Author Topic: 833A Cathode Modulation  (Read 1868 times)

Offline Twospoons

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Re: 833A Cathode Modulation
« Reply #20 on: February 09, 2021, 09:18:09 PM »
BTW, I didn't check the input polarity of the opamps carefully - you'll need to check that the grid bias moves the right way to compensate for cathode current changes.  And as mentioned before, you may need to band limit the feedback around the opamps.

Oh, and that lower opamp wasn't meant to operate as a comparator - I was just too lazy to draw in the feedback resistors to control the gain.

Offline Vdekje

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Re: 833A Cathode Modulation
« Reply #21 on: May 14, 2021, 08:58:15 PM »
Any updates on this? Two of us in a discord are looking into ramping VTTCs as well.

I'm planning on ramping the screen, as done by Lukáš Koval here
I'm using a 8890, and I have a 20kv 48µF bus cap. Since the anode idle current is relatively high without screen voltage (140mA avg with half wave doubled 4.2kv, so much higher with, say, 10-15kv smoothed DC), I'd like to cathode modulate to only have the anode on during the screen ramp.

The other person is trying to get a ramp by sawtooth modulated PWM cathode modulation. He's doing it on a mini-vttc, PL500.

Offline pc50kd

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Re: 833A Cathode Modulation
« Reply #22 on: May 16, 2021, 12:21:23 PM »
Interesting topic. I also played around with a cathode modulated VTTC a while ago.

I used a GU81M vacuum tube and interrupted the cathode with a IGBT.
For modulation I used a audio modulated PWM signal at 0.33 times the resonant frequency of the VTTC driving the IGBT.

I also made a post about it on the 4HV forum. https://4hv.org/e107_plugins/forum/forum_viewtopic.php?p=1&id=29247#post-29348
Unfortunately the schematic posted there is unreadable. But if there is interest I could dive into my archives and see if I can recover it.

« Last Edit: May 16, 2021, 12:23:45 PM by pc50kd »

Offline SalinsLV

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Re: 833A Cathode Modulation
« Reply #23 on: May 21, 2021, 07:28:55 PM »
I have built VTTC that plays music using cathode modulation, but I can't figure out why I can't tune it properly. Video of the coil in action:
/>
I have more than 15 tap points and have changed capacitors but the resonant frequency seems to be stuck unless i connect big capacitance to the primary tank or connect the primary to absurd low amount of turns. By adding 230pF capacitor i suspected the Fres to drop about 10KHz but in reality the Fres stays the same. I then added 470pF capacitor and the Fres dropped by about 60KHz! It's almost as if my coil is impossible to fine tune to exact frequency. For some reason i have to make big differences in the tank circuit for the Fres to change.

To measure Fres i use 1X probe that sits unconnected about 1m away from the coil. Maybe my method of measuring Fres sucks but i'm not sure. I don't have 1000X or 100X probes to measure the oscillations directly :(
Krisjanis Salins

Offline SalinsLV

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Re: 833A Cathode Modulation
« Reply #24 on: May 21, 2021, 07:33:26 PM »
And i am VERY very dissapointed with my coil. I use GU5B with 4 MOTs, 4Kv supply and because of the tuning problem, the sparks are no longer than 20cm.
Krisjanis Salins

Offline SalinsLV

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Re: 833A Cathode Modulation
« Reply #25 on: June 04, 2021, 09:20:46 AM »
I think i found the problem. The signal I use for modulating the coil is very distorted.
The Yellow signal is how I want the signal to look like. The Blue signal is the output to the coil modulation. As You can see, the Blue signal is high as soon as the input (Yellow) signal rises. Because of this, the coil doesn't ramp up smoothly and instead is pulsed with the distorted blue signal. To ear, the coil sounds acceptable, however the square wave pulses limit spark length.
Krisjanis Salins

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Re: 833A Cathode Modulation
« Reply #25 on: June 04, 2021, 09:20:46 AM »

 


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