More phasing problems... (Guitar amp)
Chris Berry
For the umpteenth time I've opened up my Marshall clone to see what can be
done. Just to make sure, I changed all the bottles and rebiased the thing to
boot.
I don't know if I'm hearing ghost notes or a slight phasing effect but
either way it's driving me nuts - just as the Eminent was until I decided to
relegate that to another timezone (well another country anyway).
It's not nearly as aparent with single solo lines but the moment I play
lower or power-chords it's not sounding right at all when it's breaking up.
I'm stumped as to where to point the finger but I'm suspecting the mains
transformer.
That tranny's rated at 280V at unknown current (0.02A just doesn't sound
right but that's what it says) - subtracting all the delivered VA from the
other windings, there's 0.315VA left over so a 0.2A guess can't be far
wrong.
On Standby, I'm measuring 400V across the filter caps, off standby, 373V and
at maximum signal 320V - which I understand is a "normal" sag.
Like I said, I changed the output bottles, biased for 70%, changed the input
bottles and nothing's made the phasing go away.
It's strange though that under similar conditions as the eminent, I'm
getting the same sound problems using it for the same purpose.
Any ideas?
Thanks.
cb
Dr.Tube
> For the umpteenth time I've opened up my Marshall clone to see what
can be
> done. Just to make sure, I changed all the bottles and rebiased the
thing to
> boot.
> I don't know if I'm hearing ghost notes or a slight phasing effect
but
> either way it's driving me nuts - just as the Eminent was until I
decided to
> relegate that to another timezone (well another country anyway).
> It's not nearly as aparent with single solo lines but the moment I
play
> lower or power-chords it's not sounding right at all when it's
breaking up.
> I'm stumped as to where to point the finger but I'm suspecting the
mains
> transformer.
>...
> Any ideas?
Your description is a bit weird, but could it be that you are hearing
"subharmonics" on the louder and lower chords when in "overdrive"? If
so, then this could well be the classic case of worn out PSU caps (or
filter caps for those in the USofA).
It isn't really subharmonics you're hearing, but an interaction
between your played chord and the 100Hz (120Hz) originating from the
PSU when having to work hard. The amp can be hum free (when not
playing) and still have this "phenomenon".
Replace your filter caps. I like the F&T's for this:
http://www.drtube.com/tubeshop.htm#FT (I know, shameless plug...)
Regards Dr.Tube
www.DrTube.com
Chris Berry
"Dr.Tube" <a...@Dr.Tube.com> wrote in message
news:40c01ac8$0$37789$e4fe...@news.xs4all.nl...
Got F&T's... Live in Germany...
Anyway, I've a more detailed description of the circuit...
First of all, It's running on 373V HT - according to the voltage charts, the
Marshall runs at 400V. Under serious overdrive this sags to 320V - a likely
candidate I assume is the PT but the ratings on this are unclear at best.
I'm assuming there's 200mA available.
Overall, there's 50uF of extra stabilisation in the marshall before the
choke and it's aranged slightly differently thereafter.
Next, my copy has 1K screen grid resistors where these are conspicuously
absent in the plexi.
The control grids also have 1k resistors on them and the bias seems more
stabilised on my copy.
Further down towards the input side of things, the tone stack is slightly
different and a few components have been saved (slight capacitor changes and
common cathode capacitors are used).
There's a schematic here:
http://www.drtube.com/schematics/londoncity/dea70.gif
This one's got me baffled like I said. anyone have any ideas as to where to
start? Or do you think I should just go ahead and change everything that's
not quite plexi about it?
Seing as the schematic's on your site... want some photos? This one's a
DEA70 but the chassis used was a DEA100mkIV, green logo, grey box, aerial
wire for the output transformer - even the board's wired up for 4 EL34's.
Original mains tranny burned out long ago.
Thanks...
cb
Chris Richmond - MD6-FDC ~
Traynor 100W amp that, from description, sounded similar. What
I eventually did was get a 'scope and start checking things out.
I found that the premap overdrive was really nasty looking. I
adjusted a few component values and that seemed to help. The other
wierd thing was that once in a while, after a short loud guitar
burst, would get this hum. I *finally* tracked it down to a bad
PS cap. It was a nice Sprauge Atom I had added years ago. Once
removed, the overall sound at high volume was improved. Sorry
for being vauge.
chris
--
Chris Richmond | I don't speak for Intel & vise versa
Chris Berry
> > Any ideas?
> Your description is a bit weird, but could it be that you are hearing
> "subharmonics" on the louder and lower chords when in "overdrive"? If
> so, then this could well be the classic case of worn out PSU caps (or
> filter caps for those in the USofA).
> It isn't really subharmonics you're hearing, but an interaction
> between your played chord and the 100Hz (120Hz) originating from the
> PSU when having to work hard. The amp can be hum free (when not
> playing) and still have this "phenomenon".
> Replace your filter caps. I like the F&T's for this:
> http://www.drtube.com/tubeshop.htm#FT (I know, shameless plug...)
OK, I did some experimentation to see "what if"...
First thing I did after measuring voltages last time was to put a variable
resistance from the tone network output to earth.
a) this renders the tone control "useless" in conventional terms but allows
some wierd and wonderful effects - just rename the knobs whatever you feel
like sort of thing...
b) It can greatly attenuate the volume of the preamp section.
(aside) - Having fiddled with it for a bit, I wrote down the settings and
ran them through Duncanamps tone stack calculator and found that it's a
typical Marshall EQ curve - albeit at -20dB
OK so I basically measured the same stuff as I did before - droop and
whatnot, messed with the controls until I found what I thought was a good
level and wrote it down - then repeated the process with an overdrive pedal
in the input...
So, far from a conclusion but an observation: When the HT drops to 340V from
373V, that phase sound comes into play again - that's 10% droop - not a lot.
Is it possible to draw any conclusions from this? well, I don't think so but
what I can do is test a few more things tomorrow.
1) Test hypothesis that if HT drops below 340V - bas sound occurs:
- Use the 315V winding of the mains transformer - same current.
- Use a different transformer (I think I've got a Partridge 400VA mains
transformer lying around somewhere I can try. That will probably decide the
current V Voltage story.
2) It's possiuble that I just don't like the sound of overdriven pentodes -
although all my "guitar heroes" use them aparently.
3) Check mains filter caps as DrT says - although they seem to be holding up
fine. I could put a little more filtration in there.
4) Attempt the laborious task (well it doesn't appeal to me right now) of
converting everything to Plexi standards.
Ideas? Comments?
Thanks.
cb
Chris Berry
Tried the 325V winding to see what's happening...
Frustration had me playing chromatic scales with micro-bends to see if I can
isolate whatever or wherever there's an annoying frequency causing all of
this...
200Hz!!!! Bingo. It seems that playing the octave of the mains ripple helps
me identify the cause of my troubles...
Again a 10% drop in HT brings the "ghost note" or "phasing" sound to
annoying levels. so It's not the voltage but the stabilisation.
Like I said before, the mains transformer was replaced by a previous owner
(and in these amps the trannies were a bit on the weak side) so it doesn't
surprise me that the current delivered is a little weak...
I'll have to check all the capacitors as well but I think I'm well on my way
to getting this beast to sound good after all.
Another thing I'm going to do to at least bring it under a bit more control
is to hard-wire the presence control (same as Plexi) and put a master volume
in the hole.
cb
lbrt...@aol.com
>
> Ideas? Comments?
It is diffcult or impossible to discern what this "phasing" annoyance
is as you describe it. But it sounds as though you are attacking
every aspect of its circuit rather hapazardly and in frustration
without troubleshooting it, and for a considerable length of time
reported. Do you have a 'scope and an audio generator?
Blind shots in the dark (not at all inclusive):
- the cone characteristics of the particular spkrs?
- an IM distortion characteristic that isn't in your taste?
- your guitar setup or its settings, or your technique of playing it?
- an unhappy coupling cap somewhere?
- an unrealistic expectation of the design?
A second simple audio signal source (not necessarily a 2nd generator)
would tell much with a 2-tone test, and a keyboard would do even
better as a more continuous & adjustable multi-tone source, but either
would only be meaningful when compared to the same test on an amp you
like the sound of.
What other amp(s) have you played through in a band situation (not
heard others play through) that perform(s) just as you wish or very
close in stock condition, with the same guitar & setup? If perchance
there hasn't been one, the problem may not be amp-related.
If you "maybe don't like pentode distortion", it is hard to imagine
what tube guitar amp you do like for an overdriven sound, since that's
about all there is in popular use.
Don't get rid of the grid stoppers unless you like risking disaster
(parasitics).
If this experimenting gives you recreational pleasure, do enjoy that,
but you may want to consider it is counterproductive in terms of
diagnosing & solving a problem, or even defining one in behavioral
terms which are specifically conveyable in writing.
Chris Berry
<lbrt...@aol.com> wrote in message
news:205ef942.04060...@posting.google.com...
> It is diffcult or impossible to discern what this "phasing" annoyance
> is as you describe it. But it sounds as though you are attacking
> every aspect of its circuit rather hapazardly and in frustration
> without troubleshooting it, and for a considerable length of time
> reported. Do you have a 'scope and an audio generator?
Well, let me explain... at weekends, I have access to some lab equipment -
but very limited internet access.
During the week, all I've got is a voltmeter, soldering iron and a guitar.
The phasing annoyance makes the guitar sound very much like when playing
through a phaser effect pedal - although the frequency is more intensity
dependent.
Take a look at my other recent post on this thread and you'll see that
playing a 200Hz note (detuned a little) coincided with the "phasing"
frequency in such a way that the problem went away to a great degree (and
the sustained note was not the usual octave predominant sound you'd expect
either).
Using different output level pickups also produced less power overall but as
soon as the "magic" 90% of HT level was reached - either by adjusting the
master volume or gain, the problem appeared again.
Today's about the only day I have access to a scope and audio generator so
I'll be looking at the supply under load (surprise surprise..) and also
using a different transformer which can provide a lot more current.
>
> Blind shots in the dark (not at all inclusive):
>
> - the cone characteristics of the particular spkrs?
They're old faithfuls that work well.
>
> - an IM distortion characteristic that isn't in your taste?
A possibility.
>
> - your guitar setup or its settings, or your technique of playing it?
Certainly not. 3 guitars used, nicely set up, various techniques etc...
>
> - an unhappy coupling cap somewhere?
A possibility.
>
> - an unrealistic expectation of the design?
Possibly although if any plexi clips are to go by - it's exactly the sound
I'm after.
>
> A second simple audio signal source (not necessarily a 2nd generator)
> would tell much with a 2-tone test, and a keyboard would do even
> better as a more continuous & adjustable multi-tone source, but either
> would only be meaningful when compared to the same test on an amp you
> like the sound of.
>
> What other amp(s) have you played through in a band situation (not
> heard others play through) that perform(s) just as you wish or very
> close in stock condition, with the same guitar & setup? If perchance
> there hasn't been one, the problem may not be amp-related.
Sound city 120, Plexi (although unatenuated with a DSP driving it), Marshall
2210, boogies, fenders... the one that sounded best to me was the plexi
(cranked) but it was too loud for the band situation hence playing through
the DSP.
>
> If you "maybe don't like pentode distortion", it is hard to imagine
> what tube guitar amp you do like for an overdriven sound, since that's
> about all there is in popular use.
6l6, kt66, kt88 aren't pentodes... but I see you. I find it hard to believe
that deep down this amp does want to please me.
>
> Don't get rid of the grid stoppers unless you like risking disaster
> (parasitics).
>
> If this experimenting gives you recreational pleasure,
It does...
do enjoy that,
> but you may want to consider it is counterproductive in terms of
> diagnosing & solving a problem, or even defining one in behavioral
> terms which are specifically conveyable in writing.
I understand what you mean but I've got very limited tooling during the week
(and paradoxically more time) and can resort to lots of fairly limited
tests.
From what I've discovered, the preamp's actually fine as it is - plenty of
tasty overdrive there but the power amp side of things needs looking at.
This amp wasn't stock - it had the mains transformer changed at some point
in it's life and I can't think of a way to check how much current it can
deliver at the stated voltages and even then, I've only a 200mA ballpark
figure to compare that with. I can say though that people have provided me
with a lot of good input and for that I'm very grateful. Now, I could take
it to a tech and have him "fix it" but what would I learn? and would it be
done right?
Thanks for your input.
cb
Lord Valve
If you'll just run the amp up to a soft clip across a load
(1 KHz will work fine) and eyeball the top and bottom
of the wave, you can see whether your damn filters are
shot or not. If you see fuzz or hashy-looking lines at the
+/- tips of the trace, replace 'em. The more crud you see,
the worse they are. An ESR check won't hurt either.
LV