The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

"Neil" wrote in message
...
The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

I had a VS150H and the power amp used TDA7293 that blew. The local store
wanted to sale them for me for 30$... I was able to get them online for a
few dollars. (ended up buying 10 at a think 1$ each)

In any case I replaced the component and it worked fine. I believe it blew
because I had no load hooked up(it was in a dorm and I didn't have the cab
and was running the line out to my comp).

So first off make sure you have the speakers hooked up(correct ohms). Second
makes sure that whatever the darlington's are driving isn't somehow shorted.

If the darlingtons are driving the speakers then I'd bet you have a load
mismatch. If they are not then maybe whatever they are driving has changed
impedence. Check the resistors around the darlingtons to make sure they
have the right resistance too. Also sometimes the diodes can go out and end
up causing problems so you really need to look at everything around it.
(besides the fact that if one component goes out it can take out others with
it)

Obviously 100oC is pretty hot for such things so for some reason the
darlingtons are passing a lot of current. This is either because there
configuration circuit has changed or the load has changed or the
input(potentially) has changed.

Does the 4 darlington's look like a left and right pair? What stage are they
part of? (output, input, power, pre, loop, etc...)

Did it work fine when you replaced them when it was working? Your going to
have to do a little bit more work if you want some help. If you can take a
pic and post it then it might help too.

On Thu, 16 Oct 2008 15:44:46 -0700 (PDT), Neil
wrote:

The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

Are you getting some parasitic oscillation? If any wires have moved or
capacitors are failing then some HF oscillation could cause this.



--
Regards, Paul Herber, Sandrila Ltd.
Electronics for Visio http://www.electronics.sandrila.co.uk/

In message . net, Paul
Herber writes
On Thu, 16 Oct 2008 15:44:46 -0700 (PDT), Neil
wrote:

The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

Are you getting some parasitic oscillation? If any wires have moved or
capacitors are failing then some HF oscillation could cause this.

I was just about to suggest similar, could yo get hold of an
oscilloscope and look at the output and see if there is anything there
that shouldn't be?




--
Bill

"Neil"
The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this?


** T6 ( MJF122) may be damaged - replace it.

It is the thermal sensor for the bias current.



..... Phil

On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe it blew
because I had no load hooked up(it was in a dorm and I didn't have the cab
and was running the line out to my comp).

Solid state amp: Ok with no load. Tube amp: not ok.

-DC

"Neil"

The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,


** Where did you buy the BDVs ?????



...... Phil

On Oct 16, 7:36 pm, "Phil Allison" wrote:
"Neil"

The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this?

** T6 ( MJF122) may be damaged - replace it.

It is the thermal sensor for the bias current.

.... Phil

And also, even if it's technically working fine, it might have
mechanically dislodged itself from the heat sink. Stranger things
have been known to happen.

Heck, he could just disconnect all 3 of it's leads out for testing
purposes, couldn't he? The amp would go into deep class B with
serious crossover distortion, but it might still work, and give the
added bonus that it would sound better when turned louder. So long as
it didn't go into a destructive oscillation, it might even sound cool
- built in noise gate. And his power transistors would run really,
really cool.

Ok, scratch that last post, I wasn't thinking straight. Disconnecting
it would give more bias current, definitely not less. Putting a diode
in, with anode where the collector of T6 used to be, and the cathode
where the emitter used to be, that would give a nice cold bias for
testing, wouldn't it?

"Dave Curtis" wrote in message
...
On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe it blew
because I had no load hooked up(it was in a dorm and I didn't have the cab
and was running the line out to my comp).

Solid state amp: Ok with no load. Tube amp: not ok.


Thats not completely true. The TDA7293's are designed to work more
efficiently with a certain load. By not having a load you reduce the
efficiency and it gets warmer. In general it shoudln't be an issue but in
rare circumstances it can.

Even if that wasn't the case then the chips shouldn't have blow at all
because they were not in use. since I had no speaker cabs hooked up at
all(just headphone hooked up to the headphone jack). I was playing for a few
hours and it just stopped working. took it hope and opened up and saw one of
the TDA's slightly burnt... replaced it and everything worked fine.

So it might have been just a weak chip that got a little too hot(load or no
load) but in case it's the facts are the facts.

1. It blew with no load.
2. No other component failed, if it was from a different component then when
I replaced the TDA it would have blown again(I ran a stress test on it to
see and used it extensively afterwards).
3. There was no power issue's at the time it blew.

I understand the logic that if there is no load with a solid state, since it
being direclty hooked up to the load, that no current should flow... but
thats ideally and really depend on the circuit because there usually is
always some type of load.

So I agree that in 99% of cases it is true for solid state but I disagree
that there is no possible way for all cases to be load independent. (even if
it has to do with the chip malfunctioning)

Of course I suppose it probably was something else too but I all I can say
is that the load was the only thing that changed and there is no telling
with the design how bad it is.

If you even look at the manual it says not to do it:

http://www.marshallamps.com/download..._275%20hbk.pdf

(warning I)

Neil wrote:
The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

Summats up with the claas B bias circuitry.

No direct experience of this design, but you may find a preset pot on
the board somewhere in that general area.

Assuming nothing else is blown, the off signal current draw should be in
the tens of mA range. You can check that by using a meter instead of the
fuse.

If it has a bias preset pot, adjusting that should adjust the queiscent
current smoothly. If it jumps around, replace the pot.

Dave Curtis wrote:
On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe it blew
because I had no load hooked up(it was in a dorm and I didn't have the cab
and was running the line out to my comp).

Solid state amp: Ok with no load. Tube amp: not ok.

-DC
Correct.

Jon Slaughter wrote:
"Dave Curtis" wrote in message
...
On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe it blew
because I had no load hooked up(it was in a dorm and I didn't have the cab
and was running the line out to my comp).
Solid state amp: Ok with no load. Tube amp: not ok.


Thats not completely true. The TDA7293's are designed to work more
efficiently with a certain load. By not having a load you reduce the
efficiency and it gets warmer. In general it shoudln't be an issue but in
rare circumstances it can.

No. You have it wrong. The only thing that can cause a poorly designed
amp to blow on no load, is either that its unstable and oscillating, or
that the actual voltage is too high off load, and blows the chip or
output stages

In a well designed amp this should never happen, however this IS a
Marshall, and thefore all bets I suppose are off.

FWIW I spent more than ten years designing power amps, and at lest 5
doing guitar amps.

I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design, with a
skimped on power supply and output transformer,and a lot of feedback
removed to give it more gain.

Pus a preamp that closely follows the mullard design, made by someone
who thought orange was red..and so got the valve bias wrong, thus
accidentally discovering the 'Marshall sound'

Don't get me wrong. I like the Marshall sound and Ive got one. But I
always laugh inwardly when peole talk about 'valve sound' and 'marshall
sound' as if it it was a stroke of genius, rather than the accidental
result of skimping on costs and a glorious mistake..

Paul Herber wrote:
On Thu, 16 Oct 2008 15:44:46 -0700 (PDT), Neil
wrote:

The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

Are you getting some parasitic oscillation? If any wires have moved or
capacitors are failing then some HF oscillation could cause this.



Yes. That is the other possibility. There is normally something akin to
a resistor and inductor, in series with the output, and a resistr of
about 5-10 ohms in series with a capacitor of about 0.1uf across te
output. If that capacitor is bust, you may well get oscillation,
especially with no load.

"The Natural ****** "


I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design,


** Nonsense.

The first Marshalls were clones of the Fender Bassman of 1959, the 5F6-A
schematic.

Subsequent models all followed a very similar pattern.


Pus a preamp that closely follows the mullard design,

** Pus is right.


Don't get me wrong. I like the Marshall sound and Ive got one.


** ROTFL ....




..... Phil

Phil Allison wrote:
"The Natural ****** "

I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design,


** Nonsense.

The first Marshalls were clones of the Fender Bassman of 1959, the 5F6-A
schematic.

Subsequent models all followed a very similar pattern.



Well just look at the Mullard 50W circuit with 4xEL34 and compare and
contrast.


American tube amps were all based on different valves entirely..beam
tetrodes mainly, not pentodes.



Pus a preamp that closely follows the mullard design,

** Pus is right.


Don't get me wrong. I like the Marshall sound and Ive got one.


** ROTFL ....




.... Phil

"The Natural ****** "

I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design,


** Nonsense.

The first Marshalls were clones of the Fender Bassman of 1959, the 5F6-A
schematic.

Subsequent models all followed a very similar pattern.


Well just look at the Mullard 50W circuit with 4xEL34 and compare and
contrast.

** No relevance to Marshall whatever.


American tube amps were all based on different valves entirely..beam
tetrodes mainly, not pentodes.


** All the early 60s Marshalls used beam tubes, 5881s and KT66s.

Marshall Majors used 4 x KT88s.

USA models used mainly 6550s.

EL34s were later substituted with a bias change and little else, as an a
economy measure.

Read the Wiki & check out the schems.

http://en.wikipedia.org/wiki/Marshall_Amplification

http://www.drtube.com/marshall.htm

You are totally WRONG.




....... Phil

Phil Allison wrote:
"The Natural ****** "
I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design,

** Nonsense.

The first Marshalls were clones of the Fender Bassman of 1959, the 5F6-A
schematic.

Subsequent models all followed a very similar pattern.

Well just look at the Mullard 50W circuit with 4xEL34 and compare and
contrast.

** No relevance to Marshall whatever.


American tube amps were all based on different valves entirely..beam
tetrodes mainly, not pentodes.


** All the early 60s Marshalls used beam tubes, 5881s and KT66s.

Marshall Majors used 4 x KT88s.


None of the marshalls I ever saw used KT88's

All EL34s.

Maybe that was a US export model you hade.


USA models used mainly 6550s.

EL34s were later substituted with a bias change and little else, as an a
economy measure.


They are not cheap, and never were.

Read the Wiki & check out the schems.

http://en.wikipedia.org/wiki/Marshall_Amplification

http://www.drtube.com/marshall.htm

You are totally WRONG.



Or teh Wiki is..




...... Phil

"The Natural ****** "


Well just look at the Mullard 50W circuit with 4xEL34 and compare and
contrast.

** No relevance to Marshall whatever.


American tube amps were all based on different valves entirely..beam
tetrodes mainly, not pentodes.


** All the early 60s Marshalls used beam tubes, 5881s and KT66s.

Marshall Majors used 4 x KT88s.


None of the marshalls I ever saw used KT88's


** Then you have never seen a Marshall Major

Too damn lazy too look up the schem as well.



USA models used mainly 6550s.

EL34s were later substituted with a bias change and little else, as an a
economy measure.


They are not cheap, and never were.


** Philips /Mullard EL34s were much cheaper than either KT66s or US made
5881s.

You pig ignorance is showing.

Oink, oink.....



Read the Wiki & check out the schems.

http://en.wikipedia.org/wiki/Marshall_Amplification

http://www.drtube.com/marshall.htm

You are totally WRONG.



Or teh Wiki is..


** Fuuuuck ooofffff

- you know nothing **** HEAD.




........ Phil

The Natural Philosopher schrieb:
Phil Allison wrote:
"The Natural ****** "
I reverse engineered a couple of marshals. Triumph of overrunning
components and cost reduction over correct design.

The marshal 100W valve is based on an overrun Mullard 50W design,

** Nonsense.

The first Marshalls were clones of the Fender Bassman of 1959, the
5F6-A schematic.

Subsequent models all followed a very similar pattern.

Well just look at the Mullard 50W circuit with 4xEL34 and compare and
contrast.

** No relevance to Marshall whatever.


American tube amps were all based on different valves entirely..beam
tetrodes mainly, not pentodes.


** All the early 60s Marshalls used beam tubes, 5881s and KT66s.

Marshall Majors used 4 x KT88s.


None of the marshalls I ever saw used KT88's

All EL34s.

Maybe that was a US export model you hade.


USA models used mainly 6550s.

EL34s were later substituted with a bias change and little else, as an
a economy measure.


They are not cheap, and never were.

Read the Wiki & check out the schems.

http://en.wikipedia.org/wiki/Marshall_Amplification

http://www.drtube.com/marshall.htm

You are totally WRONG.



Or teh Wiki is..




...... Phil







Hi Mr Philosopher,

it's very remarkable that you don't recognise one of the most polite
answers Phil.A ever made.

Let me assure you - his statement's correct, yours' not so...

There also were KT88 Marshalls, although they are a completely different
story. IIRC the indeed evolved from generic Mullard or the GEC designs.
But - the bread and butter Marshall began 1962 with the 5881/6L6/KT66
equipped JTM45 - an almost 100% copy of the 5F6-A - and they (Ken Bran)
have publicly admitted that they were clearly aware of that fact. The
evolvement to the usage of pentodes (EL34s) was mainly driven by
economical reasons. The similarities to the mullard designs may come
from the fact, that there ain't so many possibilities to connect a
pentode or a beam power tetrode to a circuit - and - the 5F6-A was also
based on a common available basical circuit (from WE or RCA IIRC).

The preamp of the marshall *was* the same as a 5F6-A and the future
evolvement was only incorporated by the addition/alteration of some
components to fit the sonic tastes of the upcoming rock musicians
(cathode Cs, mixer Rs, bypass Cs, different B+ filtering etc). Even the
MV Marshalls just rewired the existing preamp circuit to a cascaded
design. OK some Rs and Cs were added but that was all. Up to the generic
JCM800 the circuit still clearly showed it's origins.

regards

Jochen

"jh"

Hi Mr Philosopher,

it's very remarkable that you don't recognise one of the most polite
answers Phil.A ever made.

Let me assure you - his statement's correct, yours' not so...

There also were KT88 Marshalls, although they are a completely different
story. IIRC the indeed evolved from generic Mullard or the GEC designs.
But - the bread and butter Marshall began 1962 with the 5881/6L6/KT66
equipped JTM45 - an almost 100% copy of the 5F6-A - and they (Ken Bran)
have publicly admitted that they were clearly aware of that fact. The
evolvement to the usage of pentodes (EL34s) was mainly driven by
economical reasons. The similarities to the mullard designs may come from
the fact, that there ain't so many possibilities to connect a pentode or a
beam power tetrode to a circuit - and - the 5F6-A was also based on a
common available basical circuit (from WE or RCA IIRC).

The preamp of the marshall *was* the same as a 5F6-A and the future
evolvement was only incorporated by the addition/alteration of some
components to fit the sonic tastes of the upcoming rock musicians (cathode
Cs, mixer Rs, bypass Cs, different B+ filtering etc). Even the MV
Marshalls just rewired the existing preamp circuit to a cascaded design.
OK some Rs and Cs were added but that was all. Up to the generic JCM800
the circuit still clearly showed it's origins.


** 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

Marshall Majors used 4 x KT88s.


None of the marshalls I ever saw used KT88's

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 ****** 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..


http://www.usaelectron.com/GM







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!!!!!!!!!!!!!!!

Jon Slaughter wrote:
"Dave Curtis" wrote in message
...
On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe
it blew because I had no load hooked up(it was in a dorm and I
didn't have the cab and was running the line out to my comp).

Solid state amp: Ok with no load. Tube amp: not ok.


Thats not completely true.

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

www.kevinaylward.co.uk

Kevin Aylward wrote:
Jon Slaughter wrote:
"Dave Curtis" wrote in message
...
On Thu, 16 Oct 2008 18:05:06 -0500, "Jon Slaughter"
wrote:

In any case I replaced the component and it worked fine. I believe
it blew because I had no load hooked up(it was in a dorm and I
didn't have the cab and was running the line out to my comp).
Solid state amp: Ok with no load. Tube amp: not ok.

Thats not completely true.

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....


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.



Kevin Aylward

www.kevinaylward.co.uk

On Fri, 17 Oct 2008 15:04:19 +0200, jh
wrote:

the bread and butter Marshall began in 1962 with the 5881/6L6/KT66
equipped JTM45 - an almost 100% copy of the 5F6-A

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

On Oct 16, 6:44*pm, Neil wrote:
The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

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.....

"Neil"
The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this?



** 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

** 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

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.

"Neil" wrote
....

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.


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