Thunderbolt Discussion

Brian Carling bcarling at CFL.RR.COM
Wed Jan 19 07:49:51 EST 2005

I thought this exchange might interest you guys...

N5RVZ wrote:
have restored this amp and the only problem is the HV pops a 
fuse every time the HV Plate switch is thrown. 

I have replace the following. 
(1) Original 866's with solid state 
(2) HV Cap's with new ones 
(3) New HV wiring underneath, from HV caps to 4-400's 
(4) Replaced existing 4-400's with other sets 
(5) Got HV Plate transformer rewound 

I am at a loss what is still the problem! When I disconnect the 
HV at the caps from the rest of circuit. I have AC HV at the 
end of HV section transformer Coil L101. Doesn't blow the 
fuses at this point. Something is shorted after this point! 

Can anybody help? 

Thanks in advance. 

Keith - N5RVZ 
= = =
Keith - you should eliminate one thing at a time. 

STEP #1 - 
Try disconnecting the rectifier from the HV 
secondary of pyour plate transformer. Does it 
blow the fuse now? 

If so, then your transformer is bad or your wiring is wrong. 
Check all wiring for shorts. 
Use an ohm meter to mneasure the resistance of your 
transformer windings. Use the LOWEST ohms range. 
They should be something ABOVE zero ohms, even if they are 
only an ohm or two. 

STEP #2 - 
If it all looks OK up to that point, you should 
try connecting JUST the rectifiers (no caps or load.) 
Does it blow the fuse now? If so, then you might 
have some shorted out rectifiers. This could have occurred 
because a) They were bad to begin with. OR 
b) Because the inrush current to the capacitor(s) 
is too much for them. You nee dto limit the inrush current, 
because the original circuit had tube 
rectifiers that allow a bit of E/R current limitation 
through the 866As. 

Let us know how you are progressing after the above two 
steps. Consider adding some resistors in series 
with your rectifiers. I have never done this but 
I hear that some guys add like about 10 ohms at 20 
watts to ease things up for the circuit. Remember, 
with today's higher line voltage AND the higher plate voltage 
you get from the sand state rectifiers, you have probably 
increased the plate voltage by 10 to 20% over what it was in 
the original application! 

Brian, AF4K

This list is a public service of the City of Tempe, Arizona

Subscription control -
Archives -

More information about the Boatanchors mailing list