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21st Mar 2017, 6:30 pm | #1 |
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Using valves to amplify a DC voltage?
Just joined UK Vintage Radio as I've learned a lot over the last few months. Thanks all for this
My problem is all in the title... I've 'done' transistors and ICs for many years and recently decided to get back into valves. Since I started on this new tack I have learned a lot and built a variety of the usual stuff like an RF sign gen, various radios and amplifiers etc. etc. On trying to learn a little more about how one can use valves for different applications, I have come to a halt with some recent experiments. Quite simply, I have a very slow sawtooth waveform, sometimes with a DC level in between transistions that is too small and needs amplifying. That's about it! I could have done this with trannys with no problems but valves ~~~~ The issue with my current experiment is that there don't seem to be any circuit ideas out there I can find to adapt, learn from and utilise. If using and AC coupling then all is well, but as soon as things become DC coupled then things just don't seem to work how I want. At best I end up with nothing more than a buffer circuit (like a cathode follower). Can anybody help please? In the last couple of days I've tried duplicating front ends from the 'Y' input of various valve oscilloscopes but these don't seem to work in the 'basic' manner that I want either. Any help gratefully received. |
21st Mar 2017, 10:02 pm | #2 |
Dekatron
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Re: Using valves to amplify a DC voltage?
The big downer with thermionics is that there's no PNP valve. Life would be so much simpler if there was...
DC amplification can be done, but it's not easy. Imagine a simple two-valve RC coupled amplifier, replace the C by a resistor, and then take the second-stage grid leak to a really negative voltage (-100V or so). Get the resistor values right, and you'll have the correct bias on the second stage, with only a small amount of attenuation of the swing from the first stage anode. Low level DC amplification was often done by chopping the DC to convert it to AC, amplifying that, and then rectifying back to DC. This solves the problem of drift, which when you're trying to amplify millivolts, can be a major nuisance. But it's obviously no good if you want to amplify right from DC to ultrasonic frequencies. |
22nd Mar 2017, 12:03 am | #3 |
Heptode
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Re: Using valves to amplify a DC voltage?
It might be worth looking into valve voltmeters as they are predominantly DC amplifiers, and even evolution of the op amp. There were some valve based op amps about.
Before the advent of the op amp IC many issues occurred with transistor DC amplification too. It is only in recent years that we have the 'simple' building blocks that we have today. |
22nd Mar 2017, 12:16 am | #4 |
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Re: Using valves to amplify a DC voltage?
The better end of valve voltmeters, with good DC sensitivity and low offset voltages used chopper techniques. Sometimes using an oscillator driving neon bulbs and ORP12s, soetimes a spinning disk chopped light between a bulb and a photoconductor.
David
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22nd Mar 2017, 1:22 am | #5 |
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Re: Using valves to amplify a DC voltage?
As you've probably gathered by now, one of the big problems in designing d.c. amplifiers with valves is that the anode of one valve is at a +ve voltage that is far in excess of the grid-cathode bias required by the successive valve, which needs a d.c. connection to the first one. Old text books often invoke the idea of a battery between the two to cancel out the excessive voltage - the embedded idea is that the a.c. impedance of that battery will be negligible. Personally, I've always had my doubts about that, but have never tried the idea. However, what does spring to mind from that is the fitment of a Zener diode.
Any thoughts along those lines, anyone? Al. |
22nd Mar 2017, 6:15 am | #6 |
Dekatron
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Re: Using valves to amplify a DC voltage?
A couple of impractical ideas for completeness.
Chart recorders (which worked down to DC) often used a mechanical (vibrating reed) chopper in the input stage. It was operated by an electromagnet run from the mains transformer (often the heater winding). This meant the output of the amplifier was an AC signal in phase (or in antiphase) with the mains which could be applied to one winding of a capacitor-run motor. There was also the (infamous) Vibron electrometer which used a vibrating capacitor plate (again driven by a solenoid run from the mains transformer) to modulate the input signal and turn it into AC. |
22nd Mar 2017, 7:53 am | #7 |
Nonode
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Re: Using valves to amplify a DC voltage?
Oscilloscopes use valved DC amplifiers successfully, too, though allowing more drift than a chopper-stabilised amplifier. Have a look at the circuit diagrams of some scopes and Y amplifier plugins. The Solartron CX1441 and 1442 plugins (for the ubiquitous CD1400 scope) are basically valve DC amplifiers. Something like the Tektronix 310A is probably also a good example: decent performance but not crazily complicated.
Chris
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22nd Mar 2017, 8:59 am | #8 |
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Re: Using valves to amplify a DC voltage?
Before they had zeners, a few little Hivac neon bulbs in a string were sometimes used as voltage shifters between stages.
Despite Isaac Asimov, no-one ever made a positronic valve as a complement to the plain old electronic ones we were stuck with. David
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22nd Mar 2017, 9:48 am | #9 | |
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Re: Using valves to amplify a DC voltage?
Quote:
All a bit too simple, sure: I'm obviously missing a few fundamental factors in all that, despite my 'signature' below. Your thoughts; corrections; enlightenments, please. Thanks, Al. |
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22nd Mar 2017, 10:04 am | #10 |
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Re: Using valves to amplify a DC voltage?
Many thanks everyone for the input. Hence the lack of seemingly suitable circuits! Your ideas and thoughts are much appreciated. Will beat this one yet...
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22nd Mar 2017, 10:08 am | #11 | |
Dekatron
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Re: Using valves to amplify a DC voltage?
Quote:
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22nd Mar 2017, 10:18 am | #12 |
Nonode
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Re: Using valves to amplify a DC voltage?
Fortunately those positively charged nuclei cling rather firmly to the rest of the cathode surface, otherwise there soon wouldn't be much cathode left! The electrons meanwhile, having crossed to the anode, recirculate via the HT back to the cathode again to rejoin their nuclei.
I fear that there isn't a corresponding mechanism for those charged nuclei (e.g. thorium) to go off wandering round the circuit and return to the cathode. Martin
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22nd Mar 2017, 10:46 am | #13 |
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Re: Using valves to amplify a DC voltage?
Thanks Martin. In my simplistic approach, I hadn't thought of that! I get lots of ideas: some good; some un-good. However I draw some consolation that the History of Science shows that many important discoveries were made by accident, lateral thinking or challenging established dogma.
Al. |
22nd Mar 2017, 11:20 am | #14 |
Heptode
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Re: Using valves to amplify a DC voltage?
For general purpose DC amplification the Operational Amplifier springs to mind.
The first commercial implementation was the Philbrick K2-W in 1952 which used two 12AX7s and a couple of neons for level shifting (as David mentioned). PDF datasheet here. Jim |
22nd Mar 2017, 2:30 pm | #15 |
Dekatron
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Re: Using valves to amplify a DC voltage?
Yes - we're stuck with single-polarity valves because there just aren't positive charge-carriers which can be boiled off a hot cathode, travel to an anode, and then recirculate through wires back to the cathode. Unlike semiconductors, where P-type material really does behave as if it has positive charge carriers - witness Hall effect in P-type material!
Using gas discharge tubes as level shifters can and does work, although they are apt to introduce noise. The legendary Philbrick K2-W (whgich jimmc101 kindly gives a link to)used a resistive divider to the second-stage, level-shifting but also losing signal, and the gas discharge tube to the final (cathode-follower output stage). |
22nd Mar 2017, 3:15 pm | #16 |
Dekatron
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Re: Using valves to amplify a DC voltage?
There was a technique used to allow the anode of one valve to be DC-coupled to the grid of a second valve to make a two-stage amplifier:
Search for references to the "Loftin-White" circuit. Kinda like a "Darlington valve" - special valves were designed specifically for this [2B5 and 6B5 and 6N6]. It was used in some audio applications too [Be warned - the biasing is somewhat 'interesting'] |
22nd Mar 2017, 3:30 pm | #17 |
Nonode
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Re: Using valves to amplify a DC voltage?
I've just had a glance at the circuit diagrams for my Tektronix 535A scope. It's interesting to see how they solve the level-shifting problem by using lots of power supply rails. For example, in the trigger amplifier, the first stage has the valve anodes fed from +100V, cathodes returned to -150V, and the second stage has the anodes fed from +225V and the cathodes returned to ground. Similarly, the vertical amplifier first stage is strung between +225V and -150V, and the second is between +350V and +100V. Generally all the amplifiers are differential so the absolute DC level of the output doesn't matter all that much (it's going straight to the CRT plates).
So many power supply rails are probably overkill for a simple DC amplifier, though! Chris
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23rd Mar 2017, 7:12 am | #18 |
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Re: Using valves to amplify a DC voltage?
Don't think this will help in this case but here's a link to some interesting ideas using toob's - http://w140.com/tube_circuits_in_tek.pdf as . It was pointed out to me by someone on this forum and has come in handy.
Andy.
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23rd Mar 2017, 8:32 am | #19 |
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Re: Using valves to amplify a DC voltage?
Circuit number 11 should look rather familiar to audio people. It's what Arthur Bailey added to Arthur Radford's amplifiers as an improved phase-splitter.
David
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24th Mar 2017, 3:37 pm | #20 |
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Re: Using valves to amplify a DC voltage?
Some seriously interesting circuit ideas there!
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