for the fun of it screen volume control

All the parts have to be put together at some point. Have fun!
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Maturin
Posts: 22
Joined: Thu Jul 19, 2012 5:36 am

for the fun of it screen volume control

Post by Maturin »

I am in awe of Dave's inductive volume controls - perfection. What I'm going to buy for my next project.

This thread is just for the fun of it. I freely admit this circuit is a terrible idea...but it's MY terrible idea LOL. Actually, it's an old idea; using the screen to control gain - I just wanted to see if I could come up with something that COULD conceivably be used as the front end for a phono preamp. Now, that doesn't mean you WOULD use this - just that you COULD. Kinda like hitting yourself in the head with a hammer...you COULD, but why WOULD you?

Actually, there are some days when hitting myself in the head with a hammer sounds like a GREAT idea....

This circuit is a tube example of a terrible idea I had in 1969. Op amps were then new, and was planning to build a new radio station control console for my college radio station. I proposed using opamps and changing the gain (feedback) using plain old pots for the DJ's to twirl. The advisor, who was an EE, promtply and thoroughly cussed me out and told me to leave the damn gain alone, and just buy good quality attenuators. Which I did - remember the old attentuators, balanced, with the perfectly ground copper contacts? - and the console turned out beautifully. Worked like a charm for years.

As you'll notice, I use lots of voltage sources and current sources in LTSpice to make mocking up a circuit easy. In real life, I'd have to figure out how to do all these fancy-pants things.

I could not figure out how to make the voltage source for the screen ramp up with a concave slope rather than convex - thus, as you'll see, it doesn't work right...the ramp up of the voltage has to be exponential, but it has to be concave, not convex. Arrrrrggggg. However, this could be a GREAT chance to learn (probably from sbench) how to do this in LTSpice.

You'll notice I have a mosfet CF for the input, driving a pentode. The screen voltage is then controlled to achieve gain control. Why would I do such a dumb thing? Well, partly because I CAN LOL. Partly because as I vary the screen voltage, there is a varying amount of Miller effect - the CF input shields the phono cartridge from any varying Miller effect. Also, built into this idea is that the volume control comes right up front...not the usual way of doing things, and probably not a good idea, as pentodes are noisy, but...still...oh well, it's the way I did it. There, I don't have a better answer. You should still buy Dave's inductive volume control and put it where it belongs, at the input to the power amp.

Please note you can't pay any attention to the THD numbers with a circuit where your gain varies - to look at THD you have to change the screen voltage back to just one voltage. If you do that, set the screen voltage to 140...the THD for M1source will be about 0.07 and for U1anode will rise to about 1%, I think, as I vaguely remember.

[/img]https://www.dropbox.com/s/9wn0ocqemy32j ... ge.jpg[img]

[/img]https://www.dropbox.com/s/2n85ogrc8zxe5 ... en.jpg[img]

(blast it, why didn't the images post?)

asc attached.

VB, Maturin
Attachments
mosfet CF ef86 v2.asc
(4.33 KiB) Downloaded 104 times
sbench
Posts: 296
Joined: Fri Jan 28, 2005 3:45 pm

Post by sbench »

Hi,
Actually that's pretty easily. Replace your screen voltage source (V4) with a "Behavioural source" (BV in the component list).

The function exponential function is then implemented as an equation of time. (See the "help" box for circuit elements and scroll to BV.

The equation I used to test it varies the screen from 60 to 140 volts in 1 second. The equation is

v=(60+(exp(time)-1)*46.2)

That should work nicely.

Warmest Regards,
Stephie
Maturin
Posts: 22
Joined: Thu Jul 19, 2012 5:36 am

Post by Maturin »

Stephie,

Thanks so much!! Now I have a new tool to use in LTSpice...heh heh heh.

I have attached the waveform graph of U1anode vs U1screen.

I can't figure out how to post a link that will display IN the post using dropbox; anybody know what I did wrong in the first post?

Well, that was fun.

Very Best, Maturin
Attachments
sbench bv equation screen gain control outputs.jpg
sbench bv equation screen gain control outputs.jpg (155.88 KiB) Viewed 3306 times
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