Marshall Guitar Amp Transistors overheating - help

[Phil Allison]

"jh"

** The ECC83 phase splitter with 100k/82k plate loads and the use of 220k bias feeds to the EL34's grids are standard across nearly all Marshalls, plus the cause of problems with bias runaway. The direct coupled ECC83cathode follower driving the tone stack is also a common feature across nearly all models too, as is the presence control in the NFB path.

All come direct from the Fender 5F6-A schematic.

Fender wisely improved their basic design as time went by - ie changing to a

12AT7 phase splitter and 68k output tube bias feeds.

But Marshall never got the hint, even to this day.

..... Phil

[JJTj]

Almost all UK Majors had KT88's. The first few Major's (via Rose Morris n UniCord) had 88's, then they switched to 6550's. This allowed the amps to loft along, due to the less demanding needs of 6550's. A Major with 6550's sounds very little like one with KT88's. Of course, bias needs to be tweeked. Alot of folks who dislike Major's only heard them with 6550's. And today's KT88's are not the same. Thank God I have a good supply of real GEC's. Put aside for old age, ya know. :)

Besides, a Major on a slant 4-12 cab looks awesome.

Head is as deep as the cab top (11" I believe)

Last killer Major 1/2 stack I saw, because he paid me danger $ for the KT88's ($200 each in 98) had a cab full of old white cap'ed SRO speakers. He had to put bricks behind the wheels, because the amp would roll backwards when played. He liked the cab being off the ground, but hated to chase it every other song.

Newest GP shows 'KRANK' amps using surface mount resistors in the POWER AMP stage ?????????? Flocking wanker design.

Major's had chassis mounted round power resistors.

Transformers that weigh more then today's amps, cab n all.

Guess the times, they are a changing...

JJTj

FS: Old press/Fan Club kit for The GrandMothers, (Mothers of Invention original members) by 'Panda Records' SIGNED by Band in 2000 !

#522 of about 700 made

About as RARE as it gets, boyz n girlz..

She's 200 years old So mean she couldn't grow no lips She's 200 years old So mean she couldn't grow no lips (Boy, she'd be in trouble if she tried to grow a mustache . .)

She's 200 years old Squatting down And poppin' up In front of the juke box Like she had true religion

Boy

She's 200 years old Squattin' down Poppin' up Front o' the juke box Just like she'd had true religion, now,

Boy

Boy, boing,, it's 200 years Half of this, none of that Was 50 . . . Oh squat, yeah, oh, now, yeah!

She got religion now, boy

Oh, she's 200 years old Oh, she told me She just, she just can't grow no lips

Squat

Down

So mean she Can't grow no lips

200 years old Whaddya mean she can't grow no lips . . .

Squattin' down Poppin' up 'n down at the juke box

OWW!

She got the true religion, boy

Boing!!!!!!!!!!!!!!!

[Kevin Aylward]

Of course it is. The only way a transistor amp can blow up with no load, is if it is designed incorrectly. Like, it oscillates off load, or the power supply has such bad regulation that it raises its voltage etc....

Kevin Aylward [e-mail]

[The Natural Philosopher]

Well that is what I SAID, and in the context of MARSHALL, its quite possible.

Ive seen power amps blow from having transistors unable to handle the rail to rail voltage they saw off load before.

First job in one company was to work that out and select transistors that COULD.

[Dave Curtis]

Which is why the switches are (still?) upside-down, and the inputs are on the opposite side.

[boardjunkie]

You likely have leaky drivers. Never just replace output devices without checking upstream to see if other components were damaged when the outputs shorted. It happens.....

[Phil Allison]

"Neil"

** Have a very careful look at R29 ( 0.33 ohms 7 watt ) and make sure the soldering of the legs is OK.

This resistor is CRITICAL as it conducts speaker current to ground and supplies a take off point for the POSITIVE feedback loop.

If the soldering is cracked, the amp will oscillate at supersonic frequency - probably intermittently.

..... Phil

[Neil]

Sorry its been a while since my last message!

I've just popped R29 out of circuit to test. Allowing for the error in my DVM (doesn't go to Zero) the resisitor value is fine (as are the other 0.33 ohm resistors).

All solder joints seem oaky and theres no sign of cracks or burning.

An earlier message suggested the MJF122 darlington so I popped this out too and there are no short circuits. With the dvm set to diode function on the ohm scale and the neg probe on the centre pin and pos probe on left pin the reading is 0.73 & 0.64 with the pos probe on the right pin.

[Stephen Cowell]

Measure the bias currents into the 'base' leads... somehow, somewhere, there's too much, usually a leaky, perforated driver. __ Steve .