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Discussion Starter · #1 ·
I'm looking at a friend's 1959/60 5E3 Deluxe that is randomly blowing the 2A fuse. It has happened often enough that there is most certainly a problem however not often enough to pin down any sort of pattern.
I've tested the tubes and all seem ok. I've run the amp for an hour or two without issue. Voltages check out ok. Poked around with my chopstick thinking I might find something. Nada.
I do have one "tell" that might help. The last time the fuse went the owner said he got a shock off the chassis. The ground switch and death cap have previously been removed from the circuit so now plug wire goes directly to the fuse then to the on/off switch.
Of course with a 60 year old amp any number of components could be failing but I'm kind of suspicious of the on/off switch?
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Monitor the mains current flow during test, I use a Kill-a-watt. 2 amps mains is high for an amp of that power.
Try to duplicate the conditions when it occurs, including the power outlet that it fails from.
 

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Do yourself a favour and spend $20.00 and half an hour and built a current limiter using an incandescent light bulb...plenty of diagrams online. I find it almost impossible to locate an intermittent high current draw/ short without one. Bonus? You won't be going through fuses by the truck load while diagnosing.
 

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Ok...a few questions first:
Does the fuse blow instantly or after 5-10 seconds or minutes/hours later?
Have you tried powering it up with the power tubes or rectifier removed?
You can usually reduce a blowing fuse issue to about 4 things in these amps. Rectifier, power tube, bad filter cap or dare I say it, power transformer.
One other thing worth mentioning is those Astron (yellow) coupling caps are notorious for leaking DC.
 

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Discussion Starter · #6 ·
Ok...a few questions first:
Does the fuse blow instantly or after 5-10 seconds or minutes/hours later?
Have you tried powering it up with the power tubes or rectifier removed?
You can usually reduce a blowing fuse issue to about 4 things in these amps. Rectifier, power tube, bad filter cap or dare I say it, power transformer.
One other thing worth mentioning is those Astron (yellow) coupling caps are notorious for leaking DC.
I haven't witnessed it blowing a fuse but from what I understand it's within seconds of powering up.
Before I applied power I pulled all the tubes, tested them and plugged back in one at a time.
The Spragues are probably more than 10 years old so it's not impossible that one is bad.
Getting a shock off the chassis as he powered it up (and fuse blows at same time) had me wondering if the switch might be shorting.
 

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Discussion Starter · #7 ·
Do yourself a favour and spend $20.00 and half an hour and built a current limiter using an incandescent light bulb...plenty of diagrams online. I find it almost impossible to locate an intermittent high current draw/ short without one. Bonus? You won't be going through fuses by the truck load while diagnosing.
Kinda like this?

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Try powering-it-up without the 5Y3 rectifier (isolate B+ from circuit). If it still fails, it's power TFM back to plug...knowing the AC current flow value would help.
Check for signs of arcing at the power TFM, any carbon deposits?
Parasitic arcs are viewed easier in the dark...lights out and keep your eyes peeled on the target.
 

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Discussion Starter · #15 ·
Just to be sure, he is using slow-blow type fuses?
Fast blow will do exactly what is happening (blow occasionally at turn-on) due to the current surge when powering up.
That's a good point. It has a regular fuse in it now but he had just bought these a day or two ago. Not sure what was in there before.
I'll make sure it has slow-blow going forward!
 

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The center-tap for the HV winding should be terminated at the first high-current ground (first filter-cap in this case). The hydro ground point should not share it's termination with any other grounds. Where the CT for the HV winding is grounded now, creates a current flow in the chassis to the first filter-cap.

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Discussion Starter · #17 ·
The center-tap for the HV winding should be terminated at the first high-current ground (first filter-cap in this case). The hydro ground point should not share it's termination with any other grounds. Where the CT for the HV winding is grounded now, creates a current flow in the chassis to the first filter-cap.
That's interesting Paul. The transformer looks original. Would this centre-tap have been moved at some point or would that be where the factory would have placed it? It does look like kind of a sloppy soldering job.
 

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The center-tap for the HV winding should be terminated at the first high-current ground (first filter-cap in this case). The hydro ground point should not share it's termination with any other grounds. Where the CT for the HV winding is grounded now, creates a current flow in the chassis to the first filter-cap.

View attachment 378326
True but in this case, (unless the ground lug has a resistance across it), the reference is still to earth.
At worst this would cause some kind of noise from the charging cap.
Electric shock on a properly earthed chassis is not possible....blowing a fuse is. Therefore, you might want to look at continuity of the 3 pronger. Paul's blow-up photo indicated a rather sloppy job of fastening the ground wire. It's possible there might be an incomplete connection to the chassis which can definitely cause problems with a shock.
The fuse type mentioned by jb welder, is definitely something to consider. That said, you might try gently tapping the retifier or power tubes with a pencil while it's on to see if you get an arc. Found many intermittently bad tubes that way.
As I said earlier, while you're in there, best to check those Astron caps. I've had to replace many over the years as they have a propensity to start conducting DC which apart from possibly causing the fuse to blow, can cause other serious problems.
PS.... Might be a good idea to replace the cathode bypass cap on the power tubes while you're in there. I suspect it's waaayyy off spec by now and probably reading twice it's rated capacitance and possibly having an unacceptable DC leakage which in parallel with the cathode resistor, would stress the output tubes.
 

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where the factory would have placed it?
I have never viewed the innards of an original 5E3. If you reference the schematic, the HV CT may have been terminated at the ground point of the isolation cap:
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