mark,
sorry i can't explain it. i don't have much knowledge of tubes. but i
built it thrice and spent many hours testing and measuring. i
basically used trial an error to choose my resistor values for the
circuit. i have no idea how the voltage at the input grid went down
below the rail. that part is what confuses me, but the behavior i was
seeing was just like what is described in that article i posted. the
voltage no longer drops so low with the resistors i changed.
i have it on a soldered breadboard currently. hopefully next week i
can do schematic capture and pcb layout. i'll post schematics within a
few weeks. for now here is a description:
taking ken's vca circuit. i insert a sallen-key filter node like this:
http://mypeoplepc.com/members/scottnoanh/sitebuildercontent/sitebuilderfiles/scotts_mutant_filter.pdf
but without the feedback path created. i keep the opamp to buffer in
between the RC and the tube input. that opamp might be removed but it
was easier to just leave it. then the tube amp follows set for gain of
3. following the tube amp is the opamp buffer again. connecting from
its output is a pot used to divide the output before inserting back
into the feedback capacitor of the sk-filter node. the 1Meg resistors
in the posted schematic are really vactrols, vtlc2/3. so it is not a
fully tube filter but a tube is at least used in the most important
place. maybe an all tube version could be made. it might be better to
make one using only tubes in the feedback path? another option i tried
which is actually used in that sound example was to put the pot
divider before the tube amp instead of after it.
i removed the amplitude control. that could be used for resonance but
i found a bit more even overdrive clipping when fixing it to +15v with
a 27k resistor. the output is limited this way to something like +-6
or 7V but with the fixed gain of 3 in the tube amp, it is easy to
overdrive the filter.
the vactrol current source was copied from scott stites mutant vactrol
filter (which was came from rene schmitz 4096 vco). it has exponential
response, but vactrols don't react linearly so cv is not precise.
http://mypeoplepc.com/members/scottnoanh/birthofasynth/id17.html
sorry i can't explain it. i don't have much knowledge of tubes. but i
built it thrice and spent many hours testing and measuring. i
basically used trial an error to choose my resistor values for the
circuit. i have no idea how the voltage at the input grid went down
below the rail. that part is what confuses me, but the behavior i was
seeing was just like what is described in that article i posted. the
voltage no longer drops so low with the resistors i changed.
i have it on a soldered breadboard currently. hopefully next week i
can do schematic capture and pcb layout. i'll post schematics within a
few weeks. for now here is a description:
taking ken's vca circuit. i insert a sallen-key filter node like this:
http://mypeoplepc.com/members/scottnoanh/sitebuildercontent/sitebuilderfiles/scotts_mutant_filter.pdf
but without the feedback path created. i keep the opamp to buffer in
between the RC and the tube input. that opamp might be removed but it
was easier to just leave it. then the tube amp follows set for gain of
3. following the tube amp is the opamp buffer again. connecting from
its output is a pot used to divide the output before inserting back
into the feedback capacitor of the sk-filter node. the 1Meg resistors
in the posted schematic are really vactrols, vtlc2/3. so it is not a
fully tube filter but a tube is at least used in the most important
place. maybe an all tube version could be made. it might be better to
make one using only tubes in the feedback path? another option i tried
which is actually used in that sound example was to put the pot
divider before the tube amp instead of after it.
i removed the amplitude control. that could be used for resonance but
i found a bit more even overdrive clipping when fixing it to +15v with
a 27k resistor. the output is limited this way to something like +-6
or 7V but with the fixed gain of 3 in the tube amp, it is easy to
overdrive the filter.
the vactrol current source was copied from scott stites mutant vactrol
filter (which was came from rene schmitz 4096 vco). it has exponential
response, but vactrols don't react linearly so cv is not precise.
http://mypeoplepc.com/members/scottnoanh/birthofasynth/id17.html
On Fri, Jul 18, 2008 at 1:57 PM, Mark <yahoogroups@...> wrote:
> On 7/7/08, ryan williams put forth:
>>
>> a couple of questions and comments: Has anyone noticed the tubes going
>> into cutoff with larger input signals? I built the amp as ken's
>> circuit twice with 3 different 1j17b's and got the same result. the
>> problem was that as the input increased, the voltage at the control
>> grid decreased until the tube cutoff. it decreased a few volts. i was
>> looking at average voltage of -18 or -19V at the grid with a 10v p-p
>> input. mine only accepted very small inputs without clipping most of
>> the signal. looking at my archived email, it seems one other person
>> has had this problem.
>
> As I built a modified version using a different tube, my circuit is
> different, but I did not notice that. It seems odd that the signal
> on the input grid would effect the voltage on the control grid. Even
> though Ken's circuit has a rather large resistor on the output (which
> I included in my version), the control grid shouldn't draw much
> current, so unless you are getting capacitive coupling, its voltage
> should be determined by the op-amp. Also, it seems odd that you are
> getting a voltage higher than the rails. I measured around -14.38V
> with the gain potted all the way up.
>
>> so, for the filter version. it sounds nice i think. the thing will
>> resonate fine but requires some input to get it started. I have been
>> testing and i'm pretty happy with it. I'll be preparing a PCB layout
>> this week, and i'll be trying with some 1j24b's soon. i'll post some
>> schematics after that. here is a sample of its sound. it starts off
>> high resonance, low input. then high input high resonance. then high
>> input lower resonance.
>> http://www.homebuilthardware.com/tmp/sk-tube-vcf.mp3
>
> That sure sounds like a resonant filter. I would love to see the
> schematics, or a description of your circuit.
>
>> for anyone interested there is a really nice article here:
>> http://www.freewebs.com/valvewizard1/Common_Gain_Stage.pdf
>> which describes these two types of tube clipping very well. i highly
>> recommend reading it!
>
> Thanks :)
>