Fender Champ AA764 I think '73

[SAG Electronics]

So, Disconnected the O.T. red wire and retried the 5Y3. Bulb lit. Boo. Looking all over for a short...

[The Dude]

What is your resistance reading from HT to ground? That may provide a clue. (Discharge caps before measuring to be sure no high voltage is stored.)

[SAG Electronics]

Sorry for my lack of proper terminology, HT? I got choke wrong already. Just learned about cap discharge (more about it) As i plugged an old sony amp into the limiter to see if it was something I wanted to have "fun" with and the bulb lit warm and the relays clicked as the caps discharged. Still foggy on HT? Power transformer? If so, why would the heater circuit work? All comes from the line.

[The Dude]

HT (High Tension) is B+ (labeled 360V on the schematic). B+ is "made" by the tube rectifier, so without the tube there is no B+. When you install the tube and it tries to make B+, if there is a short on the B+ line, the light bulb limiter will be bright. That's why I was wanting you to check the resistance of the B+ node to ground. It may tell us if there is a short or near short there. The resistance reading may also show us where to look for the short if the value is (for instance) the same as one of the node resistors.

[Leo_Gnardo]

Sorry for my lack of proper terminology, HT? I got choke wrong already. Just learned about cap discharge (more about it) As i plugged an old sony amp into the limiter to see if it was something I wanted to have "fun" with and the bulb lit warm and the relays clicked as the caps discharged. Still foggy on HT? Power transformer? If so, why would the heater circuit work? All comes from the line.

HT = High Voltage. In French "Haute Tension" but I dunno why it's mostly British amps that have HT labeled fuse holders... one of those things that crossed the Channel I s'pose. What we're askin' about is, measure resistance from chassis to high voltage power supply points. Even if the filter cap measures infinite ohms by itself, there may be some short to ground that's raising hob with your Champ. That's what your bright test lamp is pointing out in a general way. Some research with the ohm meter may make the fault more specific. Of course do your ohm meter testing with the amp disconnected from AC power, and make sure there's no DC voltage lingering on the high voltage points. I doubt there would be, given the symptoms, but discharge any DC you find lest it damage your meter at worst, or give you misleading resistance readings.

Have you had a good close look at the tube sockets? Especially the rectifier & output tube octal sockets? Sometimes an arc between chassis and a high voltage terminal leaves a carbon track and that will give you the hi voltage short circuit symptom we seem to be seeing. Sometimes on an output tube socket, the carbon track is between a filament and high voltage terminal, say between pins 3 and 2. Also examine tube bases, a carbon track can develop there too. Bright light and a magnifying glass can help.

[xtian]

Late to this thread, sorry. 75w is too "big" for your limiter. That's not going to prevent anything from burning up. Try to find a 25-40 watt bulb.

[Enzo]

Tension may come from French, but growing up, those power distribution lines were always "high tension wires" to us.

HT for high tension, B+ from the old old battery days, High Voltage. We refer to it by various names but in each case it means the several hundred volts for the tubes.

[g1]

those power distribution lines were always "high tension wires" to us.

Like in the song.

With a purposeful grimace and a terrible sound
He pulls the spitting high-tension wires down

[Jazz P Bass]

"Godzilla".

[Enzo]

OOOh no, there goes To Ke Yo
Go go Godzilla

[SAG Electronics]

Yeah, I am seeing about 300k ohms to chassis from all points along the HT line. Checking those sockets. Darn good tip! since I cannot find anything along that HT line.

[Helmholtz]

If you don't have a replacement 5Y3 and while still on the lamp limiter you could temporarily use two 1N4007 diodes instead of the 5Y3.
If that works fine, it's quite sure that your 5Y3 is the culprit.

[SAG Electronics]

Lordy Hallelujah! Rechecked connections, removed the 5Y3 socket and cleaned it. Reconnected everything and rechecked connections, then put in a new 5Y3. The bulb did not light, but glowed warm. Put the rest of the tubes in and turned it back on and it still just softly glows! I am gonna put this sucker back together for a final test and, hopefully. Get it back to my friend!

[SAG Electronics]

LOL. not quite. So excited that it wasn't blowing fuses that I put it all back together only to find 0 output through the speaker. Put it back on the bench and started checking voltages. Holy cow! seeing way too high voltages everywhere. 480DC where it says I should be reading 205VDC. How can this be? Do I need to be checking with everything connected? Speaker and instrument, but that even makes little sense. Speaker measures 3.7 ohm and cable reads continuity on both signal and ground. At least its not blowing fuses.

[Enzo]

Way high voltage wouldn't prevent it from making sound. However, way high voltage is often a sign the tubes are not conducting. And the primary reason for that would be the heaters are not working. SO then, are the little glowing heaters glowing in ALL the tubes?