New 5f6a homebrew build blows fuse right after flipping standby

[Frisco]

Divide and conquer. Lift the OT CT. Still shorted? If yes it's not the OT. Lift the choke input. Still shorted? If yes it's not the choke or anything down stream of the choke. I can't think of any way to wire the rectifier so it behaves until the standby is disengaged. So if the OT and the choke check out that would only leave the main filters or the way they are wired.

I did the test as suggested, the bulb lighted up after flipping the standby on both instances.

Could it simply be a bad switch? I am considering looking under the circuit board to see if there is anything odd under there. I was pretty careful removing wire clippings and solder pieces, but maybe there is something loose under there causing a short.

[Chuck H]

Well it's narrowed to a pretty small area of circuit now. With the above test there is only the main filters or their wiring to be suspected. Unless by some mechanism I don't know about the circuit behind the standby switch somehow stops behaving once the switch is thrown. That seems unlikely. So, using the "if/then" method this problem must be in the main filter circuit. Since you lifted each cap and the problem remained there must be a problem in the wiring. Either a mistake in assembly or an errant solder blob, etc.

Though the tests so far DO indicate a problem with the wiring of the main filter assembly, they don't rule out a problem with any of the other possibilities tested because the wiring problem in the filter circuit will have prevented effective testing of those other circuits. You need to locate and correct the wiring error first to know if there are any other problems.

[52 Bill]

What wattage light bulb do you have in the limiter? How bright does it glow during your tests?

[Justin Thomas]

How is this amp constructed? We know the circuit, but not the materials. PTP, eyelet on fiber board, epoxy, etc.? Having accidentally punched a few holes in already-stressed points in the insulating boards on a vintage Fender and causing a short, and having had solder fall through a turret that was too close to the chassis, or poke a wire too far... I wonder if there's something up with the filter cap boards - solder blob, stray wire, hole punched through... It's my WAG.

Justin

[loudthud]

Is the bias supply working? What is the DC Voltage on pin 5 of the output tubes before the Standby switch is thrown? It should be in the range of -40V to -50V.

What is the Ohm reading on both sides of the Standby switch with the power off?

[Frisco]

200 watts. Very bright, will light the room after the standby is flipped. Just a red glow before the standby is flipped.

[Frisco]

Some more detail about the bulb limiter. The light brightens up significantly upon flipping the standby switch. Afterwards (5-10 seconds?) it dims.

Also, after flipping the standby, both power tubes and the preamp tubes stop glowing and stay that way, the jewel light also dims, but ephemerally. The rectifier stays glowing before and after flipping the standby, it looks to me that it glows more after the standby, but its hard to tell with the bulb limiter brightening the room.

Not sure these observations are helpful, but I thought I would share them.

[Frisco]

Is the bias supply working? What is the DC Voltage on pin 5 of the output tubes before the Standby switch is thrown? It should be in the range of -40V to -50V.

What is the Ohm reading on both sides of the Standby switch with the power off?

The DC voltage on pin 5 of the power tubes is -27 on standby, and -8 on play.

[Justin Thomas]

Bias voltage should stay the same whether in Standby or Play... yes, it seems off in Standby mode, but that may be due to the bulb limiter. But it shouldn't be so low that it goes to -8V... that's more in line for an EL84. If you've only got -8V, then I would imagine your 6L6s would conduct a Lot of current.

That your heaters seem to go out completely also makes me wonder... check for shorts there, too. You may want to lift the Center Tap from ground to do that test - you're checking the wiring throughout the tube chain to ground - I'd check every single tube heater pin and also the pilot light terminals. I burned up a heater winding when I was futzing with my bias in my Bassman head and spun one of the pilot terminals around to touch the chassis...

I think the new info IS relevaant, but wait on others to jump in before acting on what I said.

Justin

[Chuck H]

Ok... The dimming tubes are very significant. I'm not sure how you could do it so the filaments glow until the standby is flipped and then stop, but I think there must be a mistake in your PT secondary lead allocation.

Voltage readings, both AC and DC, with the amp in standby on either side of the standby switch please.?.

[Justin Thomas]

Pictures might help, too...

Justin

[Frisco]

All, The amp seems to be working fine now. It was all due to my fault.

I looked in between the fiber boards of the main circuit board and found an errant strand of wire shorting two points in the bias section. Looks like a strand from one of the transformer wires, thinner than a hair.

I put it back together and did a prelim test with my guitar, the amp is quiet, and the tone is nice. I might work on the biasing next (or not). Right now I just have the completed chassis, and I am debating whether to put it in a combo box, or just make a head.

I did want to thank Chuck, Justin and everybody else everybody for chiming in, and will understand if you are disappointed that this problem was due to my own untidiness.


I hope to be able to use your expertise in the future.

[Chuck H]

All, The amp seems to be working fine now. It was all due to my fault.

I looked in between the fiber boards of the main circuit board and found an errant strand of wire shorting two points in the bias section. Looks like a strand from one of the transformer wires, thinner than a hair.

I put it back together and did a prelim test with my guitar, the amp is quiet, and the tone is nice. I might work on the biasing next (or not). Right now I just have the completed chassis, and I am debating whether to put it in a combo box, or just make a head.

I did want to thank Chuck, Justin and everybody else everybody for chiming in, and will understand if you are disappointed that this problem was due to my own untidiness.


I hope to be able to use your expertise in the future.

We did get to use our "expertise" (cute word, but probably overstated in my case ). I'm sure the basic troubleshooting helped clue you in on where to look for the short A lot of guy just start replacing parts until the problem goes away. That's expensive in time and parts and nothing is "learned". Now you have a light bulb limiter and some new troubleshooting skills. And that's pretty much exactly why we, as an on line community (that includes you), are here. It's been fun to see posters grow from asking for advice to giving advice. It can take years (it did in my case anyway). We all win.