Re: amplifier loss of signal strength


"Ron(UK)" <ron@xxxxxxxxxxxxxxxxxxx> wrote in message
news:raKdneXnhqaDhxDb4p2dnAA@xxxxxxxxx
ll wrote:
On Jul 3, 9:55 pm, Meat Plow <m...@xxxxxxxxxxxxxx> wrote:
On Wed, 04 Jul 2007 01:51:27 +0000, Arfa Daily wrote:

"Meat Plow" <m...@xxxxxxxxxxxxxx> wrote in message
news:12aki9.6gu.17.1@xxxxxxxxxxxxxxx
On Tue, 03 Jul 2007 13:18:52 +0000, Arfa Daily wrote:
"Meat Plow" <m...@xxxxxxxxxxxxxx> wrote in message
news:12a4tn.mg8.17.1@xxxxxxxxxxxxxxx
On Tue, 03 Jul 2007 08:28:35 +0000, Arfa Daily wrote:
"Eeyore" <rabbitsfriendsandrelati...@xxxxxxxxxxx> wrote in message
news:46899C19.A10DCF7A@xxxxxxxxxxxxxx
Arfa Daily wrote:
"Eeyore" wrote
ll wrote:
I recently installed a transformer (12VAC, 2Amp) in a musical
instrument amplifier (rated at 10 Watts RMS), and when I test
it
with
a variety of input sources (guitars, keyboards, different
cables),
for
about 30 seconds, the amplifier produces a nice healthy volume
level,
and then, consistently, the volume just drops about 50%. For
some
reason, this problem sounds like it might be heat-related. Has
anyone
else experienced this?
Are you by any chance sine wave testing it for those 30 seconds
?
I took it he wasn't, as he says that "when I test it with a
variety
of
input
*sources* ... " and then specifically mentions guitars and
keyboards.
I was a bit dozy at the time.
I've seen that effect though.
Graham
Likewise. I often deliberately test group amps with a 400Hz tone
for a
few
minutes, just to get the output stages thoroughly thrashed up to
temperature.
I made a looped test CD with me playing some leads, licks and riffs
on
my
Les Paul through my Boss GT-6 just to test guitar amps. :)
You should send me the files !!
Arfa
Probably could in MP3 form. Would have to dig it up tho. These days
when
someone brings one I just get out a guitar :). I won't work on any SS
stuff these days, just vintage Marshall, Fender, Ampeg, Traynor and a
few
stray oddball Silvertone, Alamo, Gibson, etc...Back in the day my
services were pretty heavily sought. Tried to train a couple young
apprentices because I had so much work but it was too hard to train
and do
top quality repair at the same time. Nobody wanted to do tube repair
and I
didn't want to do SS guitar, still don't unless you come groveling on
your
knees with a wad of green in your hand. I used to do my fair share SS
of
high power pro audio amps like Soundcraftsman, BGW, Crown, SCS,
Peavey.
Those I did have some help from a friend who did Yamaha warranty work
but
he didn't know shit about a tube :)
Know what you mean. None of the kids seem to have any aptitude for high
voltage stuff any more. I far prefer working on tubed stuff to SS. At
least
it's well behaved, and fixes - and better yet *stays* fixed - with a
minimum
of fuss. The circuitry is straightforward, and understandable. At a
pinch,
you can pretty well fix any tube amp without schematics, just by having
an
understanding of the principles of operation. Unfortunately, the music
shop
in my village only has one person doing their service work - me ! So I
have
to take on all-comers, although it is a proper music shop, always
having
several long-haired 'musos' hanging about talking, or jamming in the
play
test room, so a large proportion of the gear is tubed anyway.
Sounds like one of the shops I did work for. I wasn't the resident but
was
the only Fender and Marshall specialist around. I really tried to stick
with a couple brand names but always got tossed others. Back then I had
long hair tied in a pony and a face full of beard so I had the rocker
image plus I was that rocker too :)

Back in the old days, I knew Jim Marshall. Their works is just down the
way a few miles from where I live. Jim himself custom built an amp and
console for a very good friend of mine who ran a mobile disco, when
such
things were new. I used to run the light show for him. We were out on
the road 6 nights a week, and I reckon that it's got to be one of the
best periods in my life
I used to run lights and sound in between gigs for friends or customers
caught in a jamb. Lots of times I would get up and fill in on a set too.
Those were some great days of my life.



That sounds great. Did Jim do much custom work, outside of
Marshall?
I would love to learn about tube repair - my father did quite a bit of
that, as well as in organs. Speaking of that, Emerson's organ
technicians must get quite a workout!

Back to the amp in question - it's rated at 10watts RMS.
I did a few more 'listening' tests with it yesterday, and made more
comparisons. I was able to have the amp on for about 15 minutes
before the volume 'cut out' occurred (no variation in the input source
- same guitar). I then turned it off and then back on - the signal
was strong again. This isn't consistent, though, as I could turn off,
wait about 10 seconds and then turn on again, and it might or might
not have the full volume.
I tested the line out and headphone jacks, and they each produced a
healthy audio output. One item of note: when plugging into the line
out, the amp's internal speaker went back to that 'cut out'/ half
volume sound. This would occur, even if just plugging in an
unconnected guitar cable. When the internal speaker was producing
that weak output, the amp into which the line out was plugged was
producing a healthy signal. The headphone jack had the same effect.
After I had turned it off and on several times, I would hear a
'scratchy' sound (like turning dirty pots) just after turning back on.
Now I'm wondering if speaker wires/connections might be suspect?

Thanks


Sounds like a dodgy 'insert jack' Are there contacts on the line out jack?
maybe they are dirty or one of the soldered joints is fractured.

Ron(UK)

I think that the key here is that he says it works ok from the headphone
jack, when the speaker level has gone low. On a small amp like this, the
headphone jack is almost certainly straight across the output, with the
internal speaker switched by the socket's contacts, so that would leave only
bad contacts on the jack, bad wiring to the speaker, or of course, the
speaker itself. My next move would be to shove a good squib of switch
cleaner / lubricant straight into the headphone jack entry, and then work a
plug in and out vigourously for 10 seconds, then try again. If it now stays
on, then you either need to get at the contacts and clean them properly, or
better, replace the jack.

As far as Jim doing custom work goes, I really don't know if he did a lot.
This is going back like 35 years, when Marshall was a very small company. I
met him as a result of going there a number of times with the friend who
knew him well. At that time, mobile discos were a very new thing, and there
was perhaps 4 or 5 only in my town and surrounding area, which is quite
large. Jim built a custom semi-circular shaped console for my friend, with a
preamp / mixer set in the middle between the two decks. This preamp had such
innovative features ( for the time ) as slider level controls, separate mic
input and gain control, headphone socket with pre fade listen and so on. The
decks were a pair of Garrard semi autos, with the auto mech wedged, such
that they would unload at the end of a record, when you could just lift off
the current record, and drop the next one on. The deck would carry on to
self cue, at which point you flicked the deck power off, then used the PFL
to set your 'ready to roll' cue point. It took quite a bit of skill to run
it, but it was a very smooth operation when you got used to doing it.

The amp that Jim built him was a pretty meaty thing - 100 watts as I recall.
It used four output tubes in parallel push pull. I seem to recall that they
were KT66's or maybe even 88's. They are the true beam tetrodes over the
cross substitutable EL34's, which are normally accepted as being pentodes,
but are sometimes actually beam tetrodes inside.

The amp definitely had the standard gold front, and Marshall logo on it. The
preamp face plate was also gold, but I can't remember whether or not it had
the logo. I would guess that they were probably built through the factory,
at a time when any business was good business, and custom designs were taken
on and built in that little factory. That console was mighty heavy I can
tell you, and many was the time that I cursed it at two in the morning in
the rain, as we loaded it back on that old 15cwt Ford Thames van ...

Arfa


.

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