Ampeg SVT CL Classic multiple issues

[xtian]

I have an Ampeg SVT CL Classic on the bench. Owner tried to replace 10R bias sense resistor, but amp would not come off standby.

I repaired his botched job; now all 10R resistors (R35-40 on schematic) and surrounding circuit measures correctly for circuit continuity through the J11 jumper to the tube PCB.

When I turn on the amp in standby mode, the tubes all light up, the power LED is red. When I switch out of standby, the LED goes dark, and the heaters in all the tubes go dead! It's like the standby switch is a heater switch! The bias LEDs on the back panel never light up.

I cannot see anything on the schematic that would suggest why the heaters go dead when I flip the standby switch. Ideas?


http://www.bustedgear.com/images/sch...peg_SVT-CL.pdf

[Enzo]

You have something wired wrong, pretty much has to be on the small AC board.

Your schematics are incomplete, you are missing the AC board.

The way this works is the main power switch turns on power to the filament transformer. Yes, there are separate filament and power transformers. So any time the power switch is on, the tube heaters should light. The standby switch should not be involved.

There is a relay circuit, and once it detects the DC heater voltage from the preamp coming back to the AC board, the relay turns on to enable the main power transformer. The standby switch then controls that power. I expect the switch wires are on the wrong posts. The AC board has like 25-30 little push-on posts.

[Jazz P Bass]

"Your schematics are incomplete, you are missing the AC board."
Here it is: AC Terminal Bd.pdf

[xtian]

Bingo, Enzo! I had reassembled the power and standbyswitches like the owner delivered to me. But after seeing a photo of the proper wiring, realized he had put the fast-ons on the wrong terminals. So how she's powered up and working!

However, only the red LEDs on the bias control will illuminate; more as I turn the bias pots clockwise (and idle noise from speaker increases). As I turn the bias pots counter clockwise, the idle noise diminishes, eventually becoming silent, but the green LEDs never light.

[nevetslab]

Bingo, Enzo! I had reassembled the power and standbyswitches like the owner delivered to me. But after seeing a photo of the proper wiring, realized he had put the fast-ons on the wrong terminals. So how she's powered up and working!

However, only the red LEDs on the bias control will illuminate; more as I turn the bias pots clockwise (and idle noise from speaker increases). As I turn the bias pots counter clockwise, the idle noise diminishes, eventually becoming silent, but the green LEDs never light.

Were the original 10 ohm 1% cathode resistors replaced? Over the years, Ampeg has used a few different brands, but have always used tight-tolerance 10 ohm 2W resistors for R35-R40. If they were replaced with loose-tolerance parts, the next steps in checking cathode current for matching becomes more tedious, as you'll be measuring the voltage drop across each of those resistors when in the sextet.

To see if your bias LED meter's GRN LED circuit is at fault or if instead you have some power tubes not right, this is the usual procedure I use:

With the power amp and preamp chassis connected to each other, outside the cabinet, you need to stand the power amp on its' power xfmr end (using a small block to stablize the chassis from tipping over. Do the same with the preamp, orienting it so you can access the Power/Standby Switches and see the Status light on the front panel.

You need to measure the cathode current of each of our power tubes to see if you have one or more not working. It's still possible the fault is in the window comparator circuit that drives the GRN LED circuit. All six power tubes need to be installed for the bias LED comparator circuit to work. Here's a couple images to clarify this operation:



On that last photo, I had tack-soldered on some test points of each of the cathode resistors, so I could hang a scope probe on it for some additional testing, but not necessary here. I normally clip my ground lead to any of the 10 ohm leads that catch the ground bus that runs along the bottom edge of the PCB, though later generation SVT's have gobs of RTV goop on all the leads, preventing that. You do, of course, need ground. The point to this portion of the exercise is to set and check the cathode current of all six power tubes V1 thru V6, with the 'nominal' current being 22-23mA. Depending on how well balanced the tubes are, you may have to juggle them for a best-match on each half. Typically, they will match within a couple mA of each other. Having separate bias pots affords more latitude in getting a best-match for the set.

Now, if all six power tubes are working and can be matched, with an average in that 22-23mA range, you are in the GRN range. For each set of three tubes, the GRN range is from 14.7mA thru 29.4mA, and above that, the GRN goes out and the RED comes on (for each half). If you have good match/balance, and No GRN LED's are being lit, you'll have to troubleshoot the comparator circuit...IC3A, IC3B, IC4A, IC6A, IC5A, IC5B.

I use this basic set-up to test power tubes in pairs when I start into the power tube screening/matching process, and record the voltage levels across the 10 ohm cathode resistors (220mV = 22.0mA). I will normally preset the bias pots for around -45VDC, found on pin 8 of each of the 12AU7 driver tubes. That bias voltage may need to be tweaked to find the best range, then leave both the same, so you'll see the true spread of cathode current on each of the tubes. Once I have the six tubes measured, I'll group them into a best-match set, then install them accordingly and move ahead to fine-tune each half.

The Protect circuit is a God-send when you have failed tubes. Coming out of Standby, you may see a flash in one of the two tubes you're testing, which will fire the protect and shut down the HT xfmr, and fire the Protect LED circuit to flash RED & GRN. You DO have to have intact screen resistors, of course.

I maintain a rental 'fleet' of SVT-CL's and SVT-VR's, so over the years, I've accumulated 'pulls of 6550's and KT-88 tubes of various mfgrs, which come in handy when one or two of the six tubes fail. I often am able to find a suitable 'match' to the remaining four tubes to keep that set running with the added replacement 'used' tube. If money were no object, I'd always install new tubes when one fails, discard the one tube, and have even more 'pulls' to work from, but at the price of a sextet........dream on. I do always stay with the same mfgr of 6550's or KT-88's, but, have had to mix in a pair of Sovtek's with a set of Svetlana tubes, being unable to find a close match.

[xtian]

Wow, thanks for the excellent, detailed post. My client is looking to flip this amp, so no further work is authorized. But he's an electronics tinkerer, and I'm sure he'll be please to read your notes! Thanks!

[Enzo]

There are two 12AU7s and a12AX7 on the power amp. MAKE SURE you have the correcrt tubes, and they are in the correct sockets.

When I got SVTCLs that wouldn't balance the LEDs, more often than not it was 12AU7s not in the right place. The two 12AU7s go behind the two bias pots, the 12AX7 is the one closest to the center.of the board.

[xtian]

I did, thank you. The tube types are silkscreened on the rear of the chassis to help you out.