question on old schmatic

[iccaros]

So looking at diffrent ways to come at a problem, I thought what if I configured a set of tubes like and opamp. while this is not new, started in 1940, but I have this schamatic


But I do not know what the VR1 and VR2 are?

Thanks for any help

[Liquitone]

They look a bit like how the little neon lamps look in this Philcorda tube organ i recently repaired, the symbols are somewhat the same atleast,. only i cant make sense why two of those are used here. VR stands for Variable Resistor in most schems i saw, so it could be 2 trimpots, one for tuning and one for fine-tuning? they dont look like trimpots though,. im just guessing here.

[esnabez]

I agree; they are neon lamps. I believe in this case VR stands for voltage regulator. Quote from Wikipedia: "Most small neon (indicator-sized) lamps, such as the common NE-2, break down at between 90 and 110 volts. This feature enables their use as very simple voltage regulators or overvoltage protection devices."

[boogietone]

Neon lamp. http://www.google.com/m/search?oe=UTF-8&client=safari&hl=en&aq=f&oq=&aqi=g1-k0d0t0&fkt=783&fsdt=3079&cqt=&rst=&htf=&his=&maction=&q=ne-2+bulb&flip=0. You should be able to find them on eBay.

[iccaros]

thanks for the input
I am thinking of using either 12u7 or 12ae7 as they are made for 12V, so pedal territory...

here is a picture of that tube opamp for those interested

[Hides-His-Eyes]

I agree; they are neon lamps. I believe in this case VR stands for voltage regulator. Quote from Wikipedia: "Most small neon (indicator-sized) lamps, such as the common NE-2, break down at between 90 and 110 volts. This feature enables their use as very simple voltage regulators or overvoltage protection devices."

Interesting. Do they make 100V zeners?

[boogietone]

I agree; they are neon lamps. I believe in this case VR stands for voltage regulator. Quote from Wikipedia: "Most small neon (indicator-sized) lamps, such as the common NE-2, break down at between 90 and 110 volts. This feature enables their use as very simple voltage regulators or overvoltage protection devices."

Interesting. Do they make 100V zeners?

Yep. Mouser stocks up to 270V zeners.

[iccaros]

Lamps have been used to lower the voltage for tube heaters to remove the requirement to have a filament transformer. Also I have seen them used in attenuation devices. 

here is a write up on Tube opamps if anyone is interested
http://www.analog.com/static/imported-files/seminars_webcasts/Op%20Amp%20Applications%20Book%20(PDF)/P2%20ChH_final.pdf

[R.G.]

I agree; they are neon lamps. I believe in this case VR stands for voltage regulator. Quote from Wikipedia: "Most small neon (indicator-sized) lamps, such as the common NE-2, break down at between 90 and 110 volts. This feature enables their use as very simple voltage regulators or overvoltage protection devices."

They break over at 90-100, but then fall back and regulate a bit lower, often 60-70V depending on the actual gas mix and electrode metal/chemistry.

Neon lamp. ... You should be able to find them on eBay.

Mouser stocks them, last time I looked for them there.

Lamps have been used to lower the voltage for tube heaters to remove the requirement to have a filament transformer. Also I have seen them used in attenuation devices. 

Probably. Any current limiting device will do that. However, be very, very careful if you try running this like it was most frequently used - in no-transformer, directly rectified AC power line radios and such. The safety issues are huge. Also, it's a long way from several hundred volts on a plate down to 6V on a filament. Everything "attenuated" on the way down is given off as waste heat, as the heater currents - usually 150-300ma per duotriode - have to then come from the plate supply, and this is dramatically larger than the few ma of plate current used by the tube plates themselves. It's easy to get into a situation where it may work, but is remarkably impractical.

here is a write up on Tube opamps if anyone is interested

My first opamps were in fact Philbrick two-tube opamps. The whole setup was donated to the school by the Air Force. At this time, the 741 was new, and was much more expensive than a tube.

[iccaros]

It's easy to get into a situation where it may work, but is remarkably impractical.

this is what our hydrophone engineers says about tube amps....

R.G.
I was thinking of using two 12AE7's and lowering the voltage to 12volts and using a opamp distortion design, does this sound feasible, or is my headroom just way too low?

[PRR]

The vital feature of an "operational amplifier" is that it responds down to DC. This allows it to "hold" the results of a mathematical (operational) computation so that you can read it on a meter.

We are audio boys. We actually do NOT want DC to pass to the output.

> At this time, the 741 was new, and was much more expensive than a tube.

The introductory price of 741 was comparable to the going price of a new K2-W. But like you, I got mine hand-me-down from some large operation.

Very soon after, the price of 741 got MUCH MUCH lower than a K2-W, even lower than a 12AX7. Now we started using op-amps even in places where the DC response was not needed.

Another feature of a good op-amp is that the stuff inside the op-amp does NOT matter. Response is determined "only" by the external impedances (feedback resistors). Therefore it should not matter if the stuff inside the triangle is solid-state or hollow-state. And in general, a K2-W worked at a reasonable gain will NOT sound like a "tube amp", but far cleaner. (However a "K2-W" built with lame tubes working on lame voltages will be a lame opamp.)

> using a opamp distortion design

"opamp" distortions use opamps because opamps are now under 19 cents.

In vacuum-tubes, you can do the same thing with MUCH simpler AC-coupled audio amplifier design.

If you are doing diodes to distort, it probably matters very little what kind of amplifier you have driving the diodes.

[iccaros]

The vital feature of an "operational amplifier" is that it responds down to DC. This allows it to "hold" the results of a mathematical (operational) computation so that you can read it on a meter.

We are audio boys. We actually do NOT want DC to pass to the output.

> At this time, the 741 was new, and was much more expensive than a tube.

The introductory price of 741 was comparable to the going price of a new K2-W. But like you, I got mine hand-me-down from some large operation.

Very soon after, the price of 741 got MUCH MUCH lower than a K2-W, even lower than a 12AX7. Now we started using op-amps even in places where the DC response was not needed.

Another feature of a good op-amp is that the stuff inside the op-amp does NOT matter. Response is determined "only" by the external impedances (feedback resistors). Therefore it should not matter if the stuff inside the triangle is solid-state or hollow-state. And in general, a K2-W worked at a reasonable gain will NOT sound like a "tube amp", but far cleaner. (However a "K2-W" built with lame tubes working on lame voltages will be a lame opamp.)

> using a opamp distortion design

"opamp" distortions use opamps because opamps are now under 19 cents.

In vacuum-tubes, you can do the same thing with MUCH simpler AC-coupled audio amplifier design.

If you are doing diodes to distort, it probably matters very little what kind of amplifier you have driving the diodes.

Edit... sorry PRR, I took your post as to why even do this, it will not sound like  a tube amp...
which is not the point.. if it does not work, oh well, this is why I have bread board.
The point is that tubes have low input impedance and high output impedance, opamps don't, and are not effected by different cables or guitars. I do not know what you mean by lame tubes, these tubes are built for 12volts, so they are engineered very well to take a low voltage, its easy to get amplification at high voltage from a thermatic device, but at lower voltages plate resistance goes up making it harder to amplify signal.  

So in part, if you setup tubes in an opamp configuration, you should get the impedance benefits, a good clean buffer that is different than other takes on the same thing..
As a Army Radio repair man (14 1/2 year)  I have been dealing with opamps for a while, both for audio and for key tone sequencing tuning.  Tubes were a major part in the late 80's, so I have my knowledge, but with low voltage guitar pedal world.. all of that is crap.  once we get to the cable impedance, other effects and different guitars, well made designs can fail, so I am looking for something stable, different and fun.

and this looked interesting..

[iccaros]

Everything "attenuated" on the way down is given off as waste heat, as the heater currents - usually 150-300ma per duotriode - have to then come from the plate supply, and this is dramatically larger than the few ma of plate current used by the tube plates themselves. It's easy to get into a situation where it may work, but is remarkably impractical.

sorry, meant audio attenuation, like the hot plate http://www.americanmusical.com/Item--i-THD-HP8-LIST
as it acts more like a speaker than a resistor. but like you said can be impractical

[R.G.]

The point is that tubes have low input impedance

Tubes have a quite high input impedance. The impedance of the grid itself as long as it's not near the grid conduction threshold is as high or higher than a JFET. Only MOSFETs have a higher native input impedance.

So in part, if you setup tubes in an opamp configuration, you should get the impedance benefits, a good clean buffer that is different than other takes on the same thing..

You do get a good clean buffer, but the thing about feedback is that it's the great electronics equalizer, in the sense that the six-gun was the great equalizer in the old west. All amplifiers with sufficient gain to make the approximations true will sound alike and have the same impedance benefits, with possibly issues of more or less native impedance on the inputs. All opamp-derived buffers will act much the same and have much the same impedance benefits. The thing about tubes is that they will have a high native input impedance, similar to JFET input opamps and CMOS opamps, and the possibility for much higher voltage swings, if you use that.

In fact, this all-opamps-are-equal concept is the reason for opamps - it's an operational amplifier, meaning that the action is determined by the rest of the circuit, and you can largely ignore the insides, as long as the assumptions made are valid.

so I am looking for something stable, different and fun.

and this looked interesting..

That is a perfectly valid reason for experimenting.

Paul and I get a lot of questions where someone without much experience thinks that rubbing a tube or germanium transistor on a circuit will make it sound like lasers, flying saucers, Yngvie Malmsteen, whatever. Experimentation is good. Go enjoy.

[iccaros]

Thanks RG and PRR for answering my questions

[stringsthings]

so I am looking for something stable, different and fun.

and this looked interesting..

That is a perfectly valid reason for experimenting.

Paul and I get a lot of questions where someone without much experience thinks that rubbing a tube or germanium transistor on a circuit will make it sound like lasers, flying saucers, Yngvie Malmsteen, whatever. Experimentation is good. Go enjoy.