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Hi,
I'm looking for some input on where I might find the fault in this amp. It's a Mesa Boogie Road King, first version (the one with five pre-amp tubes). The pre-amp section seems to be perfectly all right, the signal on the master volume pot is nice and clean.
But the power section has some issues, you might say. There is clean sound at low volume, but when I turn up something bad happens. Soft playing gives nice clean sound, but when I play harder it will not only overdrive (that would be expected) but it distorts so much that the output cracles and loses power. I have made a video which illustrates the problem both with sound and oscilloscope, check it out:
The output stage of this amp is rather complicated, it has two pairs of 6L6 and one pair of EL34, these can be combined five different ways. The problem is the same in all settings.
The power supply also has a bunch of settings, including a switch for "spongy" or "bold" ("spongy" reduces the PSU voltage a bit), and settings for rectifier type (tube or SS). The problem is the same in all settings here too.
And its the same which pre-amp channel I use, same fault on all channels.
The schematic I have found is this:
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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 measured 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.
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. I recon this isn't the source of the problem, as the "spongy" setting would do just that: reduce the voltages a bit. The transformer is a 1000VA unit, it should be big enough, and there is no significant voltage drop when I play hard.
There is voltage drop in the output section though:
The amp was originally in for tube swap, customer figured that new tubes would solve the problem. It didn't. This behavour was with the old tubes and it is now with the new tubes (which are original Mesa Boogie tubes in the power section and rectifier, and Sovtek in the pre-amp).
So the amp on my workbench now has brand new tubes.
The tubes do not run particularly hot, nor is there any ill omen visible anywhere that I can see.
Filter caps seem nice, there is little or no ripple on the B+.
Filament voltage is stable, at 6VAC (as mentioned above)
Voltage from the power transformer is 320VAC bipolar, it drops to about 313VAC when I play hard.
So one question to those that have more experience than me: the B+ voltage drop I measure here, is it normal?
Or is the fault in the power supply somewhere?
Any ideas?
I'm looking for some input on where I might find the fault in this amp. It's a Mesa Boogie Road King, first version (the one with five pre-amp tubes). The pre-amp section seems to be perfectly all right, the signal on the master volume pot is nice and clean.
But the power section has some issues, you might say. There is clean sound at low volume, but when I turn up something bad happens. Soft playing gives nice clean sound, but when I play harder it will not only overdrive (that would be expected) but it distorts so much that the output cracles and loses power. I have made a video which illustrates the problem both with sound and oscilloscope, check it out:
The output stage of this amp is rather complicated, it has two pairs of 6L6 and one pair of EL34, these can be combined five different ways. The problem is the same in all settings.
The power supply also has a bunch of settings, including a switch for "spongy" or "bold" ("spongy" reduces the PSU voltage a bit), and settings for rectifier type (tube or SS). The problem is the same in all settings here too.
And its the same which pre-amp channel I use, same fault on all channels.
The schematic I have found is this:
FileDen.com | Free file hosting and online storage
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 measured 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.
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. I recon this isn't the source of the problem, as the "spongy" setting would do just that: reduce the voltages a bit. The transformer is a 1000VA unit, it should be big enough, and there is no significant voltage drop when I play hard.
There is voltage drop in the output section though:
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 -52 when driven hard
Grid 2 is 421VDC (pin 4) Drops to around 350 when driven hard
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 -52 when driven hard
Grid 2 is 421VDC (pin 4) Drops to around 350 when driven hard
The amp was originally in for tube swap, customer figured that new tubes would solve the problem. It didn't. This behavour was with the old tubes and it is now with the new tubes (which are original Mesa Boogie tubes in the power section and rectifier, and Sovtek in the pre-amp).
So the amp on my workbench now has brand new tubes.
The tubes do not run particularly hot, nor is there any ill omen visible anywhere that I can see.
Filter caps seem nice, there is little or no ripple on the B+.
Filament voltage is stable, at 6VAC (as mentioned above)
Voltage from the power transformer is 320VAC bipolar, it drops to about 313VAC when I play hard.
So one question to those that have more experience than me: the B+ voltage drop I measure here, is it normal?
Or is the fault in the power supply somewhere?
Any ideas?