What kills a valve rectifier?

[Ian Field]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:945i3mFlbuU1@mid.individual.net...

"Ian Field"


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.

** Quite unnecessary - any voltage spikes are clamped by the first filter
cap.

Class X caps are more likely to fail than 1N4007 diodes are.


As well as an added sag resistor,

** Not essential - there will be only 30 volts increase in the B+.

Enough for a nice blue glow around the output valve mica supports - if you
like that sort of thing.

 

[Arfa Daily]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:946m1qFcc6U1@mid.individual.net...

"Arfa Daily"
"Phil Allison" "Arfa Daily"


I've had a number of GZ34s fail short circuit, or suffer from serious
flashover, and knock out fuses over the years, but I've always put this
down to saggy filaments or whatever, where the valves are mounted
horizontally, or upside down.


** GZ34s have an indirectly heated cathode - just like most other
valves.

Sooo, the cathode is a solid tube of metal, with a heater wire inside
it.

Ergo - it simply cannot sag.

Only directly heated cathode types do that

- like 5Y3s, 5U4s, 5AS4s etc.


Agreed, but I did say "or whatever".


** Fine - but you also had no idea what was importantly different about a
GZ34 / 5AR4 compared to other rectifiers.


I've not taken that much notice of the actual failure mechanism. I think
you can spend too much time in a commercial world, agonising over exactly
what went wrong.


** Many times, such analysis pays off in the long term.

Cos failure mechanisms that are not dealt with by the service tech are
bound to be repeated.

For the most part, that's accepted, and if a component has failed in a way
that it obviously shouldn't have, then of course the reason behind the
failure should be investigated and understood. But in the case of valves
with an obvious internal mechanical failure, it's just that - a dud valve
....

Suffice to say that in 'modern' amps that use a GZ34, I have seen more
than a few with a vapourised fuse and no obvious short causing it. Often,
a replacement fuse holds up, until you clout the valve with the butt end
of a screwdriver, whereupon a miniature fireworks display is launched
inside, and the fuse goes out again.


** Seen that scenario many times myself - especially with Chink valves.

Sometimes, it's spontaneous, and only needs the voltage to be present.
More often than not, it seems to occur when the valves are not upright.
This would imply that gravity acting on the structure has something to do
with it.

** Valves are generally not upright when servicing is going on - and a
great many amps have the power valves mounted horizontally or inverted in
normal use.

Agreed, but that does not make it 'right' in terms of minimising the stress
on the valve's internal structure. I guess that upside down is better than
on their sides, but I still prefer to see power output valves and
rectifiers, being run upright.

Owners of same can operate the amp any damn way up they like - and do.


Popped fuse - new fuse - clouted valve - fireworks - new valve - new
fuse - soak test - write bill - phone customer. Job done. Cash in bank.

** As posted by me earlier - the best way to fix recurring problems with
crappy GZ34s is called a 1N4007.

Works a treat in Quad Mk2s, long as you get rid of those dreadful oil
filled filter caps.



... Phil

 

[Arfa Daily]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:irldqp$lto$1@dont-email.me...

So no thoughts on generator use , followed by rectifier failure.

It seems the mains fuse I have here was not necessarily the original one.
Someone looked at the amp and replaced the fuse, probably with this
probably
previously mechanically broken one (knocked about in a toolbopx). The
original may not have failed but assumed to have fused and so just
rectifier
failure.
Could generator brownout or spikes or something weaken a valve rectifier
so
it is ok while warm but gives up the ghost on the next powerup from cold?

I would have said not, but who knows ?

Arfa

 

[Meat Plow]

On Wed, 25 May 2011 02:16:06 +0100, Arfa Daily wrote:

"Meat Plow" <mhywattt@yahoo.com> wrote in message
newsan.2011.05.24.21.25.01@emutt.macspoofer.lmao...
On Tue, 24 May 2011 12:04:48 +0100, Arfa Daily wrote:

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:irg1t5$bvd$1@dont-email.me...
Amp used for the first time, in 2 years of regular mains use, with a
generator. Works fine with no problems. Coincidence ?, next gig back
to usual mains supply, at power up, blows the mains fuse. The GZ34 is
failed both sides, 0 percent output . Inspecting the fuse it has
failed at an end ,
not the central (T) section and otherwise complete wire shows no sag
or discolouration. So possibly mechanical failure of fuse. If fast
make/break intermittant action of the fuse wire at an end , would
that knock out an RR
valve? What in general happens in the failure of RRs ? I've only ever
seen 75 percent , same as new, or 0 percent "goodness" at prescribed
current settings of the Avo valve tester.



I've had a number of GZ34s fail short circuit, or suffer from serious
flashover, and knock out fuses over the years, but I've always put
this down to saggy filaments or whatever, where the valves are mounted
horizontally, or upside down.

Arfa

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

and

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

The 5U4 appears to be able to handle more current, but at a lower max
plate voltage. Also, the heater requires substantially more current.
Might be an issue for the transformer supplying it ?

Arfa

If the iron is able to handle it I've known it to be a preferred
replacement for those who want some more sag. Especially harmonica
players.



--
Live Fast Die Young, Leave A Pretty Corpse

 

[Meat Plow]

On Wed, 25 May 2011 11:38:28 +1000, 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.

Not for those looking for more sag. I have a couple clients running
variacs to get that kind of sag sound with a GZ tube.



--
Live Fast Die Young, Leave A Pretty Corpse

 

[Meat Plow]

On Tue, 24 May 2011 21:25:50 -0700, spamtrap1888 wrote:

On May 24, 6:38 pm, "Phil Allison" <phi...@tpg.com.au> 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've installed several SS diode replacements for a valve rectifier.
Most notably the RTSSR from Ruby. Some musicians like sag in their
amps and you certainly won't get it from the RTSSR or a 5AR4.

It TOTALLY depends on the musician and I always cater to their ears.


--
Live Fast Die Young, Leave A Pretty Corpse

 

[Phil Allison]

"Arfa Daily"

The 5U4 appears to be able to handle more current, but at a lower max
plate voltage. Also, the heater requires substantially more current. Might
be an issue for the transformer supplying it ?

** Very unlikely to matter.

Say the winding is rated at 5 volts and 2 amps to suit a 5AR4 and the copper
wire is gauged to give about 10 % regulation factor. Then the heat loss in
the 5 volt winding is 10% of the load power or about 1 watt.

If a 5U4 is subbed - the extra 1.1 amps will mean that 5.0 volts drops to
4.7 volts - well within specs for the valve.

The extra heat in the winding amounts to 0.8 watts - which is trivial in a
tranny designed to lose 15 to 30 watts as heat in normal operation.


...... Phil

 

[Meat Plow]

On Fri, 27 May 2011 17:37:43 +1000, Phil Allison wrote:

"Arfa Daily"

The 5U4 appears to be able to handle more current, but at a lower max
plate voltage. Also, the heater requires substantially more current.
Might be an issue for the transformer supplying it ?


** Very unlikely to matter.

Say the winding is rated at 5 volts and 2 amps to suit a 5AR4 and the
copper wire is gauged to give about 10 % regulation factor. Then the
heat loss in the 5 volt winding is 10% of the load power or about 1
watt.

If a 5U4 is subbed - the extra 1.1 amps will mean that 5.0 volts drops
to 4.7 volts - well within specs for the valve.

The extra heat in the winding amounts to 0.8 watts - which is trivial
in a tranny designed to lose 15 to 30 watts as heat in normal operation.


..... Phil

Good to see the real Phil back for a little while.



--
Live Fast Die Young, Leave A Pretty Corpse