Mesa Boogie MkII

[dirkwright]

I need some advice on a Mesa Boogie MkII (I think it's a B). What's in the amp doesn't really match any schematic that I have been able to find. It does have a lot in common with the attached schematic though. Anyway, the problem I have right now is that the screen voltage on the output tubes (there are only 2, not 4) is way over the maximum rating for the 6L6GC, which is 450 volts (screen to grid). The screen to grid voltage in the amp is 512 volts. Screen to ground is still over, at some 475 volts, the same as the plates. One thing I know is that the line voltage coming out of the wall has changed since this amp was built in 1970. Back then it really was only about 110 VAC, whereas now 125VAC is more common, so all of the voltages in the amp are above what is shown in the schematic. (I already lowered the voltage on the preamp stages).

So, I need some ideas on how to reduce the voltage on the screens without affecting the preamp voltages. I thought about using a voltage divider (which would be high impedance), or taking from the half plate voltage (since the first filter stage in the power supply has stacked capacitors - voltage there is about 245 but that may be too low for the output tubes). Any ideas are appreciated, as long as they not very complicated. I prefer the most simple solution that works well, thanks.

 

[gingertube]

This is not uncommon in many Fender, Mesa Boogie and other amps. The best thing to do is to simply replace those 470 Ohms screen resistors with 1K 5 Watt. That will give a little additional protection to the screens.

I have a MKII C 60/100W on my bench right now for a new set of output tubes and this mod is what Im doing to it before fitting the new tubes.

The screen stress is quite evident in the old tubes I took out. Screens grids should be aligned with the control grid. When the screen grids over dissipate they get really hot and tend to sag. This is quite evident when putting the old tubes on teh tube tester (Avo MKII). To run the test you dial up a bias voltage to get a particular anode current, on these old tubes with stressed screens the anode current just will not settle down but fluctuates around a mean value. The size of the fluctuation is generally a guide to how badly the tube screens have been stressed. This fluctuation in the anode current at a fixed bias voltage is one of the things I look for in deciding whether to replace output tubes or not and which of teh ones I replace can be kept as "emergency" spares and which go straight in the bin.

Cheers,
Ian

 

[dirkwright]

Thanks! How did you decide on 1k ohms?

 

[gingertube]

I did what most do - copied some of the amp techs who have been around for many years.
The change of the screen resistor from 470 Ohms to 1K is something "old" amp techs have been doing to every Fender which comes across their service bench for many years. Mesa output stage is pretty much identical with a Fender (coz. that is where Randall copied it from) so treat it the same way.

If you are looking for a specific source then Kevin O'Connor (London Power) is one of the amp "gurus" who lists this screen resistor mod as "essential".

Cheers,
Ian

 

[dirkwright]

OK, thanks. I happened to have some 10k 3W resistors lying around and tried them. It helped lower the voltage a little, but not enough. So, I also happened to have some 20k 3W resistors also, and put them in. The screen voltage is still high @ 462 V to ground (480 something to grid), but at only 12 V over I think it's better than nothing. There isn't much screen current so it's difficult to use a screen resistor to lower the screen voltage. I measured a 12 V drop across the 20k resistors.

 

[gingertube]

dirkwright,
No need to obsess over voltages in any tube circuit, it is power dissipation (voltage x current) that you need to manage.

There isn't much screen current at idle (as you state) but it goes up significantly when pushing signal current. The screen resistor acts to reduce voltage a little, but reduces screen current more significantly, thus reducing screen dissipation which is what you really need to control. Being a bit overvoltage is not a problem.

I would suspect that 10K and 20K would change the sound quite a bit and probably would not recommend larger than 2K2. Some guys use up to 10K for EL84 and 1K is the absolute minimum for EL34 (More fragile screens).

What most people use for 6L6 is 1K.

By all means try some larger values if you like but listen carefully to make sure you are not castrating the amp.

There was an interesting thread on another forum about voltage dividers for screens and their effect etc:
For your possible interest:
http://ampgarage.com/forum/viewtopic.php?t=18855&highlight=screens+voltage+divider


Cheers,
Ian

 

[dirkwright]

OK thanks. I didn't know all of that, obviously. I never learned a lot about pentode output stages since back in the day I was always using the ultralinear circuit. Oh well, I can take those resistors out, no problem.