I'm working on a mid-90s Mark IV. It came to me with a blown screen resistor on one of the Class A power tubes. Fixed that and fired it up but now the tubes want to pull way too much current (they start to red plate as soon as it comes off standby). I've already tried a couple other sets of tubes and they all do the same. I checked voltages and everything looks right except the negative voltage on pin 5, which looks way too low to me. I'm reading between -14v and -18v. The schematic I have shows -60v coming off the filter caps and that reads right, but doesn't show what it should read after going through the bias resistors and getting to pin 5. I'm guessing -18v is way low for 6L6s (I'm used to seeing something in the -40v range on other amps). Anyone know what it should read?
Mark IV bias voltage seems off, what should it be?
- Thread starter fjs1962
- Start date
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Shep
Well-known member
check what the voltage is futher back around the power supply.it may be supply caps or diode or winding on your transformer.
Boogiebabies
Well-known member
Probably around -51 inner and -46 outer.
Shep said:
I did, and I'm getting the proper (acording to schematic anyway) -60v at the filter cap, then I trace that to the resistor going to pin 5. After going across that resistor I get ~ -18v, which is what I'm reading at pin 5. This is with no tubes in the circuit, because they will run away if I fire it up with tubes in the circuit. I'm pretty sure it's a bias issue but the schematic I have doesn't show a spec for negative bias at pin 5, so I'm wondering what it's supposed to be. -18v has got to be way low...
Boogiebabies said:
that's what I was thinking, so it looks like I need to troubleshoot my bias supply a little more...
Boogiebabies
Well-known member
fjs1962 said:
It would be a massive leak to -18mA. These things only couple 2-10V and are rated at least 400VDC/250VAC.
I can only assume the amp's bias supply resistors have been modified to lower the bias on a particular set of
cold tubes. A lot of Mesa yellow labeled 6L6's only draw -17mA at -51V.
Look at R233 for 3.3K, R204 at 150 K, R105 at 825 Ohm, R81 at 82K and R81 at 47K.
Restless Rocks
Well-known member
The screen resistor blew for a reason: probably a shorted tube in the class A socket. Did you replace the tubes or just the screen resistor? If the tube is still shorting the grid it will read a very low bias voltage and it would still red plate and will blow the new resistor in a short time.
Restless Rocks said:
The guy who brought it in is the original owner. He said he didn't play the amp for a couple of years then retubed it with fresh Mesa tubes and that's when he noticed the problem. I've since subbed in a set of known good 6L6s i have here with the same results. The measurememnts I'm checking now are with the tubes out of the sockets. I'm guessing the messed up bias voltage made the tube run hot which is what took out the screen resistor.
Boogiebabies said:
Thanks, that helps. I just checked and all resistors are (at least according to the bands) the right specs.
I checked voltages again and I'm getting -57v from the bias supply (my schematic says it should be -60 so I'm OK here) right up til it hits the 220K resistors R321 and R322. on the other side of R321 I'm reading -18v and R322 I'm reading -22v. This gives me pin 5 readings of V8: -15v, V7: -20v, V6: -17v and V7: -22v.
Boogiebabies
Well-known member
You sure that the bias supply is hooked up instead of the EQ supply ?
Boogiebabies
Well-known member
Off the two 221K resistors brown and blue bias feeds to the terminal strips are you seeing 150K resistors to pin 5 on the outer
sockets and 2.2K resistors to the inner pin 5 ?
sockets and 2.2K resistors to the inner pin 5 ?
Boogiebabies said:
Sort of. That's one of the places where it starts to deviate from what I have on the schematic I got from Mesa. There ARE 150K resistors from the bias feeds to pin 5 on the outer sockets, and 2.2M from pin 5 to ground, but there are solid wires from the bias feeds to pin 5 on the inner sockets.
My schematic shows 220K from the feeds to pin 5 outer, 2.2K from the feeds to pin 5 inner. The screen resistors on the outer sockets are also different, they are 1K where the schematic says 2.7K. The screen resistors on the inner sockets are 470 like on the schematic.
The owner bought the amp new back in the 90s and swears no one has ever been inside it before me.
Boogiebabies
Well-known member
fjs1962 said:
Totally normal for an Early-Mid MK IVB. Mesa had a service bulletin that later added the 2.2K to the inner sockets.
The 1K outer and 470 Ohm inner screen grids are normal as well. I would not chase that schematic too far, Mesa is known
to be a little shady about offering correct ones.
Boogiebabies said:
I wonder if I should add the 2.2K resistors to the inner sockets while I'm in the amp?
Regardless of that, I'm at a loss as to why the voltae is droppin so much past the
221K resistors. The voltage looks ood all the way to them. I hate to start unsoldering things, what do you think the chances are that both resistors went bad? I think something else is going on, but I'm at a loss at this point. if this were an old Fender or Marshall it would be a lot easier to figure out.
Boogiebabies
Well-known member
If they were bad it could easily explain the issue. One good spike could have cooked them both enough to lose the resistance
to lower the voltages. At this point it would be easy enough to snip them out, pull the old leads out and surface mount two new
220K .5W's. 8 out of 10 times you try to be too clean or professional with these PCB's and they bight you in the *** by lifting the pads.
When I work on Boogie's I begin by telling myself not to be a Hero, just fix the **** thing.
to lower the voltages. At this point it would be easy enough to snip them out, pull the old leads out and surface mount two new
220K .5W's. 8 out of 10 times you try to be too clean or professional with these PCB's and they bight you in the *** by lifting the pads.
When I work on Boogie's I begin by telling myself not to be a Hero, just fix the **** thing.
Boogiebabies said:
Yeah, i think that's probably the next step. Everything else checks out. I guess it wouldn't be a bad idea to calculate how much the voltage should drop across 220K so I'll know what I'm shooting for.
Update:
OK, I decided to remove one of the 220K resistors from the circuit to test. Out of the circuit it reads 220K on my Fluke. But here's where it really gets weird, at least to me. With R322 out of the circuit, I still get -11.4V on pin 5 on that side of the circuit. Reading on both sides of coupling cap C244 (the only thing between the PI plate and pin 5 now that R322 is gone) I get 244V on the PI side (which sounds right) but -11.4V on the pin 5 side. Where is the negative voltage coming from?
Any help is much apreciated, I'm about ready to throw in the towel on this one!
OK, I decided to remove one of the 220K resistors from the circuit to test. Out of the circuit it reads 220K on my Fluke. But here's where it really gets weird, at least to me. With R322 out of the circuit, I still get -11.4V on pin 5 on that side of the circuit. Reading on both sides of coupling cap C244 (the only thing between the PI plate and pin 5 now that R322 is gone) I get 244V on the PI side (which sounds right) but -11.4V on the pin 5 side. Where is the negative voltage coming from?
Any help is much apreciated, I'm about ready to throw in the towel on this one!
Sucess! Even thought the 220K resistors tested good I decided to try a couple of new ones in the amp. I guess the old resistors were failing under load, because I now have -45V on pin 5 of all tubes! Fired it up with the tubes in and all is well.
