GUITAR BOARD

Just a quick question,
Does anyone own a plexi round here? The reason is Im trying to troubleshoot
an amp which I think has a problem and its close to a plexi design (London
City DEA70).
Closest to the set-up Im using would of course be best which means:
Fullrange mic (NOT SM57 but thats OK if its what youve got)
Fixed Bridge guitar with Seymour Duncan JB Bridge Mahog body maple/rosewood
neck (Ill settle for any bridge humbucker though)
Sound - overdriven. Mild and heavy sound needed
The problem occurs when playing a 5th chord so just give me a G5 for a
couple of secs, then the octave G5 and Ill be in heaven...
Im not a tech but Ive tried this amp on 2 different power supplies so far
and seem to be getting some kind of ghost note coming through at anything
over mild overdrive.
If it makes sense to anyone, at 373V measured on the HT (Quiescent)
overloading the amp progressively, it starts to sound a little clapped
outonce the HT drops to 340V and its just reached a MILD overdrive sound by
then. Increasing the HT to 470V made it louder but the sweet point (before
it noticeably sounded bad) was 440V...
Id be really grateful for the soundclip and any advice trying to get this
one to its former beauty (even as a copy..)
Thanks.
cb

Chris Berry wrote a question about a plexi
exhibiting a fault that I would describe as symptomatic of bad filter caps
in the power supply.

FWIW, the original Marshall plexi (JTM45) has an anode voltage based on
300 - 0 - 300 power transformer secondary into a GZ34 rectifier. That pans
out to 390V on the B+ rail, give or take.

I presently have a Marshall on the bench with a B+ voltage of 425V. Right on
the money for a Marshall with a solid state rectifier. (actually four 1N4007
diodes)

HTH

Trev :)

Trev Ridney wrote in message
news:rErxc.1648$WX7.14380301@news-text.cableinet.net...

The filter caps have been changed already - extra 100uF added(didnt have a
spare 50uF.
0-290V gives 373V rectified on the rails. 0-315 gives 475V rectified... Its
a bit of a mystery this one and Im having no luck with it as yet...
Scope time maybe...


What does it have on the B+ under load can you tell me?
cb

Chris,

This problem sounds like it could be naturally occuring. All amps
generate what is known as intermodulation distortion. This happens when
more than one note is being played at the same time. However, when you bend
one of the notes you will hear two others... one raising in pitch and the
other lowering. Nothing can remove this. Some amps will be worse than
others and it will be particularly noticeable with heavy distortion. Sadly,
you may be wasting your time trying to cure it... which is when it makes you
even more aware of it presence.

If this is just noticable at home use, then try to ignor it. These amps are
for stage use only really and youd probably never hear it at a gig. Also,
try not to use complexed chords where the notes incorporated will beat
against each other in a peculiar way.

In fact, its how those 70s trim phones made their destinctive ring-tone
(ring-tone not as we know them today of course!). They used two sine wave
tones playing at almost the same pitch. They went in and out of phase
causing the warbling sound you hear. You hear the sum, the difference and
the original two tones mixed into a complexed signal. All totally natural.

--
Stewart Ward
www.Award-Session.com

Tel. 01256 477 222 Intl. +44 1256 477 222
Guitar Amps, Guitars, Speakers, Pickups, Cables & Amp Repairs.
Chris Berry wrote in message
news:ca5a9m$do1$03$1@news.t-online.com...

Stewart Ward wrote in
message news:ca6i1e$uhc$1@newsg4.svr.pol.co.uk...

I dont think Its IM. and its present even with mild overdrive - almost
clean.
It sounds like theres a very fixed distorted sound/character that isnt
note related whenever the output stage is overdriven by a certain amount.
If I could describe the sound better...
Each power chord sounds like its got the same SHHhhhhh overdriven sound
superimposed on it.
I played chromatically until I could localise the superimposed frequency and
it seems like it interferes least with a 200Hz slightly sharp G.
Id have expected 100Hz from the bridge rectifier... but 200Hz isnt quite
unexpected.



Its a plexi right... needs to be cranked and thats what Im doing (with a
hotplate of course...)
Its not the notes beating with each other thats the problem but all of
them beating with what sounds to me like a dragged down supply.

natural.


Thats the reason I want a reference tone to compare how the break-up
changes in character but Ill be putting a scope and signal generator on it
soon enough...

cb

is this why extended chords often sound mushy or just plain wrong when
distorted, whereas anything thats only got a 1, a 3 , a b3, a 5 or b7 in it
is fine? even just a maj7 chord can sound nasty

also, isnt hexafuzz supposed to solve this problem, where there is
effectively a distortion unit per string, as featured often on old guitar
synths, korg x911, rolands gr300, gr100, arp avatar (?) and no doubt some
emulation in the modern roland gr and vg boxes

jim

Chris Berry wrote in message
news:ca6pcj$kvk$00$1@news.t-online.com...

I understand. However, that is often the sound of slewing. That means
the output stage cannot respond quick enough to the changing state of the
signal fed into the grids of the output valves. What happens then is that
the high frequencies get converted into triangular waveforms, which are
grossly unpleasant and produce the edgy sound you explain above.

Ringing caused by the inductance of the output transformer might also
contribute... although not completely responsible. Very trebly amplifiers
add to the Shhhhhh sound greatly. Confirming the route of the cause.

At high gigging levels youd not normally hear this too much (Twins do it as
they just go into clipping), but with a Hotplate effectively turning down
the volume, these odd tones become much, much more audible (log hearing and
all that stuff). I first noticed this when I imported a Tom Schotz Power
Soak back in 1980 for testing amps in the workshop at full bore.

The Hotplate can also upset the way the output stage behaves too. Making
the problem more prominent or even being the possible cause. Its not a
speaker after all. Each amp will respond differently to different load
types. Ahhh, the wonder of valve output satges... although trannies can be
more of a headache until you get em right too!


This may be an additional effect caused by the simple power supply and an
under-rated transformer. The only way to really pinpoint it would be to use
a sig gen and scope. Any 100Hz ripple on the HT would beat with the near G
note being played... and intermodulate with it. Particularly at just
clipping, when the power supplys 100Hz ripple is very large in amplitude.
As the signal beats with the ripple it would generate several other notes as
it goes in and out of just clipping.

Chris, Im just trying to exclude red herrings and save you time. It
seems to me that there are probably two things happening at the same time.

Good luck anyway.

--
Stewart Ward
www.Award-Session.com

Tel. 01256 477 222 Intl. +44 1256 477 222
Guitar Amps, Guitars, Speakers, Pickups, Cables & Amp Repairs.

Not really, because each note played will be mixed into a composite signal
for the amp. So ID is still a potential problem. Reducing the treble
content of a sound prior to the distortion circuitry lessens the Shhhhhhh
(not ID, but slewing) problem to great effect. This is why (regular)
humbuckers distort more pleasantly than plain trebly single coils. Hence
HSS configured guitars.

You can try it for yourself. Distort your Strat and turn the treble control
down.... hear the Shhhhh disappear!

Ibanez Gem style guitars exploit these facts with good result. Weve had
this conversation before last year. You may be able to Google it. Its
also in our Gear Talk off the website as a PDF.

--
Stewart Ward
www.Award-Session.com

Tel. 01256 477 222 Intl. +44 1256 477 222
Guitar Amps, Guitars, Speakers, Pickups, Cables & Amp Repairs.




jim harris wrote in message
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