Help with dead Ampeg SVT-3 PRO

[Barry Wiseman]

Just bought this unit on ebay, and it looks to be in near-mint
condition. It powers up (fan and lights come on) but I get no signal
from either the pre-amp out or any of the speaker jacks.

Removing the cover, I see that the two tubes on the rear circuit board
light up, but the three on the front board do not. Swapping the 12AX7
from the rear section with one of the front ones shows that the tubes
are not the problem. All connectors appear to be firmly seated.

Any ideas? I'd hate to get involved with trying to return the unit to
the seller if it's some easily fixed problem. Thanks.

[Fred Marx]

Post by Barry Wiseman
Just bought this unit on ebay, and it looks to be in near-mint
condition. It powers up (fan and lights come on) but I get no signal
from either the pre-amp out or any of the speaker jacks.
Removing the cover, I see that the two tubes on the rear circuit board
light up, but the three on the front board do not. Swapping the 12AX7
from the rear section with one of the front ones shows that the tubes
are not the problem. All connectors appear to be firmly seated.
Any ideas? I'd hate to get involved with trying to return the unit to
the seller if it's some easily fixed problem. Thanks.

knowing that sometimes the small things get over looked.... you checked
the mute switch on the front panel right? :) Something I learned is
that Ampeg has fairly good coverage when It come to factory authorized
service outlets, it might be worth checking to see if one is near you
and having them look it over.... there are schematics available at
Ampeg.com which I can't get to currently.....

[Barry Wiseman]

Post by Fred Marx

Post by Barry Wiseman
Just bought this unit on ebay, and it looks to be in near-mint
condition. It powers up (fan and lights come on) but I get no signal
from either the pre-amp out or any of the speaker jacks.
Removing the cover, I see that the two tubes on the rear circuit board
light up, but the three on the front board do not. Swapping the 12AX7
from the rear section with one of the front ones shows that the tubes
are not the problem. All connectors appear to be firmly seated.
Any ideas? I'd hate to get involved with trying to return the unit to
the seller if it's some easily fixed problem. Thanks.

knowing that sometimes the small things get over looked.... you checked
the mute switch on the front panel right? :) Something I learned is
that Ampeg has fairly good coverage when It come to factory authorized
service outlets, it might be worth checking to see if one is near you
and having them look it over.... there are schematics available at
Ampeg.com which I can't get to currently.....

Fred, thanks for the reply, and exactly the kind of dumb but simple
solution I was hoping for. :-) Unfortunately the mute switch ain't it.
I will look into local authorized service, I'm in the NYC area, there
must be one around. As for ampeg.com, the site appears to be down right
now, as I can't pull it up either.

[Fred Marx]

Post by Barry Wiseman

Post by Fred Marx

Post by Barry Wiseman
Just bought this unit on ebay, and it looks to be in near-mint
condition. It powers up (fan and lights come on) but I get no signal
from either the pre-amp out or any of the speaker jacks.
Removing the cover, I see that the two tubes on the rear circuit board
light up, but the three on the front board do not. Swapping the 12AX7
from the rear section with one of the front ones shows that the tubes
are not the problem. All connectors appear to be firmly seated.
Any ideas? I'd hate to get involved with trying to return the unit to
the seller if it's some easily fixed problem. Thanks.

knowing that sometimes the small things get over looked.... you checked
the mute switch on the front panel right? :) Something I learned is
that Ampeg has fairly good coverage when It come to factory authorized
service outlets, it might be worth checking to see if one is near you
and having them look it over.... there are schematics available at
Ampeg.com which I can't get to currently.....

Fred, thanks for the reply, and exactly the kind of dumb but simple
solution I was hoping for. :-) Unfortunately the mute switch ain't it.
I will look into local authorized service, I'm in the NYC area, there
must be one around. As for ampeg.com, the site appears to be down right
now, as I can't pull it up either.

You said the fans powers up, does it power up and stay up or power up
and stop? If it keeps runing it may indicate a poroblem with thermal
regulaton. As far as the three apparently dead tubes. I suspect there
is a problem there. I have a schematic out, one set of tubes is tied
preamp stage and the tone shaping section. The others are tied to the
post amp stage and are contolled by the tube gain knob. The Bass Mid
and Treble controls are pass/no pass as opposed to boost/cut, so iff
they were all down (0) you'd have no sound. You could also try the
balance output (it's a DI) at the back panel, it might help you trouble
shoot... Also try sending a signal to the power amp input on the back
panel, it should bypass the pre amp section and help eliminate the post
as the problem

[Barry Wiseman]

Post by Fred Marx

Post by Barry Wiseman

Post by Fred Marx

Post by Barry Wiseman
Just bought this unit on ebay, and it looks to be in near-mint
condition. It powers up (fan and lights come on) but I get no signal
from either the pre-amp out or any of the speaker jacks.
Removing the cover, I see that the two tubes on the rear circuit board
light up, but the three on the front board do not. Swapping the 12AX7
from the rear section with one of the front ones shows that the tubes
are not the problem. All connectors appear to be firmly seated.
Any ideas? I'd hate to get involved with trying to return the unit to
the seller if it's some easily fixed problem. Thanks.

knowing that sometimes the small things get over looked.... you checked
the mute switch on the front panel right? :) Something I learned is
that Ampeg has fairly good coverage when It come to factory authorized
service outlets, it might be worth checking to see if one is near you
and having them look it over.... there are schematics available at
Ampeg.com which I can't get to currently.....

Fred, thanks for the reply, and exactly the kind of dumb but simple
solution I was hoping for. :-) Unfortunately the mute switch ain't it.
I will look into local authorized service, I'm in the NYC area, there
must be one around. As for ampeg.com, the site appears to be down right
now, as I can't pull it up either.

You said the fans powers up, does it power up and stay up or power up
and stop? If it keeps runing it may indicate a poroblem with thermal
regulaton. As far as the three apparently dead tubes. I suspect there
is a problem there. I have a schematic out, one set of tubes is tied
preamp stage and the tone shaping section. The others are tied to the
post amp stage and are contolled by the tube gain knob. The Bass Mid
and Treble controls are pass/no pass as opposed to boost/cut, so iff
they were all down (0) you'd have no sound. You could also try the
balance output (it's a DI) at the back panel, it might help you trouble
shoot... Also try sending a signal to the power amp input on the back
panel, it should bypass the pre amp section and help eliminate the post
as the problem

Fan powers up and keeps running, although quietly. As for DI out and
power in, those are good suggestions but I have no gear here to try 'em
with. Will have to bring this unit over to my rehearsal space in a few
days.

[Fred Marx]

This post might be inappropriate. Click to display it.

[Fred Marx]

Post by Barry Wiseman

Post by Fred Marx

Post by Barry Wiseman
Just bought this unit on ebay, and it looks to be in near-mint
condition. It powers up (fan and lights come on) but I get no signal
from either the pre-amp out or any of the speaker jacks.
Removing the cover, I see that the two tubes on the rear circuit
board light up, but the three on the front board do not. Swapping
the 12AX7 from the rear section with one of the front ones shows that
the tubes are not the problem. All connectors appear to be firmly
seated.
Any ideas? I'd hate to get involved with trying to return the unit
to the seller if it's some easily fixed problem. Thanks.

knowing that sometimes the small things get over looked.... you
checked the mute switch on the front panel right? :) Something I
learned is that Ampeg has fairly good coverage when It come to factory
authorized service outlets, it might be worth checking to see if one
is near you and having them look it over.... there are schematics
available at Ampeg.com which I can't get to currently.....

Fred, thanks for the reply, and exactly the kind of dumb but simple
solution I was hoping for. :-) Unfortunately the mute switch ain't it.
I will look into local authorized service, I'm in the NYC area, there
must be one around. As for ampeg.com, the site appears to be down right
now, as I can't pull it up either.

one other thing is that these want to run with master wide open, don't
know why but that's the way they are designed...... I run mine wide
open adjust the pregain to set my volume.

[Mark Marsh]

Post by Fred Marx
one other thing is that these want to run with master wide open, don't
know why but that's the way they are designed...... I run mine wide
open adjust the pregain to set my volume.

Fred-
What about the SVT2-pro and SVT4-pro - do they also want to run with the
master full up? How did you determine that they want that - by ear, schematic,
et cetera?

Thanks!
-Mark

[Ryze Edup]

Just bought this unit on ebay, and it looks to be in near-mint condition.
It powers up (fan and lights come on) but I get no signal from either the
pre-amp out or any of the speaker jacks.
Removing the cover, I see that the two tubes on the rear circuit board
light up, but the three on the front board do not. Swapping the 12AX7
from the rear section with one of the front ones shows that the tubes are
not the problem. All connectors appear to be firmly seated.
Any ideas? I'd hate to get involved with trying to return the unit to the
seller if it's some easily fixed problem. Thanks.

blown fuse for the tube heaters ?

[Barry Wiseman]

Post by Ryze Edup

Just bought this unit on ebay, and it looks to be in near-mint condition.
It powers up (fan and lights come on) but I get no signal from either the
pre-amp out or any of the speaker jacks.
Removing the cover, I see that the two tubes on the rear circuit board
light up, but the three on the front board do not. Swapping the 12AX7
from the rear section with one of the front ones shows that the tubes are
not the problem. All connectors appear to be firmly seated.
Any ideas? I'd hate to get involved with trying to return the unit to the
seller if it's some easily fixed problem. Thanks.

blown fuse for the tube heaters ?

I like that. Where would I look for such a fuse?

[Ryze Edup]

Post by Barry Wiseman

Post by Ryze Edup

Post by Barry Wiseman
Just bought this unit on ebay, and it looks to be in near-mint
condition. It powers up (fan and lights come on) but I get no signal
from either the pre-amp out or any of the speaker jacks.
Removing the cover, I see that the two tubes on the rear circuit board
light up, but the three on the front board do not. Swapping the 12AX7
from the rear section with one of the front ones shows that the tubes
are not the problem. All connectors appear to be firmly seated.
Any ideas? I'd hate to get involved with trying to return the unit to
the seller if it's some easily fixed problem. Thanks.

blown fuse for the tube heaters ?

I like that. Where would I look for such a fuse?

I'm not familiar with the internals of this amp, i would start by tracing
the heater wiring.

The 12AX7 heater can run at 6.3v or 12.6v depending on how you
wire up the heaters. Pins 9,4 & 5 are for the heater.

Be carefull, there will be 3-500 volts inside.

[Gregory Rochford]

Post by Ryze Edup
I'm not familiar with the internals of this amp, i would start by tracing
the heater wiring.
The 12AX7 heater can run at 6.3v or 12.6v depending on how you
wire up the heaters. Pins 9,4 & 5 are for the heater.
Be carefull, there will be 3-500 volts inside.

I had a tube amp once (Fender Super) that got hot enough to melt
the solder off one of the connections of the heater wiring to
a tube socket, causing an open circuit.

You don't need to have the amp plugged in to check continuity
of the heater wiring, but you still need to be careful, as
the capacitors in the amp can store a pretty good charge.
A schematic is helpful as well ;)

best
gr

[Benj]

Post by Barry Wiseman
Removing the cover, I see that the two tubes on the rear circuit board
light up, but the three on the front board do not. Swapping the 12AX7
from the rear section with one of the front ones shows that the tubes
are not the problem. All connectors appear to be firmly seated.
Any ideas? I'd hate to get involved with trying to return the unit to
the seller if it's some easily fixed problem. Thanks.

It is very unlikely that this is a problem serious enough to consider
returning to seller.

Clearly heater power is not getting to the front board somehow. The
first possiblity is a bad tube. Swapping one tube won't do it. You
really have to test all the tubes. It all depends on the circuit wiring
which I can't talk about because the Ampeg site is down! If the
filaments are in series then if one tube burns out they all go out. If
they are wired in parallel then if one burns out, only it goes out.
Easiest thing is to just test them all. You can do it with an ohm meter
if you know which pins on the tube connect to the heater.

If the tubes are all OK, then you have to dig deeper and that takes the
ampeg schematic. Using that you (or a technician) will trace back
looking for supects like loose wires/connectors and fuses. Finding the
problem is usually quite simple by starting at the power transfomer and
following the voltage through the circuit with an AC voltmeter until
you locate the place where it's not getting through.

Even if the problem is the worst possible thing that can happen (power
transformer has had a winding open up) it's easy to just bolt in an
additional filament transformer somewhere in the unit to run the three
tubes. The key to all this is the circuit from Ampeg. Perhaps an
authorized dealer has the info if the website stays down.

Good luck!

Benj

[Fred Marx]

Benj wrote:

<snip>

Post by Benj
The key to all this is the circuit from Ampeg. Perhaps an
authorized dealer has the info if the website stays down.
Good luck!
Benj

the circuit in question is unfortunately a "black box" that just says
"tubes" in the manual schematic, which is the same as what is on the
site. You'll want to find a service rep who should have the "black box"
diagram....

[Nigel Goodwin]

Post by Benj
Clearly heater power is not getting to the front board somehow. The
first possiblity is a bad tube. Swapping one tube won't do it. You
really have to test all the tubes. It all depends on the circuit wiring
which I can't talk about because the Ampeg site is down! If the
filaments are in series then if one tube burns out they all go out. If
they are wired in parallel then if one burns out, only it goes out.

It would be EXTREMELY rare for heaters to be in series in an amplifier,
I've never seen one? - that was more ancient TV's and radios.

[Benj]

Post by Nigel Goodwin
It would be EXTREMELY rare for heaters to be in series in an amplifier,
I've never seen one? - that was more ancient TV's and radios.

I wasn't thinking the old radios, but rather high end preamps where
heaters are powered by DC to reduce hum and are in series to reduce
current and filtering requirements. I don't know much about SVT gear,
but since the thing in question is a "black box" I figured I might as
well cover that case too. Yeah, it is rare in normal gear.

Benj

[Barry Wiseman]

Much thanks to all who've replied. I never expected such a wealth of
practical guidance. agb rules!

I will rule out a bad tube by swapping the two others I hadn't tested.
Failing that, now that ampeg.com is back up and I see there's Ampeg
authorized service a mere 6 miles from me, I will let a pro delve
further. My hardware forte is computers not audio electronics, and I
can read a bass clef but not a circuit schematic :-(.

[svtbass]

Be sure the two 1/2 amp glass fuses on the main power board are OK.. they
supply the Preamp section... they are hidden by the black filter caps
...also be sure the two multi wires usually red wires with while molex
connectors are seated properly on both ends...
I had a similar problem with my SVT3 ..
hope that helps

Post by Barry Wiseman
Much thanks to all who've replied. I never expected such a wealth of
practical guidance. agb rules!
I will rule out a bad tube by swapping the two others I hadn't tested.
Failing that, now that ampeg.com is back up and I see there's Ampeg
authorized service a mere 6 miles from me, I will let a pro delve further.
My hardware forte is computers not audio electronics, and I can read a
bass clef but not a circuit schematic :-(.