Ampeg SVT-CL erratic behaviour

[Giaime]

Hello all,

I've got an SVT-CL on my bench with very strange behaviour and I can't find what's wrong with it.

It's one of the newer ones, made in Korea, probably 2010 or 2011.

The amp did the classic "I won't go off standby with the tubes in" so I removed all the tubes.

Now the back panel leds don't light up regardless of the standby switch position, and the switch doesn't seem to have an effect on the front panel led, that turns red->green when he wants to, without closing the relay (no sound from it).

However, some other times, the amplifier turns on with the green led lit, no red led and no effect of the standby switch.

Some other times it does red, then the relay closes, then after some seconds it goes green. When I turn off the standby again, the relay flickers between red an green. Very strange.

I tested:
- the relay - with 6Vdc on the contacts it does click fine
- all the 10ohm 6550 cathode resistors that are prone to fail
- redid most of the soldering joints in the amp, even in the preamp

What could be the cause of this behaviour?

BTW, in the power amp schematic, I see C13 and R50 (near IC2B) with a very strange connection, should they be reversed? The schematic matches the amp as far as I can see for those two components.
And C12 was missing from the board, I added it (220uF 16V) with no results.

[Enzo]

I am not sure what the standby problem you refer to might be.

The rear biasing lights all measure tube current. With no tubes, ther is no tube current to measure, so no lights.

The small tubes are two 12AU7 and a single 12AX7. MAKE SURE you hqave the correct tube types there, and also make sure they are in the correct sockets.

In my experience, a very common problem with these is failed 12AU7 driver tubes, and that makes it difficult to adjust the bias successfully.


Please don;t start changing the circuit. The amp worked before it failed, so we should not assume it was built wrong.

C12 was a bit of timing, and they probably decided the amp was better without. If you reversed C13 and R50, that would put +15v on the IC pin, then the tube current sense voltage coming through R47 would have to exceed 15v to toggle the IC into fault mode. That would render the fault circuit useless.

[Giaime]

Hello Enzo,

thank you very much for your reply.

I have removed the noval tubes from their sockets, to test the standby circuit alone, the tubes are the correct ones and in the correct sockets (they appear to be the original ones, as I said the amp is very new).

Since the +15/-15V power supply seems to be ok, and I managed to trigger some movement in the relay by tapping the preamp chassis near the switches, I assume I have a bad IC or a bad solder joint in the preamp chassis. Other possibilities could be bad IC in the power amp board, or a missed bad solder joint, since I retouched almost all (both boards) of them. The front panel led sometime flicker and this can be a bad IC.

What is the correct behaviour of the front panel light? Power on / standby on -> red, then standby off -> green, then standby on again -> red?
Because no matter what I do, once the led becomes green it stays green, even toggling the standby switch (thus removing the B+ voltage), this is without tubes. Maybe the led stay green because the HV is still there, no tubes to discharge it.

About C13, I assume the stage is a Schmitt trigger, so probably C13 is there to smoothen out the reference voltage. Why connecting it to the +15V line, instead of ground?

Or the circuit works by considering the small leakage current of the electrolytic capacitor?

[Enzo]

I assume the initial charging current of C13 puts a small positive bias on the IC input pin, but then that settles down a moment later to a reference amount so the circuit can act as a comparator. A power-on reset circuit more or less. A timing cap, not a smoothing cap. At least that is how I see it.

To be honest, I don;t recall the panel indicator lights. They generall display condition. I would wager that once proper operation of the amp is restored, those indicator colors will become normal.

[MarkusBass]

What is the correct behaviour of the front panel light? Power on / standby on -> red, then standby off -> green, then standby on again -> red?
Because no matter what I do, once the led becomes green it stays green, even toggling the standby switch (thus removing the B+ voltage), this is without tubes. Maybe the led stay green because the HV is still there, no tubes to discharge it.

The LED on on the front panel is described in the User Manual:
>>>>>>>>
STANDBY/POWER/FAULT INDICATOR LED: This is a multifunction LED. In Standby Mode, it glows red. In the On mode (when the high voltage comes on) it glows green. If it does not turn green in
the On mode, there is no high voltage present and the unit needs servicing. If the amp detects a fault in the power tube circuit, the high voltage is turned off and the LED flashes between red and green.
>>>>>>>>
It stays green because +300V is not discharged. You can tell it only by looking at the schematic (voltage divider on pin #5 IC1). There are also voltages specified (STANDBY, OPERATE). I think that you should start from checking power supply for Q1 (that powers up the relay). The transistor is powered from the preamp. An the time delay voltage - base of Q1 (from 0V to 15V) is from the power amp.

Mark