Need 5E3 Build Troubleshooting Help

paul-e-mann

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Interchangeable

What’s your actual measured mains voltage?
I can think of a valid rationale for 110V, unless yours is that low.
Generally, from reports of typical North American voltage levels, the amp should run much happier with it set to 125V.
Wall voltage is 120V. Not that I doubted Chris youre my 2nd opinion, I'll rewire to 125V and run the variac at 120V.
 

paul-e-mann

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1/ As you were feeding less volts in by using the variac, so you get less volts out. Transformer windings just put out a proportion / ratio / % of what’s put into them.
2/ you were only measuring half the voltage, so your meter only displayed half the expected value.

If the resistor still measures ok, give it a go, with the amp powered via a 60W light bulb limiter.
If it starts to smoke turn off immediately. But if not, measure the VDC between the chassis common and both ends of that resistor.
With the rectifier installed only I get about 430V on both sides of the resistor.
 

paul-e-mann

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Ok, my guess is that points to the output valves as the cause of the trouble.
Either they’re shorted, you wired the sockets up incorrectly, or :shrug:

So following Rob Robinette's startup procedures I determined the rectifier and wiring is good and the preamp tubes and wiring are good. With the power tubes in standby its good but with standby off the bulb limiter lights and the resistor starts to smoke. As per Robinette I tried to isolate by pulling the power tubes and running them one at a time in each socket with the hopes of determining a bad tube or bad socket, the problem exists in all scenarios, there's a universal problem affecting both power tubes. Are the 3.3k ohm input wires on the OT interchangeable?
 

Im247frogs

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Are you still horse-whipping that fried resistor? Or did you change it and the new one is smoking?
 

paul-e-mann

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Are you still horse-whipping that fried resistor? Or did you change it and the new one is smoking?
Horse whipping, it measures good gosh darn it, the new one wont come in the mail for a couple days.
 

Pete Farrington

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Are the 3.3k ohm input wires on the OT interchangeable?
Yes
As per Robinette I tried to isolate by pulling the power tubes and running them one at a time in each socket with the hopes of determining a bad tube or bad socket
Hmm, I don’t like that suggestion; to avoid extremely high over dissipation, amps with shared cathode bias require all output valve sockets to be populated with functioning valves.
 

paul-e-mann

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Yes

Hmm, I don’t like that suggestion; to avoid extremely high over dissipation, amps with shared cathode bias require all output valve sockets to be populated with functioning valves.
I suppose the lit bulb limiter suggests theres a short. I still have to pull the board to check the bottom wiring, but after that I'm running out of ideas.
 

paul-e-mann

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At least the the ‘one 6V6 at a time’ test indicated the issue seems (to me) to be miswired 6V6 sockets.
I wish thats what it was, I would have caught that a long time ago, I've checked the socket wiring a million times!
 

william vogel

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I’m betting there’s a under board wire that is pinched between a board mounting screw that’s grounding the power supply chain.
 

paul-e-mann

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Dammit, I was hoping to find something but everything is clean, in the right place and solid as a rock. I'm out of ideas other than the power tubes are bad. Whats the chance of 2 bad power tubes?? Unfortunately I have no way to test them. Anybody have some good 6V6 tubes they dont need I can borrow or buy cheap to test with?

BD9oodl.jpg
 
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Pete Farrington

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With the rectifier installed only I get about 430V on both sides of the resistor.

I’m betting there’s a under board wire that is pinched between a board mounting screw that’s grounding the power supply chain.
If it was a HT short on the board, surely that resistor would still be smoking with the 6V6 removed?

With the 4k7 resistor removed, do the 6V6 warm up, ie with heater power?
 

paul-e-mann

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I found some cheap Russian tubes online, should have them in a couple days. :yesway:
 

Im247frogs

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I've gone back to the beginning of the thread and I just don't see it. Much smarter people here haven't spotted it either. It doesn't look like anything could be shorting to ground on the power section end of the board due to how fat the cloth pushback wire is, but I could be wrong.
If it was me, and I've done this more than once, I'd just de-solder everything on that (power section) end of the chassis, trim the PT and OT wires to manageable lengths, and start over. Just desolder all your put connections and start over. You're bound to have a eureka moment and realize the thing you think you've checked 10 times is botched. It happens to everyone.

Usually what I do when there are that many under -board connections is to have a small copy of the diagram and check continuity at the top of the board to make sure everything is going where it's supposed to go, and then check it off on the diagram so I know I did it.

Also, if it were me, and this would only be a minor pain in the ass, would be to lose the ground bus on the back of the pots and arrange it off the turrets. You can have a separate bus for the power section and another one for the preamp, and run your potentiometer grounds down to the bus rather than running grounds from the board up to the backs of the pots. Run it all the way down to your first bypass cap and then to an input jack ground or better yet directly to a chassis bolt. Some guys will use solid core house wire for this but it's a tight fit in a 5E3 chassis.
If you have a foot of solid core pushback wire, just pull the wire out and use it like that. I like to put my bus wire in a drill chuck and hold the other end w a pair of pliers to twist it up real nice and straight.

You'll get it.
 

myersbw

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So...when you pulled the power tubes and tried one at a time, did you actually try the scenario of just leaving both power tubes out? That's a fast test that would've told you if at least one power tube is bad if the limiter bulb was now softly glowing. ? I've forgotten what I've read, but you've verified your speaker and connections since you need one connected with all tubes in, right? Also, a Fender usually has you wire the output jack so it shorts should the speaker cable be jerked out. Kind of a failsafe for the output transformer vs. the open circuit which you really never want at the speaker jack. Check everything,...measure it.

Also, I can't get a good visual on the rectifier socket with the stray wiring. Can you post a photo of that? Specifically peek and verify what transformer wire colors are on pin 2 & 8...should be black and white going direct from the transformer for the 5vac according to the transformer wiring chart on the transformer.
 
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