"N_Cook" wrote in message
...
spamtrap1888 wrote in message
...
On May 24, 6:38 pm, "Phil Allison" wrote:
"Arfa Daily"



What do you think about subbing the 5AR4 with a 5U4?

Never tried it, but looking at the specs, they look rather different
from
one another, see -

http://www.r-type.org/exhib/aaa0997.htm

http://www.r-type.org/exhib/aaa0504.htm

** Useless info.

The big difference between the 5AR4 and the 5U4 is in amount of voltage
drop
across each diode when conducting.

The 5AR4 is uses an indirectly heated cathode, close coupled to each
plate
and the voltage drop is very low, about 35 volts at 700mA peak.

The 5U4 uses a directly heated cathode with lotsa space between the
plates
and the cathodes - so there is a much bigger drop in voltage, more like
100 volts at peak current.

So, if you drop a 5U4 in place of a 5AR4 - expect to lose 60 to 70 volts
DC
straight off the B+.

Also, the 5AR4 is rated to work reliably with a large filter cap on the
cathodes ( 60 uF) compared to the 5U4 ( 40uF).

So the proposed sub is bad news all around.


Considering that the vacuum tube rectifier was the first tube to be
eliminated (in receiving sets at least), why not replace it with a
semiconductor diode circuit? What effect would it have on the
amplifier's sound?


+++++

I totally agree a couple of 1N4007 and sag Rs would be fine by me but we
are
talking musos here and the "valve sound" would magically dissappear. Of
course if I told them nothing about swapping to SS, the valve sound would
not disappear - strange old world isn't it.


I'd go for 2x 4007's in series to be on the safe side - not forgetting equal
value x-class capacitors in parallel with each diode to damp & equalise any
spikes.

As well as an added sag resistor, you need to make sure each rectifier path
is protected by a fuse, when silicon rectifiers fail short circuit - they do
it for keeps.