Bias sticker on my Mark III chassis?

[Jeff R]

I picked up what I think is my sixth-over-the-years Boogie Mark III a couple weeks ago. Green stripe, January 1990. As usual, sounds stellar. Since the shop bought this one as a test-drive platform for clients, it's a keeper

I know Marks are fixed bias from the factory, I assume so they can keep you in their $$$ loop by selling Boogie-stamped tubes lol. This head has this sticker on the back of the chassis (attached photo because my Photobucket is still acting up).

The power tubes in it right now are non-Boogie-stamped Sovteks EL34s and 6L6s (it's Simulclass, chassis date is penned by "Roth" 1-1-90). Does that sticker mean the amp may have had a bias setter/whatever added to it?  I don't know shit about amps really. I see no added exterior controls or flathead screwdriver twisty thingies. I don't know what to potentially look for inside. I know all the transformers are stamped Boogie and all the blue (I assume filter caps) tubular thingies with wires coming from them inside the chassis are Boogie stamped too.

I'm mainly trying to figure out if I can stick and go with Boogie glass or some other reputable glass in it, or if I have to get someone to bias the head when the retube occurs, or if I need to tell the tube peddler what that sticker says. I don't even know what the hell a Bias King is or does. I do know the last time I stuck non-Boogie tubes in a Mark (V), a $2 resistor failed and cost me a helluva lot more in service from Petaluma and shipping to and fro. Ain't going there again - I'd rather just ID and use safe glass, whatever that safe glass may be.

I have some photos of the guts if anyone would like to see or if anyone thinks they can determine what that sticker means to me. I can pop the chassis again for more detailed photos if need be.

[Thundernotes]

A Bias King is simply a plug which you temporarily install between a power tube to check the bias (idle current) of a power tube.  It has a precision resistor in the cathode leg to allow measurement of the current.   It's hard to say if any adjustment was made to the amp itself in terms of modifications.  Usually, within a certain range, any tube of the same model will work fine.  Boogie likes to spec their stuff out so as to encourage you to buy their replacement tubes.  

If you want to snap a pic, I can take a look inside.

[Jeff R]

Try these on for now ... I can get more close-ups of more specific areas ...

 

[Dave Scepter]

Good question~ I have the same amp was wondering that myself~ 

[triodecr]

The area circled in blue below looks odd.  I think the two gray capacitors are the bias supply filters but it's hard to tell without it right here in front of me.  Can you get a pic from the left side looking at the caps and whatever that gray thing to the left of them is ("cement" brick resistor or ???)?

 

[triodecr]

These four resistors also look suspect.  Can you get a better view of those?

 

[triodecr]

Not trying to butt in on Thundernotes BTW... just curious.  

[Jeff R]

I don't have the first one handy and I put the chassis back in the box after I shot those (shop amp, test drives), but I have this one from the initial wave of photos. I saw that and I HAD to take a picture of it - not saying it's hokey, just hadn't noticed anything like that in an amp before.

 

[Jeff R]

Just blew the first suspect area up w/photoshop ...

[triodecr]

That all appears to be factory.  I was scratching my head trying to sort out the resistors, thinking someone had done a mod to run the preamp tube heaters on DC voltage because the schematic I found didn't show the 39ohm blue resistors but then I found this schematic that shows Mesa did exactly that, but only for V1 (large red rectangle in the schematic).  The gray thing is a 330ohm resistor in the bias supply (small red rectangle).  The blue resistors to the right of the bias supply filter caps set the bias voltage and were selected at the factory to give the appropriate bias voltage (blue oval).  I'm guessing that the tech who put the sticker there either documented the current draw (as set by the factory) with a new set of tubes or they changed the factory selected resistors to get 36ma current draw with the matched set of tubes they installed.

[triodecr]

I found the schematics on the Boogie Board forum in threads where people were adding bias pots to their amps so I bet some of them would know what the stock cathode current draw range is.  Or you could email or call Boogie and see if 36ma is a reasonable current draw for a factory Mark III.  

[Jeff R]

I'm guessing that the tech who put the sticker there either documented the current draw (as set by the factory) with a new set of tubes or they changed the factory selected resistors to get 36ma current draw with the matched set of tubes they installed.

I think the e-mail to Boogie asking if 36ma is reasonable for stock/factory is my best bet. This amp is the best sounding Mark III I've owned, VERY harmonically alive and touch sensitive. Modded or not, I kinda don't want to fuck with it if I don't have to, I'd prefer to just use glass in it, Boogie-stamped or whatever, that ain't going to blow it up haha. Thanks for the eyeing and the heads up!

[Lockbody]

If it's SimulClass, shouldn't it have two bias supplies?

[Jeff R]

I was curious about this also ... my amp is, and I'm running EL34s outer, 6L6s inner

[bubs_42]

2 hours ago, Jeff R said:

I'm guessing that the tech who put the sticker there either documented the current draw (as set by the factory) with a new set of tubes or they changed the factory selected resistors to get 36ma current draw with the matched set of tubes they installed.

I think the e-mail to Boogie asking if 36ma is reasonable for stock/factory is my best bet. This amp is the best sounding Mark III I've owned, VERY harmonically alive and touch sensitive. Modded or not, I kinda don't want to fuck with it if I don't have to, I'd prefer to just use glass in it, Boogie-stamped or whatever, that ain't going to blow it up haha. Thanks for the eyeing and the heads up!

From Randy Aikens site:
A general rule of thumb is that class AB amplifiers are usually operated at no more than 70% of the maximum plate dissipation of the tube (to account for the higher dissipation that occurs under signal conditions), while true class A amplifiers generally run right at the maximum plate dissipation (the dissipation at full power is lower than the dissipation at idle in a true class A amplifier).  

For example, the aforementioned EL34 tube has a plate dissipation of 25W, so at 400V class AB operation, it should be biased no higher than (0.7 * 25/400) = 44mA.  At 500V class AB operation, it should be biased no higher than (0.7 * 25/500) = 35mA.  

This doesn't mean you should automatically bias all tubes to 70% of max dissipation! 

They can be biased at any lower current if desired, and many people prefer a point of around 50% to 60% of the max plate dissipation, which contributes to longer tube life.

 .

[triodecr]

7 hours ago, Lockbody said:

If it's SimulClass, shouldn't it have two bias supplies?

There's a series resistor going to V8 and V9 grids that drops their bias voltage to -45V.  This is in addition to the normal 220k resistor in the bias circuit.  There's no extra resistor on V6 and V7 so they get -56V bias. 

[Jeff R]

Finally got around to e-mailing M/B an inquiry. My question was is 36ma a reasonable draw for a factory MIII that appears to be unmodded, and am I safe running Boogie-stamped EL34 and 6L6 pairs in simulclass. The reply is below. Is this saying I'm safe running Boogie's tubes? I have some in my glass stash.

Hi Jeff, 

Hope you had a nice weekend! I hit up our chief tech on the old Mark Series amps about your inquiry and he said... If the amp is not modded on the biasing, then the tubes must have been matched to the class a and a/b sockets to get  36ma at all 4 sockets.  grn dot is always a simul class.

Hope this is of help, 
Trent

MESA/Boogie - Product Specialist
My Hours: Monday - Thursday, 9am - 6pm CST

 

[tomteriffic]

Looks like that's what he's saying is that you're good to go, if not modded.

[Jeff R]

Trent e-mailed me back almost immediately (before 8 a.m. his time!) and yep, he said run with the Boogie-stamped tubes. I think I'm good!

[Jeff R]

Okay, thought I'm good ... lead channel went kaput a couple days ago LOL. R1 fine and bold, R2 fine and bold, lead channel DOA, rotary controls on front fine, EQ DOA. Tube swaps did nothing, which I anticipated considering that fried innard stench that suddenly enveloped the room haha.

Just got an RA from Petaluma, chassis, head box and reverb tank ships out tomorrow. They are doing an A-to-Z check and unscrewing and unfucking anything and everything hokey, from her DOA features to scratchy pots and stuff and undoing any post-MB mods/asshat-fuckery they may find.

They are also adding a rear-panel volume just for R2 and they are making some tweaks on the reverb circuit based on a list I provide of my preferred control settings to make it a little more lush.