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Vox N.T. 15H power transformer not suitable for the job intended? (Consumer Law?)
Hope I'm not starting another Night Train thread that will drag on forever so I'll try to be succinct.
The problem -?- I'm looking at is the ac heater voltage for the 2 x EL84's is too low at 5.35VAC.
Ways to fix it
1. Ignore it. It's only R&R... owner probably thrashes it to death anyway...
2. Complain to Vox .. threaten a pox on them and mention consumer law - unfit for purpose
3. Mention it on MEF and go into pedantic detail and bore everyone 'till Enzo suggests "Get a Life" !
So you see I've chosen option 3 .
Surprisingly the HT is over voltage - in comparison with 2 other independent readings from "the net".
The mains power here in Australia is supposedly 230VAC but at my outlet (due to step down transformer down
the street) varies around 242VAC.
"In 1983 Standards Australia adopted a 20year plan to convert Australia from the nominal 240 volts to 230 volts,
to align with European Standards – IEC38. The aim was to align Australian manufactured products with our
main trading partners."
"Australian Standard AS/NZ3000.2 Australian Standard AS/NZ60038
High limit 253VAC Low limit 216VAC"
I guess I could sue if my million dollar digital workstation (230v) was supplied with 254volts
and went up in a cloud of magic smoke! Proving it another matter.
From what I can see the rules/specs have changed but not the secondary tappings on the 220kVAC step downs.
So in reality we are still 240volts AC.
The Vox NT 15H was brought to me as unreliable/cutting out after it had been taken to another tech.
Inside were all the signs of resoldering all the joints in sight, tube sockets and the inrush current limiter
the Ametherm SL22 10008 a 8 amp Thermistor with a nominal resistance at normal state of 10 ohms.
This thermistor proved to be the problem as I measured about 16vAC across it initially and after about
half an hour it went open circuit but then worked after switching the amp off and back on again.
I am assuming it got wrecked by over heating (very short leads) when resoldered previously.
Got another which worked fine then checked the voltages (see attached) .
I was dismayed to find the ac heater voltage to the output tubes was 5.64VAC which also meant
the DC supply for the pre-amp tubes was even lower ! aprox 5.3VDC.
I recall some debate around this topic where it seemed over 5.7 was close enough.
It seems to me the o/p tubes do not last very long in this amp.
It also occurs to me not all venues will have like me, a generous 242 volts at the outlet
so the heater voltage would be even lower depending on the outlet.
The back of the amp states 240v as does the manufactures sticker on the transformer.
Most others on the net seem to be 220/230v apart from the ones for the US market.
Quote from the Mullard Bible (Mullard Valve & Service Reference Manual 1951)
available here :- https://frank.pocnet.net/other/Mulla...anual-1951.pdf
"Heater (indirectly-heated valves)
Heater voltages should be maintained within +/- 7 per cent. of the rated values.
Under-running the heater may cause as much damage to a valve as over-running.
Where it is permissible to operate heaters in series, this is clearly stated in the
data. When heaters are so operated the heater current should be maintained
within +/- 5 per cent. of the rated value."
Am now thinking I'm getting too pedantic and should adopt the Auzzie vernacular..
"jeeze mate she's #%&@ed ! better get another one."
I now think the original tubes wore out prematurely and strained the thermistor.
My thoughts so far... replace the 10 ohm with a 4 ohm thermistor say a SL15 4R008
and use very low voltage drop diodes to increase the DC volts to the pre-amp.
Of course that's a tiny change bordering on the ridiculous so perhaps the way to fix it is option 2
"2. Complain to Vox .. threaten a pox on them and mention consumer law - unfit for purpose
(Thanks to Blinker for his original schematic.)
Hope I'm not starting another Night Train thread that will drag on forever so I'll try to be succinct.
The problem -?- I'm looking at is the ac heater voltage for the 2 x EL84's is too low at 5.35VAC.
Ways to fix it
1. Ignore it. It's only R&R... owner probably thrashes it to death anyway...
2. Complain to Vox .. threaten a pox on them and mention consumer law - unfit for purpose
3. Mention it on MEF and go into pedantic detail and bore everyone 'till Enzo suggests "Get a Life" !
So you see I've chosen option 3 .
Surprisingly the HT is over voltage - in comparison with 2 other independent readings from "the net".
The mains power here in Australia is supposedly 230VAC but at my outlet (due to step down transformer down
the street) varies around 242VAC.
"In 1983 Standards Australia adopted a 20year plan to convert Australia from the nominal 240 volts to 230 volts,
to align with European Standards – IEC38. The aim was to align Australian manufactured products with our
main trading partners."
"Australian Standard AS/NZ3000.2 Australian Standard AS/NZ60038
High limit 253VAC Low limit 216VAC"
I guess I could sue if my million dollar digital workstation (230v) was supplied with 254volts
and went up in a cloud of magic smoke! Proving it another matter.
From what I can see the rules/specs have changed but not the secondary tappings on the 220kVAC step downs.
So in reality we are still 240volts AC.
The Vox NT 15H was brought to me as unreliable/cutting out after it had been taken to another tech.
Inside were all the signs of resoldering all the joints in sight, tube sockets and the inrush current limiter
the Ametherm SL22 10008 a 8 amp Thermistor with a nominal resistance at normal state of 10 ohms.
This thermistor proved to be the problem as I measured about 16vAC across it initially and after about
half an hour it went open circuit but then worked after switching the amp off and back on again.
I am assuming it got wrecked by over heating (very short leads) when resoldered previously.
Got another which worked fine then checked the voltages (see attached) .
I was dismayed to find the ac heater voltage to the output tubes was 5.64VAC which also meant
the DC supply for the pre-amp tubes was even lower ! aprox 5.3VDC.
I recall some debate around this topic where it seemed over 5.7 was close enough.
It seems to me the o/p tubes do not last very long in this amp.
It also occurs to me not all venues will have like me, a generous 242 volts at the outlet
so the heater voltage would be even lower depending on the outlet.
The back of the amp states 240v as does the manufactures sticker on the transformer.
Most others on the net seem to be 220/230v apart from the ones for the US market.
Quote from the Mullard Bible (Mullard Valve & Service Reference Manual 1951)
available here :- https://frank.pocnet.net/other/Mulla...anual-1951.pdf
"Heater (indirectly-heated valves)
Heater voltages should be maintained within +/- 7 per cent. of the rated values.
Under-running the heater may cause as much damage to a valve as over-running.
Where it is permissible to operate heaters in series, this is clearly stated in the
data. When heaters are so operated the heater current should be maintained
within +/- 5 per cent. of the rated value."
Am now thinking I'm getting too pedantic and should adopt the Auzzie vernacular..
"jeeze mate she's #%&@ed ! better get another one."
I now think the original tubes wore out prematurely and strained the thermistor.
My thoughts so far... replace the 10 ohm with a 4 ohm thermistor say a SL15 4R008
and use very low voltage drop diodes to increase the DC volts to the pre-amp.
Of course that's a tiny change bordering on the ridiculous so perhaps the way to fix it is option 2
"2. Complain to Vox .. threaten a pox on them and mention consumer law - unfit for purpose
(Thanks to Blinker for his original schematic.)