Mesa Boogie Strategy 400 -- why mismatched screen resistors?

[xtian]

Each channel has six 6L6GC tubes. Four of them have 470R/2W screen resistors; two have 2K7. Why?

Schematic attached.

[xtian]

I'm asking, because one of the 2K7 screen resistors is burned, and I need to replace.

[xtian]

Huh, found a likely explanation: https://www.thegearpage.net/board/in...class.2359616/

You can replace the last row of 6L6 power tubes with EL-34 power tubes for more grind.

This is supported by the manual https://mesa-boogie.imgix.net/media/...0%20Stereo.pdf

All twelve sockets should be fitted with STR 6L6's when the Strategy is being used in any application other than live guitar. We have specially adapted 4 of the sockets-- those along the rear edge-- to accept either 6L6's or STR 416 type 6CA7/EL34. This tube is characterized by more prominent harmonic output when driven hard which produces a brighter, more grinding high-end, usually preferred for guitar. No internal changes are required; the two type of tubes are immediately interchangeable.

[xtian]

Channel A failure is stumping me. Something is drawing excessive current. I'm getting very low voltage on the plates of V1. (I tried new tubes already). With V1 and V2 pulled out, I see a 100v drop across the 22K/2w resistor, which is weird, because there's nothing to pull current. Comparing with Channel B (also with V1/V2 removed) there is only 1.5v drop across that resistor. I tried lifting one leg of the 30u/500v cap thinking it might be shorted--no change. Voltage drop across the 1N5368 Zener is the same on both channels, about 47v. The only path I see for current to leak from B+ to ground is via the 0.1uF post-PI coupling caps--but there is zero voltage on pins 2 & 7 of V2.

Any ideas?

[The Dude]

It's been quite a while since I worked on one of these. Do both amps share the same power supply?

[Helmholtz]

The schematic doesn't look cpmplete.

Does each channel have its own 22k/2W dropper (and zener)?
If so, does it measure good?

[xtian]

Yeah, I cannot find a better schematic. It has a single PT, single rectifier, and B+ is tied together for both channels. After that, it has individual filtering and the rest of the downstream circuit. Each channel has it's own 22k/2w and Zener, and both Zeners are dropping 47v.

[Helmholtz]

Did you measure the 22k dropper of the bad channel?

Is there maybe another filter cap after the zener?

What happens if you switch PI supplies between channels?

[xtian]

Found the issue! Fully shorted 0.1uF coupling cap between V1 and V2. Replaced, issue solved! Amp is running and sounding OK. Thanks, friends.

And here's a happy little thermal photo for your amusement.