doug wrote:

I finally succumbed and added a feedback network to my PSE 6l6 amp
just put a 22K resistor between the OPT and the K of first triode in the
driver. Took out the WAAAY too prominent mids that I just couldnt stand but
I still have to decide how much feedback to use.
not sure if I like the result yet or not.
much flatter but seems to have lost a lot of the high end that I liked so we
will have to see after some trial and error

would love to hear some inout from the group though

Doug

A straight connection of 22k between OPV anode and IPV cathode
may upset the biasing of the IPV, since DC flows down the 22k,
which displaces the working point of the tube.
A 10 uF cap in series with the 22k could be tried, but then you have to worry
about
LF stability.

You DO NEED to work out what reduction of sensitivity of the amp
occurs after FB is applied, so if you need 1v input instead of 0.25v
to get the same output level, then that's a gain reduction of 4 times,
or an applied 12 dB of NFB.

If the Ro without FB is high, then the accoustic output of the speaker
varies up and down in ways ther designer didn't allow for, since
it has been made to suit low Ro amps.
The prominent mids could be due to the speakers making more than normal
mid levels due to the high Ro.
Prominenet mids means an absense of bass and treble.
So what is the response of the amp with no FB?

FB tends to extend and flatten the response, as well as lowering Ro from say
4 ohms down to 0.5 ohms with 12 dB of applied FB.
Maybe you only need 6 dB.

Patrick Turner.

Thanks Patrick,
Time to break out the meter and not be so lazy. I will start measuring this
weekend and report back what I find

Doug

"Patrick Turner" wrote in message
...


doug wrote:

I finally succumbed and added a feedback network to my PSE 6l6 amp
just put a 22K resistor between the OPT and the K of first triode in the
driver. Took out the WAAAY too prominent mids that I just couldnt stand
but
I still have to decide how much feedback to use.
not sure if I like the result yet or not.
much flatter but seems to have lost a lot of the high end that I liked
so we
will have to see after some trial and error

would love to hear some inout from the group though

Doug

A straight connection of 22k between OPV anode and IPV cathode
may upset the biasing of the IPV, since DC flows down the 22k,
which displaces the working point of the tube.
A 10 uF cap in series with the 22k could be tried, but then you have to
worry
about
LF stability.

You DO NEED to work out what reduction of sensitivity of the amp
occurs after FB is applied, so if you need 1v input instead of 0.25v
to get the same output level, then that's a gain reduction of 4 times,
or an applied 12 dB of NFB.

If the Ro without FB is high, then the accoustic output of the speaker
varies up and down in ways ther designer didn't allow for, since
it has been made to suit low Ro amps.
The prominent mids could be due to the speakers making more than normal
mid levels due to the high Ro.
Prominenet mids means an absense of bass and treble.
So what is the response of the amp with no FB?

FB tends to extend and flatten the response, as well as lowering Ro from
say
4 ohms down to 0.5 ohms with 12 dB of applied FB.
Maybe you only need 6 dB.

Patrick Turner.

doug wrote:

I finally succumbed and added a feedback network to my PSE 6l6 amp
just put a 22K resistor between the OPT and the K of first triode in the
driver. Took out the WAAAY too prominent mids that I just couldnt stand but
I still have to decide how much feedback to use.
not sure if I like the result yet or not.
much flatter but seems to have lost a lot of the high end that I liked so we
will have to see after some trial and error


It may be easier to just try different values of resistance for that 22K
resistor.
Temporarily connect a pot of value of around 100K and vary it to see if you
find a "sweet spot". Also be aware that the triode's operating point may
shift, and that you may need to adjust upwards its cathode resistor to
get the operating point back to where it was. Also a small amount of
capacitence to ground across the cathode resistor may bring back some of
the highs you lost. Again trial and error is the only real way, as you
probably
don't know exactly what frequency you're starting to lose the highs. Do
this
after setting the feedback resistor.

"doug" wrote in message
. ca...
I finally succumbed and added a feedback network to my PSE 6l6 amp
just put a 22K resistor between the OPT and the K of first triode in the
driver. Took out the WAAAY too prominent mids that I just couldnt stand
but
I still have to decide how much feedback to use.
not sure if I like the result yet or not.
much flatter but seems to have lost a lot of the high end that I liked so
we
will have to see after some trial and error


would love to hear some inout from the group though

Doug


Hi, Doug,
You stuck in the 22k and it sounded better or different. Some bits were
better. Some bits were worse. Feedback is not penecillin. It doesn't kill
all known germs and like most medicines, there are side effects.... It
cannot make a crap design sound good. It may make a good design sound
better... It is not always the path to personal happiness.... It could
turn your amp into a transmitter and NASA will be banging on your door
'cause you just ****ed up the $70 zillion Mars mission and Voyager 9 is now
heading for a quiet suburban street in Alabama !!
Patrick is dead right in that you can't just hang a 22k from the OPT
secondary back to V1 cathode because it's in parallel, down to ground, with
the V1 cathode resistor. What effect this has on the working point of V1
needs to be established. V1 may now be in meltdown !!
You need, ideally, a scope, an AF generator, a voltmeter, a pile of paper
and a pencil.
You draw out the schematic, run the amp in it's original state amd note down
all the fixed voltages at the valve pins. If you're happy with this no
load/idle state, that's fine....
Then run the AF generator and scope together to establish a frequency/gain
graph comparing the input to the OPT secondary. Ideally this will be a
flat straight line from 0Hz to 100kHz
It won't be, but at the moment you are just collecting information
No scope ? No generator ? Not a problem. You are talking about how it
sounds. Ears are pretty good. If you can't measure it, listen to it.
Come back and tell me what you can generate and what you can measure.
If you only have a $10 Walmart multimeter, I can point you in the right
direction
regards
jim