by EWBrown » Fri Nov 09, 2007 1:59 pm
First thing, with the power off and unplugged, and the capacitors all totally discharged, try the resistance theck as described in the assembly instructions, this will show if any of teh electrolytic or coupling caps have gone bad, look for unusually low readings.
If all that looks good then:
Two simple tests, and then a couple more:
First, pull out the rectifier tube, and check if the fuse still blows. This isolates everything in the B+ stream, but still lights the six tubes' filaments. If the fuse pops then most likely the power transformer may have a problem - the tube filaments draw a combined 22 VA @ 6.3VAC / 3.64 A. which is only about 1/10 if the 1A fuse's capacity.
The rectifier tube takes another 10 VA (5V / 2A) approximately. Total of 32VA filament consumption with all tubes inserted.
If the fuse survives, then replace the rectifier tube and pull out the other six tubes, and see if the fuse blows. If it does, then check the PSU's electrolytic capacitors, look for low resistance measurements, and bloated or leaking caps.
Third, replace the 12AX7 and 12AU7 (ECC83 and ECC82) and one channel's 6BQ5 / EL84s , aand check to see if the fuse survives.
If it does, then Last, replace the other set of 6BQ5s. and see what happens.
NB: If one or more of the cathode bypass caps (470 uF / 35V) is bad / shorted, it would draw the cathode voltage down towards zero volts, which would increase the plate and cathode current rather dramatically, and this could pop the fuse.
Last but not least
Possibly the 1 amp fuse is too close to the "edge" and perhaps a slow-blow 1A or a 1.25 to 1.5A fast blow could be substituted.
HTH
/ed B in NH
Last edited by
EWBrown on Tue Nov 13, 2007 7:01 am, edited 1 time in total.
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