Ampeg SVT CL

[bsco]

Received this and none of the tubes would light up....Power would come on and stay in protect mode....I opened it up and somebody had gone in and removed the wires from the stand-by switch and connected them to the un-used section of the main power switch...tore it all apart and checked all the traces to the heater pins on the output sockets...also measured heater voltage from the two white heater wires......plugged the tubes in a tube tester to verify that they at least light up....everything was fine....Re-connected the standby wiring back to the standby switch...and this switch also good...on both sections...the tubes all light up now....when turned to run mode, there is a hum in the speaker which signafies the tubes are not balanced....adjusting the bias pots don't do much...the relay is cycling every few seconds...I am going to rip it apart again and do some more component testing to see what I can find....any comments kindly welcome......
Cheers
Bernie
P.S. Found a ribbon cable that was not fully seated......the one that goes to the main board......Now I hsave it powering up and have a mis-match is a tube or two...so the process of elimination begins......

[Leo_Gnardo]

I guess "somebody" didn't want to bother waiting a minute to bring the amp out of standby.

In the power amp, the 220 ohm screen grid resistors often fuse open, also the 10 ohm cathode R's. There's a circuit that monitors output tube bias, with a comparator chip attached to each 10 ohm resistor. If they see too high or too low a voltage on any of those Rk's, it will put the amp into auto-shutdown and blink the pilot lamp at you. So it pays to have a more or less matched set of output tubes and Rk's besides.

There are some more monitoring systems in there too, and I have to admit I'm not familiar with all of them. But if they don't like what they see, blip you're in auto-shutdown.

[nevetslab]

The hum in the speakers doesn't necessarily mean a bad tube. I didn't read that the hum disappeared when you found the unseated ribbon cable. Was that the grey ribbon in the preamp?

When I service these SVT's, quite often, actually....and if the fault light is flashing on the front panel, it's inhibited turning on the HT/screen/bias supply for the amp (heaters & bipolar supplies are on the other pwr xfmr), and there's trouble within. Might just be a tube, but also associated parts with the tube. After you've removed the hold down clamps for the power tubes, mark each tube as to it's position as you remove them. Then remove plate exposing the power tube PCB. Disconnect it so you can inspect that board for any open screen resistors. Originally, these were 22 ohm resistors, but they were changed to 220 ohm flame-proof metal oxide parts, Res # R2, R4, R6, R7, R9 & R11. I use 1W MO in those locations. Also a diode at each tube gets removed in that upgrade...diodes D1, D2, D3, D7, D8 & D9. You'll want to verify those changes have been made. Also check to see that none have been burned open, including the 47k grid stopper resistors.

And, while the board is out, look close at all the tube socket soldering, to make sure there aren't solder fractures lurking about. When all is ok there, check on the main board for the six 10 ohm 1% cathode resistors, four of which are located together at the end of the board, while the remaining two are in a ways past the resistor/diode network for the comparator section. If you have the parts placement guide for the SVT-CL, it makes life so much easier.

What I do next, having checked for open resistors, burnt parts and such, I'll re-install the power tube board, screw each socket to the plate (one screw is adequate), then mount that assembly back onto the chassis, but leave off the hold down clamps. I set the power amp chassis on it's end, with a short block under the power transformer to stablize it, so I can have access to the bottom of the main PCB and will install one pair of tubes at a time to first see what the plate current is for each tube....measuring across the cathode resistor. You need the 'road map' to tell you where the cathode resistors are, and which one is which. I clip lead my DMM's gnd lead to one of those resistors along the edge, then I'll go thru and check each pair of tubes, they now having a position number, and I can see what the nominal plate current is. Once I have the list, I'll sort the tubes for a best match for each triad of the sextet, and put them all in....or at least one pair at a time.

V1, V2 & V3 is the upper triad, V4, V45 & V6 is the lower triad. They're positioned on that tube PCB in a triangular pattern.

If your rear panel bias pots are not positioned nearly the same, you'll probably want to start over with a single pair, and bring the bias up for each to register 20mA per tube (200mV across the 10 ohm resistor) for starters. Then,with the bias controls set for a nominal starting point, you can begin matching the tubes within your sextet...or discover you've got one or two tubes that are way high or way low.....like 22mA, 23mA, 45mA, & 8mA, 18mA, 20mA. You might be able to group the 20mA, 22mA & 45mA on one half, and the other three on the other half and adjust the bias controls accordingly to see how that balances out. Invariably with that sort of imbalance, I'm digging thru my collection of SVT tube pulls to see if I have one that will match the other five, or other four, as in this instance, two tubes are way off from the rest.

The SVT's Bias Light system on the rear panel will not work until you have 6 tubes installed. It will still indicate tube problems if they exist at that point (like always RED, won't go GRN) But, it does allow you to use the chassis as a test fixture to sort tubes. Their protection system still works with one or two pairs....it will shut down if you put a bad tube in while sorting.

One thing I do as a regular routine with SVT's is.....replace that bloody garden hose they put over the two sets of cables between chassis and preamp. I cut that apart, and replace it with Tech Flex, so you can easily position the preamp relative to the power amp chassis during this part of service. The clear thick garden hose insulation instead dictates where the two chassis members will be on your bench.

Loud hum in the speakers.....thoroughly inspect the main PCB for solder fractures....particularly along the rear panel edge....the bias pots, phone jacks, XLR connectors, etc break free from the board, and develop tiny annular fracture rings where there used to be a solid solder joint. You can find them all over the main board, as well as on the output board, AC mains input board, and on the preamp board. There is a critical screw on the main PCB that anchors the system ground to Chassis...it's one of the support screws, and has a mating screw that ties the standoff to the chassis at the opposite end. If that is loose,the amp will hum, and until I discovered that, I was chasing my tail for hours.

And, I wouldn't be a bit surprised if the power transformer and output transformer's mounting hardware is loose. They're designed to loosen, I think. Once those mounting screws are loose, the vibration during travel shakes the rest of the amp, breaking down panel-mount hardware that doesn't use mounting brackets. There's a silly notion amongst manufacturer's that solder terminals are as good as welded support brackets.

[TimmyP1955]

Excellent post nevetslab.

Also check the solder joints on the IEC power inlet - I've had a few SVTs that were wonky because of it. Also check that the FastOn connectors are all a tight fit. The multipins on the output tube board are a real pain - they couldn't have made them an inch longer?!

[nevetslab]

I did mention that....the AC Mains Input board. Nearly all the SVT's I've serviced have solder fractures on the IEC-320 connector. Probably flow solder fault.

Yup....getting that board re-connected is a real test of patience. And, the FB 2-cond header isn't polarized. Ever find yourself asking....uh.....now which way did this come off??? Those multipins on the output board are a common solder fracture fault. Again, production flow solder issues. And, as stupidity demands, another board supported by the solder connections of panel connectors! Done that way to keep us employed.

[bsco]

Wow!!! Thanks for all that info...to start with the hum was caused by tube imbalance. Before I removed the tubes I marked them just like you had suggested.....any cables that I had removed I had them marked as well so I would know where they went and in which direction as well.....I had checked all the boards for bad connections and I had to re-tighten a bunch of spade connectors on the power supply board...

I had checked the output board for burnt resistors and everything measured fine....the resistors had been upgraded and the diodes were also removed...I had also checked those 10 ohm resistors as well....there were some jumper wires on the output board that send the heater voltage to the tubes...some of them were not the best so I fixed all that up...as sometimes the tubes would not light up...

then I went to the heater transformer and every wire that had a spade connector seemed loose, so I removed them, crimped them tighter and re--installed them for a real tight fit...that solved that issue...that grey ribbon cable was causing the amp to cycle.....it wasn't plugged all the way in...and yes...it is the one from the pre-amp board....

I installed two tubes at a time to check them and by doing so, I managed to come up with a set that were pretty close..the bias controls are pretty much set at the same position in relation to each other....one side switches from green to red a bit faster than the other when adjusting but there is still a delay between the color change..........so now I was able to actually listen to the amp...


one thing I did not think off was to remove that clear plastic hose......that is a real pain.....I also cleaned up the fan, checked for any loose screws and let the amp run for a couple of hours. then I would shut it off, leave it till it cooled down and fire it up again to make sure I had the heater issue resolved....The amp worked great.....

Then I reassembled it, installed the tube retainers and put the chassis back in the cabinet......and performed the testing again.....run it for awhile, then turn it off to cool down and then run it again.....I think I spent more time testing it than I did in repairing it as the guy who owns this is a four hour drive from me....so I don't want it to act up on him once he gets it back home...but thank you guys for your input.......as always I learn something new every time I come here. I am very happy to have joined this forum.....

[nevetslab]

Always a pleasure to pass along knowledge and experience. These beasties do demand respect and attention to detail I thought they did an excellent job in the refinements to their protection circuits. I have plugged bad tubes into it during the screening tubes process, without damage to the system....finger on the Standby switch, just in case a light show emerges, but their protection shuts down the HT quickly.

Interesting the hum was caused by the tube imbalance. I'll add that attribute to my on-going SVT Service Notes file, as I haven't run into that. Cyclic hum, yes.

Yeah, removing the clear plastic hose is no fun at all. Material is thick enough to fight back.

Sounds like you got it nailed down for your customer, and glad I was able to assist.

Cheers,

Steven

[bsco]

Always a pleasure to pass along knowledge and experience. These beasties do demand respect and attention to detail I thought they did an excellent job in the refinements to their protection circuits. I have plugged bad tubes into it during the screening tubes process, without damage to the system....finger on the Standby switch, just in case a light show emerges, but their protection shuts down the HT quickly.

Interesting the hum was caused by the tube imbalance. I'll add that attribute to my on-going SVT Service Notes file, as I haven't run into that. Cyclic hum, yes.

Yeah, removing the clear plastic hose is no fun at all. Material is thick enough to fight back.

Sounds like you got it nailed down for your customer, and glad I was able to assist.

Cheers,

Steven

Thanks Steven...the hum could be adjusted somewhat when the bias controls were moved...I had forgot to mention that...that is the reason I suspected bad output tube balancing...plus all the bad connections, loose spade connections, etc....thank you for your input.....
Cheers,
Bernie