first off, fast cycling of tube-rectifier-based amp causing failure is different than too much first
stage capacitance causing failure. good that we've eliminated the first as the root cause.
next. the Chinese 5z3 is not the same as the old American 5z3 - the later is a
4-pin base and the former is an octal base and mostly equivalent to a gz34/5ar4.
do note that some octal rectifier tubes do not come with all 8 pins of the octal base
populated so some folks think it's a 4-pin base. count the populated/non-present
holes and check for a guide pin with a locator tab in the middle of the tube. I am
assuming the tube manufacturer takes liberties with their designs.
from memory the Chinese 5z3 is indirectly heated (like the 5ar4) and only because it
is indirectly heated that there's a slow ramp of B+, and to some people, good for less
stressful start-up operations.
the GZ34/5AR4 is designed with 40-60uf max at the first cathode tap of the tube.
this number varies depending on the manufacturer. it would wise to use the lowest
in case your future replacements cannot tolerate the upper end of this first cap range.
then if you really need large capacitance values in the amp, then use as much as
you need after the first cap stage. IOW shift anything over 40ufd from first to second
stage in the PS.
in all cases, anything close to 100uf will cause a short on power up, simply the
nature of caps as they charge, the larger the value, the higher the current draw
the higher the probability of an arc. think crowbar across auto battery.
replace that 100ufd cap with 40UFD (less is OK), then use it elsewhere. repeated
use (power on) will cause eventual failure. I replace those e-caps with a 10ufd film
cap but that's me.
lastly, and most important - you mention the use of pin 1. I would research this
before anything else. on GZ34/5AR4s pin 1 is i.c. internally connected and
I would not be happy if, in an amp the designers did not design for American/European
GZ34/5AR4, they decided to use a Chinese tube with or without this pin1 feature.
between the tube difference, pin1, and first cap value, I'd think about changing the
PS circuit to use the more common rectifier tube (I recall there was no Russian
equivalent so to use a Chinese 5z3 you are limited to only the Chinese 5z3).
with more (descriptions, links perhaps, pictures, schematics, and even
a datasheet for the Chinese 5z3 even if it's in Chinese) the PS can be
changed to use American/European GZ34/5AR4 rectifier tubes which
will give you more choices, cheaper costs, and perhaps better reliability
and SQ. there are numerous threads on the SQ of the 5u4, GZ34/5AR4,
and others regarding SQ. your unit joins the big boys threads like the ST70, etc.
if you decide to go down this route, I'd highly recommend looking at the
Dynaco ST70. very classic. and depending on the Chinese 5z3 heater
draw (which in turn tells you the transformer current for the rectifier tube)
then you can even use the 5u4 variants.
an alternative to modifying the PS circuit for use with GZ34/5AR4 and possibly
the 5U4, is to convert to SS, either by way of using a SS-rectifier equivalent
in the rectifier tube 8-pin socket, or simply to wire SS diodes on the socket
pins. More research is mandatorily required because the SS diode output
is higher than the tubes and may exceed voltage ratings.