SVT CL Protection circuit one more time

[maxfixesthings]

I apologize for bringing this up again, but in my searching I wasn't able to find a solution to my specific problem. When I turn on the amp with all tubes installed, the relay clicks on and off and doesn't stop, although it slows slightly after a few seconds. The green light comes on in this condition. When I test the amp with the power tubes removed the relay clicks on like normal and all the voltages look correct.

I haven't tried swapping the 12AU7s yet, but I have replaced some bulging caps in the power supply and the 10 ohm cathode resistors just cause. Also replaced Q1 and tested all the components on the AC term board. All 6550s are a new matched set.

Does anyone know what condition could be causing the relay to go nuts when turned on with the tubes in but act fine with them removed?

using schematics found here:
http://www.tangible-technology.com/s...g/SVT/Classic/

[Jon Snell]

The protect works by detecting over current, via the cathode voltage, of the output valves.
The main cause of this is the ECC82 (12AU7) driver valve failing and giving one or more KT88s (6550s) the wrong bias voltage.

[maxfixesthings]

With the 6550s removed I measured approx -45V on each grid pin. Is it possible that voltage is affected by having the power tubes installed? In this scenario it makes sense to me that one or more 6550s is not receiving the correct bias voltage which would cause the protection relay to kill power to the transformer, but what would create the condition that the relay closes again immediately and continues cycling between open and closed 5-10 times per second? I would think the protection circuit would stay in protect mode if there was a consistent voltage problem. Is it safe to bypass the relay to try to get measurements while power is on? I run through a dim bulb current limiter.

[nevetslab]

Try with just one pair of Power Tubes plugged in. If you don't know the pattern for which tubes are where on the board, facing the rear of the amp, from left to right, it's V4, V2 and V3 on the front row, and V5, V6 and V1 on the back row. V1, V2 and V3 are the upper triad, and v4, V5 and V6 are the lower triad. You won't see Bias LED's lit with only one pair installed

I always mark the base of the tubes as I remove them, so I know where they go. I usually use Tube Sockets V3 and V4 when checking pairs of tubes (mostly for convenience) when I want to see what the plate current of each tube is, with the bias voltage set the same on each driver.

[maxfixesthings]

Back with a bit more info. I tried nevetslab's suggestion and tried it with just two tubes at a time. I tried it with pairs of 1/4, 2/5, and 3/6, and in all positions the amp turned on and the light turned green, with sound coming through the speaker. Using a tube socket probe I measured the following plate voltages/currents:
V1 558V 10.5mA
V2 558V 10.5mA
V3 558V 10.5mA
V4 559V 11.5mA
V5 555V 10.6mA
V6 548V 10.2mA

First, I noticed that the plate voltage is lower than 660V with even two tubes installed. Just for kicks I measured the AC mains power that the amp is plugged into and it's sagging around 106-108VAC. Makes sense I guess. So I checked the rest of the power supply voltages, expected voltage versus measured:
+660V | +564V
+470V | +462V
+343V | +294V
+345V | +342V
+380V | +323V
+365V | +309V
-180V | -158V

It seems weird to me, given the sagging mains voltage, that some parts of the PSU are putting out significantly less B+ while other parts are putting out close to the expected value. Could this be a power supply issue then? Would that explain why the amp won't turn on with 4 or all 6 power tubes installed but will work with just two? I've been going crazy trying to diagnose the protection circuit but it might not be the issue at all?

Thanks again for the help so far

[g1]

Are you off the lamp limiter?
If not, repeat readings without.

[maxfixesthings]

Without lamp limiter and in a different AC main circuit (114.6VAC) measurements as follows:

Expected | Measured
+660V | +625V
+470V | +503V
+343V | +319V
+345V | +373V
+380V | +358V
+365V | +341V
-180V | -176V

Same issue just more volts

[nevetslab]

I've attached a road map of the SVT-CL wiring along with schematics to follow.

SVT-Classic-Audio-Schematic.pdf
AC Terminal PCB Schematics and Parts List (31904p1_).pdf
Ampeg SVT-CL Power & Heater Xfmrs.pdf
Ampeg SVT-CL, -AV, -VR Output Transformer.pdf
Chassis Assembly Final Assembly Drawings (16-412-XX_15).pdf
Power Amplifier PCB Schematics (419xxh2).pdf
PWA Power Amplifier Parts List & PCB Layouts (419xxp2).pdf
PWA Preamp PCB Layout (51912PWA_0).pdf
PWA Preamp PCB Schematics (51912SCH_0).pdf

In looking at the SVT-Classic-Audio-Schematics drawing, you'll find all the interconnects between the PCB's. The AC Mains/Relay PCB may be where your problem is, though it also may be on the Main PCB where the source that feeds Q1 MPS A13 that pulls in the relay on the AC Mains board. That comes in on J35 from J12 on the Main Power Amp PCB along the back edge where all the power connections are made. IC2B is the comparator circuit the looks at the DC output levels from all six power tubes and compares that to the voltage at Pin 5 of that TL072 op amp. Sometimes C13 goes bad and doesn't charge up. IC2B basically controls the Relay and also sends a DC signal to the Preamp board's Fault Detector Status display circuit IC1A / B. There's also the DC supply for the relay that comes from the Preamp PCB via J6 (+VFIL/-JFIL), which is a rectified/filtered DCV from the AC heater supply that comes from the J10 on the Power Tube PCB and lands at J5 on the Preamp PCB. I've had issues with the C18, the tall 4700uF/25V cap's solder joints, as well as the solder joints of J5 and J6 on the Preamp PCB.

Solder Joint fractures are often the major cause of small failures of this amp from working correctly over time. I pull the Preamp PCB out for close inspection of all solder joints, as I do on the Main Power Amp PCB, while it's still mounted in the chassis. The 0.062 square Pin terminals along the back edge where multi-pin connectors attach are a common fault for solder joint fractures. Also, the same type connectors on the Power Tube PCB. Another source of grief is the main Heater Connections (the heavy white wires from the Heater/Bias Xfmr) that connect via right-angle 1/4" faston terminals that plug onto the foil side of the Power Tube PCB. If your terminals insulators are darkened from over-heating, and there's oxidation on those terminals, you probably need to chop those off and hard-solder the heavy wires directly to those male Fast-on terminals, as THAT is the source for the heater supply for the amp.

The two bias pots on the main Power Amp PCB are also a source of fracture. All those rear panel components were chosen by Ampeg's engineers to support that PCB (Stupid is as Stupid Does), with the firm belief that solder terminals make fabulous cost-savings to eliminate mechanical support brackets. Who cares if the result over time is solder joint fracture on your bias pots anyway?

I've posted numerous threads here on Music-Electronics-Forum on the Fault Detection circuitry as well as on the Bias LED circuits and loads of other threads in repairing and keeping these amps running. I maintain a fleet of Ampeg SVT-CL, -AV and-VR Bass Amps at CenterStaging, LLC in Burbank, where my shop is located.

[maxfixesthings]

Thank you nevetslab, I will examine all these points. I definitely have several other tabs open regarding this problem, so I've read many of your posts and I'm very happy you are willing to lend your expertise here.

[Purgasound]

I had and influx of these SVT's about 8 or so years ago and every single one was having problems with the protection circuit. It got so bad I stopped working on them. At the time I didn't know about the service bulletins but I doubt that would fix the inherit design issues.
Just throwing this out there but recently someone reached out asking me to work on another one. I almost said no but I thought, "why not just bypass all this crap and wire the bias supply like they were done in the 70's?"
Something to consider. I really need to see one of those boards again to know how feasible this really is. On the schematic it looks doable. Just changing a couple values here and there, maybe the bias pot values and so forth...