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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
Tube VCF fine-tuning help? audio files included
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Gonecat



Joined: May 02, 2009
Posts: 43
Location: Rome, Italy
Audio files: 4

PostPosted: Sun Oct 09, 2011 6:49 am    Post subject: Tube VCF fine-tuning help? audio files included Reply with quote  Mark this post and the followings unread

I am building this tube vcf on the CGS site:
http://www.cgs.synth.net/tube/vcf.html

I have the unit almost complete and it is 'semi' working.

I am using a 6CB6 Tube which Mr. Barbour told me was cool, all other parts are exactly what is in the schematic.

As is, the filter mostly acts as an oscillator (a super fat one, by the way).
I like this a lot, but would prefer to have it operate primarily as a filter naturally and when i want the oscillation i would crank up the resonance or flick a switch to alternate caps.

Mr. Barbour says that if the range of the frequency isn't correct, you can "scale" the .01 caps. I have increased the values on these incrementally up to .05. At that point, i cannot get it to oscillate, but the filter isn't really happening. I can hear some sweep, but its not at all strong. At this value i also hear the original signal passed through to the output no matter what frequency is swept.

Check out the audio examples - first one is with the .01 caps, next with 5nf more added in parallel. Third one is 56nf added to the .01s.

My questions are these:

Do i need to also scale the 3rd .01 cap which goes to ground?

I haven't checked tube bias yet, I assume this is critical to do so I will check it out today. Could that account for what i'm getting?

Has anyone built this filter who might be able to post a sound clip as a reference point?

Cheers for any input...


2011-10-09 14.26.51.jpg
 Description:
Built in the classic oven pan chassis...
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2011-10-09 14.26.51.jpg



original-caps.mp3
 Description:
Original .01 caps.

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 Filename:  original-caps.mp3
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5nf-caps.mp3
 Description:
.005 caps installed in parallel with two .01 caps.

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56nf-caps.mp3
 Description:
.056 caps installed in parallel with the .01's.

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andrewF



Joined: Dec 29, 2006
Posts: 1176
Location: australia
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PostPosted: Sun Oct 09, 2011 5:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

In this vid you can kinda hear the bandpass filters. The two tube VCOs were hardwired to two BPFs then to tube VCAs. Its a bit hard to tell when the other synth is making such a racket, but you can tell which noise is from the tube synth when i tweak the pots

pinky vs. incubator

look up twin-t bandpass filter for some ideas on how to tune it. Changing any of the components in the feedback circuit will change the tuning. These tube VCFs are a bit subtle, don't expect to get any sharp honky wah sounds out of them, or if you do it will be in a narrow freq range. As Eric wrote in the description - "... this is not a "sharp" sounding filter, and many people who have heard it have consistently complained that it wasn't "what they expected". Although all agree it can be used as a "formant" filter, the expectation is for the familiar sound effects."
I have built 4 BPFs but these days turn to the very capable TS-22 pentode filter bank when I want a tube filter.
IMHO with tube gear, the VCOs have so much meaty character that a filter is unnecessary and the beam modulators are so good, they always get chosen 1st.
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Gonecat



Joined: May 02, 2009
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Location: Rome, Italy
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PostPosted: Mon Oct 10, 2011 2:55 am    Post subject: Reply with quote  Mark this post and the followings unread

hey thanks a lot for your post, and those are some wicked, wicked synths you have there.

I was aware going into it that it wasn't your typical filter sound, though the way i read that same paragraph he was referring to "The older filter design, shown at the Synthfool web site" and not this one. Not sure now!

I will check out some info on tuning a Twin-T and see what i can come up with since what i'm really trying to find out right now is which direction to go in order to at least get the deepest filter sound this thing can do.

thanks again for the info Smile
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Gonecat



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PostPosted: Mon Oct 10, 2011 2:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

One thing I'm noticing as i research Twin-T's is this one seems to be different. Looks like most Twin-T's have the cap to ground as twice the value of the two caps on the other side of the T.

C1 = C2 = C

C3 = 2C

Reference:
http://www-k.ext.ti.com/SRVS/Data/ti/KnowledgeBases/analog/document/faqs/tt.htm


Reckon i'll try a .05 to ground instead and see where it gets me...
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Gonecat



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PostPosted: Wed Oct 12, 2011 7:53 am    Post subject: Reply with quote  Mark this post and the followings unread

I haven't tried adjusting that cap yet, since I figure I should get the voltages correct first. So, I checked them and here's what I'm seeing:

Plate: 143V
Cathode: 317mv
Grid 1: -88mv
Grid 2: 17v


Do these seem ok? It's a 6CB6 tube.
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andrewF



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PostPosted: Wed Oct 12, 2011 5:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

the measurements are what you should expect.

I had a listen to your samples again through better speakers (than my laptop), the 2nd one sounds about as good as it is going to get, #1 is not bad either.
It would be interesting to hear again with a signal modulating the cutoff.
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metasonix



Joined: Dec 28, 2003
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Location: CALIFORNIA

PostPosted: Wed Oct 12, 2011 8:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

[quote]Do i need to also scale the 3rd .01 cap which goes to ground?[/quote]

[size=18]YES. You have to scale ALL the capacitors in the feedback circuit together.
If you change 2 of them and not the third, you won't get resonance.
They all have to be the same value.

The bias resistor on the cathode might need to be different for a 6CB6.
Try 2.7k ohms instead of 1.5k.

Do you understand that adjusting the screen-grid voltage will change
the Q or resonance of the filter? And that it also affects the tube bias?
And that changing the bias will change the behaviour?
And that different tubes will behave differently in the same circuit?[/size]
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Gonecat



Joined: May 02, 2009
Posts: 43
Location: Rome, Italy
Audio files: 4

PostPosted: Thu Oct 13, 2011 6:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you both for the input and advice, I truly appreciate it.

I upped the cathode resistor to 2.7k.
(Removed extra caps so it's only the .01's)

Fired it up and that really seems to have helped a great deal, i can hear the filtration much better now. I am a little curious what 'lowering' those .01's would sound like instead of raising them (which lessens the depth it seems).

Plate Voltage: 160v
Cathode: 800mv
Grid: -6mv
Grid 2 changes with the Q pot so not tested


Quote:
Do you understand that adjusting the screen-grid voltage will change the Q or resonance of the filter?
And that it also affects the tube bias?
And that changing the bias will change the behaviour?
And that different tubes will behave differently in the same circuit?


Yes, No, Yes, Yes

The 'no' is that i thought the bias was being set by the first grid (as it is running negative, and grid two is positive) - so thank you for pointing this out.



Quote:
It would be interesting to hear again with a signal modulating the cutoff.


Next up is the CV circuit for this using the optocoupler which Mr. Barbour recommended over the tube version. I'm looking forward to that myself!

Attached is a clip of me running through different settings on the filter. At the end you can hear the Q cranked up and how cool the oscillator sounds.


orig-caps-back.mp3
 Description:
Upped the cathode resistor, used only original .01 caps. Starts with low resonance, then mid, then high. Freq is swept at each 3 resonance settings.

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 Filename:  orig-caps-back.mp3
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