses negative feedback circuits.
In guitar world the negative feedback is commonly used in conjunction with rc circuit(s) as the presence tone control.


beyond the pre stage and power, the rest of the amp is standard stuff. The power supply has options instead of it being one power supply, there is two. also different B+ selections for phase splitters as well.
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Old 13th March 2013, 03:11 PM   #6
DF96 is offline DF96  England
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Power supply problem? Something is causing the large DC droop.
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Old 14th March 2013, 03:57 AM   #7
midmoe is offline midmoe  United States
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there are many ways to get a similiar result, a schematic plus standard diagnostics should give a more pinpont location to start looking. The low b+ is a tip-off...are the tubes getting too hot? Check neg bias....a too low voltage would be a problem in ruining your tubes. Until you check bias or have a working knowledge of "how hot" is too hot, don't leave amp on for very long...let it cool off a minute between tests of a minute or less duration. This will help protect you new tubes. Without looking at a schematic, if it has a normal 6L6 or similiar type output, the bias should be -40 to -50 on the output tubes as measured from cathodes to control grids. Of course, as a seasoned tech I've learned to see first if the amp has been messed with...previous mistakes of attempted repairs or mods, before I even plug the amp in......get a schematic and half the battle is won.....being aware of the repair history of the amp is important. midmoe
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Old 14th March 2013, 06:24 AM   #8
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You can check rectifier diodes, lousing one it end.
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Old 14th March 2013, 06:36 AM   #9
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Negative feedback or some supply problem, that dip in B+ makes me wonder
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Old 14th March 2013, 09:08 AM   #10
elros is offline elros  Norway
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Hello again and thanks for the tips so far.
I have now measured (again) the voltages in the output section. They are as follows:
B+ is 400V (schematic says it should be 455V)
Grid on EL34 pair is -33V (schematic says it should be -38V)
Grid on 6L6 pairs is -47V (schematic says it should be -51V)
The voltages are a bit low, but proportionally so.
One thing I haven't mentioned is that the amp is a US 120V model, it is used with a step-down transformer which gives about 105VAC, this could account for the lower voltage compared to the numbers in the schematic.
But the transformer AC voltage doesn't drop when I play hard, not the way the B+ drops.
So I wonder, is the drop in B+ voltage normal / acceptable in an amp like this?
Is it possible that the power supply filter capacitors are defective even if there is little or no ripple, can this cause the drop in B+ voltage?

The schematic I have found is this:
http://www.fileden.com/files/2008/12...0King%20II.pdf
It is for the revised Road King 2, but it seems most of the important things are the same. As far as I can tell it is only the reverb circuit that is radically different, the revised version has a separate tube for the spring reverb, the one I have here doesn't.
There are probably other differences which I haven't yet discovered.
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