Post by Rich KoernerNote the line current spec on this Marshall Major print for setting bias.
http://www.drtube.com/schematics/marshall/200w.gif
1.25 amps is a lower line current bias adj. spec than the 2.5 amps line spec for a single pair of
6550's switched on draws in a 400-PS.
The Major was, from Pig days on, made for the GEC/GL KT-88. Add to that, the
Iron could (a whole lot of times) support the hand written scribe off that
schematic. Maybe (and there are other factors involved) a good 6550 would
survive the numbers, the design allowed KT-88s to loaf, and 6550s 2 try harder.
Take a 6550 SVT and use GEC KT-88s. Adjusting bias, and removing that stupid
screen diode etc, and you got more power/sound/MoJo than any of us could use.
But 6550's in a Major..or DR405..you have Bizzaro-world. KT-90 no work either.
..."..you had to of been there..."...
Post by Rich KoernerWithout the pre stand by mode line current figure, you don't know what portion of the 1.25 amps
belongs to the four 6550's in the Major.
Best mod is to isolate the preamp B+, cause no matter what it needed, the Power
section just piss'd it off. Add a separate heater tranz to any amp allows the
old PT to give all that spew to the (now drawing more spew)re-designed Class-a
hope to be. It really is all fish AND vinegar, ALL da smart boyz..they know
why.
Post by Rich KoernerHowever, a factory line current spec was originally arrived at with the tubes that the amp was
designed to used. That line current is original design tube specific.
ADD to that, the x-factor of DECADES of use. Only recently, rubes had to
be convinced that oldfartdays(.com) caps died. Whatever data those days
had to deal with, is long..long..long..gone by 1984. Some ?? doing a 'bias'
adjustment when replacing tubes..? Fuck, yer IcOn (and/or sound) you want is
just not something you are willing to do right, and pay the price 2 BE there.
Post by Rich KoernerWhen a DIFFERENT tube from the original is used, it is most often found that the original line
current spec is too hot for the replacement tube. What is one to conclude from this?
OldFart friend Rowdy Piper said, and I'll quote:
"..whenever you think you have all the answers, I change the questions.."
Your decades old toy has NOS (marked with awesome print) tubes, they r nuked.
YOU want the icon's sound. You want cheap tubes. You expect your IcOn amp
has not changed <chuckle> since, at best 198-de-O. No need to bias..at all.
hehehehehehehehe Shooting fish in a paper cup...
Post by Rich KoernerThen, with the original factory designed full power *before clip* line current spec, you can
determin by the line current increase, and the actual output on the load, how well the output tubes
pass signal current to the load.
...and adjust the amp to be able to live that lifestyle...
Major's were never..ever..a 'Klipp' type of amp..which is why
they bombed <back then> Bass players loved them, Guitar..no..
Post by Rich KoernerLoading differences and electrical energy pulled from the line not reaching the load, which usually
indicates electrical energy converted to wasted heat energy trapped somewhere, can be discovered
from line current and output load observations.
...never mind, the simple crap NOS stock part that (when built) is 1K, now
measured at 75ohms, yet looks great, never gets hot, cause it's dead.
Yet you like the sound (we all have been there). Don't rock the boat.
When I fixed Edge's AC-30 (ripped grill cloth and all) the reason it
died was simple screen resistor failure. I looked at it, inside, and
told the guy "..paying me.." that it needs major work. We both smiled,
knowing all they wanted was a band-aid fix. Seems the tech couldn't
get there in time..I was the Vox tech. He used it that nite, and even
remembered me years later, thanking me, and saying he keeps that @ home.
WHERE am I going with all this..?
"...learn..or be sold to..."
JJTj
I've been a drag racer on LSD,
and I rode bare-assed on top of the shpinx,
I even had a gorilla on the slopes of kismet,
and man, that was fun for a while you bet but...
Bikini girls with machine guns,
Bikini girls with machine guns,
that stuff will kill ya, it's loaded with fun,
Bikini girls with machine guns
well I savored many foriegn kinds of delicacies,
intoxicated til I can't tell what the hell I could see,
had all the violence and liquor within close reach,
but all bars, pills and threeways lead me back to the beach...
Bikini girls with machine guns,
Bikini girls with machine guns,
that stuff will kill ya, it's loaded with fun,
Bikini girls with machine guns
now they say that virtue is it's own reward,
but when that surf comes in I'm gunna get my board,
got my own ideas about the righteous kick,
you can keep the rewards, I'd just as soon stay sick...