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
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
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.
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.
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.
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.
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
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/2008/12/25/2237836/Mesa Boogie Road King II.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.
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/2008/12/25/2237836/Mesa Boogie Road King II.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.
Moved to I & A. Please post all music amplifier questions to the Instrument & Amps forum per forum policy. Make note of the sub-headers in the forum title. and
Tubes / Valves All about our sweet vacuum tubesThreads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum
Thanks!
According to the schematic, it should be 345-0-345 VAC. I measured it to 320VAC, and it drops to about 313VAC when I play hard at high volume.
No I haven't checked current through each valve, I'm not sure how to do that. I assume I would have to disconnect the plate? Or the cathode? to measure current directly with my DMM.
I haven't seen any HF in the output signal.
But I am preparing a video which may be of some help, it shows the scope screen when the bad stuff is happening.
Here is a link to my video showing the behaviour of this amplifier.
I hope it can be of some enlightenment.
Youtube video here
I hope it can be of some enlightenment.
Youtube video here
1) You should have stated this important information from the beginning
That said, you must start by feeding proper 120V to this amp before worrying about voltages.
And the transformer you use must be a 500 VA capable one.
*At least* use a 500 VA "220/110V" transformer but add in series with the "110V" tap the secondary of a 12VAC/6A regular transformer.
Check the proper phase to get as close to 120VAC as possible.
In theory you will get 117V, which is close enough.
2) the amp has a maddening set of multi-voltage options:
>> Bold/spongy
>>Tube/SS rectifiers.
Which one are you using?
Best (or less bad, until you solve the 120VAC problem) is:
Bold+SS rectifiers
Please post +V , both bias voltages, and "6.3V AC" filament voltage (measure at power tubes pins 2-7).
3) once you have done that, we'll start troubleshooting that amp.
Consider having a special autotransformer made, with proper voltage and current rating.
1) You should have stated this important information from the beginning
I know. Apologies.
The transformer is a 1000VA iron, should be big enough. It is the one supplied by the owner.
The voltage, alas, can not be helped at present.
I've been trying out all settings; both tube and SS rectifier, bold and spongy (spongy seems to lower the voltages a tad, schematics indicate that spongy setting connects to a longer primary winding on the power transformer).
It behaves pretty much the same at all settings.
Voltages measured on active EL34 tube, amp in Bold, 4x6L6 + 2xEL34, SS rectifier mode:
Filament voltage is 6.01 VAC (pin 2-7)
B+ is 424VDC (pin 3) Drops to around 350 when driven hard
Grid 1 is -34,7VDC (pin 5) Drops to around -51 when driven hard
Grid 2 is 412VDC (pin 4) Drops to around 300 when driven hard
Voltages measured on active 6L6 tube, amp in Bold, 4x6L6 + 2xEL34, SS rectifier mode:
Filament voltage is 6.01 VAC (pin 2-7)
B+ is 424VDC (pin 3) Drops to around 350 when driven hard
Grid 1 is -49,7VDC (pin 5) Drops to around 350 when driven hard
Grid 2 is 421VDC (pin 4) Drops to around -52 when driven hard
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