Mesa 2:90 Pop when going to standby

[AmpFix]

Like the title says, my Mesa 2:90 makes a moderately loud POP when I switch it to standby mode. The problem seems to have gotten worse lately.
What could be causing this?

[booj]

chopstick it with the amp out of standby. Connections are an issue with mesa amps. that's not a fix, but it's a start. rubber hammers are real nice too

[Enzo]

I don;t consider it a problem, it is not something I would "fix."

[AmpFix]

IMO, an amplifier should not pop when you take it off standby. I'll have a go at the connections and switches to see if I can find anything.

[rpbale]

Mesa would probably agree with you jcmjmp. My Mesa does not pop at all when moving in or out of stand by.

[AmpFix]

The problem was due to one of the 6L6GCs red plating. Swapped it with an older GE 6L6GC and all is good. I just have an odd tube in now and its probably not matched to the others already in there

Now that I think about it, a bias mod would be cool on these amps.

[AmpFix]

Looking at the 2:90 schematic, the bias mod doesn't seem to be as trivial as I originally thought.

http://www.tubefreak.com/290pwr.gif

the bias voltage is taken from the "+" side of C24 at the bias supply... but I wonder where to put the bias adjust.

It almost seems like it would be better to remove the 1k5 resistors (R237/238) going to pin 5 of the two A/B inner tubes and replace them with a 5k pot (see below for ref)

http://www.tubefreak.com/2901.gif

Would I have to do anything for the class A portion of the amp or will the bias auto adjust itself - It looks like maybe I'd have to replace the 220k resistors with a trim pot to adjust the tubes for class A

[defaced]

I would yank the 3k3 resistor in the bias supply and replace it with a 1.5k resistor in series with a 5k linear pot. This will give you some swing on either side of what the 3k3 is giving you now.

These are ball park values, the method is good, but the values may need some tweaking once you get in there.

[AmpFix]

Yeah, I thought of that (changing the 3k3) but that will affect the bias for all tubes.

Since this amp is simul-class with a class A and Class AB section, it would be nice to be able to tweak both sides of the amp independently, which is why I was thinking of changing the values of the other resistors - but I'm not sure if that would work.

It would be a good idea to change the 3k3 resistor so I can get more range overall and then tweak the values for each tube/section individually.

[defaced]

There's a member, Paul B maybe, had a thread about dual bias supplies that's worth digging up. You can extend this for more bias (3, 4, etc) if you'd like with little effort. Basically you'll just add parallel legs where the 3k3 resistor is. You will need to also modify the power tubes to have their own coupling caps though, otherwise you'll get the individual bias supplies interacting because of the shared coupling caps. I don't know enough about this amp to know if that would disrupt the simulclass operation.

[Enzo]

The bias is not taken from the + end of the filter caps. The schematic simply has the caps drawn in backwards.

Those 1.5k resistors to the grids of your power tubes are their grid stopper resistors. Changing them will not change the bias voltage.

The only real way to drop the bias voltage - and I assume you don;t want to increase it - is to make a voltage divider. the transformer winding here is designed to provide the voltage they wanted right from the start, no divider needed. But you want it adjustable which means voltage changing, which in turn means at least a divider unless you want to build a variable regulated supply.

The outer tubes in fact have their own dividers off the main bias. The 220k over 2.2M resistors drop the voltage by 1/10 so the -52v from the supply becomes -47 at those two grids.

Changing the 3.3k to a pot won;t get you where you are going either, because with no current flowing, no voltage is dropped. The 3.3k is fine, but you would have to add a resistor to ground from the righthand end of it. That resistor would be variable for adjusting.

You want two separate bias adjustings? Fine. Connect another diode to the transformer AC, and duplicate the existing circuit. COuple resistors and a cap or two, not bad.

Good point about the extra coupling caps.

[AmpFix]

My worry was with the Class A section. I'm not too familiar with how these things are biased but it does look like the Class A section is just biased warmer and is NOT cathode biased (auto-biased). That means that adjusting the bias will affect the Class AB section, as well as the A section.

My understanding is that when adjusting the bias value, I'd be adjusting it for the Class A & Class AB sections at the same time, meaning that I would need a matched quartet.

I'm assuming that the class A section is the one that is biased at 52v while the AB is biased at 47v? I guess that I could live with having control over only the right vs left side of the amp as opposed to the Class A and class AB side of each amp.

Where would I need to add the coupling caps if I were to add a duplicate bias circuit for each side (left/right) of the amp?

[defaced]

http://www.schematicheaven.com/boogi...boogie_290.pdf

You will need two more of C22 and C21, so a total of four 0.1u caps. If use independent bias, you will need to give each tube it's own coupling cap, bias feed resistor, and grid stopper. See the attached schematic for a diagram. If you just add the coupling caps and bias feed resistors (each tube already has it's own grid stoppers), I think everything will work as it's designed but give you side-to-side adjustability.

I've also included the schematic from Paul's thread I mentioned earlier about dual bias supplies. Enzo is correct, you need a path to ground to allow current to be drawn to make an adjustable supply, which is an important detail I had forgotten earlier. The values aren't necessary important, it's the way everything's hooked up that is.

[jmaf]

The bias is not taken from the + end of the filter caps. The schematic simply has the caps drawn in backwards.

I've seen this more than once. The official Laney LC-15R schematic that is available online has the BIAS caps inverted as well.

I wrote Laney and they were very kind to reply within less than 24 hours acknowledging the error and sending me a revised LC-15R schematic.

[AmpFix]

Thanks for that, Defaced.
I think that as a first step, I'll will try to add an indenpendant adjustable bias for the Class A and Class AB sections and when I have that working, I'll attempt to separate it out for both left/right channels