Mesa Boogie MKIV bias issue

[Gregg]

Enzo is right. The two simul class tubes (regardless of their type) should have higher (less negative) bias voltage so check voltages at pin 5 of all tubes and report back please.

[Ampzone]

Sorry for delay, got hit with the flu! I have voltage readings but note I changed the one one .1uf cap a few days back and it was still reading -50v and -52v with b+ on and -52v ~ -52v on idle. So today fired it up and got the following readings v6-v9.
-48 -52 -52 -48. Ok so no idea why it evened out with B+ on, strange, but if this is the correct reading why can it not be just -52 on each input grid? I'm replacing with a matched set of 6l6GC's so one needs even distribution.

The amp is NOT set to "simuclass". But to class A.

[The Dude]

See Enzo's post #14. The amp (if g1's schematic is the correct one) has different grid setup for V6 & V7 than is the configuration for V8 & V9. Presumably so that one can run a pair of EL34's and a pair of 6L6's. The simul-class switch only grounds the cathodes of the 6L6's. It's not going to change the grid voltages. In other words, IMO, the amp is working as it should and is designed for 2) 6L6's and 2) EL34's.

[g1]

That is the design. It can be run with four 6L6 but the bias will be slightly different for the inner and outer pairs. This is normal for this model. But note that the screen resistors are also different for the inner and outer pairs.

[Leo_Gnardo]

See Enzo's post #14. The amp (if g1's schematic is the correct one) has different grid setup for V6 & V7 than is the configuration for V8 & V9. Presumably so that one can run a pair of EL34's and a pair of 6L6's. The simul-class switch only grounds the cathodes of the 6L6's. It's not going to change the grid voltages. In other words, IMO, the amp is working as it should and is designed for 2) 6L6's and 2) EL34's.

Many times I find 6L6's in the "outer" pair of output tube sockets in simulclass Mesa's. Sometimes the customer claims they came from the factory that way. The grid bias voltage is set by a voltage divider, often the "lower" resistor, between grid and ground is 820Kohm. Of course the plate current runs sky high, 60 to 85 milliamps is not unusual. You can alter the value of that resistor to reset the bias current to something more reasonable. Simulclass amps usually leave this shop with 2M2 resistors in that position, 6L6GC tubes in the "outer" pair and nary a customer complaint.

[The Dude]

Yep. It would be easy enough to mod the amp for 4 of either tube with a few resistor changes if that's what a person wanted to do.

[bob p]

Many times I find 6L6's in the "outer" pair of output tube sockets in simulclass Mesa's. Sometimes the customer claims they came from the factory that way.

Your customer probably knows that they did come from the factory that way.

I just pulled my Mk IV B out of storage -- where it's been neglected for 15 years. It still has the original Mesa STR 6L6 in all 4 sockets, the way it came from the factory. It still has it's original power tubes because I use it for it's preamp and it never gets played very loud.

The difference in bias between the inner and outer tube pairs is intentional. That's the point of the whole "simulclass" thing. I wouldn't go trying to "fix" things to make the inner and outer tube bias votlages equal or you'll bugger up the amp.

In case anyone is not familiar: There's an operating class selector switch on the back panel to select between "Class A" and "Simul-Class". In the "A" setting the inner pair cathodes are lifted so that only the outer pair is enabled. That's supposed to provide 30W in triode mode, 50W in pentode mode. In the "Simul" position both pairs are enabled and the amp provides about 70 (triode) or 85 (pentode) watts.

The tube swapping thing confuses a lot of people.

The Mk IV shipped from the factory with 4x6L6. That's the standard config. 5881 are also an option. Like Enz said, you have the option to put EL34 in the outer sockets with 6L6 in the middle, but the Mk IV never shipped that way. The final option is to use a quad of 6V6 with the low-voltage transformer tap and Simul-Class mode and switching to the low Z OT tap.

IME the whole tube rolling thing in the output section is sort of an illusory gain, at least for me -- as the amp is really designed to be more of a preamp gain flexibility animal than anything else. I never found output tube swaps to be all that worthwhile as most of the tone shaping happens in the front end and the back end is biased cold to preserve the tone of the amp as it gets louder.

that's my two cents worth.

[bob p]

My schematic is from a different source, but it's the same circuit, with the 2M2 grid leaks and the 1k screens on the outside pair.

Here's the Mark IV Owner's Manual. See pages 12-13.

Mark 4 Owners Manual.pdf

[Gregg]

I never found output tube swaps to be all that worthwhile as most of the tone shaping happens in the front end and the back end is biased cold to preserve the tone of the amp as it gets louder.

There's an audible difference in tone from the power amp when switching the outer pair in triode mode. Some people like the that "rounder" tone some don't. Also when using EL34s there bear in mind that some types will run hotter than others. For example if you put regular JJ EL34s they will run colder compared to JJ EL34 II.

[bob p]

There's an audible difference in tone from the power amp when switching the outer pair in triode mode. Some people like the that "rounder" tone some don't. Also when using EL34s there bear in mind that some types will run hotter than others. For example if you put regular JJ EL34s they will run colder compared to JJ EL34 II.

I'm not kidding when I say that I put my Mk IV away 15 years ago and haven't powered it up until just recently. Having only powered it up a few times in the past month, now I'm remembering why I set it aside. To me it's more of a studio amp than a live performance amp. It produces some really great tones, but it's a convoluted pain in the ass to get there. Dialing in the amp requires a lot of tedious knob tweaking, and until you get get it dialed in, the amp really doesn't sound all that great. A lot of the time it can sound really BAD until you get it dialed in. There are a lot of other good sounding amps that work a lot better as far as "plug-n-play" is concerned. With almost any other amp I think it's easier to find a good tone quickly than it is with the Mk IV. IME it takes a long time to find the sweet spots on the Mk IV, but they're worth it when you finally find them.

I'm one of those guys who doesn't like how pentodes sound in triode mode. I think that all amps that have triode/pentode switching don't sound that great in triode mode, so I don't use it. My comments were intended to be about swapping power tube types, not operating modes with the same power tube. The way I use the amp it's never been worth power tube swapping, for a couple of reasons:

First, you have to play the amp damned loud to hear the difference in output tube clipping -- as in you have to push an 85W amp that's intentionally biased cold to the point of clipping in simul-class mode. That's pretty loud.

Second, the amp is not particularly useful outside of a home or studio setting where you have a lot of time to get it dialed in. The MK IV takes a lot of tweaking to produce great sounds, and it's very easy to mis-tweak the knobs with the result being an amp that sounds like ass. Although i really like using the amp for dialing in different preamp sounds, I won't consider gigging with it. All of those knobs are a double-edged sword -- it's too easy to make the amp sound bad in a live setting, and nobody wants to listen to you tweak the knobs to get it dialed-in.

IMO those First and Second conditions are sort of mutually exclusive, so I never use the Mk IV in a setting where it gets turned up loud enough to really enjoy power tube clipping. I tend to use the front end of the amp for tone shaping more than the back end..

I think that "not-so-friendly-for-live-use" problem is the reason that Mesa discontinued the Mk IV and kept the Rectifier series in production. With the Rectifiers it's pretty easy to dial in a good tone and pretty hard to dial in something really bad. with the Mk IV I think it's too easy to dial in a bad tone, so the amp was never quite as popular as the Rectos. The Rectos sold a lot better and the Mk IV got discontinued.

Because the Mk IV has non-adjustable fixed bias, you really do have to select tubes that meet the bias spec. A lot of people put on their cheepskate hats and rebelled against the concept of buying current-rated tubes from Mesa, and they preferred to just pop-in whatever tubes they had without paying close attention to the bias spec. If someone does that, they WILL encounter tonal changes. OTOH if you follow the manufacturer's recommendations and go with the Mesa-spec tubes then the bias will be cold enough that the tone of the amp will be preserved as the volume gets turned up. That's part of the Mk IV design.

Of course, we can't stop anyone from not following the directions and doing stupid things... I think that's where most of the tonal variation comes from, from not being vigilant about proper tube selection.

[Ampzone]

Thank you guys a lot of info here, bottom line I just want the amp to work safely, and not burn up the outer tubes and screens as it did. I put new 1k screens on the ends. So should I order a set of El34's for the ends or 6l6GC to match the remaining 6l6GC in the middle? And will -48 and -52 be a safe range?
And what should I select "class A" or "simuclass" on the switch?

That's all I need to know?

[Leo_Gnardo]

And what should I select "class A" or "simuclass" on the switch?

Depending on how loud you want to be. In "class A" the inner pair of tubes add nothing to your power, and the outer pair produce 15 watts more or less. If that's enough for everything you do, you could remove the inner pair entirely. EL34 or 6L6 for the outer pair? You really should experiment with both, find out which suits your ear better. I find it's hard to tell the difference at low volumes, but there definitely is a difference when you push the amp into clipping. In simulclass generally you'll have over 50W of clean power (regardless that the amp claims 100W - trust me it doesn't go there). Again you should audition 6L6 vs EL34 to suit your own tone needs. FWIW several of my customers have simulclass/class A Mesa/Boogies, most of them park the switch on class A and leave it there. Which flummoxes me a bit. Such a heavy amp, big transformers, huge EV speaker, for what? 15 watts? Well these days so many of us drive around in big suburban assault vehicles, same thing...

[Ampzone]

Depending on how loud you want to be. In "class A" the inner pair of tubes add nothing to your power, and the outer pair produce 15 watts more or less. If that's enough for everything you do, you could remove the inner pair entirely. EL34 or 6L6 for the outer pair? You really should experiment with both, find out which suits your ear better. I find it's hard to tell the difference at low volumes, but there definitely is a difference when you push the amp into clipping. In simulclass generally you'll have over 50W of clean power (regardless that the amp claims 100W - trust me it doesn't go there). Again you should audition 6L6 vs EL34 to suit your own tone needs. FWIW several of my customers have simulclass/class A Mesa/Boogies, most of them park the switch on class A and leave it there. Which flummoxes me a bit. Such a heavy amp, big transformers, huge EV speaker, for what? 15 watts? Well these days so many of us drive around in big suburban assault vehicles, same thing...

Its not my amp. Just trying to make it work safely and this whole vary bias is odd. What is the safest tube arrangement given this amps circuit El34/6l6 or all 6l6? and what setting "class a" or not?

[bob p]

I tested mine for output with a distortion analyzer a long time ago. I was able to get 85W into a resistive load with 5% THD, but I had to run in the whole quartet in pentode mode at full power to get there. (Class switch in the Simulclass position). That Tweed Power (low voltage), Class-A, and triode mode really does take the power down to about 15W if you use 6V6. But that's one of those cases where it's easy to make the amp sound like ASS with some of those ridiculous settings. Part of the problem with that amp is that Mesa used to market some really stupid switchable features. They wanted to market the amp as a jack of all trades. Doing that made the amp sound great on paper, not so good in real life. This amp has a bit of a learning curve. It takes some experimentation to determine which of those switch settings are just worthless and need to be avoided. If you want a 15-20W amp then you really want something else, like an AC15 or a Deluxe, not a big behemoth with heavy iron and 21 knobs and as many switch positions.

I tried playing my Mk IV with Celestions last night instead of the EV. Didn't like it. There's just no way to get full power loud and clean with a 2x12 V30 celestion cab. Whenever I tried to get that earsplitting screamy SRV volume that the EV do so well, the V30 just went to hell long before I got there. I really like the amp better with the EV. I can't say that I've ever liked that amp with Celestions. I think there's a thread around here somewhere from the time that I killed a pair of greenbacks with it.

Ampzone, what makes the whole bias thing confusing is that you can't just order any pair of 6L6 or EL34 and drop them in there because the bias is not adjustable. Some tubes require a hotter bias than others, and what you really need is a set of tubes that will be on the cold side at the operating voltages you measured. If you can do that, you'll have better luck keeping the tone of your preamp presets so that the amp's output sounds consistent as you go up in volume. If you end up using power tubes that bias too hot, the tone of the amp changes as you raise the master and all of your preamp presets go out the window. Then you're left on-stage with bad tone and not knowing which knobs need to be fine-tuned to get you back where you wanted to be. IMO not the best gigging amp.

This is one amp where I think it's worth rolling several sets of power tubes until you find the ones that allow the amp to keep it's character without changing as you bring it up to stage volumes. If you don't have a pile of tubes to test then it'd be worth it to order the proper current spec tubes from Mesa, or another supplier who knows how to pick tubes that work with Mesas. IMO this is one amp where the manufacturer supplied tubes are worth the effort.

At one time I had given some thought to rebuilding the amp with two bias circuits, so that I could dial in the inner and outer pairs manually, but I hate working inside of that amp so much that I decided that it wasn't worth the effort.

[Ampzone]

I tested mine for output with a distortion analyzer a long time ago. I was able to get 85W into a resistive load with 5% THD, but I had to run in the whole quartet in pentode mode at full power to get there. (Class switch in the Simulclass position). That Tweed Power (low voltage), Class-A, and triode mode really does take the power down to about 15W if you use 6V6. But that's one of those cases where it's easy to make the amp sound like ASS with some of those ridiculous settings. Part of the problem with that amp is that Mesa used to market some really stupid switchable features. They wanted to market the amp as a jack of all trades. Doing that made the amp sound great on paper, not so good in real life. This amp has a bit of a learning curve. It takes some experimentation to determine which of those switch settings are just worthless and need to be avoided. If you want a 15-20W amp then you really want something else, like an AC15 or a Deluxe, not a big behemoth with heavy iron and 21 knobs and as many switch positions.

I tried playing my Mk IV with Celestions last night instead of the EV. Didn't like it. There's just no way to get full power loud and clean with a 2x12 V30 celestion cab. Whenever I tried to get that earsplitting screamy SRV volume that the EV do so well, the V30 just went to hell long before I got there. I really like the amp better with the EV. I can't say that I've ever liked that amp with Celestions. I think there's a thread around here somewhere from the time that I killed a pair of greenbacks with it.

Ampzone, what makes the whole bias thing confusing is that you can't just order any pair of 6L6 or EL34 and drop them in there because the bias is not adjustable. Some tubes require a hotter bias than others, and what you really need is a set of tubes that will be on the cold side at the operating voltages you measured. If you can do that, you'll have better luck keeping the tone of your preamp presets so that the amp's output sounds consistent as you go up in volume. If you end up using power tubes that bias too hot, the tone of the amp changes as you raise the master and all of your preamp presets go out the window. Then you're left on-stage with bad tone and not knowing which knobs need to be fine-tuned to get you back where you wanted to be. IMO not the best gigging amp.

This is one amp where I think it's worth rolling several sets of power tubes until you find the ones that allow the amp to keep it's character without changing as you bring it up to stage volumes. If you don't have a pile of tubes to test then it'd be worth it to order the proper current spec tubes from Mesa, or another supplier who knows how to pick tubes that work with Mesas. IMO this is one amp where the manufacturer supplied tubes are worth the effort.

At one time I had given some thought to rebuilding the amp with two bias circuits, so that I could dial in the inner and outer pairs manually, but I hate working inside of that amp so much that I decided that it wasn't worth the effort.

Its not my amp, I just need to get it safely operational. I have a new pair of mathed JJ 6l6GC tubes now and can order another set of Mesa. So which position would be THE SAFEST for this JJ pair, center or outside since I cannot test or adjust the bias? (I assume the inside -52 with higher neg voltage)