NTC Thermister and light bulb current limiter issues?

has anybody had an issue with using a light bulb in series for current limiting on an amp that has a NTC thermister in series with the incoming AC power.

I have a 200W linear bass amp, in an Ampeg BA115HP. It had a shorted opamp that I was trying to isolate, and though I could leave the preamp board disconnected to see which opamp was causing the short. But my 16V zener diodes were on that board, so my +/-16V went to +/- 50V. ooops!

So i replaced the mains FET transistors, and decided to use the light bulb limiter. But as I ramp up the AC voltage the light bulb gets bright, at around 30 vac and gets getting brighter. My +/-50V buses are clamped at +/-4V as if by design?

I can't find anything wrong. The output from the transformer is good with no load. But i noticed a big black NTC in series to limit the in rush current. Probably a good idea since the 1st filter caps are 5600uF each.

But I noticed the NTC thermister isn't getting hot. Just slightly warm. In that state it still has a 5K resistance, so perhaps it is limiting my voltage?

I am going to try and take it out of the circuit, but I don't expect it to matter.

Any ideas on what else to try? I have almost no voltage across the 0.33 ohm resistors in series with the main power transistors, so I don't think my amp is causing the high current draw.

Here is the schematic.

115b by Dennis Kelley, on Flickr

115a by Dennis Kelley, on Flickr
 
You wonder about the inrush thermistor. You indicate you are already using both a variac and a bbulb limiter, so just short across the part and find out if that is the issue.

And for that matter, measure AC volts ACROSS it to see if there is a large portion of your mains voltage dropping there.

I don't see a thermistor on the schematic.

5000 ohms sounds way off to me, 120v dead across it would only draw like 24ma from the mains.

What do you mean by " output from the transformer is good with no load." The only thing this could mean to me is that with a full 120v on the primary, you get the full expected voltages on all the secondaries. I could be wrong, but that doesn't seem like what you intend? Those 50v rails would expect about 35vAC from the transformer, is that what you get?
 

Tikysty

Member
2018-02-12 9:41 pm
Hi Dennis,
I've got a problem with the same amp model (Ampeg BA115HP), the NTC is exploded and I cannot read model on the body to replase it. Can you kindly provide me with a picture of it or with its model number? Is your amp 110Vac or 230Vac mains? Maybe I'm wrong, but it could be important for the NTC choise.

Many thanks in advance
Stefano