Marshall JCM900, 4100, of 2006

N

N_Cook

Guest
Dead amp. It never looks right,to me, to use 1/3W anode dropper
resistors, even if high voltage type. Did Marshall stop using HV types,
these are light green body if suggestive of anything?
100K open circuit one to V2p2. 2x seriesed 47K 0.5W will go back in
there and its mate.
No discolouration to the coating, scraped back coating and normal under
the axial sampling I scraped off ,dark grey MO spiral, failed at one
end. To last but 2 turn measured 63K, to last turn before end measured
110K. Presumably failure by gradual oxide breakdown by micro-arcing and
thinning and resistance increase.
Another problem waiting to happen, especially if rough handled/knocked
on front. There is a sub-front panel in these , rivetted to the pcb.
Unnecessarily long landing tongues protrude across the pcb and their
edges are about 1mm average gap to component leads, gap to one resistor
about half a mm.
 
"N_Cook" <diverse@tcp.co.uk> wrote in message
news:lp6424$a2t$1@dont-email.me...
Dead amp. It never looks right,to me, to use 1/3W anode dropper resistors,
even if high voltage type. Did Marshall stop using HV types, these are
light green body if suggestive of anything?
100K open circuit one to V2p2. 2x seriesed 47K 0.5W will go back in there
and its mate.
Click to expand...

Very common for 100k anode load resistors in the preamp stages to fail on
any make or model that uses 12Axx / ECC8x twin triodes ...

Arfa
 
On 14/07/2014 02:25, Arfa Daily wrote:
"N_Cook" <diverse@tcp.co.uk> wrote in message
news:lp6424$a2t$1@dont-email.me...
Dead amp. It never looks right,to me, to use 1/3W anode dropper
resistors, even if high voltage type. Did Marshall stop using HV
types, these are light green body if suggestive of anything?
100K open circuit one to V2p2. 2x seriesed 47K 0.5W will go back in
there and its mate.

Very common for 100k anode load resistors in the preamp stages to fail
on any make or model that uses 12Axx / ECC8x twin triodes ...

Arfa
Click to expand...

In the days of carbon resistors yes ,going high and then being
overstressed to failure ,but failing with no telltale discoloration of
resistor body or pcb?
Does no one use HV spec'd anode MO resistors
 
"N_Cook" <diverse@tcp.co.uk> wrote in message
news:lpvs54$nbb$1@dont-email.me...
On 14/07/2014 02:25, Arfa Daily wrote:


"N_Cook" <diverse@tcp.co.uk> wrote in message
news:lp6424$a2t$1@dont-email.me...
Dead amp. It never looks right,to me, to use 1/3W anode dropper
resistors, even if high voltage type. Did Marshall stop using HV
types, these are light green body if suggestive of anything?
100K open circuit one to V2p2. 2x seriesed 47K 0.5W will go back in
there and its mate.

Very common for 100k anode load resistors in the preamp stages to fail
on any make or model that uses 12Axx / ECC8x twin triodes ...

Arfa



In the days of carbon resistors yes ,going high and then being
overstressed to failure ,but failing with no telltale discoloration of
resistor body or pcb?
Does no one use HV spec'd anode MO resistors
Click to expand...

Don't know. I do, but as I said, a very common problem still, no matter what
type of resistor it now is. Plus consider that many people don't even know
that resistors have a voltage rating. You would expect designers to know
this, but judging by some of the design work that I see ...

Considering the generally poor quality of some tubes now, I always put a new
one in along with the resistor. One of the shops that i do work for, has a
tube tester, and you wouldn't believe some of the things that it finds wrong
with what you would have thought were some of the most robustly constructed
types, like twin triodes. Over the last year perhaps, I have had a number of
12Axx types with one half completely open circuit. Presumably down to an
electrode weld having given way, but something that you would never have
seen thirty years ago, or probably even 10 years.

Arfa
 
Arfa Daily wrote:
"N_Cook" <diverse@tcp.co.uk> wrote in message
news:lpvs54$nbb$1@dont-email.me...
On 14/07/2014 02:25, Arfa Daily wrote:


"N_Cook" <diverse@tcp.co.uk> wrote in message
news:lp6424$a2t$1@dont-email.me...
Dead amp. It never looks right,to me, to use 1/3W anode dropper
resistors, even if high voltage type. Did Marshall stop using HV
types, these are light green body if suggestive of anything?
100K open circuit one to V2p2. 2x seriesed 47K 0.5W will go back in
there and its mate.

Very common for 100k anode load resistors in the preamp stages to fail
on any make or model that uses 12Axx / ECC8x twin triodes ...

Arfa



In the days of carbon resistors yes ,going high and then being
overstressed to failure ,but failing with no telltale discoloration of
resistor body or pcb?
Does no one use HV spec'd anode MO resistors


Don't know. I do, but as I said, a very common problem still, no matter what
type of resistor it now is. Plus consider that many people don't even know
that resistors have a voltage rating. You would expect designers to know
this, but judging by some of the design work that I see ...
Click to expand...

It may be the OEM's purchasing department substituting a cheaper
part, after the design is released for production. I went through that
with inductors. They switched vendors, without having the new parts
tested in house. They then bragged that the new parts were not only
cheaper, but they were 5%, instead of 10%. The problem was the SRF for
each value was different between the two vendors. The $200 purchasing
saved, cost us around $3000 in rework costs in production to find and
replace every one of the unacceptable parts. The problem was when the
SRF was critical in IF and RF stages.


Considering the generally poor quality of some tubes now, I always put a new
one in along with the resistor. One of the shops that i do work for, has a
tube tester, and you wouldn't believe some of the things that it finds wrong
with what you would have thought were some of the most robustly constructed
types, like twin triodes. Over the last year perhaps, I have had a number of
12Axx types with one half completely open circuit. Presumably down to an
electrode weld having given way, but something that you would never have
seen thirty years ago, or probably even 10 years.

Arfa
Click to expand...

--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.
 
On Mon, 14 Jul 2014 10:09:55 +0100, "Arfa Daily"
<arfa.daily@ntlworld.com> wrote:

Considering the generally poor quality of some tubes now, I always put a new
one in along with the resistor. One of the shops that i do work for, has a
tube tester, and you wouldn't believe some of the things that it finds wrong
with what you would have thought were some of the most robustly constructed
types, like twin triodes. Over the last year perhaps, I have had a number of
12Axx types with one half completely open circuit. Presumably down to an
electrode weld having given way, but something that you would never have
seen thirty years ago, or probably even 10 years.
Click to expand...

Ah - but then we used proper valves and not these tubular devices of
which you speak...
 
"Geo" <nhhu-o3hu@dea.spamcon.org> wrote in message
news:tcoas91un3vmmq6glbr4l4nio97ilpn0tm@4ax.com...
On Mon, 14 Jul 2014 10:09:55 +0100, "Arfa Daily"
arfa.daily@ntlworld.com> wrote:

Considering the generally poor quality of some tubes now, I always put a
new
one in along with the resistor. One of the shops that i do work for, has a
tube tester, and you wouldn't believe some of the things that it finds
wrong
with what you would have thought were some of the most robustly
constructed
types, like twin triodes. Over the last year perhaps, I have had a number
of
12Axx types with one half completely open circuit. Presumably down to an
electrode weld having given way, but something that you would never have
seen thirty years ago, or probably even 10 years.

Ah - but then we used proper valves and not these tubular devices of
which you speak...
Click to expand...

If I say valve, I don't communicate properly with the leftpondians :)

And everyone in the UK knows what a tube is. And actually, I guess "vacuum
tube" is just as good a description of them as "valve" ...

Arfa
>
 
On 16/07/2014 03:26, Arfa Daily wrote:
"Geo" <nhhu-o3hu@dea.spamcon.org> wrote in message
news:tcoas91un3vmmq6glbr4l4nio97ilpn0tm@4ax.com...
On Mon, 14 Jul 2014 10:09:55 +0100, "Arfa Daily"
arfa.daily@ntlworld.com> wrote:

Considering the generally poor quality of some tubes now, I always
put a new
one in along with the resistor. One of the shops that i do work for,
has a
tube tester, and you wouldn't believe some of the things that it
finds wrong
with what you would have thought were some of the most robustly
constructed
types, like twin triodes. Over the last year perhaps, I have had a
number of
12Axx types with one half completely open circuit. Presumably down to an
electrode weld having given way, but something that you would never have
seen thirty years ago, or probably even 10 years.

Ah - but then we used proper valves and not these tubular devices of
which you speak...

If I say valve, I don't communicate properly with the leftpondians :)

And everyone in the UK knows what a tube is. And actually, I guess
"vacuum tube" is just as good a description of them as "valve" ...

Arfa
Click to expand...

I don't know what the term tube to the general public in the USA means
but in the UK it means an open ended cylinder. You would not retain much
vacuum in an open ended pipe.

As for losing the craft and skill in manufacturing, I've been
researching mathematical tiles recently. One way of discovering recent
examples is if the "brickwork" looks terrible, ie not co-planar
http://diverse.4mg.com/math_tiles.htm
the secret of how to retain flat surfaces during the air-drying and
kiln-firing has been lost over time
 
And everyone in the UK knows what a tube is. And actually, I guess
"vacuum tube" is just as good a description of them as "valve" ...

Arfa


I don't know what the term tube to the general public in the USA means but
in the UK it means an open ended cylinder. You would not retain much
vacuum in an open ended pipe.
Click to expand...

I think I would dispute that the description "open-ended" is in any way
implied in the definition of a tube ...

Arfa

>
 
On 16/07/2014 15:11, Arfa Daily wrote:
And everyone in the UK knows what a tube is. And actually, I guess
"vacuum tube" is just as good a description of them as "valve" ...

Arfa


I don't know what the term tube to the general public in the USA means
but in the UK it means an open ended cylinder. You would not retain
much vacuum in an open ended pipe.

I think I would dispute that the description "open-ended" is in any way
implied in the definition of a tube ...

Arfa
Click to expand...

yep a test-tube has only one open end. Capsule or phial seems a more
appropriate term if describing the appearance (USA) of such a thermionic
device, rather than its function (UK)
 
Arfa Daily wrote:
??
?? And everyone in the UK knows what a tube is. And actually, I guess
?? "vacuum tube" is just as good a description of them as "valve" ...
??
?? Arfa
???
?
? I don't know what the term tube to the general public in the USA means but
? in the UK it means an open ended cylinder. You would not retain much
? vacuum in an open ended pipe.

I think I would dispute that the description "open-ended" is in any way
implied in the definition of a tube ...
Click to expand...

'Tube' simply means that it is round, with straight sides. For
example, copper tubing. It isn't called pipe, even though they are
similar. Pipe requires a bending tool, but tubing can be formed by
hand. :) We also get toothpaste in tubes. ;-)




--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.
 
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