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tv crt as amplifier?

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throbscottle

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In theory it should be possible to use a CRT as an amplifier valve. Any thoughts on this?
 

ronsimpson

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Certainly possible.
(assuming no deflection coils) If you keep the electron beam focused, like in a TV, then the beam will heat up one spot. This will burn out that spot. I have seen holes punched through the glass. A large CRT might put 30 watts in a spot as small as one pixel.

If you de-focus the beam, then I don't see a problem.

Next thought: I have always used the CRT in a current mode. (where the anode voltage is fixed and I am adjusting the current) If you move to a voltage mode where the anode voltage varies then the anode capacitance will be a major problem with AC frequency response. I have used the electron gun at 100mhz. I can vary the current that fast. If you are trying to build a amplifier, thousands of pF of capacitance will slow the amp down.
 

dr pepper

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Interesting idea.
You can run a Crt gun without Eht, as in a Crt tester, dunno if that would do what you want though Crt's werent meant to amplify.
If you really want to run a tube with Eht and get a spot you could put mains ac from a step down transformer on one or both deflection coils (assuming the tube is magnetic) and just draw a line or a circle.
Theres a post here somewhere where I messed with a radar screen, and before that a vector monitor from an old telly.
 

throbscottle

Well-Known Member
This arose from a conversation with my brother in law who is a musician, he was talking about people using "TV tubes" as guitar amp valves - without realising he really meant valves from TV's back in the day. But anyway it got me thinking about it.
So my two trains of thought were running it with no EHT as dp says, or with EHT but very unfocussed so the whole screen gives a gentle glow, which might visibly vary. I know when I messed about with CRT's in the long and distant past I had full screen glow happening when I wanted a spot.
I think I saw your radar screen thread dp.
Lissajous figures would be cool
The next question is, how do you run a valve in current mode? It had never even occurred to me as a thing...
My OTHER other thought on visualisation was to generate a raster and see if any visible patterns develop. Of course then you have the problem of the scan signal getting back into the audio.
I wonder how well A1 and A2 would be at acting like pentode grids G2 and G3?
But first I have to get the old TV off him...
My next other other other thought is it's a colour tv so I get 3 guns to play with :D
 
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ronsimpson

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The next question is, how do you run a valve in current mode?
There is no voltage gain. The Anode is fixed at 25kv and never changes. Current changes and lights up the screen.
In voltage mode there is a anode resistor. You get voltage to change when current flows. Output voltage might be 5kv to 22kv with a load that can not pull more than 200uA.
The electron guns were built for 1mA or so...
I have two "amplifiers" that do 1kv to 40kv output. They probably have no use now days. The "valve" is about 10 inches tall. Probably much like a CRT but with out the focus grids and with a small anode.
 

dr pepper

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This isnt something I've played with much, however I have an old radio that uses the screen grid of one of the tubes as a diode as part of the agc, I wonder if you could use the grids of the Crt to do a similar function, sort of a diode clipper, as I guess a guitarist is only really interested in 'tube' distortion.
Dunno if this'll work on one of the grids of a Crt.
 

atferrari

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I have used the electron gun at 100mhz. I can vary the current that fast. If you are trying to build a amplifier, thousands of pF of capacitance will slow the amp down.
What was the use Ron?
 

ronsimpson

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What was the use Ron?
The first TVs and CRT monitors I designed were based on 15,750hz by 60/30hz. The vertical scanned at 60hz buy only 1/2 the picture was display at a time so the entire picture came at 30hz. The video is updated at about 4 to 8mhz.
I later designed 640x480 monitors. Then 1240x768. 1600x1200=(2M), 3M and 5M.
These monitor did not take 30hz to display a picture but 70 or 80hz.
If you double the pixels horizontal and double the pixels vertically and triply the display rate then the video rate goes up by (2x2x3=12)
Toward the end we had special color guns made that did not use 3 video amplifiers but 6 amplifiers. One set had three guns to cut the video rate by 1/3.
Many monitors had horizontal scan rates of 120khz using conventional scan, and then I learned to scan non-conventionally much faster.
So we did draw pictures very fast.
 

throbscottle

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I've learnt a whole thing I'd never thought about - thanks Ron!
 

schmitt trigger

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There are some threads in the DIY Audio forum which talk about using video amplifier tubes as the output stages in an an audio amplifier.
 

dr pepper

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Interesting idea, I have a videcon tube from a camera, I wonder if that'd make a sound to light to sound amp.
 

throbscottle

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Don't see why it couldn't do the light to sound part
 

throbscottle

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This isnt something I've played with much, however I have an old radio that uses the screen grid of one of the tubes as a diode as part of the agc, I wonder if you could use the grids of the Crt to do a similar function, sort of a diode clipper, as I guess a guitarist is only really interested in 'tube' distortion.
Dunno if this'll work on one of the grids of a Crt.
CRT has only 1 grid AFAIK - the rest are anodes. I was speculating they will still behave as grids with appropriate biasing.

Soon there will be musicians queuing up to get that "beam modulation sound"...
 

dr pepper

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Correct however you might be able to use them other than designed.
Yes it has the capability of becoming 'trendy', guitarists can get pretty obsessive, I've repaired amps that were 'improved' by musicians.
 

ronsimpson

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CRT has only 1 grid AFAIK
I have seen, and did, drive a signal into G1 of a CRT.
BUT
99.9% of the designs we held G1 at a voltage and drove the Cathode with a signal.
...Something like a grounded grid amp, grounded Base, grounded Gate amp
...The difference between G1 and Cathode is what it takes.
 

throbscottle

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Oh yes of course that's how they're normally driven - I forgot.
I'm wondering if I leave the CRT in situ in the TV for the supply voltages, at least at first, feed audio somewhere into the video amp and see if I can get an output at the first anode. Or would I get more gain feeding the grid?
 

unclejed613

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I have seen, and did, drive a signal into G1 of a CRT.
BUT
99.9% of the designs we held G1 at a voltage and drove the Cathode with a signal.
...Something like a grounded grid amp, grounded Base, grounded Gate amp
...The difference between G1 and Cathode is what it takes.
IIRC in most TVs and computer monitors, G1 is grounded, and the cathodes are driven. you could use the focus electrode as a plate instead of using the dag anode there's also a G2 supply, and that's usually used for setting the beam current. G2 voltages used to be.... 200-500V, and the 1st anode (focus electrode) anywhere from about 1kV-5kV. in a 3-gun CRT. the G1 for all 3 guns are usually tied to one single pin.

it's been a while since i've worked on CRT monitors, but most PIX tubes are pinned out the same after 1990 or so.
 

Nigel Goodwin

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IIRC in most TVs and computer monitors, G1 is grounded, and the cathodes are driven.
It varied with the design, for a LONG time it was common (probably back before IC's were more common than transistors?) to feed the three colour signals to the cathodes and the luma signal to the grid, thus combining them actually in the CRT. This made for a simpler design, but once IC's took over the combining took place inside an IC, and usually fed the cathodes only. Another common option was using the grid for feeding the blanking pulses, again this simplified the design.
 

dr pepper

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Thats how my ecko radar doofer does it, minimum brightness control goes to the cathode, & the reflected rf goes to one of the grids, at least I think thats how it was.
At least you dont have to play around to much with deflection, every time I've played with a Crt the deflection has been a bugbear.
 
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