Ampeg SVT CL

[sunshine_bassman]

SVT CL turns on, green light flashes then goes away. Standby on or off, no green or red status light. Bias lights sometimes shine red and green, sometimes you can adjust the red out, sometimes you can’t, sometimes the bias lights don’t shine at all.
Tubes light up, horrible hum with no guitar signal coming through. No guitar signal plugged into FX loop.
New 12AU7s, power tubes are supposedly brand new.
However, within five minutes of being turned on, the humming stops and high voltage disappears (monitoring the big filter caps). Only the bias voltage can be read on the power tubes at this point. I assume this is some kind of protection.
No cracked solder joints to be seen, 220ohm resistors are there as opposed to 22ohm but the diodes are still there. No open screen resistors that I’ve found.

[Leo_Gnardo]

The combination of big hum and squirrely bias LED behavior indicates a major imbalance of output tubes. One or more are out of match with the rest. I know it's a major expense, but the bias system only works with a well matched set of output tubes. The amp is too "smart" to cheat it into working as you expect it to, unless they're matched. Plus, the high voltage shutting down on its own, yes that's a safety feature that is keeping you out of worse trouble. Perhaps one or more of the output tubes is going into runaway bias. That would usually be indicated by the well known ominous "red plate glow" but the CL doesn't let that happen.

Regular tube testers may not reveal which tube(s) are deficient. They ordinarily apply about 200V to the plate. Your SVT runs the plates at 700V. What may show "good" on a typical tester, could easily be breaking down at 700V.

SVT's of all eras are frustrating to work on. But soooooo good sounding when they're working right.

[nosaj]

SVT CL turns on, green light flashes then goes away. Standby on or off, no green or red status light. Bias lights sometimes shine red and green, sometimes you can adjust the red out, sometimes you can’t, sometimes the bias lights don’t shine at all.
Tubes light up, horrible hum with no guitar signal coming through. No guitar signal plugged into FX loop.
New 12AU7s, power tubes are supposedly brand new.
However, within five minutes of being turned on, the humming stops and high voltage disappears (monitoring the big filter caps). Only the bias voltage can be read on the power tubes at this point. I assume this is some kind of protection.
No cracked solder joints to be seen, 220ohm resistors are there as opposed to 22ohm but the diodes are still there. No open screen resistors that I’ve found.

Never trust should be new. New can fail as quick as old.
Suspect your filter caps check ripple.
Nosaj

[sunshine_bassman]

So could I reposition the tubes to be matched better?

[nevetslab]

So could I reposition the tubes to be matched better?

The short answer on this specific question is YES. But, that won't solve your basic problem of seeing changes in bias level as being reported by the two sets of bias LED's. The GRN indicator range with ALL SIX Tubes installed is from 14.7mA to 29.4mA, assuming matched tubes. Above 29.4mA, the RED LED turns on. As you've seen in operation, NORMAL bias LED indicators will have GRN on continuous, with the RED indicators lighting during signal envelope (signal current flowing thru the tubes).

With the preamp and power amp chassis's removed from the cabinet, turn the power amp chassis over or set on one end, with block to stabilize the chassis from falling, and with high intensity light & magnification.....inspect the rear panel's solder connections. The two Bias Pots, along with the phone jacks and XLR DI output connectors are used as mechanical support for that main Power Amp PCB. Over time, the solder joints on the bias pots develop fracture rings which now yield intermittent behavior in controlling the power tubes. If you've never done surgery on SVT-CL's, SVT-AV's and SVT-VR's, their biggest problem stems from solder joint failure. It occurs on the 5-pin right-angle xfmr secondary and I/O harness connector headers J20 and J19 (towards the front of that PCB) all the time, as well as on the 2-pin feedback header J10 next to the ribbon cable connector to the Power Tube PCB.

You'll also find on the preamp control PCB, solder joint fractures on the pot terminals, along with the input jacks. Not as common, but certainly have seen it is fractures on the power tube sockets on the Power tube PCB. While looking on that PCB, look to see if there's blackened insulation on the two heavy heater wire terminals. THAT is the distribution point from the Heater winding secondary to the entire amp, and also a source of misbehavior.

I service SVT's on a regular basis, and it's typically a 5-6 hour operation completely going thru one to find all of the issues. In general, what you have is a chassis with two very heavy transformers, whose mounting hardware is shaken loose over time from being transported in the equipment truck from gig to gig, or on tour, and that weight, inside a road case or not, serves with with road vibration, the xfmrs become a shaker table to inflict heavy vibration onto solder joints being used as PCB supports, as well as mounting hardware that begin loosening, and then eventually unthread. Biggest culprits are the power and output xfmr mtg hardware. You can't get at all of them without lifting out the main PCB, which IS a PITA to put back into place. AND, the front three standoffs that support the PCB to the chassis towards the front of the amp....the Output Xfmr side corner standoff IS the GND Reference path from the power amp PCB to Chassis GND. If it's loose, hum becomes a companion.

The core bolts on the two large xfmrs also arrive loose from the factory, which allow the two xfmrs to shift their weight in transit. Those core bolts can be a problem, as it's not difficult to snap one in half while attempting to tighten them.

ONCE you've found ALL of the solder joint fractures, de-soldering and re-soldering is the proper method. THEN you can set up the power tubes for a best plate current match, done with the power amp chassis on end.



I manage a fleet of SVT's for rental, and have replaced the clear plastic 'garden hose' I/O cable between the preamp and power amp chassis with Tech Flex, which then allows standing the preamp vertical next to the power amp chassis without the harness disallowing that. Then, after pre-setting the bias voltage at pin 8 on the two 12AU7's for a common value, say -48VDC.....I'll measure two tubes at a time from the sextet of 6550's or KT-88's. My target bias to begin with is 23mA on the tubes, measured across the cathode resistors. The third photo shows where those resistors are per tube. You may need to trim the two bias pots for same voltage while getting your starting point. Then, record the voltage reading across the 10 ohm cathode resistors per tube. You'll end up with the current values, which you can then group into best plate current balance for both upper and lower halves. Putting all six tubes back in, then you can do final bias trim with target for 22-23mA. 220mV across 10 ohms is 22mA.

I do this operation on a regular basis....often have to find replacement tubes of the same mfgr when one or two of the tubes from the sextet fail. From the years of servicing these, I end up with a collection of pulls, which come in handy to keep the fleet alive, without having to replace sextets every time. I also go thru this procedure without the hold-down clamps in place.

I also have shock-mounted the hold-down clamps to prevent breakage of the large glass bottles.



Once I've eliminated all of the solder joint fractures thruout the amp (make sure you look on the AC mains PCB to find fractures on the IEC connector & fuse holder, as well as on the output PCB where the slide switch and phone jacks are culprits), these amps then become stable and will run well, with then just the usual tube maintenance issues to contend with.

[BobK]

You can probably reposition the tubes to get them matched better, or worse!
I'd start just by removing a single tube from each side, and continue to do that to isolate the potential culprits.
One pair out... that pair back in, different pair out, etc.

And, yes, the killing of the HV is a protection mechanism. You can see it in the bias section of the schematic. You can temporarily override that by jumpering around the relay, but understand the havoc that might ensue from doing that.

Ah, I see while typing my short answer that you've been given a great response above!

[Leo_Gnardo]

What nevets said ^^^. Plus, keep in mind there's 700V of DC as well as the usual shock hazards from the AC line & hi voltage secondary leads all awaiting their chance to shock the bejeezus out of you. So - be very very careful! Concentrate on what you're doing & allow no distractions.

[Enzo]

First: I am not disagreeing with any of the above. But I did have an immediate thought. The tube LEDs all rely on a good low voltage power supply to work. In this case the +/-15v supplies. If one of those is collapsing or really full of ripple, it can confuse the circuit as well as cause big hum. Simple to check first.

[nevetslab]

First: I am not disagreeing with any of the above. But I did have an immediate thought. The tube LEDs all rely on a good low voltage power supply to work. In this case the +/-15v supplies. If one of those is collapsing or really full of ripple, it can confuse the circuit as well as cause big hum. Simple to check first.

Very good point! I've had several SVT's +/- 15V supplies fail. It would also be a good idea, after searching for, finding and repairing any solder joint fractures thruout the amp, to then power it up without any power tubes installed. That would give you the opportunity to verify the protection circuit is working, all of the supplies are working. When I've had protect issues that persist even with the power tubes removed, you have to go thru that system, which takes you to circuits in the preamp chassis, the main PCB as well as the AC Mains PCB. Do you have all of the schematics for the amp? Essential, of course. And, as Leo cautioned, there is lethal potentials all over the place.

[sunshine_bassman]

I took all the power tubes out and started testing the +/-15vdc rails.
The -15v rail is reading around -2.3vdc on C28 and R61 in the power supply. The protection circuit is kicking in in a matter of seconds tho so I’m not able to check voltages for long.
Yesterday after I checked all the solder joints and fixed the questionable points, I put it back together without power tubes and a relay clicked for a few minutes before going into protection mode.

[nevetslab]

I took all the power tubes out and started testing the +/-15vdc rails.
The -15v rail is reading around -2.3vdc on C28 and R61 in the power supply. The protection circuit is kicking in in a matter of seconds tho so I’m not able to check voltages for long.
Yesterday after I checked all the solder joints and fixed the questionable points, I put it back together without power tubes and a relay clicked for a few minutes before going into protection mode.

Sorry, I missed this. You can disconnect the two-pin plug that connects to J36 on the Relay board, which will prevent the relay from pulling it, but it should allow the output from the heater xfmr to still run, which is where the +/- 15V regulators are fed from. With J36 disconnected, the HT won't come on. The Fault LED will light, since the 300V potential applied to R34 in the preamp is needed to indicate all is well with the supplies. That shouldn't prevent you from addressing all of the bipolar voltages, and find what's causing the -2.3VDC on C28/R61 in the power supply. So, besides that issue, you still have to find what else is triggering the protection circuit. OH....fixing the -15V regulator should fix the Fault Detector circuit, since the op amp IC1 is needed, and there's R5 which requires -15V (and +300V via R36) to achieve GRN LIGHT status (no Protection fired).

[lowell]

I don't see J36.. where is that in the schematic?

[BobK]

nevetslab posted it here:
https://music-electronics-forum.com/...d.php?p=503616
in that first zip you'll see the relay board.