Hi there,

I've recently bought a second hand SET DHT 300b amplifier, a Fi X3,
around 9w per channel.

I'm using it connected to Lowther Acousta's fitted with EX3 speakers and
I can hear a fairly hum (100hz?) from the speaker, from the amplifier,
at my listening position, around 10' back. The hum drops off only
slightly in the central position of the hum potentiometer.

I believe the Acousta's are 96dB/W/M efficient.

I've tried lifting the earth by inserting a 66ohm resistor (3x22ohm) in
line but with little effect.

I did intend to construct a Hedlund horn but am now worried about it
being of increased efficiency and therefore even louder background hum.

Any suggestions as to hum cures, was 66ohm enough of an earth lift,
should I use more inefficient loudspeakers.

Any advice appreciated.

--
Stoatgobbler

Stoat wrote:

Hi there,

I've recently bought a second hand SET DHT 300b amplifier, a Fi X3,
around 9w per channel.

I'm using it connected to Lowther Acousta's fitted with EX3 speakers and
I can hear a fairly hum (100hz?) from the speaker, from the amplifier,
at my listening position, around 10' back. The hum drops off only
slightly in the central position of the hum potentiometer.

I believe the Acousta's are 96dB/W/M efficient.

I've tried lifting the earth by inserting a 66ohm resistor (3x22ohm) in
line but with little effect.

I did intend to construct a Hedlund horn but am now worried about it
being of increased efficiency and therefore even louder background hum.

Any suggestions as to hum cures, was 66ohm enough of an earth lift,
should I use more inefficient loudspeakers.

Any advice appreciated.

If you are getting 100Hz hum, that's the frequency of the rectifiers working

with a 50Hz mains supply, so your noise problem suggests
that the B+rails are insufficiently filtered and that the earth paths are
not optimal.

Using a horn capable of going down to 50 Hz will make things worse.

But ppl still use 300B tubes with horns and with no hum.

Some I know have used CD power to the cathode; its not hard to make a
floating
DC supply of 5V x 1.2A, and then any hum caused in the cathode circuit
dissappears.

What's your amp schematic?
Have you had anyone with a CRO check out your amps?

Patrick Turner.






--
Stoatgobbler

On Mon, 27 Jun 2005 20:29:59 +0100, Stoat
wrote:

Hi there,

I've recently bought a second hand SET DHT 300b amplifier, a Fi X3,
around 9w per channel.

I'm using it connected to Lowther Acousta's fitted with EX3 speakers and
I can hear a fairly hum (100hz?) from the speaker, from the amplifier,
at my listening position, around 10' back. The hum drops off only
slightly in the central position of the hum potentiometer.

If the hum can't be nulled by the pot, then it's not at line
frequency. Leaving the power supply as the guilty party.

Magnetic coupling from power supply iron to the output
transformer is a strong contender, followed by grounding
and filter caps, neck and neck.

Good fortune,

Chris Hornbeck
"I can build you a test that will show either one. Which
would you prefer me to demonstrate?"
--scott

If the filament "centering" does not reduce hum, it is not due to these
being AC powered. The 300B is a low-mu tube requiring a bias of some 90V,
therefore the 5V AC appearing on the cathode do not introduce much noise, if
the filaments are properly grounded from their centres, 'cos the residual
"noise voltage" is very small compared to grid signal.
Moreover You wrote "100Hz noise", ie. it must be coming from a rectified
supply - the problem is in the power supply. Generally speaking, replacing
the caps will fix it, and it is not unusual with old amps (a common
practice, indeed).
Do not increase capacities unless rectifier load has been carefully
calculatd and found OK. If You can, use a higher voltage and temperature
rating for the new caps, it will add extra safety and longer life.

Ciao

Fabio


"Stoat" ha scritto nel messaggio
...
Hi there,

I've recently bought a second hand SET DHT 300b amplifier, a Fi X3, around
9w per channel.

I'm using it connected to Lowther Acousta's fitted with EX3 speakers and I
can hear a fairly hum (100hz?) from the speaker, from the amplifier, at my
listening position, around 10' back. The hum drops off only slightly in
the central position of the hum potentiometer.

I believe the Acousta's are 96dB/W/M efficient.

I've tried lifting the earth by inserting a 66ohm resistor (3x22ohm) in
line but with little effect.

I did intend to construct a Hedlund horn but am now worried about it being
of increased efficiency and therefore even louder background hum.

Any suggestions as to hum cures, was 66ohm enough of an earth lift, should
I use more inefficient loudspeakers.

Any advice appreciated.

--
Stoatgobbler

Fabio Berutti wrote:

If the filament "centering" does not reduce hum, it is not due to these
being AC powered. The 300B is a low-mu tube requiring a bias of some 90V,
therefore the 5V AC appearing on the cathode do not introduce much noise, if
the filaments are properly grounded from their centres, 'cos the residual
"noise voltage" is very small compared to grid signal.
Moreover You wrote "100Hz noise", ie. it must be coming from a rectified
supply - the problem is in the power supply. Generally speaking, replacing
the caps will fix it, and it is not unusual with old amps (a common
practice, indeed).
Do not increase capacities unless rectifier load has been carefully
calculatd and found OK. If You can, use a higher voltage and temperature
rating for the new caps, it will add extra safety and longer life.

Ciao

Fabio

In principle you are quite right about all this.
However, in practice when nulling out the hum with a CT pot there
may be some residual hum still left and at double the mains F and from the
filament AC.
Nulling out hum is a nice but not perfect idea.

If you have 5mV at the cathode which is un-nullable hum, and the tube gain is
4, then 20mV must appear at the anode, and if the voltage ratio of the OPT is
25:1, then you have around
0.8mV at the speaker secondary, and this can be heard with a horn speaker.

Similarly, if RL = 5k, and Ra = 800 ohms, and the plate supply hum
at the OPT connection is say 50mV, the 5k and 800 ohms act as a divider to
produce 43 mV across
the OPT primary, and that may add to what the cathode hum produces.
Therefore, with horns, you get the sound of a distant fog horn if the map is
that damn noisy.

So its essential that plate supply hum be reduced to less than 2 mV if possible.

I like to use large 470 uF electrolytic caps and a choke to reduce the hum.

A suitable CLC supply would have the following :-

Potted mains tranny with ss rectifiers well away from signal paths.
A 50 ohm series R between rectifiers and C1 which is 235 uF using
series 470 uF each rated for 350V.
This reduces the peak charging currents into such a large and useful value of
capacitance.
The ripple voltage at 75 mA will be 0.7 vrms.
C2 will be four x 470 uF caps in series and parallel
to make a total of 470 uF.
We want the ripple to be 0.002vrms, so we want
the attenuation factor by the choke to = 0.002 / 0.7 = 0.0028.
Or in other words, the choke impedance = 1 / 0.0028 = 351 x the impedance of C2.

So ZC2 = 3.4 ohms, so choke impedance at 100 Hz = 351 x 3.4 = 1,193 ohms,
so since 1 Henry has Z = 628 ohms, about 2 Henrys would give us what we want.
However, we may be able to find a suitable 5H choke at 75 mA with less than 100
ohms DCR,
and this would be even more suitable, and thus reduce hum at C2 to less than a
mV.

The 5H choke and 470 uF have a resonant F of 3.28 Hz, which is what I consider
to be low enough.
Any higher, and the transients in music may excite the B+ to dance up and down
at the Fo,
and if Fo is too high, it worsens the music.

In addition to the huge amount of power supply capacitance I think is essential
for an SE amp with no NFB there should be 0.47 uF polypropylene caps across
the electro C1 and C2.

In the fifties ppl used caps of perhaps up to 50 uF,
more was rare, but they also tended to use 10H chokes.
Such chokes are very expensive compared to buying capacitance.

The larger the C2 is, the lower the power supply output impedance
at signal freqencies and the lower the imd.

Patrick Turner.



"Stoat" ha scritto nel messaggio
...
Hi there,

I've recently bought a second hand SET DHT 300b amplifier, a Fi X3, around
9w per channel.

I'm using it connected to Lowther Acousta's fitted with EX3 speakers and I
can hear a fairly hum (100hz?) from the speaker, from the amplifier, at my
listening position, around 10' back. The hum drops off only slightly in
the central position of the hum potentiometer.

I believe the Acousta's are 96dB/W/M efficient.

I've tried lifting the earth by inserting a 66ohm resistor (3x22ohm) in
line but with little effect.

I did intend to construct a Hedlund horn but am now worried about it being
of increased efficiency and therefore even louder background hum.

Any suggestions as to hum cures, was 66ohm enough of an earth lift, should
I use more inefficient loudspeakers.

Any advice appreciated.

--
Stoatgobbler

Many thanks for your kind replies, you've given me a bit to think about.

Again, thanks.

Stoat.


--
Stoatgobbler

100hz or 120hz is almost surely B+ filtering issue.

If you can get a copy of the schematic or have someone comfortable with
tracing the B+ origin you can try adding additional filtering to the
B+. You could add a 5H choke and 10uF cap with correct voltage rating
to the B+.
Of course you do this at your own risk to life.