1971 Ampeg SVT problem

[boroman]

Yes, pin 4 of the molex is the only ground connection from pre to pwr amp.

Isn't there another one, of the signal (hot+ground)? It also should link with common ground, right?

[g1]

Isn't there another one, of the signal (hot+ground)? It also should link with common ground, right?

That is pin4 of the molex.

[boroman]

Okay, amp is ready. I will write down everything I dones

- About the ground from molex - yes, pin4 is the ground, but not only. Pin 2 also connects to cap ground.
- Loud hum was caused by one of 2 issues I found, but I don't know which it was, because I corrected two of these at the same time. Well, the input jack signal was shorted to ground (resoldered at the inputs to make it better) and also few caps that were replaced by previous owner was attached to separate ground on the chassis. I resoldered them to the original cap ground.
- Also, made conversion to 240V, and important: AMPEG IS WRONG on the official schematic about the colors of the wires to rewire it from 120 to 240V - MIND YOU!
- With 240V the fan was getting 240V and was supposed to run at 120, so I changed the fan for a new one.

I did it and everything is well now.
Super quiet, beefy sounding amp, no hum, no problems. Pure sound. Love it

ONE MORE QUESTION
As I turned the amp, I saw that pilot light is super bright now... voila, it's the light that connects stright to the wall voltage, so 240V now. What resistor should I use to turn it dow to 120?

[Delta362]

Since you now have the PT wired for 240, the old 120V wire will have 120 on it. Consider using the 120V tap on the PT for the existing light instead.

[boroman]

Since you now have the PT wired for 240, the old 120V wire will have 120 on it. Consider using the 120V tap on the PT for the existing light instead.

There's no way to make this happen (if you see gow its designed) - separate preamps and power amps. etc.

[g1]

There's no way to make this happen (if you see gow its designed) - separate preamps and power amps. etc.

No, you are mistaken. Both the 120V and 240V versions used 120V fans and pilot lights. Look at the schematic you posted in post #1 (rev.D). In the lower right corner of pg.1 the 240V transformer wiring is shown. Whether the colours match or not, they still use the same principle of parallel primary windings for 120V and series windings for 240V.
If you do not understand the wiring then there is a concern about safety. You did not require a 240V fan, you just didn't have the wiring right. With the series primary windings for 240V, the fan and light are connected across half the primary (series) winding.

As far as pin2 of the molex, at the preamp end it only connects to the polarity switch cap. So if pin4 of the molex gets disconnected, there is no preamp signal ground. In later models with no polarity switch, pin2 of molex has no connection at preamp side.

[boroman]

No, you are mistaken. Both the 120V and 240V versions used 120V fans and pilot lights. Look at the schematic you posted in post #1 (rev.D). In the lower right corner of pg.1 the 240V transformer wiring is shown. Whether the colours match or not, they still use the same principle of parallel primary windings for 120V and series windings for 240V.
If you do not understand the wiring then there is a concern about safety. You did not require a 240V fan, you just didn't have the wiring right. With the series primary windings for 240V, the fan and light are connected across half the primary (series) winding.

As far as pin2 of the molex, at the preamp end it only connects to the polarity switch cap. So if pin4 of the molex gets disconnected, there is no preamp signal ground. In later models with no polarity switch, pin2 of molex has no connection at preamp side.

Sorry for that quick and undetailed reply from my side - was writing from my phone... I know I could use other tap to provide 120V for the fan, but since it was almost 50 years old noisy fan, I just replaced it to a new 240V quietest one I could get. And for the pilot light, I'd need to send the separate wire up to the preamp for the light which I don't want to mess up that way, I soldered 100k resistor to the light, and it gets now almost exactly 120V, so perfect now.

[g1]

for the pilot light, I'd need to send the separate wire up to the preamp for the light which I don't want to mess up that way

No, that's not how ampeg did it. The wiring changes in the power supply (pwr amp side of molex), and the same voltage (120V) goes through the same wires in the molex.
You now need an extra wire because you are running a 240V fan, and the light and fan use the same molex wires.

But you have made it work, so as long as it is safe you are good.

[bdy]

Okay, amp is ready. I will write down everything I dones

- Also, made conversion to 240V, and important: AMPEG IS WRONG on the official schematic about the colors of the wires to rewire it from 120 to 240V - MIND YOU!
- With 240V the fan was getting 240V and was supposed to run at 120, so I changed the fan for a new one.

Please, can you share more on the conversion? I have a '75 V9 that shares an SVT power amp running on 120V and I will be happy to use it on 240V here in Europe without a step-down transformer. Thanks!