[Your Name]

will changing a 500k pot to a 100k or less have any negative effects on an
amplifier, specifically?

http://users.rcn.com/fiddler.interport/HF14.HTM

thank you

[Peter Wieck]

On Jan 29, 9:43 am, Your name wrote:
will changing a 500k pot to a 100k or less have any negative effects on an
amplifier, specifically?

I am assuming you are writing of the bias pot (this amp does not
appear to have a volume pot)? Uh, yes, it will change things.
Typically a pot is both a fixed resistor-in-circuit and a variable
resistor. Depending on where you position a fixed 400Kohm resistor (or
two @ 200Kohm; 1 @ 300, 1@ 100; whatever), you may be able to get away
with a 100K pot to do the necessary adjustments. But you do need to
make the in-circuit resistance correct.

If you have an open-ended volume pot, you will be LOUD and LOUDER, as
you are already 80% up the scale before you even start. Most volume
parts are also not linear, so you will be more like 95% up the pot as
to its actual effect on the volume. Similarly with a bias pot, your
range of adjustment will be very limited and already at the upper/
lower end of the scale.

Peter Wieck
Wyncote, PA

[John Byrns]

In article 6,
Your name wrote:

will changing a 500k pot to a 100k or less have any negative effects on an
amplifier, specifically?

http://users.rcn.com/fiddler.interport/HF14.HTM

thank you

Changing the 500k pot to a 100k pot will have no adverse effects on the
amplifier, and can have several beneficial effects. It will however
load the source more heavily which can result in a loss of bass response
if the output coupling capacitor is to small, and it may also result in
increased distortion if the output stage of the source is not designed
to drive the lower load impedance.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[tubegarden]

On Jan 29, 8:59?am, "Peter Wieck" wrote:
On Jan 29, 9:43 am, Your name wrote:

will changing a 500k pot to a 100k or less have any negative effects on an
amplifier, specifically?I am assuming you are writing of the bias pot (this amp does not
appear to have a volume pot)?

Hi RATs!

Peter, R1, upper left corner of schematic. This is typical of
schematic representations of the period.

It is, indeed, a 500K volume pot. Audio Taper is assumed in this
posion of an amplifier.

The value of this pot will impact the device driving it more than the
amp itself.

If a CD player, anything above about 20K will probably work.

Pots are relatively inexpensive. Try one, then, if you like it, keep
it.

Happy Ears!
Al

[Patrick Turner]

Your name wrote:

will changing a 500k pot to a 100k or less have any negative effects on an
amplifier, specifically?

http://users.rcn.com/fiddler.interport/HF14.HTM

thank you

The 500k pot is the input gain attenuating pot for the amplifier.

As it exists it has some effect on the high F response of the amp which
due to the
effect of the amplifier input capacitance and the pot resistance and the
10k
also shown on the schematic between the pot arrow and the input tube
grid.

Pots always create an effective RC low pass filter.

As in the case of all pots, the series resistance before the C can vary
between 0 ohms
when the pot is wound to lowest gain position, to maximum R when wound
to the top position
of maximum gain.

However, nothing is simple, and not even an input pot, and we MUST
consider the
signal source resistance and cable capacitance effects and how these
interact with pot and amplifier input capacitance.

Just about all preamps or cd players will have cathode follower or opamp
outputs
which are construsted to have
an output resistance of less than 1,000 ohms, which means that if you
connect a resistance to ground
from the pre amp/other device output, you would get a reduction of
output by half, which is -6dB,
and if you connected 500k ohms, you'd get a very tiny fall in output,
because the 500k load
connected to a 1k ohm source has a negligible loading effect.

Now because the cable C may be 500pF to ground, it will cause negligible
attenuation effects
to the HF in the audio band, and a 1,000 ohm plus 500pF RC filter has a
pole, ( -3dB point )
at 318kHz, something you don't need to worry about at all.

However, where you have a 500k pot connected at the input to the power
amp,
the amp sees the 1k Rout of the source only when its is turned up to
maximum.
However, we have already established that the cable C has negligible
effect on the signal due
to the low source resistance of the signal from the preamp etc.

But then there is still that 10k series R, so even with volume at max,
you have 10k plus say 100pF amplifier input capacitance.

This makes a pole at 159kHz, and is nothing you have to worry about
either, since the amp
probably has a pole of 50kHz when fed with perfect low source resistance
of unlimited bandwidth.

But when the pot is adjusted for -6dB, you would have 250k ohms from 0V
to the arrow, and
250k ohms from arrow to the source ( plus 1k source resistance which we
can neglect ),
so effectively, the RC filter then comprises of these two R in parallel
and the 100pF of
amp input C. This is due to the series R of 250k, and shunting R of 250k
being effectively
in parallel, and so you have pot R = 125k, and you must add to this the
that 10k
so you have 135k series R and the pole could be only 11,777Hz, and well
below 20kHz, the top of the audio band.

In actual fact though, the amp input C is probably less than 100pF
because the global NFB
has the effect of reducing input capacitance and raising input impedance
and resistance.

Neverthe less the poorest bandwidth is available about when the gain
control is set for
1/2 volume, or -6dB.

In fact also, the gain position most likely to be used is at the 12
o'clock position,
and since the pot is most likely to be a logarithmic pot, it
will then be a divider with 450k from source to arrow,
and 50k from arrow to 0V, and the pot output resistance is 450k in
parrallel with 50k = 45kohms,
and to this we add that 10k to get 55kohms, and with 100pF the pole is
28.9kHz, still on the rather low side.

One could improve the bandwidth by reducing the pot resistance.

reduction of the resistance to 100k would reduce the R by 5 times at all
positions mentioned above, and therefore
lift the HF pole to a region where you didn't have to worry.
The loading effect of 100k on the cd player or preamp is also going to
be negligible,
and the bass response unaffected if there is a large dc blocking cap on
the preamp output;
a 0.47uF with 100k loading R causes a LF pole at 3.4Hz.

If the blocking cap was increased to 2uF, then the amp input pot could
be 50k or 25k without any worry.


One tends to get less noise with a the lower resistance pots, and they
are easier to find.
Alps make some nice carbon track 100k log pots.

To sumarize, remove the 500k pot to the bin where it belongs! and
replace with at least a 100k pot
or perhaps down to 25k if the blocking cap on the source is a large
enough value.

Don't worry about cable capacitances and pot resistance since the cable
C only affects the source resistance, not the
pot output resistance, unless you have a pot at the preamp and a cable
from the arrow of the pot to the power amp.

The input pot has nothing to do with biasing the amp.


The eico amp is NOT a Williamson type of design because it only has 2
triodes in the input
driver stages, whereas a Williamson has 4 triodes, because it has an
extra balanced gain stage
needed because the Williamson triodses are low gain and the output stage
requires twice the the drive voltage
that the Eico does.

The two halves of 12AX7 act so that the first gives a gain of about 60
and the second is what is called a
concertina phase inverter, CPI, which produces two opposite phases of
signal from the input tube anode,
and each has an amplitude just below the input tube anode, so the stage
has no gain.

This gain and global NFB structure plus general amplifier schematic has
been used on 1,001 amplifiers and sounds well, but
the input sensitivity tends to be a bit low, ie, you may need about
1Vrms input for clipping,
which is probably quite OK and you should not need any preamp with a cd
source.
The low sensitivity means the signal to noise ratio should be good.

I wonder about the claims made about the stability and HF performance of
this Eico.
Usually such claims are slightly ficticious if not downright lies from
many manufacturers of the past, present,
and future.

Patrick Turner.

[Your Name]

Your name wrote in
4.196:

will changing a 500k pot to a 100k or less have any negative effects
on an amplifier, specifically?

http://users.rcn.com/fiddler.interport/HF14.HTM

thank you


Thank you all.