I've been using a DYNA MKIII that just started blowing the fuse, but not until around 90volts AC while being brought up slow with a variac. All caps are new, KT-88s are new, GZ-34 Rectifier is new, 6AN8 is new. Doesn't draw more than 0.25 amps at lower voltages, but "pops" real quick at 90VAC. All filaments heat up. It was working yesterday, but I'm stumped. Thanks for any ideas. I don't see any obvious shorts, smoke, sparks. I haven't done any voltage checks yet. That's tomorrow.
Oh, then you must need the diode mod.
New 5AR4 tubes have poor reverse blocking.
New 5AR4 tubes have poor reverse blocking.
I haven't done tubes in a long time. If the diodes in the bridge make up for
the shortcomings of the 5AR4, why have the tube at all? Is it just the series
resistance of the tube to limit the inrush current? Wouldn't a series resistor
2 Watt 180 ohm for each diode fix the problem permanently? Is it the time
delay of the tube warming up?
G²
the shortcomings of the 5AR4, why have the tube at all? Is it just the series
resistance of the tube to limit the inrush current? Wouldn't a series resistor
2 Watt 180 ohm for each diode fix the problem permanently? Is it the time
delay of the tube warming up?
G²
No, it's the reverse blocking voltage that is the problem. This happens every cycle, not just on start up.
Only the added diodes will fix it. The original Mullard 5AR4 works perfectly in my amp, but not newer types.
The 5AR4 drops a lot of forward DC voltage, unlike a silicon diode (which would raise the B+ excessively).
Only the added diodes will fix it. The original Mullard 5AR4 works perfectly in my amp, but not newer types.
The 5AR4 drops a lot of forward DC voltage, unlike a silicon diode (which would raise the B+ excessively).
Could be a lot of different things.
OP´s post reminded me of one of the most annoying faults, I had a long time ago.
An old Philips CTV that blew fuses instantly. When turning the variac up slowly (EU - 230volt country), the TV turned on
around 140 volt, and when the variac hit 165, the fuse blew. After a lot of frustration, we found the mains PSU electrolytic to
be the culprit. Too curious to this behaviour, I opened it up and started to unwind the roll of foil. And behold.......
When opened up 2/3 there was a black, burned spot through the foil only visible for 6-7 windings, then the foil was again flawless to the end.
Never seen before, and never seen since, but I can´t possibly be the only one, who´ll experience this in a lifetime 🤣 .
When measuring with DMM, it behaved like any other electrolytic, so took far too many hours to nail that MF.
OP´s post reminded me of one of the most annoying faults, I had a long time ago.
An old Philips CTV that blew fuses instantly. When turning the variac up slowly (EU - 230volt country), the TV turned on
around 140 volt, and when the variac hit 165, the fuse blew. After a lot of frustration, we found the mains PSU electrolytic to
be the culprit. Too curious to this behaviour, I opened it up and started to unwind the roll of foil. And behold.......
When opened up 2/3 there was a black, burned spot through the foil only visible for 6-7 windings, then the foil was again flawless to the end.
Never seen before, and never seen since, but I can´t possibly be the only one, who´ll experience this in a lifetime 🤣 .
When measuring with DMM, it behaved like any other electrolytic, so took far too many hours to nail that MF.
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The point of keeping the tube rectifier is to 1) provide a slow warm-up, 2) keep the correct voltage drop and 3) to preserve the pleasant sonic effect of a tube rectifier.
My guess is on the can cap. Do you have access to an external DC supply? If so apply an increasing DC while watching
current.
Alternative is the GZ34, that should be observable with a bluish flash in the tube at the time of fuse blow. This could
be fixed by applying the "yellow sheet mod"
Just to be clear...
You're saying use the tube and the silicon diodes both at the same time? Let the the silicon manage reverse breakdown so it never stresses the tube rectifier?
You're saying use the tube and the silicon diodes both at the same time? Let the the silicon manage reverse breakdown so it never stresses the tube rectifier?
Thanks for all the suggestions. The zener diode was NOT replaced. Should I consider doing that? I'll be checking voltages today....carefully.
I replaced the can cab with separate caps. It worked for a month.......something let go.
