I am looking for inspiration!

My very good #45 triode amplifier is blowing fuses like hell. The
weird thing is it only blows fuses when it is switched off and
switched on after some seconds. When it plays it plays and runs about
200mA. The fuses blowing are 2 or 3 A @230V. The amp has been working
for 2,5 years now with a 1,5 A fuse.

The filaments are AC heated, and the rectifier is a tube (5AR4). The
capacitors are PP types, 47 mF after the rectifier and some more 100mF
after chokes.

With Wil Blaauw from AE transformers I discused the possibility of the
capacitors spiking off their electron load when reswitching on fast
(when thei are still loaded). Is this possible through a tuber
rectifier? by the way I replaced it two days ago sice the old sovtec
one didnt work anymore (possible killing by 3 A currents?).

the other strange thing is that this hapens after 2,5 years, are the
capacitors now played in so that thei can delivber these curents now
that thei didnt when new? That is a serious running in period then.

Doed anyone have suggestions?

Hubert Pellikaan

"hubert pellikaan"

My very good #45 triode amplifier is blowing fuses like hell. The
weird thing is it only blows fuses when it is switched off and
switched on after some seconds.


** Using slo blow fuses ????

Is there a "T" in front of the amp rating ??




.......... Phil

Does this occur when the rectifier tube is not heated yet or taken away?

(hubert pellikaan) said:

I am looking for inspiration!

My very good #45 triode amplifier is blowing fuses like hell. The
weird thing is it only blows fuses when it is switched off and
switched on after some seconds. When it plays it plays and runs about
200mA. The fuses blowing are 2 or 3 A @230V. The amp has been working
for 2,5 years now with a 1,5 A fuse.

The filaments are AC heated, and the rectifier is a tube (5AR4). The
capacitors are PP types, 47 mF after the rectifier and some more 100mF
after chokes.

With Wil Blaauw from AE transformers I discused the possibility of the
capacitors spiking off their electron load when reswitching on fast
(when thei are still loaded). Is this possible through a tuber
rectifier? by the way I replaced it two days ago sice the old sovtec
one didnt work anymore (possible killing by 3 A currents?).

the other strange thing is that this hapens after 2,5 years, are the
capacitors now played in so that thei can delivber these curents now
that thei didnt when new? That is a serious running in period then.

Doed anyone have suggestions?

A possible explanation may be a short between 45 filament and grid,
causing the anode current to rise.
Does it happen as well with the 45s removed?
I don't think the supply caps are at fault.

--
Sander de Waal
" SOA of a KT88? Sufficient. "

A simple consideration: the 5AR4 is indirectly heated, and when it's cold it
takes 15 seconds to begin conducing, then the current goes up in a
relatively slow way. This is a major advantage of these rectifier, 'cause
it provides a natural "soft start" avoiding voltage and current surges. The
45 is a DHT, therefore it is ready to "suck" current in a snap. If You turn
the amp off, the 5AR4 takes some seconds to cool down, too, therefore, when
You switch it on again after only a few seconds, the 5AR4 cathodes are still
red hot and it starts immediately. I suppose that the inrush of current
needed to re-charge the condensers and to re-magnetise all the iron (power
transformer, choke and OPTs, 'cause as we said the 45 conducts immediately
and the OPTs too will need to pass current in an eye's wink) is probably too
much. The sudden death of the 5AR4 is a clue for this.
By the way, 200 mA are A LOT for a couple of 45s: the data I have say max.
36 mA per tube, ie. 72 for a stereo SE DHT class A, plus say 30 mA for the
drivers and the bleeder resistor = 100 mA, not 200. Unless it uses some
sort of "turbo" tubes, I'd check it.
In any case, don't switch it on and off that way. If a boxer is not fallen
at the first blow, it does not mean he will withstand the 100th fist on the
nose...


"hubert pellikaan" ha scritto nel messaggio
om...
I am looking for inspiration!

My very good #45 triode amplifier is blowing fuses like hell. The
weird thing is it only blows fuses when it is switched off and
switched on after some seconds. When it plays it plays and runs about
200mA. The fuses blowing are 2 or 3 A @230V. The amp has been working
for 2,5 years now with a 1,5 A fuse.

The filaments are AC heated, and the rectifier is a tube (5AR4). The
capacitors are PP types, 47 mF after the rectifier and some more 100mF
after chokes.

With Wil Blaauw from AE transformers I discused the possibility of the
capacitors spiking off their electron load when reswitching on fast
(when thei are still loaded). Is this possible through a tuber
rectifier? by the way I replaced it two days ago sice the old sovtec
one didnt work anymore (possible killing by 3 A currents?).

the other strange thing is that this hapens after 2,5 years, are the
capacitors now played in so that thei can delivber these curents now
that thei didnt when new? That is a serious running in period then.

Doed anyone have suggestions?

Hubert Pellikaan

Is it still doing it with the new rectifier?

When the rectifier fil is hot, switching the amp off, then on means
the rect tube is still hot, and the rectifier operates instantly. So
if the caps discharge a bit just after the amp is turned off, then the
amp is turned on, the caps will recharge instantly since the rect
filament is hot. Maybe the rect was arcing internally?

If not, maybe something else has weakened, and is arcing, or a wire
has been next to something very hot and the insulation has been
damaged, causing it to arc when high voltage spikes.

Bob Hedberg




(hubert pellikaan) wrote:

I am looking for inspiration!

My very good #45 triode amplifier is blowing fuses like hell. The
weird thing is it only blows fuses when it is switched off and
switched on after some seconds. When it plays it plays and runs about
200mA. The fuses blowing are 2 or 3 A @230V. The amp has been working
for 2,5 years now with a 1,5 A fuse.

The filaments are AC heated, and the rectifier is a tube (5AR4). The
capacitors are PP types, 47 mF after the rectifier and some more 100mF
after chokes.

With Wil Blaauw from AE transformers I discused the possibility of the
capacitors spiking off their electron load when reswitching on fast
(when thei are still loaded). Is this possible through a tuber
rectifier? by the way I replaced it two days ago sice the old sovtec
one didnt work anymore (possible killing by 3 A currents?).

the other strange thing is that this hapens after 2,5 years, are the
capacitors now played in so that thei can delivber these curents now
that thei didnt when new? That is a serious running in period then.

Doed anyone have suggestions?

Hubert Pellikaan

Bob H.

Just grab that plate in one hand, the chassis in the other,
and FEEL the power of tube audio!!!
(not literally, of course, just kidding. DON'T DO THAT!)

"Fabio Berutti" wrote in message ...
A simple consideration: the 5AR4 is indirectly heated, and when it's cold it
takes 15 seconds to begin conducing, then the current goes up in a
relatively slow way. This is a major advantage of these rectifier, 'cause
it provides a natural "soft start" avoiding voltage and current surges. The
45 is a DHT, therefore it is ready to "suck" current in a snap. If You turn
the amp off, the 5AR4 takes some seconds to cool down, too, therefore, when
You switch it on again after only a few seconds, the 5AR4 cathodes are still
red hot and it starts immediately. I suppose that the inrush of current
needed to re-charge the condensers and to re-magnetise all the iron (power
transformer, choke and OPTs, 'cause as we said the 45 conducts immediately
and the OPTs too will need to pass current in an eye's wink) is probably too
much. The sudden death of the 5AR4 is a clue for this.

This remagnetising is interesting. The DC resistance of the power
transformer is 4,5 Ohm, and this is normal!
I will check by switching off after 0,1 second and by waiting untill
the capacitors have completely discharged. see what happens..


By the way, 200 mA are A LOT for a couple of 45s: the data I have say max.
36 mA per tube, ie. 72 for a stereo SE DHT class A, plus say 30 mA for the
drivers and the bleeder resistor = 100 mA, not 200. Unless it uses some
sort of "turbo" tubes, I'd check it.

there is also the heater currrent

In any case, don't switch it on and off that way. If a boxer is not fallen
at the first blow, it does not mean he will withstand the 100th fist on the
nose...

"Krzysiek Slychan" wrote in message ...
Does this occur when the rectifier tube is not heated yet or taken away?


No, i took it away, and no problems.

(hubert pellikaan) wrote in message . com...
"Fabio Berutti" wrote in message ...
A simple consideration: the 5AR4 is indirectly heated, and when it's cold it
takes 15 seconds to begin conducing, then the current goes up in a
relatively slow way. This is a major advantage of these rectifier, 'cause
it provides a natural "soft start" avoiding voltage and current surges. The
45 is a DHT, therefore it is ready to "suck" current in a snap. If You turn
the amp off, the 5AR4 takes some seconds to cool down, too, therefore, when
You switch it on again after only a few seconds, the 5AR4 cathodes are still
red hot and it starts immediately. I suppose that the inrush of current
needed to re-charge the condensers and to re-magnetise all the iron (power
transformer, choke and OPTs, 'cause as we said the 45 conducts immediately
and the OPTs too will need to pass current in an eye's wink) is probably too
much. The sudden death of the 5AR4 is a clue for this.

This remagnetising is interesting. The DC resistance of the power
transformer is 4,5 Ohm, and this is normal!
I will check by switching off after 0,1 second and by waiting untill
the capacitors have completely discharged. see what happens..

OK switching on after 0,1 second gives no problems.

switching on when the caps are discharged mostly (10 seconds) gives a
spark from the fuse.

So the hot rectifier theory has won, and explains why the old
rectifier blew out! they dont like 2 -10 Amp currents. Not too often
anyway.

Funny thing is that my bedroom amp, a modified lafayette bocama using
peter van willenwaard fabulous schematics does not have the same fuse
blowing phenomenon. The difference I see is that it uses an
electrolytic cap after the rectifier. actually it is an old rubycon,
probably the currents will be a lot less than with PP caps, which are
very fast indeed.

A consideration: since this kind of amp is getting more regular as are
PP powercaps (or paper in oils) this problem/typicallity will happen
much more often then only at my place; interesting!

thanks for helping me chaps!

Hubert Pellikaan