Early Mark IIB coliseum serials

  • Thread starter Thread starter dr teeth
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I've seen the opinion that 7581A's passed stricter QA therefore capable of 35w rating and otherwise went into the 387 bin. I highly doubt mesa would have paid up for the 7581A whether it's the same tube sorted out in QA or slightly beefed up tube. And Mesa didn't use the hot ones. I can't remember if there is more mica in the 7581A but they should be lower noise for sensitive military comm, etc.
It was the military contracts Phillips bought into in buying Sylvania.
If the 415's are and/or the pre 387 6L6GC that could explain the single / double getters. But that wouldn't explain if users here can't hear a difference why Watford states the 387 has better tonal response than its predecessor. It's the ears, not the nose that always knows.
 
For all you that send old marks to petaluma, do you know the bias point on the 6L6 sockets from the factory is set at? I can't remember the mesa rainbow mA translations but off the top of my head I'm guessing the tube range from mid teens to mid 30's. Correct me if I'm wrong but the bias circuit is meant for much colder bias on the tubes than 70%
 
The rainbow from coldest to hottest: Red > Yellow > Green > Gray > Blue > White. I think your recall on ranges is right, from mid-teens to ~ 40mA. I stick with STR415’s (or JAN 7581A’s) in my Mark II’s, and have seen tubes anywhere between 24-42mA @ idle. Generally, I try to select tubes around 39mA +/- 3mA in pentode amps. In SimulClass, those same tubes can get upwards of 70mA in the outer sockets, which is way hot, while the inner sockets will run the same as in a pentode amp.
 
In that case the mid-point would be in the upper 20's. Do you think the factory bias target is set at the mid-point of the range or the high end? When mb takes all these old Mark's in for service I assume he is checking bias and adjusting (with resistor) to a target that "covers" the range of the rainbow.
 
Yes, I would suspect that target around 25-30. I’ve never asked MB about it, though.
 
Jeremy, You still have a C+ in the land of towering magical redwoods. There's 2 questions in the queue:
A} What does mb target as the bias point to cover the range of mesa labelled tubes?
B) Can the STR-430 handle coli plate volts?
 
It depends on the amp. Rectos are all based the same, but Mark II, III and IV are biased differently- progressively colder. Also the newer amps are ice cold. I've documented all of them. So, you have to learn what colors are right for what amp.

Interesting note though- I have this cold set of JJ 5881 now. In the Recto on 6L6 mode they're running about 14 ma. In EL34 tube rect mode they're running about 34. I dare say, the tone & harmonics are better at 14. Fast forward to the Badlander- the one I had shipped new at around 18 mA with GREY. I spoke to the engineer at Mesa about this and he said they did that on purpose because that's where the best tone is.

Matt, I'll ask.
 
Cool- I'd be interested in II-III mostly. So if they got progressively colder would the IIA or the IIB be voiced for hottest tubes? Is that why the power section of the IIB is more beef than the IIC+?
The IIA is the hole in my Marks and I've never played one or the OG but I'll finish up the OG project one of these days.
 
Ok I asked Mike B., here's what he had to say-

Q- What do you target as the bias point to cover the range of mesa labelled tubes? Does it vary by amp model or fairly universal?
A- "Bias voltage is set to achieve a correct amount of dissipation when the amp idles. You had a lot more leeway with the tubes of the old being more robust. The plate voltage is what you can't change w/o a different transformer, so you go from there."

Q- Can the STR-430 handle the 105 & coli plate volts?
A- "The 430 tubes we had 10 yrs ago did not seem robust enough for those big amps. And of course no 6L6GC is rated for over 500 volt, that I have seen or by the book, except the big JJ and the big bottle TAD "STR" 6l6 claim 550 volts plate rating if I recall."
 
Thanks Jeremy.

In reverse Q order,
The interesting thing is the marketing iirc for Sovtek's 6L6 WXT+ states 20% more power handling than their 5881 WXT which is the STR-425 that mb gives blessing to for high plate V's. I know power doesn't = volts and if it can handle 20% more power but the plate volts handling is lower it must let a hell of a lot of current through. Or the capacity to. If the marketing is true. Or maybe the 5881WXT is under-rated and it's that simple.

As for in circuit bias target mb managed to tell us everything and nothing all in a short sentence. A follow up Q would be awesome (to clarify a hypothetical number) but here is how I interpret the answer:

In ideal terms according to Weber's handy bias calculator the 6L6 in an AB class circuit is at 70% as follows:
plate V idle mA
550V 24.1mA
500V 26.6 mA
450V 29.5 mA

Who knows how much variance from tranny to tranny and that may be all that matters but the circuit itself is full of resistors, caps and inductors (relays, diodes, transistors, etc) at 10 and 20% tolerances and the reason no 2 amps are the same in low production hand made runs. After putting the amp together someone has to check the bias circuit and while I can't say I look at the bias R to see if they are the same or different in my amps the bias circuit is the final tuning of the amp. If the same value R is used in production for final bias then inherently each circuit will be biased a little different but I doubt that is the way it's done.

With my red stripe at 460 Vplate the point is 29 mA. With Johnny B Rotten's early black stripe with the 105 at 500 it's 26.6mA. Now if Mesa uses something other than the 70% standard the numbers would change but not the concept.
 
dr teeth said:
Thanks Jeremy.

In reverse Q order,
The interesting thing is the marketing iirc for Sovtek's 6L6 WXT+ states 20% more power handling than their 5881 WXT which is the STR-425 that mb gives blessing to for high plate V's. I know power doesn't = volts and if it can handle 20% more power but the plate volts handling is lower it must let a hell of a lot of current through. Or the capacity to. If the marketing is true. Or maybe the 5881WXT is under-rated and it's that simple.

As for in circuit bias target mb managed to tell us everything and nothing all in a short sentence. A follow up Q would be awesome (to clarify a hypothetical number) but here is how I interpret the answer:

In ideal terms according to Weber's handy bias calculator the 6L6 in an AB class circuit is at 70% as follows:
plate V idle mA
550V 24.1mA
500V 26.6 mA
450V 29.5 mA

Who knows how much variance from tranny to tranny and that may be all that matters but the circuit itself is full of resistors, caps and inductors (relays, diodes, transistors, etc) at 10 and 20% tolerances and the reason no 2 amps are the same in low production hand made runs. After putting the amp together someone has to check the bias circuit and while I can't say I look at the bias R to see if they are the same or different in my amps the bias circuit is the final tuning of the amp. If the same value R is used in production for final bias then inherently each circuit will be biased a little different but I doubt that is the way it's done.

With my red stripe at 460 Vplate the point is 29 mA. With Johnny B Rotten's early black stripe with the 105 at 500 it's 26.6mA. Now if Mesa uses something other than the 70% standard the numbers would change but not the concept.
Click to expand...

If running 6L6GC, you would be looking for 42mA for 70% @ 500PV, and 46.67mA @ 450PV. I ”generally” target my tube selections to run 39-43 mA, using either 30W 6L6GC or 35W 7581A’s.
 
Thanks for catching that so quickly IIB. I used 6L6 (19w) literally in the calculator, not 6L6GC.

What IIB said for mA per 6L6GC plate V!
 
Thanks for the technical discussion fellas. On a more basic note, can anyone help me lay to rest the question of blue vs green stripe? I'm aware of the pentode wiring in the green, but otherwise are they identical? Lots of varying info out there like less gain on green, reworked lead channel on green, etc.
 
I'd have to see if I have pics of my former green stripe head guts. I don't remember the pre amp board. The blue stripe DRG I have has CP1G with a 1984 copyright. A purple stripe I had was CP1F also 1984 copyright.
The DRG blue stripes predate the green stripe that took over as D "option" only, so it may be necessary to distinguish between blue stripe and later S, H, K blue stripe/ green stripe D idk. The S, H or K blue stripe could predate the green stripe or could have been made along side the last green stripe made. As a result the blues had a long run for a company known to make small changes at any point.

Come to think of it there was a bridge rectifier near the lead master pot iirc so the board layout of the green definitely changed. So one question is did the factory use a different blue pcb (staying with the CP1G) when making the green (CP1H???).

Someone out here may know specific intentional changes though.

To me the power section (pentode on all 4) was the big difference and tho I regret it I sold the green stripe because my IVA covers basically the same ground as the green stripe.
 
dr teeth said:
I'd have to see if I have pics of my former green stripe head guts. I don't remember the pre amp board. The blue stripe DRG I have has CP1G with a 1984 copyright. A purple stripe I had was CP1F also 1984 copyright.
The DRG blue stripes predate the green stripe that took over as D "option" only, so it may be necessary to distinguish between blue stripe and later S, H, K blue stripe/ green stripe D idk. The S, H or K blue stripe could predate the green stripe or could have been made along side the last green stripe made. As a result the blues had a long run for a company known to make small changes at any point.

Come to think of it there was a bridge rectifier near the lead master pot iirc so the board layout of the green definitely changed. So one question is did the factory use a different blue pcb (staying with the CP1G) when making the green (CP1H???).

Someone out here may know specific intentional changes though.

To me the power section (pentode on all 4) was the big difference and tho I regret it I sold the green stripe because my IVA covers basically the same ground as the green stripe.
Click to expand...
I've heard others mention the similarities between the green stripe and IVa as well. Been thinking about picking one up to compare to my IVb but current prices aren't very motivating. Some people really hype the IVa but I'd bet it isn't worlds different from the IVb, which has grown on me despite being a bit darker and more compressed. Mike B actually just finished servicing it, stoked!

I have a pair of green stripes that to my surprise sound fairly different in the mids and lows despite extensive tube rolling. They compliment each other really well so we'll see if they both stay...hoping to ++ one of them if possible. This IIb discussion has me on the lookout for sure!
 
glpg80 said:
I’m a little confused though. In post 46 you mention you’d take a loop modded IIB over a III but then mentioned you regretted loop modding a IIB in post 52?

For 3500 it better sound righteous. I have no use for the tubes or the cab/EV and would be selling them both in the classifieds to recoup some of the cost. I’d likely get a head cabinet to convert.

Do they play nicely with greenbacks? How about taking pedals well? I’ve read some marks are picky with both?

I’ll definitely take a Les Paul, an SD-1, and 3500 cash. If I could sell the power tubes for $700 and the cabinet for $300 Id be happy.

My question is why is this model fetching so much more than other marks I see online? Is it the features? Just seems steep…I’m hoping it gives up the goods and has been refreshed but honestly given it’s condition it likely has not been seen by Mike B at Mesa. The damage on the front was from the original owners dog.
Click to expand...
This is an example of how I would figure out if I like a IIB:

https://www.musicgoround.com/produc...sa-boogie-mark-iib-1x12-combo-tube-guitar-amp
By the time you parted out what looks like a protected Black Shadow 12L, the cab you don't need and the road case you'd be around 1k after tax with a nice looking IIB. My eyes aren't good enough but the brown based tubes has me wondering if it's loaded with winged =C=. If that was the case and the tubes were good you'd have an amp for $400. Or just keep it all together and crank the shit out of it.
 
Conversely I always meant to pick up the IVb. But for the same price reason I don't feel compelled.
No good reason to grab one. Just because.
I didn't know there was a IVa fad, justified or not.
 
dr teeth said:
This is an example of how I would figure out if I like a IIB:

https://www.musicgoround.com/produc...sa-boogie-mark-iib-1x12-combo-tube-guitar-amp
By the time you parted out what looks like a protected Black Shadow 12L, the cab you don't need and the road case you'd be around 1k after tax with a nice looking IIB. My eyes aren't good enough but the brown based tubes has me wondering if it's loaded with winged =C=. If that was the case and the tubes were good you'd have an amp for $400. Or just keep it all together and crank the shit out of it.
Click to expand...
That's a really clean looking example! To your point, if you don't like the combo form, nor the EVM 12L, you could part out those and recoup part of the cost. Nice that it comes with the case, too. They do look like <<C>>'s in there. Couldn't detect the serial, but, it looks like an X101 tranny without the voltage selector switch (I could be wrong, though) - that leads me to think it might be a very late RP9C board. Mesa seemed to be using a lot of X101's without the selector switch in late IIB and early IIC production - they had a bunch of X101's laying around at that time.
 
IIBornotIIB said:
That's a really clean looking example! To your point, if you don't like the combo form, nor the EVM 12L, you could part out those and recoup part of the cost. Nice that it comes with the case, too. They do look like <<C>>'s in there. Couldn't detect the serial, but, it looks like an X101 tranny without the voltage selector switch (I could be wrong, though) - that leads me to think it might be a very late RP9C board. Mesa seemed to be using a lot of X101's without the selector switch in late IIB and early IIC production - they had a bunch of X101's laying around at that time.
Click to expand...
Well I called the shop and they weren't offering shipping on it yet. I'm a 2 hour drive away so I pulled the trigger, will pick it up Friday! They said I could "return it" on the spot for a refund if I didn't like it.

I'm not nearly as tuned in to these earlier Marks, but it sounds like from the pics you can confirm it's a IIB? Anything else I should scrutinize when I go to pick it up? Thanks for the heads up dr teeth!!

Edit - shop says the serial# is 10011
 
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Yes, that's def a IIB. 10011 probably built sometime around Sep-Oct 1982, with RP9C board. Just give is a through going through and play test. Not sure if they'd let you pull the chassis for a visual - I've had some places say OK, and other NO. Probably a good idea to anticipate a visit to Petaluma for a service to get it set for the next few decades.
 
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