5F1 Fender Champ Problem

[glowysourworm]

I have another quick newby question: The 12AX7 numbering - looking from the un-numbered socket - is the same as the 6V6 / 5Y3 correct? counter clockwise 1 through 9?

[loudthud]

Your profile does not specify your location. Do clocks run the other way in the southern hemisphere? From the side where you connect the wires, the pins are numbered Clockwise 1 - 9 (or 8) on tube sockets. Look at almost any layout.

[glowysourworm]

sorry.. i'm actually in the antarctic - so it's probably the fact that my fingers are freezing!

I was just double checking because it's the last thing I can think of that could be wrong.. So from the socket (outside the chassis) the they run the
same as the other two tubes then. got it.

[Bruce / Mission Amps]

Tube pins and socket lug numbering is clockwise and always as viewed from the lugs and wires side... not the tube side.

[glowysourworm]

Time to check the output transformer for continunity and shorts. There should be just four wires. disconnect them all. From the red to blue the resistance should be about 280 ohms. They should be infinite to ground or the transformer frame. When measuring very high resistances, you must not touch both leads with your fingers. Use some kind of clip lead.

Between the yellow and black wires the ohms should be very low, about 0.3 ohms. They both should be infinite to ground or the transformer frame. Resistance values taken from the Hammond data sheet. There have been plenty of examples where a perfectly good transformer just had the wire colors mixed up.

So, the heater wires have a center tap to ground; and I used the "hum reduction" strategy by twisting them together and connecting them to 2/7 and 9/4 of the sockets. Do you think there could be a problem using the twisted pair heater wires to the 6V6 and 12AX7?

[Tom Phillips]

So, the heater wires have a center tap to ground; and I used the "hum reduction" strategy by twisting them together and connecting them to 2/7 and 9/4 of the sockets. Do you think there could be a problem using the twisted pair heater wires to the 6V6 and 12AX7?

1) Twisting would not be a problem unless you damaged the wires and shorted them together in the process.
2) If you only connect between 9/4 of a 12A_7 then you are only powering one of the two heater sections of the tube. Pin 4 should be connected to pin 5 to energize the other heater. That could explain your "I don't have anything coming through from the guitar." comment in post #15.

[glowysourworm]

1) Twisting would not be a problem unless you damaged the wires and shorted them together in the process.
2) If you only connect between 9/4 of a 12A_7 then you are only powering one of the two heater sections of the tube. Pin 4 should be connected to pin 5 to energize the other heater. That could explain your "I don't have anything coming through from the guitar." comment in post #15.

Thanks Tom - I wish it were the problem! Unfortunately I do have pins 4 and 5 tied together. I ended up trying to boost the heater voltages by removing the center tap (green / yellow) and grounding the other green wire. The heaters are now at 6.4VAC; but still nothing at the output..

The DC voltage at pin 8 of the PT is 400V and 294V on the other side of the 10k resistor tied to that node.

As a last ditch effort - I'm just gonna do a quick continuity test to look for shorted socket nodes. Then probably just buy a PCB kit and new board components.... GAH!

[g1]

There is not too much to this circuit. Checking voltages on the preamp tubes should give some popping sounds if you have the volume up. (pins 2 & 7).
How about listing your DC voltages for both tubes.

[glowysourworm]

There is not too much to this circuit. Checking voltages on the preamp tubes should give some popping sounds if you have the volume up. (pins 2 & 7).
How about listing your DC voltages for both tubes.

Ok, so I just went and checked the DC voltages and think I found the culprit finally.

Pin 8 of the PT was high: 400VDC

Then the voltage ladder through the filter resistors was also a little high: 320VDC (pin 4 6V6), 300VDC (other side of 22k resistor attached to pin 4)

I also found that the voltage on the other side of the 2 100k resistors was near 0VDC; and pin 1 of the 12AX7 was at 250VDC (which is 100V higher than the normal operating point)

So I think the joint from those 2 100k resistsors to the 22k resistor in the filter section is bad. The soldering iron is heating up right now so I'll repost if that fixes it. This info has to be useful to someone out there right?

[g1]

Ok, I'll try this again: please list the DC voltages for both tubes.
12ax7 pins 1,2,3,6,7,8
6V6 pins 3,4,5, & 8

[Randall]

You were right - the output transformer was shorting because the chassis cut the wire coating and shorted it out!!!

One word. Grommets.