Mesa Boogie MKIV bias issue

[Ampzone]

All I have a friend who brought by a Boogie MK1V later issue, possibly a B version. Fuse had blown, 6l6GC with the switch option set to "Simuclass" I found V9 & V8 outside screens toast (one starting to burn the other burned). Schematics for this amp seem to vary. Went with 1K 5w replacements and set the amp to class A. Fired up no valves, the bias feed from the splitters read -52neg on side and -45 on the other (problem) and -57neg upstream above 221k bias split. Resistors read in circuit 201k and 199k. Since there is no bias adjust pot...kinda odd, or much info on bias voltage not sure what the Boogie bias fixed spec in neg volts is, colder/hotter.

Any ideas?

[g1]

They show -57V where you say you have it.
Try lifting the coupling cap from the PI to the bad side.

[Ampzone]

They show -57V where you say you have it.
Try lifting the coupling cap from the PI to the bad side.

Cool, yes its -57 upstream but on the grid resistors side is -52 and -45 on the other. This version pcb is a rats nest and hard to navigate unfortunately but I will try and hunt for a way.

[Ampzone]

Ok, I removed PI .1 on each side, reads -49 when removed -52 when removed on the other side (stays the same here in or out). Note yesterday it was reading -45-46ish so a slight drift up. Also note when the amp is on standby you get -52 on both bias resistors (at junction between pin5 grid resistors). Turn off standby and its back to -49 and -52 before grid pin5.

[The Dude]

That, more or less, confirms g1's theory. Removing the PI is not the same as lifting the coupling caps. There is B+ on one side of the cap whether the tube is there or not- both sides if it's leaky. By putting the amp in standby, you are removing B+ from the amp, so if the cap is leaky, it won't effect bias. The fact that bias is the same for both sides in standby all but confirms a leaky cap.

[Ampzone]

That, more or less, confirms g1's theory. Removing the PI is not the same as lifting the coupling caps. There is B+ on one side of the cap whether the tube is there or not- both sides if it's leaky. By putting the amp in standby, you are removing B+ from the amp, so if the cap is leaky, it won't effect bias. The fact that bias is the same for both sides in standby all but confirms a leaky cap.

Ok coo. I think you both solved this! Bravo! .1uf Replacement cap on order.

[The Dude]

Did you lift one end of the cap and re-test first? There is always the slight possibility that the leak is from something else.

[Leo_Gnardo]

The fact that bias is the same for both sides in standby all but confirms a leaky cap.

Last couple of years I've run across several cases of orange drops gone leaky in 90's era Mesa amps. We can put away that tired old notion that "orange drops never go bad." Maybe "seldom" instead of "never."

[Ampzone]

Last couple of years I've run across several cases of orange drops gone leaky in 90's era Mesa amps. We can put away that tired old notion that "orange drops never go bad." Maybe "seldom" instead of "never."

Ok wait... I found two .1 orange drops in my parts drawer, just put in the replacement reading with b+ is -50 and -52, standby off -52 -52. So no big change after all... hmmm.

[Ampzone]

Did you lift one end of the cap and re-test first? There is always the slight possibility that the leak is from something else.

I lifted both .1uf caps at the 221k junction. with B+ -52 -50ish, without b+ -52 -52 on both lifted caps at 221k at the junction.

It appears moving upstream PI there is another two .1uf caps.

[Ampzone]

the other sides of the PI caps read 273v and 265v (the 265v side is the -50v with B+ applied.

[g1]

I don't know if you have a way of measuring idle current?
The difference between -50V bias and -45V is significant. And the one that showed -45V at one time may have been unstable, so it may have been even worse at other times.
If you have a consistent bias now with tubes installed, you may have fixed it. What is it at the junction upstream, still -57V ?

[Ampzone]

I don't know if you have a way of measuring idle current?
The difference between -50V bias and -45V is significant. And the one that showed -45V at one time may have been unstable, so it may have been even worse at other times.
If you have a consistent bias now with tubes installed, you may have fixed it. What is it at the junction upstream, still -57V ?

We'll its still -57 upstream before the bias splitters. I don't think I changed anything to fix really. Even with the new orange drop its still -50v and -52v with B+ at the bias split and grid junction. without b+ it is even -52 on both sides.

No power tubes are in while Im getting the voltages, the one 6l6gc tube was pretty cooked, I ordered a new set for the outside sockets with the 1K screens that were burned

[Enzo]

Isn't this intended as 6L6 in the center and EL34 on the ends? The end sockets have the 2 meg grid return resistors. SO for the end sockets only, the grid voltage is reduced 220k/2200k or 10%. (2.2 meg is 2200k) 10% drop from -50 is -45. And the EL34 likes a lower grid voltage than 6L6. SO maybe this is on purpose?

[Ampzone]

Isn't this intended as 6L6 in the center and EL34 on the ends? The end sockets have the 2 meg grid return resistors. SO for the end sockets only, the grid voltage is reduced 220k/2200k or 10%. (2.2 meg is 2200k) 10% drop from -50 is -45. And the EL34 likes a lower grid voltage than 6L6. SO maybe this is on purpose?

I dunno - So the map has class a and simuclass settings, but it had all 6L6GC tubes. Its set to Class A now but was on Simclass. Whatever the case why one bias side is -50 and the other -52neg is whats troubling me. It was reading -46 a day ago so something drifted. I just wanna see an even bias split