Ampeg SVT-CL hums hums hums...

[Wasnex]

With the last two weeks I've been trying to power it on - no success.
I have recapped whole power section to be sure it's not there.
All the resistors OK. I don't see any problems in the mainboard (I looked over it hundreds of times...)

The front light stays red.

Does not go to green when I hit standby, does not flash. Just nothing happens when I hit "standby". The two 12AX7 preamp tubes are not glowing. The same with two different CL preamps (with different tubes). The J4 connection cable is OK. What could be next? The gray ribbon?

IMHO the first thing to do at this point is find out why there is not any heater voltage. The Chassis Assy Wiring diagram shows all of the ins and outs between the various modules. You have a bunch of wires going from the AC Termination PC Assy to the Filament Transformer.

The Filament Transformer also has five wires going to J19 on the Power Amp PC Assy and two wires going to J8 and J9 of the Power Tube PC Assy. J19 feeds full wave rectifiers for the -180V bias supply and also a balanced supply that provides +/-15V which is used by the ICs.

J8 and J9 on the Power Tube PC Assy feeds the output tubes AC heater voltage and loops back out on J10 and goes to J5 of the Preamp PC Assy. The bottom of the preamp schematic shows a full wave rectifier that serves as a DC heater supply for the small signal tubes. A sample of the DC heater voltage loops out on J6 and goes back to the AC Termination PC Assy on J36, which appears to feed a relay control circuit tied into the Standby relay. I.E. the actual Standby switch is mechanical, but the Standby relay has to be energized or the switch will not have any effect. Standby relay circuit also takes a feed from the Power Amp PC Assy. So a fault in the Bias sense circuit will also force the amp into standby.

Unfortunately my perception is you are in way over your head, so I suggest taking the amp to a technician.

 

[FunkHead]

I have on my workbench early 00s SVT-CL that was given to me in the "original" shape. I'm sure it wasn't even opened once. The amp has steady 50/60 cycle hum that is not the "end of the world" but it's two or three times louder than my other CL and Heritage I have here. This hum is not related to the preamp controls. It's constant. In the last few days, I have checked everything - from replacing tubes to solder joints, checking bias, resistors, grounds, replacing one 100uf cap on the mainboard and one input jack.

What I've found is:
- when I plug in to SLAVE OUT hum goes away.
- When I plug into PREAMP OUT hum DOES NOT goes away.
- When I join together PREAMP OUT & POWER AMP IN I hum goes away by 50%, the same as plugging only to POWER AMP IN.

So I assume the hum is coming from the preamp area? Tapping on the board components does nothing. I'm still wondering if there's anything out I could have missed here? Again, solder joints tested, tubes tested, measured and replaced (2 times to be sure).

Any resolution?

 

[boroman]

Any resolution?

I was busy restoring two B15Ns last week, but will get back to that amp early March. Will post here what I've found.

 

[lamplificateur]

I was busy restoring two B15Ns last week, but will get back to that amp early March. Will post here what I've found.

I Boroman, Do you find any solution with your amp ?
I have the same problem.
Thank for your reply.

 

[boroman]

Hi. Nope. This one was humming more than other CL's. I have repaired few ones, recapped few ones, but this one was/is still a mystery. I tried to trace everything: tubes, resistors, diodes, caps, jack grounds, etc. Nothing helped. I passed this one to another tech but he just said "these amps hums like that" so it went back to me

 

[lamplificateur]

Hi. Nope. This one was humming more than other CL's. I have repaired few ones, recapped few ones, but this one was/is still a mystery. I tried to trace everything: tubes, resistors, diodes, caps, jack grounds, etc. Nothing helped. I passed this one to another tech but he just said "these amps hums like that" so it went back to me

Oh ! Bad news, did you find any solution for the blown R14 ? I have this problem too

 

[lamplificateur]

Hi @boroman

I'm still reparing the Ampeg SVT. And i have the same problem as you, the resistor R14 blown up, do you have any information to avoid this problem ?

Best regards,

Kevin

 

[boroman]

That was long time ago. I repaired R14, recapped whole amp, changed tubes but it still had more hum than others. I eventually sold it but never found solution for that hum (even passed it to two other techs, so a lot of money spent on nothing).

 

[CandyBoxAndy]

Hi Bassists!

Just wanted to throw in my experience in this topic... in my case it was the solution to the blown R14 issue.

Had the same problem with the R14 resistor after i had reassembled my SVT CL (got it broken from a friend) for the first time.

So the reason why i had to put it appart in the first place was that it always went into fault mode after a few seconds (the main problem of course), and when the tubes got heated up well it immediately went into fault mode after going out of standby.

It was my first repair on a SVT, so i took photos of all the connectors an reassembled it the same way, i thought that the blown screen resistor (that i had already replaced now + filter caps) was the main issue...

BUT!!! after powering it on again i first heard strange HF noise from the speaker -> then it went into fault mode again -> then i smelled some of the magic smoke.

That's strange, i couldn't figure out why it got worse now. the same screen grid resistor was blown again and also that mighty R14, i told myself i really must have connected something terribly wrong.

So i traced back if all the connector that i possibly could wire up wrong are correct. it turned out that i wasn't sure if i had connected the 2 red wires (going from the power amp PCB to the speaker output PCB - see pictures) correct again.

It turned out they were wrong, and it DOES matter how you connect them, and i can't figure out why it is even possible to connect them flipped 180°, normally those pin sockets do have those plastic noses (don't know how to call them) for at least 2 good reasons: 1. you can't connect them wrong and 2. the connection won't get loose. In my case (SVT CL built in US 2001) those pin sockets on the PCB are simply naked, so you can connect the wires both ways and the connector is quite loose when you push it on them. i think in most of the SVTs out there it is not possible to connect those 2 red wires wrong because they have that plastic-nose-socket-things on the pins on the PCB.

As far as i understood :
The reason why you can't connect the 2 red wires the other way is because the trace of one of them (i marked it green on the pictures) is the feedback loop and has to go from the output transformer secondary winding through a RC pair directly back into the first gain stage of the power amp and, OHH there is that R14 to ground...
The 2nd one is the common ground of that secondary winding for the speakers, this trace has to go nowhere else than directly to ground and NOT to the power amps 12AX7 cathode and or or to ground via R14 *BURN*

And please correct me if i mixed up something here, i also just tried to figure that stuff out myself and i'm no tubeamp guru yet :')

And yeah, by the way, i also recognized that the grey ribbon cable next to the 2 red wires was causing the amp to go into fault mode because it was also connected 180° wrong (by the guy who tried to fix it before me). That's the one coming with all the signals from the preamp.

All the best,
Andy

 

[Wasnex]

Hi Bassists!

Just wanted to throw in my experience in this topic... in my case it was the solution to the blown R14 issue.

Had the same problem with the R14 resistor after i had reassembled my SVT CL (got it broken from a friend) for the first time.

So the reason why i had to put it appart in the first place was that it always went into fault mode after a few seconds (the main problem of course), and when the tubes got heated up well it immediately went into fault mode after going out of standby.

It was my first repair on a SVT, so i took photos of all the connectors an reassembled it the same way, i thought that the blown screen resistor (that i had already replaced now + filter caps) was the main issue...

BUT!!! after powering it on again i first heard strange HF noise from the speaker -> then it went into fault mode again -> then i smelled some of the magic smoke.

That's strange, i couldn't figure out why it got worse now. the same screen grid resistor was blown again and also that mighty R14, i told myself i really must have connected something terribly wrong.

So i traced back if all the connector that i possibly could wire up wrong are correct. it turned out that i wasn't sure if i had connected the 2 red wires (going from the power amp PCB to the speaker output PCB - see pictures) correct again.

It turned out they were wrong, and it DOES matter how you connect them, and i can't figure out why it is even possible to connect them flipped 180°, normally those pin sockets do have those plastic noses (don't know how to call them) for at least 2 good reasons: 1. you can't connect them wrong and 2. the connection won't get loose. In my case (SVT CL built in US 2001) those pin sockets on the PCB are simply naked, so you can connect the wires both ways and the connector is quite loose when you push it on them. i think in most of the SVTs out there it is not possible to connect those 2 red wires wrong because they have that plastic-nose-socket-things on the pins on the PCB.

As far as i understood :
The reason why you can't connect the 2 red wires the other way is because the trace of one of them (i marked it green on the pictures) is the feedback loop and has to go from the output transformer secondary winding through a RC pair directly back into the first gain stage of the power amp and, OHH there is that R14 to ground...
The 2nd one is the common ground of that secondary winding for the speakers, this trace has to go nowhere else than directly to ground and NOT to the power amps 12AX7 cathode and or or to ground via R14 *BURN*

And please correct me if i mixed up something here, i also just tried to figure that stuff out myself and i'm no tubeamp guru yet :')

And yeah, by the way, i also recognized that the grey ribbon cable next to the 2 red wires was causing the amp to go into fault mode because it was also connected 180° wrong (by the guy who tried to fix it before me). That's the one coming with all the signals from the preamp.

All the best,
Andy

Unfortunately the connectors are not polarized. Not the first time I have heard of this.

If I understand correctly, it sounds like the connections to the output transformer were reversed. Normally the feedback loop is out of phase, but if the transformer polarity is switched, the feedback loop becomes in phase. This causes the feedback loop to increase gain, ran than decreasing it. Your description suggests the amp was oscillating at a high frequency. The oscillation can even occur at in the ultrasonic range; so the amp can crank out max power and you won't even hear it. Not a surprise that the magic smoke was let out.

Good on you for figuring it out .

 

[byacey]

Unfortunately the connectors are not polarized. Not the first time I have heard of this.

If I understand correctly, it sounds like the connections to the output transformer were reversed. Normally the feedback loop is out of phase, but if the transformer polarity is switched, the feedback loop becomes in phase. This causes the feedback loop to increase gain, ran than decreasing it. Your description suggests the amp was oscillating at a high frequency. The oscillation can even occur at in the ultrasonic range; so the amp can crank out max power and you won't even hear it. Not a surprise that the magic smoke was let out.

Good on you for figuring it out .

Exactly what I was going to suggest.

 

[agedhorse]

If people (including wannabe techs) working on this stuff would leave connectors alone, these issues wouldn’t exist. It goes back to my often made comments about not creating new problems where problems don’t exist.

Unfortunately it happens often enough that any amp with damage from a previous repair attempt does not qualify for flat rate service and becomes an (expensive) hourly job.