Orange AD 15 Combo ...

[Arfa Daily]

Anybody got any experience of this little fella ? Although it carries the
Orange name, it was actually manufactured by Trace Idiott. It is a very
straightforward amp comprising ECC83 preamp, ECC83 phase splitter, pair of
EL84 outputs and a GZ34 reccy.

The complaint is "turn it up and listen to it howl". Well, with nothing
plugged in, so that the input jack is shorting both the input, and the grid
of the second half of the first ECC83, it's as quiet as the grave. However,
as soon as you plug anything in, it gets very 'hum-y'. Now I would be the
first to admit that the workshop test lead is not of the highest low-noise
quality, neither is the test guitar an expensive one, but both have always
worked fine enough in any other of the several amps per week that I repair.
When I say that this amp becomes hum-y when you plug anything in, anyone who
repairs these things will know what I mean. I don't mean that it sounds in
any way actually faulty - as in open grounds or bad joints on the input
socket or any of the other usual culprits - it just doesn't feel 'right'.
The more you wind the gain and master up, the noisier it gets, until it
bursts into oscillation at a couple of kHz or so. You might be half inclined
to think that the noise from the speaker is rattling the valves, but none
are in the least microphonic, tap-wise, and I have tried another set. If I
terminate the input lead with a 47k R instead of a guitar pickup, it still
seems to pick up a lot of hum on the lead, but you can then get it up to
flat out on gain and master, without it actually starting to howl, which
might suggest that the guitar is mechanically picking up the hum from the
speaker and resonating, but it still howls, even with the strings damped by
a hand across them.

One thing is that the input stage is a little unusual in its design -
although not a lot. Input socket is DC coupled straight to the grid of the
first ECC83 half, via a 68k grid stopper in series. Normal enough. It has a
1M grid leak R, which is normal enough, but unusually, this is connected on
the 'hot' side of the stopper resistor, rather than on the grid side,
although in practice, this shouldn't matter. The cathode bias R is 1.5k.
Again, normal enough, but unusually, it is un-bypassed. The schematic shows
an electrolytic across it, as you would normally find, but it has no value
shown, and is just marked "o/c" presumably for "open circuit". It has a
position on the pcb, but nothing fitted. Anode load R is 100k, as normal.
Anode is AC coupled to the top of the gain control. Wiper of said control is
AC coupled to the grid of the second half of the first ECC83. No stopper. No
grid leak. 1k5 cathode R, bypassed this time with 22uF. Anode load again
100k. 3-way tone network connected between this anode and phase splitter
first-half grid. Other grid driven from tap on split cathode R. Each anode
AC coupled to a dual gang control that is the master, and then into a
totally conventional cathode biased push-pull output stage. So whole thing
pretty conventional. Except that it all seems to have too much gain for its
own good, although given the very typical values, I can't see how it can
have.

So, anyone ever had a play with one of these ? Is this behaviour 'normal'
for it - perhaps overall lousy design of the pcb layout ? Alternatively,
anyone got any other thoughts on what is going on here. Nothing
component-wise reads faulty. Every voltage is pretty much spot on to what it
says on the test points on the pcb. All four HT rails have their own R-C
decoupling network, each following on from the previous one, and none of the
rails has excessive ripple. Heater winding is 6.3v centre tapped. Outputs
hooked straight across whole winding. ECC tapped heaters paralleled up and
connected across whole winding also.

An amp as simple as this should not represent any kind of puzzle, but at the
moment, it does ...

Arfa

 

[Baron]

Anybody got any experience of this little fella ? Although it carries
the Orange name, it was actually manufactured by Trace Idiott. It is a
very straightforward amp comprising ECC83 preamp, ECC83 phase
splitter, pair of EL84 outputs and a GZ34 reccy.

The complaint is "turn it up and listen to it howl". Well, with
nothing plugged in, so that the input jack is shorting both the input,
and the grid of the second half of the first ECC83, it's as quiet as
the grave. However, as soon as you plug anything in, it gets very
'hum-y'. Now I would be the first to admit that the workshop test lead
is not of the highest low-noise quality, neither is the test guitar an
expensive one, but both have always worked fine enough in any other of
the several amps per week that I repair. When I say that this amp
becomes hum-y when you plug anything in, anyone who repairs these
things will know what I mean. I don't mean that it sounds in any way
actually faulty - as in open grounds or bad joints on the input socket
or any of the other usual culprits - it just doesn't feel 'right'. The
more you wind the gain and master up, the noisier it gets, until it
bursts into oscillation at a couple of kHz or so. You might be half
inclined to think that the noise from the speaker is rattling the
valves, but none are in the least microphonic, tap-wise, and I have
tried another set. If I terminate the input lead with a 47k R instead
of a guitar pickup, it still seems to pick up a lot of hum on the
lead, but you can then get it up to flat out on gain and master,
without it actually starting to howl, which might suggest that the
guitar is mechanically picking up the hum from the speaker and
resonating, but it still howls, even with the strings damped by a hand
across them.

One thing is that the input stage is a little unusual in its design -
although not a lot. Input socket is DC coupled straight to the grid of
the first ECC83 half, via a 68k grid stopper in series. Normal enough.
It has a 1M grid leak R, which is normal enough, but unusually, this
is connected on the 'hot' side of the stopper resistor, rather than on
the grid side, although in practice, this shouldn't matter. The
cathode bias R is 1.5k. Again, normal enough, but unusually, it is
un-bypassed. The schematic shows an electrolytic across it, as you
would normally find, but it has no value shown, and is just marked
"o/c" presumably for "open circuit". It has a position on the pcb, but
nothing fitted. Anode load R is 100k, as normal. Anode is AC coupled
to the top of the gain control. Wiper of said control is AC coupled to
the grid of the second half of the first ECC83. No stopper. No grid
leak. 1k5 cathode R, bypassed this time with 22uF. Anode load again
100k. 3-way tone network connected between this anode and phase
splitter first-half grid. Other grid driven from tap on split cathode
R. Each anode AC coupled to a dual gang control that is the master,
and then into a totally conventional cathode biased push-pull output
stage. So whole thing pretty conventional. Except that it all seems to
have too much gain for its own good, although given the very typical
values, I can't see how it can have.

So, anyone ever had a play with one of these ? Is this behaviour
'normal' for it - perhaps overall lousy design of the pcb layout ?
Alternatively, anyone got any other thoughts on what is going on here.
Nothing component-wise reads faulty. Every voltage is pretty much spot
on to what it says on the test points on the pcb. All four HT rails
have their own R-C decoupling network, each following on from the
previous one, and none of the rails has excessive ripple. Heater
winding is 6.3v centre tapped. Outputs hooked straight across whole
winding. ECC tapped heaters paralleled up and connected across whole
winding also.

An amp as simple as this should not represent any kind of puzzle, but
at the moment, it does ...

Arfa

WAG A poor ground on one of the output bottles !

 

[Gareth Magennis]

Anybody got any experience of this little fella ? Although it carries the
Orange name, it was actually manufactured by Trace Idiott. It is a very
straightforward amp comprising ECC83 preamp, ECC83 phase splitter, pair of
EL84 outputs and a GZ34 reccy.

The complaint is "turn it up and listen to it howl". Well, with nothing
plugged in, so that the input jack is shorting both the input, and the
grid of the second half of the first ECC83, it's as quiet as the grave.
However, as soon as you plug anything in, it gets very 'hum-y'. Now I
would be the first to admit that the workshop test lead is not of the
highest low-noise quality, neither is the test guitar an expensive one,
but both have always worked fine enough in any other of the several amps
per week that I repair. When I say that this amp becomes hum-y when you
plug anything in, anyone who repairs these things will know what I mean. I
don't mean that it sounds in any way actually faulty - as in open grounds
or bad joints on the input socket or any of the other usual culprits - it
just doesn't feel 'right'. The more you wind the gain and master up, the
noisier it gets, until it bursts into oscillation at a couple of kHz or
so. You might be half inclined to think that the noise from the speaker is
rattling the valves, but none are in the least microphonic, tap-wise, and
I have tried another set. If I terminate the input lead with a 47k R
instead of a guitar pickup, it still seems to pick up a lot of hum on the
lead, but you can then get it up to flat out on gain and master, without
it actually starting to howl, which might suggest that the guitar is
mechanically picking up the hum from the speaker and resonating, but it
still howls, even with the strings damped by a hand across them.

One thing is that the input stage is a little unusual in its design -
although not a lot. Input socket is DC coupled straight to the grid of the
first ECC83 half, via a 68k grid stopper in series. Normal enough. It has
a 1M grid leak R, which is normal enough, but unusually, this is connected
on the 'hot' side of the stopper resistor, rather than on the grid side,
although in practice, this shouldn't matter. The cathode bias R is 1.5k.
Again, normal enough, but unusually, it is un-bypassed. The schematic
shows an electrolytic across it, as you would normally find, but it has no
value shown, and is just marked "o/c" presumably for "open circuit". It
has a position on the pcb, but nothing fitted. Anode load R is 100k, as
normal. Anode is AC coupled to the top of the gain control. Wiper of said
control is AC coupled to the grid of the second half of the first ECC83.
No stopper. No grid leak. 1k5 cathode R, bypassed this time with 22uF.
Anode load again 100k. 3-way tone network connected between this anode and
phase splitter first-half grid. Other grid driven from tap on split
cathode R. Each anode AC coupled to a dual gang control that is the
master, and then into a totally conventional cathode biased push-pull
output stage. So whole thing pretty conventional. Except that it all seems
to have too much gain for its own good, although given the very typical
values, I can't see how it can have.

So, anyone ever had a play with one of these ? Is this behaviour 'normal'
for it - perhaps overall lousy design of the pcb layout ? Alternatively,
anyone got any other thoughts on what is going on here. Nothing
component-wise reads faulty. Every voltage is pretty much spot on to what
it says on the test points on the pcb. All four HT rails have their own
R-C decoupling network, each following on from the previous one, and none
of the rails has excessive ripple. Heater winding is 6.3v centre tapped.
Outputs hooked straight across whole winding. ECC tapped heaters
paralleled up and connected across whole winding also.

An amp as simple as this should not represent any kind of puzzle, but at
the moment, it does ...

Arfa

You could try checking the grounding arrangements.
I've come across weird hum problems caused by someone having replaced an
input socket with one that grounds to the chassis where it shouldn't, or
vice versa, and also amps that rely partly on grounding through the control
pot bodies making good contact with the chassis, where the nuts are loose or
again a different pot has been fitted.
Try running/cutting ground links to and from various points, particularly
around the input.


Does it have a reverb tank? Fitted the right way round?



Gareth.

 

[Arfa Daily]

You could try checking the grounding arrangements.
I've come across weird hum problems caused by someone having replaced an
input socket with one that grounds to the chassis where it shouldn't, or
vice versa, and also amps that rely partly on grounding through the
control pot bodies making good contact with the chassis, where the nuts
are loose or again a different pot has been fitted.
Try running/cutting ground links to and from various points, particularly
around the input.


Does it have a reverb tank? Fitted the right way round?



Gareth.

Hi Gareth. All regular stuff, and what both of us come across all the time,
I'm sure, so basically all already checked. Everything looks original. The
input socket is on it's own little sub-pcb, and there is no evidence at all
that it has ever been disturbed. This is connected to the main pcb by a
short piece of screened cable, again undisturbed from factory original. All
pot bodies are making firmly to the chassis, and no mounting hardware is
loose. There aren't really any external ground links to cut, and adding
grounds doesn't seem to make a lot of difference. Caveat. There is a
difference when a mains protective ground is connected, against when it's
not i.e. with it running on a standard bench isolation transformer. It is
better when the ground is on - that is with the amp plugged into raw mains.
Second, it is worse when the 'scope's probe ground is hooked onto the
chassis. Both of these observations suggest a ground issue, but I'm still
tending towards poor design, rather than a fault as such. No reverb tank
fitted. Thanks for suggestions.

Arfa

 

[Arfa Daily]

Arfa Daily wrote in message ...


The grid has to have a DC path to ground, otherwise the coupling cap won't
have a path to charge and discharge. Eventually , the tube will be cutoff
from the stream of electrons charging the grid negative
There has to be a resistor from grid to ground or perhaps the wiper is not
coupled through a capacitor. In any case , I don't think think that this
would be the cause of your feedback problem.

Yes, absolutely true. I didn't put it very well. The grid has a DC return
path, as you say, via the gain pot. The stage does not have additional AC
coupling between the pot and the grid, with a separate fixed grid leak, as
you might reasonably expect. So if the gain pot is at minimum, the grid will
actually be DC grounded.

All four HT rails have their own R-C

The decoupling RC networks prevent audio from passing through the B+. The
high voltage is common to several stages, if audio couples from one stage
to
the other through the B+, the amp will break into oscillation.The ripple
might look low but when the amp breaks into oscillation, there is a good
possibility that an audio signal is present on the high voltage. Many cap
meters only test at power line frequencies. It is possible that the
decoupling caps are not working as caps at higher audio frequencies, and
it
is also possible that the caps might be borderline. I would substitute
those
caps with any junk laying around just to rule them out.

That is a possibility. I checked the caps with an ESR meter, which operates
at an equivalent frequency of around 100kHz, so I was not just declaring
them good by value, or the fact that there was little in the way of 100Hz
ripple. Just as a matter of interest, none of the individually filtered HT
rails are 'shared' as such by more than one stage. First filter nearest the
reccy, feeds just the output anodes. Second, just the output screen grids.
Third, just the phase splitter and fourth just the preamp valve, although I
suppose that you could argue that represents two stages. I'm pretty sure
that when it was howling, I did 'scope the four HT rails, and there was
nothing significant on any of them, but perhaps not. It's just the sort of
check that you make automatically in these cases without even thinking about
it. I'll take a better look at this angle on Monday.

Another possibility is that the ground for the preamp and the ground for
the
power amp might share a common path. That path might haved aged to a point
where it is no longer well grounded. Tighten any hardware, like jacks ,
grounding screws etc...
If possible, seperate the two grounds by running seperate wires from those
stages back to the negative terminal of the power supply.


bg

Not especially easy to do, as the power supply is all part of the same PCB
with the whole of the rest of the amp. There are no signs that any hardware
is loose, or that any mechanically made grounds, such as via pot bodies etc,
are not good. I repair a lot of valve amps for a living, and have done for
many many years, so these are all suggestions that are basic second nature
to me, hence why I'm finding this an 'odd' problem. Still, thanks for all
the suggestions anyway.

This is one of those situations where you would really like to have another
one to compare by, which is why I asked if anyone had any direct experience
of this amp.

Arfa

 

[Gareth Magennis]

Hi Gareth. All regular stuff, and what both of us come across all the
time, I'm sure, so basically all already checked. Everything looks
original. The input socket is on it's own little sub-pcb, and there is no
evidence at all that it has ever been disturbed. This is connected to the
main pcb by a short piece of screened cable, again undisturbed from
factory original. All pot bodies are making firmly to the chassis, and no
mounting hardware is loose. There aren't really any external ground links
to cut, and adding grounds doesn't seem to make a lot of difference.
Caveat. There is a difference when a mains protective ground is connected,
against when it's not i.e. with it running on a standard bench isolation
transformer. It is better when the ground is on - that is with the amp
plugged into raw mains. Second, it is worse when the 'scope's probe ground
is hooked onto the chassis. Both of these observations suggest a ground
issue, but I'm still tending towards poor design, rather than a fault as
such. No reverb tank fitted. Thanks for suggestions.

Arfa

Well if I had a penny for all those pieces of equipment I'd tried to find
the fault on that actually had no fault but was some sort of
shitty/quirky/misunderstod design shennanigan, I would have several pennies
in my pocket and lots more time on my hands to spend on finding faults that
actutally weren't faults but some sort of shitty/quirky/misunderstood design
shennanigan ...........








Gareth.

 

[N_Cook]

Anybody got any experience of this little fella ? Although it carries the
Orange name, it was actually manufactured by Trace Idiott. It is a very
straightforward amp comprising ECC83 preamp, ECC83 phase splitter, pair of

Arfa

Where is the mains ground connected into the circuit ? checked for mains
transformer P/Sec/ground leakage ?

 

[Arfa Daily]

Well if I had a penny for all those pieces of equipment I'd tried to find
the fault on that actually had no fault but was some sort of
shitty/quirky/misunderstod design shennanigan, I would have several
pennies in my pocket and lots more time on my hands to spend on finding
faults that actutally weren't faults but some sort of
shitty/quirky/misunderstood design shennanigan ...........

Gareth.

LOL. Yeah, likewise ! This trade sure doesn't get any easier, does it ...?
d;~}

Arfa

 

[Arfa Daily]

Where is the mains ground connected into the circuit ? checked for mains
transformer P/Sec/ground leakage ?

Good thought. I'll have a look tomorrow. Thanks.

Arfa

 

[William Sommerwerck]

I'm inclined to agree that the problem is "bad design".

When J Gordon Holt reviewed the classic Futterman output-transformerless
amplifier, he reported a hum problem when using a Decca pickup (which had a
common L/R ground), despite the fact that the pickup was "isolated" from the
amplifier by the preamp! Amazingly, Julius Futterman tracked down the
problem and fixed it.

PS: I would love to hear the Futterman. I've heard systems with later
versions, but never the original in my own system.

 

[Arfa Daily]

Oscillation means feedback. There has to be a feedback path, and the gain
has to be high enough to sustain oscillation. I found a web page that
mentioned something about a 1000p cap placed across a plate load resistor
specifically to avoid oscillation. The schematic didn't appear to be from
an
ad 15 combo. Some people were removing that cap as a mod to get a brighter
sounding amp.
Another thought - the schematic I downloaded for this amp shows a
connection
from the 2nd stage grid back to the first stage input jack. It appears
that
the purpose of this is to ground the second grid when nothing is plugged
in.
If something is a bit bent in that jack, it might be possible for this 2nd
grid to be connected to the 1st grid, which will make a feedback loop
when
the volume control is turned up.
Positive feedback will also increase the noise. Something has to have
changed because I don't think they would have been able to sell a crappy
design and stay in business.

Good thoughts. I will check them out Monday. Interestingly, I did try
hanging 1000pf across both of the load resistors, and they did produce a
significant improvement, but not enough to actually totally stop it hooting.
You could also definitely hear these caps starting to knock the 'brightness'
on the head. As you can now see from the schematic, one or two aspects are a
little 'unusual' compared to the norm - that additional connection for
grounding the second stage grid for instance, or the dual gang master pot.

I must admit that if it is down to poor design, the effects do seem a little
extreme, even given that the "Orange" name is just a badge, and it *is* a
Trace Idiott design ... It's certainly not the best constructed item I've
ever seen. Thanks all for the continued interest and suggestions. I'll post
back if I get anywhere further with it as a result.

Arfa

 

[Arfa Daily]

Arfa Daily wrote in message ...
I tried to find the website again but had no luck. The cap appeared to be
connected across r4 of the phase inverter. You could try yanking V2 out
and
see what happens with the gain cranked up.

Hmmm. Interesting. I'll give it a go, and let you know.

Arfa

 

[[email protected]]

Hmmm. Interesting. I'll give it a go, and let you know.

Arfa

Hi,
Sorry to butt in but did you ever find the problem? I have an AD30
with similar symptoms and would be very grateful if you could let me
know what you discovered and what you had to do to rectify it.

Many Thanks

Thomas