National Thunderball popping fuses

Jeff Gehring

Gold Supporting Member
Messages
7,692
So the Thunderball is the same as a Harmony 420? If you came up with a Thunderball schematic, please post a link.

The fuse is blowing when the tubes are all pulled, and the power is switched on.
  • One thing to bear in mind -- if the amp has been worked on recently (as yours has), the first areas to look at are where that work was done. You re-capped it, are the caps installed with correct polarity? Do any of them show a short to ground with an ohmmeter (power off)?
  • If nothing shows as a hard fault on the filter cap side of the power transformer, the power transformer itself needs to be checked.
  • Disconnect and individually tape off all power transformer secondary leads and if there are any primary side to ground connections (like a 'death cap') disconnect those, then try powering the amp on. If the fuse still goes, it may be that your PT has failed.
 

Rob s

Member
Messages
1,159
Yeah harmony 420 or Gretsch 6170.
For whatever reason the 6170 is easier for me to follow.
It was popping fuses before recap but the amp hadn't been on in a long long time.
So I recapped it straightaway.
They show no fault.
Gonna check PT next.
See what happens.
And it's solid state rectified.
 

zenas

Member
Messages
8,871
Light bulb limiter is handy. To check the PT you just hook up the LBL and plug it in, dead short the bulb is full bright. Then start pulling secondary wires one circuit at a time and trying it.
Unplugging while desoldering is a real good idea!
Get all the secondarys unhooked and still have a full bright bulb, your PT is now a paper weight.
Or you have short in the primary wires? Only had that once, a badly replaced plug.

Having typed all that I should ask. This is one with a PT right? Not one of those widow makers?
 

Tone Meister

Member
Messages
3,266
Yeah harmony 420 or Gretsch 6170.
For whatever reason the 6170 is easier for me to follow.
It was popping fuses before recap but the amp hadn't been on in a long long time.
So I recapped it straightaway.
They show no fault.
Gonna check PT next.
See what happens.
And it's solid state rectified.

Why not check the rectifier diodes first?
 

Rob s

Member
Messages
1,159
Having typed all that I should ask. This is one with a PT right? Not one of those widow makers?

This one has a Power transformer.
The diodes all tested fine on the fluke in diode mode.
Is that a good enough test?
And Has anyone ever heard one of these amps?
 
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Rob s

Member
Messages
1,159
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zenas

Member
Messages
8,871
Sorta looks like the schmatic for Wards Airline GIM 9111. Same rectifier set up without the B+ center tap, cathode biased 6L6s, same 1 1/2 12ax7. Leaves an unused triode that could be a gain stage. . . .
Anyway if you're as lucky as me it's probably a fried PT. The LBL will tell you as fast as anything for zero dallors if you're handy. An ohms meter across the two prongs on the plug in will show about the same on a good or bad PT.
If you're luckier than me you'll find a short in the secondary side, filiment wiring or filter cap for instance.

Did it emit any magic smoke when it started blowing fuses?
 

Rob s

Member
Messages
1,159
And thanks guys for the help.
Looking thru some bins in the basement to make the lightbulb limiter.
 

Rob s

Member
Messages
1,159
Haven't made the light bulb limiter yet.
I disconnected the secondary leads and turned it on and the lamp lit everything is honky dory.
Hook the secondaries back up and pop goes the fuse.
Still no tubes in the amp.
 

zenas

Member
Messages
8,871
Then the problem is in one of the secondary circuits. Not much to go wrong in the filiment wiring besides a short. In the B+ a shorted filter cap or other possibilities exist.
 

Blue Strat

Member
Messages
30,723
Reconnect the PT to the diode bridge, but disconnect the output connection to the power supply caps. See if the fuse holds.
 
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VaughnC

Silver Supporting Member
Messages
19,051
Yup, I'd suspect a bad rectifier. Sometimes they will test good at the meters internal battery voltage but not handle the power supply voltage. But an ohm meter on the 1k scale usually shows if a diode is shorted.

Is the pilot light working? It's after the fuse and I've seen shorted lamp sockets. Might be worth trying disconnecting it.
 
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Tone Meister

Member
Messages
3,266
Yup, I'd suspect a bad rectifier. Sometimes they will test good at the meters internal battery voltage but not handle the power supply voltage. But an ohm meter on the 1k scale usually shows if a diode is shorted.

Yep, I suspect shorted rectifier as well.

Those are good points on testing diodes. I've had my Fluke lie to me in diode mode only to find shorted diodes using the ohm meter function.
 




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