SVT-CL - Ticking

[TimmyP1955]

About every 1/2 second, varies a bit with temperature. Usually when one of these ticks, it's because the tube board plate is not making electrical contact with the chassis. This one is OK in that regard. And I've checked all of the chassis grounds that I can find. The ticking is coming from the speaker out, preamp out, and slave out. So far the scope hasn't shown me anything. Any ideas before I dive back in?

[xtian]

If it's an intermittent connection (cold/cracked joint, broken wire, etc.) a chopstick is a good troubleshooting tool. Put pressure on various places to see if you can change its behavior.

[Jazz P Bass]

The noise being at the Preamp Out pretty well indicates that it is at that jack or before it.
The preamp.

I would start by checking the power supplies.
See if it shows up on any one (or all) rail.

If it is not a supply problem then start at the Pre Out jack & work your way back to the input.
You have to find out if it is global or does it start somewhere.

[g1]

Also, have you ruled out all the tubes as possible sources of the noise? I seem to recall some mention of certain preamp tubes causing "geiger counter" noises.

[TimmyP1955]

It's not the preamp - if I disconnect the PSU lines or the ribbon, the ticking stays the same or gets louder (depending on which I disconnect). I also swapped in a different preamp just to be sure. Sigh.

[TimmyP1955]

Solved!

Two of the tube retainers were not continuous to ground. I fixed that and all seems well.

[Jazz P Bass]

You lost me on that one,
What does tube retainers have to with it?
Other than the fact that you may want to consider cleaning (DeOxit) & retensioning , at least, those two sockets, if not all of them?

[audiopete]

Is there a chance this is arcing in a failing power tube or output transformer primary? Bad socket?

[audiopete]

If it's an intermittent connection (cold/cracked joint, broken wire, etc.) a chopstick is a good troubleshooting tool. Put pressure on various places to see if you can change its behavior.

You can also try a can of FreezeIt. Watch out don't hit any hot tubes or components, just solder joints and component leads.

[nevetslab]

Tube Retainers?

Like Jazz P Bass, I too don't follow this one. Is this the recent style SVT-CL like this:



I've never come across any with this kind of repetitive ticking on these amps, and only the power tubes have retainers, none of the preamp / driver tubes do. Or is this an older SVT Classic where there are a bunch of preamp tubes, all having the spring-loaded shields?

And the tube sockets on the recent generation boards....the PCB mounting standoffs aren't grounded that I can recall



I do come across fractures sometimes on this board, as can be seen on that tube socket. But never heard the ticking you describe. Add another gremlin to the files. Interesting.

[Alex R]

if it's irregular then it could be a transformer winding. Heater windings can do it, very hard to trace. Tapping the transformer windings should produce motion sensitive noise if it is that.

[TimmyP1955]

When we bought a number of SCV-CLs before my time here (10.5 yrs), a number of them ticked like this. The problem per Ampeg was (as mentioned in my OP) that the center plate was insulated from the chassis by the paint (which apparently caused a charge/discharge). We scraped the paint at all the mounting screw locations and the problem was solved on every amp.

This problem would seem to be an extension of that - the tube retainer was insulated from the center plate. Same charge/discharge.

[nevetslab]

That's very interesting, and one that I'd never have suspected as a cause.

I did have one SVT-CL some time back have random noises, static & clicking.....probably the same ticking noise, but it came in spurts, and not cyclic. I had narrowed it down to the power amp section, and finally the sextet of power tubes, and swapped them out, ending the problem. Earlier in the servicing of that amp, I had done a full tear-down to get at the solder fractures on the main PCB, tightened up the XFMR mtg screws while that board was lifted up, and had the power tube PCB removed for replacing the screen resistors. I had fully reassembled the amp, with the hold-down clamps back into place before I ended up swapping out the sextet of power tubes.

In my previous post, I had mentioned not recall seeing the mounting hardware of the octal sockets being grounded. I'm just finishing up servicing an SVT-VR, and those socket's flanges ARE TIED IN TO Pin 1, which on some 6550's & KT-88's that connects to the metal base of the tube, while on others, it's no connection. I did see in JJ/Tesla's KT-88 data sheet Pin 1 could be tied to Grid 3 (suppressor grid), though that's normally tied to the cathode. The chassis of the SVT-VR has masked off 3/4" circles around the cover plate mounting screw locations, with the back of the cover plate having 1/2" circles around the thru-holes for mounting the plate to the chassis, so there IS ground continuity on that plate, and all the way to the metal base of the power tubes, as can be seen in the photos below. The wide masked off area on the back of the top plate picks up the metal flange of the power tube socket, completing the ground path of Pin 1 to the plate and on to the chassis.



During the last bit of service on this SVT-VR, having found one of the power tubes had an open heater, and after de-soldering and re-soldering Pins 2 & 7 with no success, I was forced to select thru that sextet of 6550 Sovtek tubes for a matching tube to balance out the sextet (measuring across the 5 ohm plate resistor thru my insulated top plate made for servicing the SVT-VR's). I was apprehensive about selecting a tube out of that batch that was the noise source before, but it's dead quiet.

I only had four photos of the SVT-CL power tube PCB, and those being close-ups of the tube sockets to show the solder fractures in my records. I didn't see Pin 1 being connected to the hex spacer as I see on the SVT-VR PCB assy. I don't recall seeing any attempt on the SVT-CL chassis nor power tube cover plate for insuring ground continuity.

It's only by chance, in tightening down mounting screws that you'll radius the head of the screw thru the painted surface of the plate to actually ground that plate to the chassis (by way of the pressed-in pem nuts receiving the machine screws). Paint does make a good insulator. We do have two very recent SVT-CL's that have a later generation power tube PCB, where they changed from the double-row ribbon connector to a single-row style. I'll have to have a look at that amp to see if they ended up grounding the power tubes/plate/PCB as they did on the SVT-VR.

That is one for the books for sure.

[TimmyP1955]

This has the CL style coil spring hold-down retainers, as opposed to the base clamps. Grounding indeed would appear to be by happenstance - something has to cut through the paint someplace to connect the retainer to the plate and the plate to the chassis. (I've never payed attention to the status of Pin1 on any of these things.)

[Joss]

Old thread, but for info purposes. Have an Ampeg svt Classic at workbench and pin 1 of 6550 tubes is not connected to the hex spacer. Double-row ribbon connector on this amp