Have an Ampeg M-15 that keeps blowing fuses. I have some
troubleshooting experience, but not with tubes. I remember hearing that
you will burn out a circuit without an output load. Is this true of
every stage of the amp? Aside from looking at the RC filters on the
power supply output, I wanted to unplug some of the tubes following the
power supply circuit to isolate what is causing the fuse to blow. Will
this be a problem? Any tips on how to proceed?

Thanks

in article , crevitch at
wrote on 5/4/05 9:31 AM:

Have an Ampeg M-15 that keeps blowing fuses. I have some
troubleshooting experience, but not with tubes. I remember hearing that
you will burn out a circuit without an output load. Is this true of
every stage of the amp? Aside from looking at the RC filters on the
power supply output, I wanted to unplug some of the tubes following the
power supply circuit to isolate what is causing the fuse to blow. Will
this be a problem? Any tips on how to proceed?

Thanks


It is always good practice to test a tube amp (or any amp, really) with some
kind of load. I use a 10 or 20W resistor which should stay cold unless
there is oscillation or an input signal. The wattage rating of the resistor
doesn't matter unless there is actual output . . . or you can also use any
old speaker.

You didn't say whether the fuse was being blown right away. If so, I'd
check:

1) The solid state power rectifiers on amps that use them; these sometimes
short.

2) Filter caps. These can short, too.

3) Power transformer. Measure DC resistance.

If a tube is bad, sometimes you'll see a flash inside of the tube when power
is applied, but not necessarily.

You can unplug preamp and driver tubes to check them, but do not unplug the
power tubes as this can raise the B+ voltage and destroy filter caps & etc.

If you must unplug power tubes, connect the amp to a variac and lower the
line voltage. If you don't have a variac you can use a 60 watt light bulb
wired in series with the A/C line.

Hope that helps.

Jon

Jon,

This is a big help. What DC resistance should I see on the transformer?
The fuse takes a few seconds to blow, but that may have been a slo-blo
fuse. The amp has a tube rectifier.

Thanks

Jon Yaeger wrote:

It is always good practice to test a tube amp (or any amp, really) with some
kind of load. I use a 10 or 20W resistor which should stay cold unless
there is oscillation or an input signal. The wattage rating of the resistor
doesn't matter unless there is actual output . . . or you can also use any
old speaker.

You didn't say whether the fuse was being blown right away. If so, I'd
check:

1) The solid state power rectifiers on amps that use them; these sometimes
short.

2) Filter caps. These can short, too.

3) Power transformer. Measure DC resistance.

If a tube is bad, sometimes you'll see a flash inside of the tube when power
is applied, but not necessarily.

You can unplug preamp and driver tubes to check them, but do not unplug the
power tubes as this can raise the B+ voltage and destroy filter caps & etc.

If you must unplug power tubes, connect the amp to a variac and lower the
line voltage. If you don't have a variac you can use a 60 watt light bulb
wired in series with the A/C line.

Hope that helps.

Jon

Operating a tubed amp without any load (LS) connected to its power output is
no good practice, as it would be to short it. If You do not want to connect
the LS, provide a dummy load.
Unplugging some power tubes will cause the anode voltage to soar because of
the lower DC request; this can be fairly dangerous for all caps in there.
If You want to do so, get a variac and power up @ lower voltage, then ramp
it up using common sense and patience. If You do not want to buy a variac
for 1 amp only, put a light bulb in series with the amp. Start with a 25W,
then put in a 60, then a 100, then (if nothing has blasted) connect it
"straight". Not a Variac but it can save Your amp and Your fingertips with
5$.
Signal tubes can be plugged in & out without problems (generally speaking).
First check: signal capacitors. If the power tubes grids, at zero signal,
do not rest at ground potential, You have a problem with leaking caps.

Ciao

Fabio

PS: There's no solid-state voltage in there. Better not to try any
electrifying experience.



"crevitch" ha scritto nel messaggio
...
Have an Ampeg M-15 that keeps blowing fuses. I have some troubleshooting
experience, but not with tubes. I remember hearing that you will burn out
a circuit without an output load. Is this true of every stage of the amp?
Aside from looking at the RC filters on the power supply output, I wanted
to unplug some of the tubes following the power supply circuit to isolate
what is causing the fuse to blow. Will this be a problem? Any tips on
how to proceed?

Thanks

On Wed, 04 May 2005 10:06:25 -0400, crevitch wrote:

Jon,

This is a big help. What DC resistance should I see on the transformer?
The fuse takes a few seconds to blow, but that may have been a slo-blo
fuse. The amp has a tube rectifier.

Thanks

Take everyones precautions as they suggest...

I'd bet 10 to 1 it's just a power tube that is shorted. That fact you
indicate that it takes a few seconds to blow the fuse, is the time
that it takes for the rectifier tube to start conducting and bring the
high voltage up.

Take out the power tubes only and try again.

It has a 5U4G rectifier which turns on almost immediately (direct heating).
If the fuse blows after 3 secs, there's a short in the B+ rail (maybe a
shorted tube). If it takes 20-30 secs, it probably has to do with the 6L6s
conducing too much (no bias - shorted grid coupling cap or cathode
decoupling cap).
OK, or 100+ other possibilities, but let's begin from the simplest ones.

Ciao

FB


"Gilbert Bates" ha scritto nel messaggio
...
On Wed, 04 May 2005 10:06:25 -0400, crevitch wrote:

Jon,

This is a big help. What DC resistance should I see on the transformer?
The fuse takes a few seconds to blow, but that may have been a slo-blo
fuse. The amp has a tube rectifier.

Thanks

Take everyones precautions as they suggest...

I'd bet 10 to 1 it's just a power tube that is shorted. That fact you
indicate that it takes a few seconds to blow the fuse, is the time
that it takes for the rectifier tube to start conducting and bring the
high voltage up.

Take out the power tubes only and try again.

If the power tubes grids, at zero signal, do not rest at ground potential,
You have a problem with leaking caps.

This assumes cathode bias. In a fixed bias amp, it will be at some negative
potential with respect to ground.
JamesG

"Fabio Berutti" wrote in message
...
Operating a tubed amp without any load (LS) connected to its power output
is no good practice, as it would be to short it. If You do not want to
connect the LS, provide a dummy load.
Unplugging some power tubes will cause the anode voltage to soar because
of the lower DC request; this can be fairly dangerous for all caps in
there. If You want to do so, get a variac and power up @ lower voltage,
then ramp it up using common sense and patience. If You do not want to
buy a variac for 1 amp only, put a light bulb in series with the amp.
Start with a 25W, then put in a 60, then a 100, then (if nothing has
blasted) connect it "straight". Not a Variac but it can save Your amp and
Your fingertips with 5$.
Signal tubes can be plugged in & out without problems (generally
speaking).
First check: signal capacitors. If the power tubes grids, at zero signal,
do not rest at ground potential, You have a problem with leaking caps.

Ciao

Fabio

PS: There's no solid-state voltage in there. Better not to try any
electrifying experience.



"crevitch" ha scritto nel messaggio
...
Have an Ampeg M-15 that keeps blowing fuses. I have some troubleshooting
experience, but not with tubes. I remember hearing that you will burn
out a circuit without an output load. Is this true of every stage of the
amp? Aside from looking at the RC filters on the power supply output, I
wanted to unplug some of the tubes following the power supply circuit to
isolate what is causing the fuse to blow. Will this be a problem? Any
tips on how to proceed?

Thanks