Hello all,

I'm not at all a specialist in electronics at all although I have few basis.
Moreover I'm french so I'm sorry for my bad english.

I'm the owner of a Vox AC30 guitar amp (manufacturing date : beginning of
eighties). I recently replaced the amp tubes (4 x EL84) as the previous set
was quite old (3 years) and started to sound ugly. Since I did that, I have
a problem: after few seconds of use the amp stage behaves strangely. The
output level oscillates (from nothing to max level with a frequency very
similar to a beating heart. When I have a look to the output tubes I can see
that there's an oscillation of the high-voltage current, which is not
synchronized for both pairs of tubes. Of course this oscillation has the
same frequency than the "heart-beating" phenomenon on the output of the
amplifier.
I tried to put back the old tubes, or to use other tubes, but it does not
improve the problem. I don't understand what happens...
Does anybody have an idea about that and about what I could do in order to
fix this problem?

Thank you very much for your help.

Joffrey.

"Joffrey Heyraud"
Hello all,

I'm not at all a specialist in electronics at all although I have few
basis. Moreover I'm french so I'm sorry for my bad english.

I'm the owner of a Vox AC30 guitar amp (manufacturing date : beginning of
eighties). I recently replaced the amp tubes (4 x EL84) as the previous
set was quite old (3 years) and started to sound ugly. Since I did that, I
have a problem: after few seconds of use the amp stage behaves strangely.
The output level oscillates (from nothing to max level with a frequency
very similar to a beating heart. When I have a look to the output tubes I
can see that there's an oscillation of the high-voltage current, which is
not synchronized for both pairs of tubes. Of course this oscillation has
the same frequency than the "heart-beating" phenomenon on the output of
the amplifier.
I tried to put back the old tubes, or to use other tubes, but it does not
improve the problem. I don't understand what happens...
Does anybody have an idea about that and about what I could do in order to
fix this problem?



** A visit to an amp tech seems in order - note not a TV, VCR or PC tech
!!

Or you could just **** about with it yourself and make matters MUCH worse.





............... Phil

Joffrey Heyraud wrote:

Hello all,

I'm not at all a specialist in electronics at all although I have few basis.
Moreover I'm french so I'm sorry for my bad english.

I'm the owner of a Vox AC30 guitar amp (manufacturing date : beginning of
eighties). I recently replaced the amp tubes (4 x EL84) as the previous set
was quite old (3 years) and started to sound ugly. Since I did that, I have
a problem: after few seconds of use the amp stage behaves strangely. The
output level oscillates (from nothing to max level with a frequency very
similar to a beating heart. When I have a look to the output tubes I can see
that there's an oscillation of the high-voltage current, which is not
synchronized for both pairs of tubes. Of course this oscillation has the
same frequency than the "heart-beating" phenomenon on the output of the
amplifier.
I tried to put back the old tubes, or to use other tubes, but it does not
improve the problem. I don't understand what happens...
Does anybody have an idea about that and about what I could do in order to
fix this problem?

Thank you very much for your help.

Joffrey.

Unless you have a lot of experience, leave it alone,
and take it to a tech who you know definately handles tube amp
repairs.

We cannot diagnose your problem with the info
you have given.

Patrick Turner.

On Sun, 13 Mar 2005 13:42:33 +0100, "Joffrey Heyraud"
wrote:

The
output level oscillates (from nothing to max level with a frequency very
similar to a beating heart. When I have a look to the output tubes I can see
that there's an oscillation of the high-voltage current, which is not
synchronized for both pairs of tubes. Of course this oscillation has the
same frequency than the "heart-beating" phenomenon on the output of the
amplifier.
I tried to put back the old tubes, or to use other tubes, but it does not
improve the problem. I don't understand what happens...
Does anybody have an idea about that and about what I could do in order to
fix this problem?

In general, low frequency oscillations (sometimes called
"motorboating") are caused by bad power supply capacitors.

Twenty years is a pretty good age for electrolytic caps in
a hot environment. Replacing the high voltage caps might
be a reasonable shotgun approach, and likely due soon
anyway.

Remember, safety first and last. Measure residual voltages;
don't allow children or pets into the room; keep one
hand in your pocket, etc. If you're not experienced with
dangerous voltages, get an experienced friend to oversee
the job, or hire someone to do it.

Good fortune,

Chris Hornbeck

Hi Joffrey

In my experience of AC30s I have found that power supply caps need replacing
often. Also the AC30 suffers from overheating, I have found that many
resistors increase in value after time. Its not unknown for a 220k resistor
to have the value of 500k. I suggest checking the value of each resistor as
a matter of course, replace any that differ from the installed
value......make sure all caps are discharged before testing.
This may or may not have anything to do with your problem but its always
worth checking, if in doubt take it to someone more technical.

Regards
Fox


"Joffrey Heyraud" wrote in message
...
Hello all,

I'm not at all a specialist in electronics at all although I have few
basis.
Moreover I'm french so I'm sorry for my bad english.

I'm the owner of a Vox AC30 guitar amp (manufacturing date : beginning of
eighties). I recently replaced the amp tubes (4 x EL84) as the previous
set
was quite old (3 years) and started to sound ugly. Since I did that, I
have
a problem: after few seconds of use the amp stage behaves strangely. The
output level oscillates (from nothing to max level with a frequency very
similar to a beating heart. When I have a look to the output tubes I can
see
that there's an oscillation of the high-voltage current, which is not
synchronized for both pairs of tubes. Of course this oscillation has the
same frequency than the "heart-beating" phenomenon on the output of the
amplifier.
I tried to put back the old tubes, or to use other tubes, but it does not
improve the problem. I don't understand what happens...
Does anybody have an idea about that and about what I could do in order to
fix this problem?

Thank you very much for your help.

Joffrey.

"Fox" wrote in message
...
Hi Joffrey

In my experience of AC30s I have found that power supply caps need
replacing
often. Also the AC30 suffers from overheating, I have found that many
resistors increase in value after time. Its not unknown for a 220k
resistor
to have the value of 500k. I suggest checking the value of each resistor
as
a matter of course, replace any that differ from the installed
value......make sure all caps are discharged before testing.
This may or may not have anything to do with your problem but its
always
worth checking, if in doubt take it to someone more technical.

Regards
Fox


Hi - this is the best advice you've got thus far - also, I'm not familiar
with the "vox" from the 80's - the company name has been bought and sold too
many times. Your 80's AC30 has fairly little in common with the original.
From the sound of it, it does sound like a filter cap gone bad - I don't
remember the '80's as being the high point in toob amp world.
Pull out the driver tube - does the amp still behave the same? If
oscillation stops, you *probably* have bad filter caps - don't bother
testing them - replace them all, with as close to identical values as you
can obtain. The size of PS caps in a guitar amp *does* affect it's sound.
Visually inspect the amp - is one of the output tubes glow more than the
other? swap them around (use the old tubes - one of the new ones could
have been bad & cooked your amp, or required a drastically different bias) -
is the same "socket" behaving the same way? Do this basic stuff, give us a
link to the schematics, perhaps then someone could give you a definitive
answer. BTW, when you say AC30 made in 1980's, it makes many of us
squirm...
-dim, who firmly believes that even Matchless (amps, not bikes) are a
triumph of high price over *both* engineering & craftsmanship.

Hi all,

Thank you very much for your advices. It seems the filter caps have never
been replaced so it could be a good track.
Unfortunately, I can't try the same thing without the driver tube, as this
"motorboating" requires a strong "attack" before starting. It does not
start by itself or if the output level is very low. It requires a high
output level and strong string attacks. However, once it's started it does
not stop even if the level is decreased. I need to turn the amp off.
In the case it could help here is a schematic of the amp:
https://schematicheavencom.secure.po...s/ac301978.pdf.

Thank you very much once again.

Sincerely,

Joffrey.

"shiva" a écrit dans le message de news:
YQ%Yd.1943$jt6.728@trndny07...

"Fox" wrote in message
...
Hi Joffrey

In my experience of AC30s I have found that power supply caps need
replacing
often. Also the AC30 suffers from overheating, I have found that many
resistors increase in value after time. Its not unknown for a 220k
resistor
to have the value of 500k. I suggest checking the value of each resistor
as
a matter of course, replace any that differ from the installed
value......make sure all caps are discharged before testing.
This may or may not have anything to do with your problem but its
always
worth checking, if in doubt take it to someone more technical.

Regards
Fox


Hi - this is the best advice you've got thus far - also, I'm not familiar
with the "vox" from the 80's - the company name has been bought and sold
too
many times. Your 80's AC30 has fairly little in common with the original.
From the sound of it, it does sound like a filter cap gone bad - I don't
remember the '80's as being the high point in toob amp world.
Pull out the driver tube - does the amp still behave the same? If
oscillation stops, you *probably* have bad filter caps - don't bother
testing them - replace them all, with as close to identical values as you
can obtain. The size of PS caps in a guitar amp *does* affect it's sound.
Visually inspect the amp - is one of the output tubes glow more than the
other? swap them around (use the old tubes - one of the new ones could
have been bad & cooked your amp, or required a drastically different
bias) -
is the same "socket" behaving the same way? Do this basic stuff, give us
a
link to the schematics, perhaps then someone could give you a definitive
answer. BTW, when you say AC30 made in 1980's, it makes many of us
squirm...
-dim, who firmly believes that even Matchless (amps, not bikes) are a
triumph of high price over *both* engineering & craftsmanship.

"Joffrey Heyraud" wrote

It's interesting that the problem persists with the old valves.

The extra current drawn by the fresh valves would mean the first PS
filter cap would see more ripple, so possibly an old one would
choose that occasion to die. If its effective capacitance falls,
then the LF resonant frequency of the amp rises to become within the
amp's frequency range. The consequent oscillation is called
motorboating.

It may be a good idea to replace all the electrolytics if you want a
reliable amp.

I've always wondered...does it really sound like a motorboat? I
imagine a little chugging fishing boat?

cheers, Ian

"François Yves Le Gal" wrote in message
...
On Sun, 13 Mar 2005 18:43:04 GMT, "shiva" wrote:

-dim, who firmly believes that even Matchless (amps, not bikes) are a
triumph of high price over *both* engineering & craftsmanship.

Looking at their schematics is always interesting...
https://schematicheavencom.secure.po...ss_clubman.pdf
(Hint: search fro MASTER and BRILLIANT)


Yikes... I can't open the link - what has schematichaven done? What is
that secure server doing? Problems, problems. Perhaps I'll just stick to
doing what I know best - smoking and drinking cofee...
-dim (will look it up later, though...)

To be honest, mine rather sounds like a beating heart than a motorboat.
However I understand why it's been called like that.

Thanks again.

Joffrey.


"Ian Iveson" a écrit dans le message de
news: ...
"Joffrey Heyraud" wrote

It's interesting that the problem persists with the old valves.

The extra current drawn by the fresh valves would mean the first PS filter
cap would see more ripple, so possibly an old one would choose that
occasion to die. If its effective capacitance falls, then the LF resonant
frequency of the amp rises to become within the amp's frequency range. The
consequent oscillation is called motorboating.

It may be a good idea to replace all the electrolytics if you want a
reliable amp.

I've always wondered...does it really sound like a motorboat? I imagine a
little chugging fishing boat?

cheers, Ian

"Joffrey Heyraud"

To be honest, mine rather sounds like a beating heart than a motorboat.


** Sounds just like a single cylinder, 4-stroke marine petrol engine.




............. Phil