Mesa 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!!

[floyd]

Check the screen resistors on the output tubes.

[flocentblack]

Check the screen resistors on the output tubes.

All screen resistors are healthy.

Thanks!

[floyd]

check all the jacks for effects loop, preamp out, etc. to make sure they're making a good ground connection when not in use.

[flocentblack]

check all the jacks for effects loop, preamp out, etc. to make sure they're making a good ground connection when not in use.

Everything is good there, like I said, healthy signal all the way to the output tube grids, but thanks!

[Ed_Chambley]

Have you checked the plates on the PI as well as the other tubes.  Are you getting around 275vdc on the PI plates.  I ran into the same problem with a Fender Twin.  It had a great signal, but the PI plate resistor had drifted.

[PRR]

> majorly wrong:
> 2 x 6L6 + 2 x EL34    60W
> 4 x 6L6                   65W
> 2 x EL34 + 4 x 6L6    65W

Are you sure that is wrong?

It may just be a 65W amp. For a given B+, and sag, and load, there's only so much power available. Maybe more than two tubes can pass, so that's only 40-50W; four tubes pass the full 60-65W.

Does MB post power output numbers?

[flocentblack]

> majorly wrong:
> 2 x 6L6 + 2 x EL34    60W
> 4 x 6L6                   65W
> 2 x EL34 + 4 x 6L6    65W

Are you sure that is wrong?

It may just be a 65W amp. For a given B+, and sag, and load, there's only so much power available. Maybe more than two tubes can pass, so that's only 40-50W; four tubes pass the full 60-65W.

Does MB post power output numbers?

They claim 135W with all 6 tubes engaged.  The amount of sag I'm getting (in SS Diode mode) at full power is what I would expect to see, but I'm measuring only half the power into the dummy load...

[phsyconoodler]

Meters can give false readings.And your signal generator can be set wrong too.And...the measure of output power is at what point?Maximum clean signal or....???
  What does it SOUND like?

[flocentblack]

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!

Thanks everyone for taking the time to reply with suggestions!

[flocentblack]

Meters can give false readings.And your signal generator can be set wrong too.And...the measure of output power is at what point?Maximum clean signal or....???
  What does it SOUND like?

Here's how I do it in the shop.  1Khz sine, eq set as flat as possible (using a scope and square wave) P-P voltage on oscilloscope at the point just before visible clipping, across a proper load.

(P-P voltage * .3535)²  / Load Impedance (big power resistors in oil) = W RMS Clean

[phsyconoodler]

They didn't say anything about that in the manual!

They shouldn't have to.You should know the correct load changes for two tubes vs four.

 A half power switch that removes the cathode ground on two tubes is a compromise method that Boogie has used for years.The output transformer seems to be able to take it,but then very few people crank their 135 watt boogie anyway,it seems they use the preamp distortion more than driving the snot out of the output tubes.
A 135 watt amp is freakin' loud!!!