I have an Ampeg SVT classic that when hot either loses input signal, starts
to crackle loudly, or both. Usually a good Fonzie will clear it up long
enough to get through. I just don't want to start guessing / throwing parts
at this thing. Any ideas where to start?

TIA

John

John Ray wrote:
I have an Ampeg SVT classic that when hot either loses input signal,
starts
to crackle loudly, or both. Usually a good Fonzie will clear it up
long
enough to get through. I just don't want to start guessing / throwing
parts
at this thing. Any ideas where to start?

TIA

John

Hi John,

If we're talking the *late model* SVT Classic (which has channel
switching, rear panel bias LEDs etc) instead of a 'classic' SVT
(Magnavox):

Solder. Solder. And then some more solder.

They're known to get 'rimmed' AKA 'cold' solder connections at the
multipin in-line internal connector headers (that will do just what
you've described), as well as all of the panel mounted controls (front
and rear, check the bias controls too!), FWIW the rear panel XLR direct
out is a sure bet for cold solder too (if the unit hasn't been stripped
yet for hand soldering.)

If I correctly guessed your 'vintage', FWIW it's usually not really a
big deal; just take the time to carefully take the whole thing apart,
inspecting all of the solder connection points I've mentioned under
magnification (oh, I forgot, the PC mounted tube sockets too) and get
to soldering.

HTH,
-Robert
QTS
http://www.Braught.com
Real Email Addy : (remove NoSpam to reply;
Duh!)

snip

Hi John,

If we're talking the *late model* SVT Classic (which has channel
switching, rear panel bias LEDs etc) instead of a 'classic' SVT
(Magnavox):

Solder. Solder. And then some more solder.

They're known to get 'rimmed' AKA 'cold' solder connections at the
multipin in-line internal connector headers (that will do just what
you've described), as well as all of the panel mounted controls (front
and rear, check the bias controls too!), FWIW the rear panel XLR direct
out is a sure bet for cold solder too (if the unit hasn't been stripped
yet for hand soldering.)

If I correctly guessed your 'vintage', FWIW it's usually not really a
big deal; just take the time to carefully take the whole thing apart,
inspecting all of the solder connection points I've mentioned under
magnification (oh, I forgot, the PC mounted tube sockets too) and get
to soldering.

HTH,
-Robert

Yes, you guessed correctly, and yes this is very helpful. now that you've
mentioned this, the bias LEDs never would set up right. I was assuming that
was somewhat related to the problem, but now I see that perhaps that *is*
the problem.

Thanks again,
John

John Ray wrote:

I have an Ampeg SVT classic that when hot either loses input signal, starts
to crackle loudly, or both. Usually a good Fonzie will clear it up long
enough to get through. I just don't want to start guessing / throwing parts
at this thing. Any ideas where to start?

Take it to a capable tech and get him to check all the
electode voltages and currents in the amp.

Patrick Turner.



TIA

John

Patrick Turner wrote:
John Ray wrote:

I have an Ampeg SVT classic that when hot either loses input
signal, starts
to crackle loudly, or both. Usually a good Fonzie will clear it up
long
enough to get through. I just don't want to start guessing /
throwing parts
at this thing. Any ideas where to start?

Take it to a capable tech and get him to check all the
electode voltages and currents in the amp.

Patrick Turner.



TIA

John

Hello Mr. Turner,

Have you happened to work on one of these yet? FWIW kinda neat, as *it
checks it's own* 'currents'(!) and will shut down completely (relay
click, front panel light color changes) if any one of the six 6550
output tubes short, or the negative AB bias voltage disappears. Someone
at SLM (St. Louis Music) spent some time with the slipstick to add what
I would call real improvements, including being able to set static
plate dissipation via 'go-nogo' LED indicators on the back.

The 'classic' SVT's, on the other hand, were commendable IMO due to
that fact that if one of the 6550's shorted (or 6146B's, first version)
it would burn open a plate resistor *and keep on going*! (The show must
go on!)

More than once I've had a 'new owner' of an 'original' SVT come in
here, completely unaware that they were only playing on 4 or 5 output
tubes!

Cheers,
-Robert
QTS
http://www.Braught.com
Real Email Addy : (remove NoSpam to reply :
Duh!)

" wrote:

Patrick Turner wrote:
John Ray wrote:

I have an Ampeg SVT classic that when hot either loses input
signal, starts
to crackle loudly, or both. Usually a good Fonzie will clear it up
long
enough to get through. I just don't want to start guessing /
throwing parts
at this thing. Any ideas where to start?

Take it to a capable tech and get him to check all the
electode voltages and currents in the amp.

Patrick Turner.



TIA

John

Hello Mr. Turner,

Have you happened to work on one of these yet? FWIW kinda neat, as *it
checks it's own* 'currents'(!) and will shut down completely (relay
click, front panel light color changes) if any one of the six 6550
output tubes short, or the negative AB bias voltage disappears. Someone
at SLM (St. Louis Music) spent some time with the slipstick to add what
I would call real improvements, including being able to set static
plate dissipation via 'go-nogo' LED indicators on the back.

I also fit cathode current detectors which turn off the amp if one or more
of the multiple
output tubes goes wobbly.



The 'classic' SVT's, on the other hand, were commendable IMO due to
that fact that if one of the 6550's shorted (or 6146B's, first version)
it would burn open a plate resistor *and keep on going*! (The show must
go on!)

A cathode resistor would be better, or else without a plate connection
the screen would frizzle up and die real quick.
Cathode fuses are not too bad, but active protection is so much better.



More than once I've had a 'new owner' of an 'original' SVT come in
here, completely unaware that they were only playing on 4 or 5 output
tubes!

At gigs you want to keep going if you loose tubes.
I make hi-fi amps where the aim is to not allow the amp to burn a house
down
or make a fastidious and wealthy owner want to sue me.
I use fuses AND active protection.

I can't trust the men and women of Russia to always make me a perfect lot
of tubes,
and when you have 12 x 6550 tubes in channel you have 6 times the
likelyhood of a random failure
compared to when you have two tubes.
But I use cathode bias, so no big deal if a tube fails, just plug one
another in.

Patrick Turner.



Cheers,
-Robert
QTS
http://www.Braught.com
Real Email Addy : (remove NoSpam to reply :
Duh!)

Patrick Turner wrote:
" wrote:
snip
The 'classic' SVT's, on the other hand, were commendable IMO due to
that fact that if one of the 6550's shorted (or 6146B's, first
version)
it would burn open a plate resistor *and keep on going*! (The show
must
go on!)

A cathode resistor would be better, or else without a plate
connection
the screen would frizzle up and die real quick.

FWIW I forgot to mention that they had individual screen resistors too,
that would additionally open under duress.

Cathode fuses are not too bad, but active protection is so much
better.

Agreed.




More than once I've had a 'new owner' of an 'original' SVT come in
here, completely unaware that they were only playing on 4 or 5
output
tubes!

At gigs you want to keep going if you loose tubes.
I make hi-fi amps where the aim is to not allow the amp to burn a
house
down
or make a fastidious and wealthy owner want to sue me.
I use fuses AND active protection.

I try to apply hazards analysis to the things I've designed, balancing
circuit reliability against 'worse case failure scenarios' including
immolation by fire; I'd like my stuff to stay working (safely!) under
the most dire of circumstances (read that: out in The Real World! ;-)


I can't trust the men and women of Russia to always make me a perfect
lot
of tubes,
and when you have 12 x 6550 tubes in channel you have 6 times the
likelyhood of a random failure
compared to when you have two tubes.
But I use cathode bias, so no big deal if a tube fails, just plug one
another in.

My partner here and I have joked that a smart robot arm attachment for
some of the bigger amps would sell, one that could replace tubes
automatically...


Patrick Turner.



Cheers,
-Robert
QTS
http://www.Braught.com
Real Email Addy : (remove NoSpam to reply
:
Duh!)