Wednesday, April 25, 2012

YBA 3 Custom Special Update

...or, it's not always what you suppose it is.

Armed with another variac I resumed an easy ramp up of voltage to insure everything was copasetic and wouldn't you know it, about 50 volts in I hear the "pung" of inrush current draw. Explicative deleted. So, it's NOT the test gear (except, of course, for the one with the faulty outlet). Troubleshooting ensues.

Amp was tubed with dummy load and the stand-by switch was set to play.

1st step: Remove tubes & repeat procedure... "pung"

Explicative deleted. After checking over my work and running continuity tests across the circuit to ground (both of which passed) my thoughts drift to the power transformer.

2nd step: Fire it up in stand-by to see if it's a shorted primary. Acts fine. What changed?

Hmmmm. Continuity check on switch reveals that while the pilot light side switches as intended, the center tap to ground portion (switch is pictured in the stand-by position, nearer half under scrutiny) exhibited high resistance when it should have been a dead short. Several actuations confirm the switch is garbage and needs to be replaced.

So, I theorize that the "pung" at the variac was instigated by arcing across the switch feeding inrush to the capacitor bank of the B+, and it probably wouldn't have been noticed if I cut the amp loose on full mains voltage instead of pussyfooting around. This also provides a satisfactory reason why all my voltages were wonky at the sub 50 volt position of the variac.

I did ponder the option of simply cleaning the switch contacts, since I have access to them.. but somehow every time I think along those lines I am hounded by the image of villagers with pitchforks and torches. Best to swap the part out entirely.

8 comments:

krivx said...

Just look at that thing. It's done, spend a few bucks and swap it out.

Out of interest, what kind of dummy load are you using? I have some parts for a 300W resistive load but haven't put it together yet.

crochambeau said...

Hahahaha, yeah, it's lead a full life. I'll probably give the contacts a buff an repurpose it into a noise box of some sort.

I've got a couple of these dummy load resistors. Not really something I would want to really bake in running the amp wide open, but fine for low intensity use. 300 watts would be nice to have on hand.

krivx said...

I have one of these eBay resistors: http://tinyurl.com/cf3jf7v

Mine measures pretty close to 8 ohms. I do want to run it hard with a temperature probe and see if the resistance actually remains constant but I don't actually have a power supply capable of doing that at the moment. I'll probably put a couple of fans on the enclosure.

crochambeau said...

Nice. Plus it will double as a club in case zombies attack while you're at the bench. I might have to consider something like that, I have a 1KW tube transmitter amplifier that would be fun to dumb down to audio (probably have to shed at least 80% of the dissipation in reining it in from class C VHF to avoid meltdown).

I'd guess that the temperature coefficient of a wire wound should be pretty stable, but given the origin....

krivx said...

The quoted specs (I have them somewhere...) was constant resistance up until something like 100 deg Celcius, then it starts to decrease with temperature. It *should* be pretty easy to use two multimeters to do simultaneous resistance and temperature measurements.

I just like that they fit perfectly into army surplus ammunition boxes, and are almost the exact same shade of green.

crochambeau said...

100C? Man, how cool would it be to wrap some glass tubing around that and fill it with water to have a visual indicator of when your resistance is starting to drift by the boiling of the water?

pearshapedhuman said...

And the prize for the ugliest most fucked up switch goes to...

manoj yadav said...

The quoted specs (I have them somewhere...) was constant resistance up until something like 100 deg Celcius, then it starts to decrease with temperature. It *should* be pretty easy to use two multimeters to do simultaneous resistance and temperature measurements.

I just like that they fit perfectly into army surplus ammunition boxes, and are almost the exact same shade of green.