Ampeg SVT classic bias problem

[Voske_1980]

Hi Guys,

I have this Ampeg SVT Classic brought in by a customer and I'm a little lost in finding the problem.
The customer says that the turned the amp on without a speaker load and now the red and green bias lights both stay on constantly, while a loud hum comes over the speaker.
I've fitted the amp with 6 new matched 6550's and new driver tubes.
I'm measuring a way too high cathode current and the voltage across the 10 Ohm resistors is around 1.9V which should be in the 220mV range (22mA).
The amps does not go into fault protection.

Voltage readings power tubes:
1: -
2: heater
3: 600V
4: 380V
5: -25V
6: -
7: heater
8: 1.9V

With the amp in standby the voltage on pin 5 is -45V
Screen resistors ar OK
Cathode resistors are OK
What could be wrong here so that the negative bias would rise up with 20V when the standby is switched off?

Thanks Marc!

[Voske_1980]

I also found out that the negative supply for the bias (should be -180V) only measures -130V. But stays stable when switch on and off standby

[glebert]

Did you disconnect any ribbon cables that might have gotten misaligned or swapped? Seems like -45 is about where the bias should be for normal operation.

This is the schematic I used for my SVT CL, don't know if is the same for yours. Bias circuit is on page 7.

[Helmholtz]

Pull power tubes and repair the bias problem first.

Isn't there a more complete schematic showing power tube wiring??

[glebert]

Could there be a shorted out triode in V2 or V3 (on page 7 of the schem I posted) that is trying to pull the -45 up toward a positive voltage when taken out of standby? I don't think shorted 12A_7 are very common, but these do have pretty high voltage across them. Did the original failure have shorted power tubes?

Edit: Also do both bias voltages go to -25V? I would be checking the grid voltages of the V2/V3 to see if they are getting biased correctly.

[Voske_1980]

btw, I'm using the schematics listed here:
http://www.tangible-technology.com/s...g/SVT/Classic/
These seem complete.
I've double checked the ribbon cables, they all seem to be fitted correctly.
I'll check the grid voltages on V2 and check out the 12aX7 tonight. I've measured only V3 so far and the grid voltage (pin7) was -47V
V2 and V3 are new tubes (12au7)

[g1]

I also found out that the negative supply for the bias (should be -180V) only measures -130V. But stays stable when switch on and off standby

What do you measure for AC volts (ripple) where it should be -180VDC ? Did you check the 4 diodes?

[Helmholtz]

btw, I'm using the schematics listed here:
http://www.tangible-technology.com/s...g/SVT/Classic/
These seem complete.

Please identify and extract relevant schematics (especially the power amp) and post them here.

PSD files don't open for me.

[nevetslab]

I've attached the rest of the SVT-CL Schematics adding the Power Tube Schematic, the Main Power Amp PCB Schematic etc to add to what's presently provided:

Power Supply PCB Schematics (792SCH_A).pdf
Tube PCB Schematics (41941h3_).pdf
Tube PCB Pictorial and Parts List (41941p5_).pdf
AC Terminal Board Parts List & PCB Layout (31904p1_).pdf
Ampeg SVT-CL Power & Heater Xfmrs.pdf
Ampeg SVT-CL, -AV, -VR Output Transformer.pdf
Tech Note -- Transformer connections drawing.pdf



As Helmholtz said, pull the power tubes as having -25V bias voltage applied to the power tubes is why you're getting way too much plate current flowing thru the tubes, as seen across the 10 ohm cathode resistors. That should have tripped the Protection Circuit and shut off the main Power Transformer. So, it appears you have some circuit problems on the Main Power Amp PCB as well as on the AC Mains Relay board. The voltage sense circuits are part of the GRN/RED Bias Circuit Window Comparator, and then the output thru IC2B, which is the comparator that's sampling the summed voltage across all six cathode resistors. That potential you're seeing (1.9V across 10 ohms = 190mA) should have tripped this comparator, so there may be a failure there, or in the wiring from this IC via J10 pin 1, which goes up to the Preamp to talk to the Status LED circuit, as well as the AC Mains/Relay board via J12 on the Power Amp PCB and comes in on J35. That Fault signal connects to Q1 MPSA13 Darlington Xstr via base resistor R3 (220k).

Under normal bias, the Fault Detection Buss at the resistor input to IC2B via R47 is typically 0.78V to 0.85VDC. I'd have to dig thru my notes to see what the actual trip voltage is on IC2B comparator circuit, but it should have tripped. So, as he said, you need to solve bias as well as the Fault Circuit to get that working

[Helmholtz]

Where can I see the 10R cathode resistors and where can I see how the negative supply voltage is applied to the driver tube cathodes?
Not even clear how the standby switch is wired.

Sorry, if I missed something. Unfortunately I'm not very patient.

What I can see from this confusing set of schematics is that the power tube grid bias is identical to the driver tube cathode voltage.
Driver tube cathode voltage varies with driver tube bias and cathode current, so power tube bias is adjusted via driver tube bias (trimpots P1 and P2).
If the standby switch cuts the driver tube supply, it's no wonder the cathode voltage and power tube bias changes (i.e. is more negative in standby).

[glebert]

Where can I see the 10R cathode resistors and where can I see how the negative supply voltage is applied to the driver tube cathodes?
Not even clear how the standby switch is wired.

I hate how these schems are split over so many files, that's why I use the "two revs" package I gave in post #3. The 10 ohm cathode resistors are on page 7 of that. They are not on the same board as the power tubes. I can't say that I can see how the standby switch is connected either. It looks like the screen and small tube voltages have their own PT tap and rectifier, I think the standby switch is between those.

[Helmholtz]

Thanks, so each power tube has its own cathode resistor and cathode idle current should be 24mA.

Now where do the -180V go?

[g1]

Now where do the -180V go?

Here

[Helmholtz]

Thanks, need new glasses.

[nevetslab]

The Standby switch, which can be seen in the AC Mains/Relay Board wiring, switches on/off the Power Xfmr that feeds the Plate and Screen Voltages. The Heater Xfmr has the Bias Supply, so it's always on when the AC Mains is switched on. Fault Protection switches off the High Voltage xfmr, killing the Plate and Screen supplies to the power tubes, as well as the Preamp/driver tube's HT. The -180V bias supply remains on as do the heaters. Yeah, drives me nuts not being able to see all of the circuitry in one view as well. Standby has all of the HT turned off, including the driver stage tubes.

From working on these amps for years, I don't see any change in current thru the power tube's cathodes switching in and out of Standby. It settles quite quickly and seems to stay where it was set on all the tubes (unless there's tube issues either in the driver or gain stage ahead of them, or with any of the power tubes).

Now, why there is a 20V shift from the nominal -45V bias (never really know WHAT the bias voltage will be until the tubes for a given amp have been set....I've seen over 10V variation for a nominal 23-24mA avg plate current). Finding -130V instead of -180V.....is that in Operate (NOT IN STANDBY)?. If so, then there's a circuit issue there to be resolved.