Ampeg SVT-CL bias control LED solid red all the time

[TubeGeek]

I have an SVT-CL on the bench that the bias control #1 red LED is on all the time after a few minutes of being on.


Normal power on, flip to standby and the green LED stays lit for a minute or two and then the red LED turns on and stays on.


I have tried a new set of KT88's, same issue.
With the original KT88's the same issue.


It is quite the job to get to everything to confirm.  I have measured the screen grid 220 ohm resistors and plate resistors, all test good.


Looking at the schematic, the cathodes connect to a bunch of opamps, resistors and diodes for the biasing LED's to function.  Could it be that one of these components is faulty and causing this red LED to stay on? Or is it just what it is…the tubes aren't 100% matched and this is normal for these amps?


I'd like to get green LED's while idling. This amp is worked hard all year round and has to be rock solid.


http://www.bustedgear.com/images/schematics/Ampeg_SVT-CL.pdf

[TubeGeek]

Notice when I turn the AC mains switch on, the bias control #1 red LED lights up, fades away, lights up, fades away.  After flickering on and off for 30 seconds it stays off until I flip the standby switch into play mode.


This is partly why I am thinking the problem is in the bias control led circuitry.

[TubeGeek]

Bias:


KT88 x 6


1: 25.4 mA

2: 23.4 mA

3: 22.5mA




4: 23.9 mA

5: 26 mA

6: 25 mA

[PRR]

Measure your "+15V" rail (should be 15V); also IC4 pin 6 and IC6 pin 6 (should be 0.294V, call it 0.3V).

A red LED comes on when the drop across any 10 Ohm cathode resistor exceeds that 0.294V (say 30mA). If ALL are going on at the same time, it suggests your "+15V" is out-o-whack.

Why the "+15V" is sick: we start with raw +20V and drop it on a 30 Ohm resistor and a 15V Zener. Many 1990s-era amps have trouble here. The hot resistors buried in a hot tube amp cook their PCB solder joints, and fail. Inspect the area around R60 R61 D27 D28, repair toasted PCB as needed, and replace generously with up-sized parts mounted for better air cooling.

This also has CB1 CB2 "THRMSTR"s into the +/-20V rectifier. I don't like that. Use both probes on ACV to read the voltage drop across each CB when running. Anything more than a Volt is probably wrong, the thermistor has drifted beyond spec, and is strangulating the poor +/-20V supply. I'd just jumper them, though a couple 1A SB fuses may be a wiser choice.

[TubeGeek]

Measure your "+15V" rail (should be 15V); also IC4 pin 6 and IC6 pin 6 (should be 0.294V, call it 0.3V).

A red LED comes on when the drop across any 10 Ohm cathode resistor exceeds that 0.294V (say 30mA). If ALL are going on at the same time, it suggests your "+15V" is out-o-whack.

Why the "+15V" is sick: we start with raw +20V and drop it on a 30 Ohm resistor and a 15V Zener. Many 1990s-era amps have trouble here. The hot resistors buried in a hot tube amp cook their PCB solder joints, and fail. Inspect the area around R60 R61 D27 D28, repair toasted PCB as needed, and replace generously with up-sized parts mounted for better air cooling.

This also has CB1 CB2 "THRMSTR"s into the +/-20V rectifier. I don't like that. Use both probes on ACV to read the voltage drop across each CB when running. Anything more than a Volt is probably wrong, the thermistor has drifted beyond spec, and is strangulating the poor +/-20V supply. I'd just jumper them, though a couple 1A SB fuses may be a wiser choice.

Measured the +15v rail at c29 and r67…+15.35v


Measure +15.35 on IC4 and IC6 pin 4's.  On pin 8's I measure -16.5v. Pin 6's I measure 0.3v.


CB1 & CB2: measure 0.12V VAC across both.


Voltage across all 10Ω resistors is 0.260vdc


Looks to me that the +15v rail is functioning correctly.


R60,61, D27,28 appear good. Resoldered just for good measure.  R60 and 61 measure 47Ω in circuit.

[PRR]

Wal, I am mystified.

That schematic is a blinding mess/maze.

I'm posting what I hope is the core part of the red-LED system. Maybe someone will find the flaw in my thinking.

If cathode voltage is over 0.3V, the "+" input of the opamp flings the opamp output hard positive, +14V, which flow through LED and R to -15V, and light.

You say you have 0.26V on cathode and 0.3V on 15K+150+150 voltage divider. That, into the "+" input, should fling the output hard negative. That leaves nearly no voltage across LED+R, so the LED should be dark.

You can't have six bad opamps. The 1K gives some protection against input abuse. It would take 1.5 Amps in each (and every?) tube to back-bias the opamp inputs from their + rail, and probably 2.5 Amps to violate their input fault-current rating. I don't see that happening.

[shooter]

there are 3 conditions that light the red led, check each of the opAmps both input n output to see which is driving the led on.  I have seen the steering diodes do flaky things before in logic circuits.
don't see which is #1, so i'll use D48, probe pins 12, 10, n 5 on the in's, 14, 8 n 7 on the outs, I suspect 1 will be different than the other 2 IC's

[sluckey]

Here's a drawing that 'may' be helpful to understanding the bias lights circuit. The owners manual available online has a good non-technical description of how to bias the amp using only the lights. Also has a simple troubleshooting table for various light conditions.

[TubeGeek]

You guys are so helpful. Thanks so very much. Steve and PRR, killer diagrams!

I found out a little more about the amp from the owner: the reason it's on my bench is because the road/stage crew powered up the amp with the power and standby in "on" and that's when the issues began.  I also notice that someone else has worked inside this amp and a few of the chips have been soldered or replaced.  All the pins on at least 3 chips have been soldered.

This amp had to go back to gigging.  I am pretty sure it's ok to go, as it's playing and sounding normal for at least 8 hours.  The bias on the tubes holds steady.  I will clear up this LED issue after the holiday season when it gets back to me.


I'll report back then.

[shooter]

It's always the roadies fault!  If you find a bad chip, you might talk with the customer on the pros n cons of a socket, quality ones, not the RS cheapies.

[PRR]

> powered up the amp with the power and standby in "on"

"Should" not be a problem. Power cords get kicked-out and shoved-back all the time. Any amp which died that easy would drown in warranty repair costs.

> someone else has worked inside this amp and a few of the chips have been soldered or replaced.

NOT a good sign when you are following someone else's troubles and maybe mistakes.