Mesa Boogie Road King low output

[flocentblack]

Hi everyone, I've been banging my head against this one for a while.

Mesa Road King came in sounding distorted even at low volume. Found faulty tubes in cathode follower positions V2 and V4, Replaced those, I'm getting healthy signal all the way up to the grids of the output tubes.

Problem is, in any mode that is more than a single pair of output tubes, power is much lower than it should be.

2 x 6L6 50W (RMS)
2 x EL34 40W

This is where it starts to go majorly wrong:

2 x 6L6 + 2 x EL34 60W
4 x 6L6 65W
2 x EL34 + 4 x 6L6 65W

Plate - 476V, sags down to 440V when driven to clipping
Screen - 479V
6L6 grids -50V
EL34 grids -38V

Disconnected the NFB loop.

Output tubes all test good.

All cathodes are grounded through the relays as they should in higher power modes.

Any ideas or suggestions would be greatly appreciated!!



Mesa Boogie Road King II.pdf

[Dr Kaos]

Hi

Have you checked all the grid stoppers and screen resistors?
I would select the mode that uses all the tubes and try putting them in a pair at a time to make sure all the sockets work.
This also lets you see how well the tubes work in circuit.

Dr Kaos

[Dr Kaos]

I am also working on a Road King but it doesn't have any EL34s in it.

Dr Kaos

[flocentblack]

Hi Dr Kaos,

I think I may have solved the mystery...

If you look at the schematic for the output section, the load that the output tubes "see" is the same no matter if its 2, 4, or 6 tubes. A whole lot of compromising going on.. The transformer impedance seems to be correct for maximum power transfer when using 2 x 6L6. If I half the load impedance and switch to 4 x 6L6, the power output almost doubles, like it should!

It is 135W amp, but only if you change the load accordingly! They didn't say anything about that in the manual!

[bob p]

To my knowledge, Mesa has never made any attempt to perform Z matching when turning output tubes on and off. They've always just switched tubes on and off without doing anything to compensate at the OT primary or the OT secondary, thereby relying upon the ability of tubes to tolerate a 100% loading mismatch. My Mark IV is a good example of this -- there are all sorts of tonal accommodations that you can make, including B+ changes, swapping 6L6, EL34, 6V6, triode/pentode and SimulClass/Class AB... none of which bother to make any attempt to adjust bias or to compensate by proper load matching. At least with this amp it looks like there's the ability to use the load switching relays on the OT secondary to accommodate large changes in load that come with power tube switching. I guess that's a step in the right direction.