5F1 Fender Champ Problem

[glowysourworm]

I'm wondering if anyone can help - I've just got a freshly wired 5F1 and am having a problem.

When starting the amp up with no tubes the pilot light comes on and things look ok. After inserting
the power tube (Sovtek 5Y3GT) and turning the amp on - the heater glows happiness for about 30 seconds
at which point it starts to spark before the fuse blows.

I've tried replacing both the large 16uF cap and 10k resister coming off of pin 8 of the 5y3 socket. All to no
avail.

One possible symptom I'm seeing is that the heater wires (green / green and yellow / yellow) are both shorting
to the chassis. So pin 8 of the 5Y3 socket is short to the chassis which seemed odd to me.. but I'm a tube
amp newb so don't have any experience with how these things operate.

Here's a scematic link for reference:

Fender Champ 5F1 Wiring Diagram | My Fender Champ | Vintage Amps

If anyone has seen this issue before or has any helpful advice I'd GREATLY appreciate it!

Thanks,
-Ryan

[Bruce / Mission Amps]

#1... Green is typically 6.3vac for the other tubes, not the 5Y3. Which probably should have yellow wires on lugs 2 and 8.
Also, you can't let lug 8 of the 5Y3 short out to the chassis... that is where your high voltage is coming out of the tube to the whole amp.
If your green/yellow wire of the 6.3v winding is connected to chassis ground, then the other green wires are too.... that is OK because the G/Y center tap wire is actually at zero volts with respect to the other ends.
BUT!!!.. the 5F1 uses only one green wire to the tube sockets, lugs 2 of the 6V6 and 4+5 of the 12AX7... and one green wire connected to the chassis ground... the G/Y wire is NOT used anywhere. Lug 7 of the 6V6 is grounded to the chassis as is lug 9 of the 12AX7.
A couple-three, well focused pictures would help us with this.....

[glowysourworm]

#1... Green is typically 6.3vac for the other tubes, not the 5Y3. Which probably should have yellow wires on lugs 2 and 8.
Also, you can't let lug 8 of the 5Y3 short out to the chassis... that is where your high voltage is coming out of the tube to the whole amp.
If your green/yellow wire of the 6.3v winding is connected to chassis ground, then the other green wires are too.... that is OK because the G/Y center tap wire is actually at zero volts with respect to the other ends.
BUT!!!.. the 5F1 uses only one green wire to the tube sockets, lugs 2 of the 6V6 and 4+5 of the 12AX7... and one green wire connected to the chassis ground... the G/Y wire is NOT used anywhere. Lug 7 of the 6V6 is grounded to the chassis as is lug 9 of the 12AX7.
A couple-three, well focused pictures would help us with this.....

@Bruce: Thanks for the quick response!

1) I mis-quoted the 5Y3 - you're right about the yellow leads. I hooked those up to pins 2 and 8.

2) I see where the schematic uses only one green wire for the 6V6 and 12AX7. Another I've seen uses a twisted pair between lugs 2 and 7 of the 6V6 and 4/5 and 9 of the 12AX7.

3) How could pin 8 of the 5Y3 be the HV pin when it carries the heater voltage?

I used the green twisted pair as in the separate schematic. I gather this is some kind of "hum reduction" technique?

The yellow/red center tap is grounded to the chassis - so that makes me think that that's where the short is coming in on pin
8 of the 5Y3. Should the yellow twisted pair be floating with respect to ground? (i.e. clip the yellow/red to chassis connection)

Thanks,
-glowy

Here's a couple shots off the 5Y3 / 5Y3 + 6V6

[g1]

The red/yellow is the center tap for the HV (2 red wires), not the yellow heater wires for the rect.
So the red/yellow should remain grounded.

[g1]

Could it just be a bad 5Y3? Have you tried another?

[glowysourworm]

@g-one: that makes sense for the center tap for the red HV wires. It might be a bad tube; but I don't want to try another until i understand why that pin 8 of the 5Y3 is shorting to the chassis

[Enzo]

Power off, no tubes. Ohm meter. Is there continuity to ground from pin 8 of the rectifier socket? There should NOT be.


And make sure you did bot install any filter caps backwards.

[glowysourworm]

Power off, no tubes. Ohm meter. Is there continuity to ground from pin 8 of the rectifier socket? There should NOT be.


And make sure you did bot install any filter caps backwards.

@Enzo: Yes, with no tubes there's continuity from pin 8 to ground. Also from pin 2 - the other side of the yellow lead from the PT. I can't figure out why though. Should I post more pictures of the circuit?

[Enzo]

What wires from the power transformer are currently connected to ground/chassis?

The center tap of the high voltage winding ought to be red w/yellow stripe. It should be grounded.

When describing a wire, the first color is the body color, the second color is the stripe. So red/yellow means a red wire with a yellow stripe. On the other hand yellow/red means a yellow wire with a red stripe.

Yellow wires in this case refer to the 5vAC heater for rectifier. If your 5vAC winding has a center tap, it will be yellow with some color of stripe. DO NOT GROUND a 5v center tap. Did you just automatically ground such a wire?

Look at your schematic, see the 5v rectifier heater winding is not grounded? And one side of the heater is wired to B+? The tube needs 5v from pin 2 to pin 8. It can float at 400v if it wants. Here it has to.

[Bruce / Mission Amps]

Hammond frequently makes their 5v windings with a center tap... with that one you actually use the center tap wire (typically yellow/black) as the B+ feed point for the standby switch, if desired, the first main filter cap and subsequently, the entire amp's B+ rail.

[glowysourworm]

Ok, so just focusing on figuring out that ground problem on pin 8 of the 5Y3 (which should NOT be grounded)...

I checked the center tap wires that are connected to ground. Using the corrected grammar via Enzo (thanks btw) - here's what I have:

1) Red / Yellow -> GND
2) Green / Yellow -> GND
3) Red pin 6, Red pin 4 (HV)
4) Yellow pin 8, Yellow pin 2

On pin 8 of the 5Y3 - Here's what's connected: Yellow Heater wire (5V) from PT, 10k 2 watt resistor, 16uF cap (correct polarity), low primary (red wire) from output transformer.

I disconnected the output transformer wire - and voila! No continuity from pin 8 to ground.

The blue primary lead from the output TF also has continuity to ground!!

Could it just be a bad output TF? I substituted a Hammond 1750C which is 7k -> 3.2 ohm. Any thoughts?

[loudthud]

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.

[g1]

the heater wires (green / green and yellow / yellow) are both shorting
to the chassis. So pin 8 of the 5Y3 socket is short to the chassis which seemed odd to me

with no tubes there's continuity from pin 8 to ground. Also from pin 2 - the other side of the yellow lead from the PT.

I disconnected the output transformer wire - and voila! No continuity from pin 8 to ground.

The blue primary lead from the output TF also has continuity to ground!!

It would be best if you give resistance readings on the ohms range rather than using the terms "short" or "continuity". Some meters beep continuity for anything under 200 ohms, best to use the ohms range rather than continuity.
The green wires will have a low resistance to ground because their center tap is grounded.
If you have a low resistance at the B+ rail, your yellow wires will also measure a low resistance.
It sounds like the problem may be related to the B+ or OT, rather than the heater windings.
Does the brown wire of the OT also give a low resistance reading to ground?

[glowysourworm]

I don't know what the threshold of my multimeter is in ohms.. but it's somewhere below 10 ohms i'm pretty sure.

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

I'll reply to the main thread with the latest....

[glowysourworm]

Thanks guys for your help so far!

Here's the latest progress:

1) Fixed the 5Y3 pin 8 short by finding that the OT wires were cut into by the chassis (!!!) I wound them with electrical tape and replaced the OT.
2) After hooking up the tubes and powering on - I got a squealing noise from the speaker
3) I looked up the squealing problem and found that it is sometimes due to reversed polarity of the OT secondary windings.
4) After switching the OT leads I found the squeal was LESS... But there is still a minor squeal for PART of the volume POT sweep.

So.. I'm OK with the squeal as long as there's output from the instrument. Right now I don't have anything coming through from the guitar.

I just checked input jack and added the 1M shunt resistor to the input jack (which wasn't there). Still nothing. I tried switching the speaker
leads as well and back again.. still nothing.

-glowy