Simple Circuit Questions

[dbishopbliss]

More fundamental questions from me.  Hopefully other people have these same questions.

The circuit in the diagram is often part of larger circuits.  I haven't bothered to include the rest of the circuit for now.  I'm curious about the R1-R3.

Is there a reason for choosing a particular value for R1 (assuming this is the first stage of a linestage or driver stage of an amp)?  I see R1 often has a value of 1M or 2M. 

R2 often has relatively small values like 200R.  Is this the "grid stop" resistor?  Is this resistor only effective if soldered directly to the socket?  Or, can socket be on a circuit board or terminal strip with a wire leading to the socket?

Assuming the B+ is connected to R3, do I use R3 to draw the load line for the triode?  The explanations of load lines I have seen deal with Power Transformers.

 

[Paul Joppa]

R1 is called a "grid resistor"; its purpose is to keep the grid near ground at no signal. It has to be small enough that the tiny leakage currents into or out of the grid does not lead to a grid voltage that upsets the bias of the tube. The maximum value is usually given on the tube data sheet, and is rarely greater then 1 megohm. The minimum value is determined by whatever is driving the grid. The optimum value may be anywhere within that range, depending on what you are optimizing.

R2 is as you say the grid stopper. At very high (radio) frequencies the inductance of wires and the capacitances between conductors will have resonances, and in the presence of a gain device (the tube) oscillations can develop. By placing a resistance in the line, these resonances are damped. In order to keep the resistance in the line, you must minimize any inductance between the resistor and the grid. Since wire has inductance (and any bend in a wire adds inductance - a 90 degree bend is 1/4 of a turn) you seek to minimize the distance from the resistor body and the actual tube pin. The actual value can vary widely but it's usually between 50 ohms and 10K ohms.

R3 is the plate load, assuming the other end is connected to the power supply. Its resistance determines the slope of the load line.

Hope that helps!

 

[VoltSecond]

R1 Most often is used as a safety resistor. When the wiper bounces or goes open, it keeps the grid of the tube at 0V dc so the tube doesn't go into run away.  R1 can also be used to turn a "linear" pot into an approximate "audio taper" pot. http://sound.westhost.com/pots.htm#chg-law

R2 is a grid stop resistor. It prevents parasitic RF oscillation in the tube. R2 can also be used to limit the bandwidth amplifier to prevent overloading the next stage on clicks, pops and ultrasonic junk from other sources.  R2 needs to be locates as close the to grid terminals as possible. Leave ~1/16 to 1/8" inch of lead between the body of the resistor and the solder joint to the grid to keep from breaking the resistor from mechanical stresses.

R3 is the plate load resistor.  It and R4 set the bias point for the tube.

 

[dbishopbliss]

In the example above, where R1 and R2 are receiving input from a device such as a CD Player, the input voltage should be a maximum value of 2V.  Is this the correct way to determine the power rating for these resistors?

I = V/R
I = 2/200
I=10mA

Therefore

P=I*V
P=0.01*2
P=0.02W

So, a 1/4 watt resistor will be fine. 

Also, is there a preferred resistor type for the the "grid resistor" and "grid stop" resistor (e.g., metal film, carbon comp, etc)?

 

[Paul Joppa]

That is correct for the grid resistor R1 (which is usually much much larger than 200 ohms!) There is no voltage across R2 since the grid impedance to ground is essentially infinite, so it dissipates no power. (The grid does have capacitance to ground, so a little current flows at high frequencies - I'm ignoring that for simplicity.)

R2 should have as little inductance as possible - see the discussion above for the reason. Theoretically, a bulk rod such as carbon composition would therefor be best, and a coil such as wirewound would be worst. Carbon film and metal film are spirals on the surface of a nonconductive rod, so would be theoretically inferior. However longtime Bottlehead VoltSecond, who knows his stuff, says that these work fine in low values (less than 1000 ohms?).

Carbon composition, thick film, and metal oxide types are said to be noise and might be poor candidates for R1. However, their noise is proportional to current and since there is no DC current and very little AC current, that may not be a problem in general. It is a big deal in R3 however, which should be metal film, wirewound (non-inductive), or a similarly quiet bulk-metal construction.

 

[dbishopbliss]

The maximum value is usually given on the tube data sheet, and is rarely greater then 1 megohm.

I have looked on a number of data sheets and I cannot find a value for "grid resistor" or "grid leak", etc.  Can you tell me where the value is specified in the following data sheet?

http://tdsl.duncanamps.com/pdf/5670.pdf

That is correct for the grid resistor R1 (which is usually much much larger than 200 ohms!)

I meant 200R for R2. 

 

[Paul Joppa]

Page 2, under maximum values - "grid circuit resistance"

200R is 200 ohms - did you mean 200K (200,000 ohms)?  :^)

 

[JC]

With regard to your formula, as Paul has indicated it doesn't really apply to R2.  R2 has essentially no DC connection to ground at the tube end, so there is essentially no path for signal current to flow to ground.  Without a path for current to flow through it, there will be essentially no Voltage drop across it, hence Ohm's law really cannot be applied to it.  Which means that the power corollary is also pretty meaningless.

Personally, I have always used carbon comp for this application as it should theoretically have a lesser chance of being inductive.  OTOH, I tend to use a higher resistive value on low-level amplifier inputs (line level and lower) on the theory that a higher resistance might have more of a damping effect on potential parasitic oscillations.  I have used as high as 10,000 Ohms, in fact, although I can certainly understand the argument for using lower values.

From the preceding discussion, though, you will see that the resistance value cannot have an appreciable effect on the signal level reaching the grid.

It would certainly seem possible to verify this without too much trouble.  A constant level of signal, such as a signal generator or a CD with audio tones recorded on it would be necessary.  Using a good AC Voltmeter or a scope would be ideal for measuring Voltages in this case, but a DMM could be used on the AC Voltage range, providing you use a test frequency within its capabilities.  In a pinch, 60 Hz should suffice. 

With regard to your new inquiry, I believe you are speaking of "R1" again, when referring to "grid resistor" or "grid leak".  The value of R2, the "grid-stopper" resistor is considerably more arbitrary.

 

[dbishopbliss]

Page 2, under maximum values - "grid circuit resistance"

Got it.  According to the data sheet the maximum grid circuit resistance is 0.5 Megohms, so 500K.

200R is 200 ohms - did you mean 200K (200,000 ohms)?  :^)

With regard to your formula, as Paul has indicated it doesn't really apply to R2.  R2 has essentially no DC connection to ground at the tube end, so there is essentially no path for signal current to flow to ground.  Without a path for current to flow through it, there will be essentially no Voltage drop across it, hence Ohm's law really cannot be applied to it.  Which means that the power corollary is also pretty meaningless.

I think that is why I confused Paul when I asked about power rating for R2... I meant 200 Ohms for the Grid Stopper resistor.  Earlier he said the following:

The actual value can vary widely but it's usually between 50 ohms and 10K ohms.

So, would 200 ohms be a poor choice?

 

[JC]

No, I certainly did not mean to imply that at all. 

When I chose 10 k for certain applications, I did so on the theory that the higher value might offer more damping to potential parasitic oscillations.

But, it was only theoretical.  In fact, the parasitic oscillations themselves were only theoretical!  I never actually observed any, I just provided against them if they should happen to appear.

Another school of thought is that larger resistance values may be more prone to introducing resistor noise at the grid given the infinitesimally tiny amount of grid current that could flow.  Hence, the opposite end of the spectrum Paul laid out is the preferred approach of others.

My purpose was to point out that along that range of 50 Ohms to 10 kOhms, you would not see any appreciable difference in the level of signal that reached the grid. 

 

[Paul Joppa]

We have used 220 ohms carbon composition grid stoppers for years. They are unfortunately fragile mechanically, especially if overheated, but they have always worked. I actually have used them as plate stoppers as well, in the Seduction.

Some years ago I was struggling with a buzz and distortion problem in a 2A3 amp - it might have been the earliest Paramour prototype, so at least a decade ago. After several frustrating hours with measurements that just would not make sense, I noticed that the row of LEDs (temperature indicator) on my solder station were flickering. Thinking there might be a power problem, I immediately switched off the 2A3 amp and the flickering stopped. Naturally I repeated to experiment to confirm that a few seconds after switching the amp on, the lights would flicker - every time. Apparently, it was radio frequency oscillation bursts, synchronized with the power line! I installed a grid stopper and the problem disappeared completely. (The amp sounded great after that, too.) I have included them in every circuit I build, ever since.

 

[JC]

I'm curious, Paul, did you attempt to see this on a scope at all?  I ask because I've never been able to catch one on a scope, even when there were other indications that an oscillation was taking place.

 

[dbishopbliss]

I have my Foreplay I sitting on the shelf here and I turned it over to look at the circuit.  I've noticed a few things:

• There are no grid stopper resistors (from volume pot to pin 2, or pin 1 to pin 7)
• There are no grid (leak) resistors (from volume pot/pin 2 to ground)

From what I understand the grid stoppers are optional, but I didn't get the impression that the grid leak resistors are optional.  Or, does the volume pot serve this purpose?

I won't bother asking about how the triodes are tied together in this thread.  I think that goes beyond Simple Circuit questions.

 

[Doc B]

The volume pot is the grid leak resistor.

 

[dbishopbliss]

The volume pot is the grid leak resistor.

Does the grid leak resistor define the input impedance?  That is, if I didn't have the volume pot, but instead R1 = 100K, would the input impedance be 100K? 

What if you had a pot and a grid leak resistor as shown in my diagram.  Do you basically calculate the impedance as if there were two resistors in parallel.  For example, if the pot were 100K and R1 = 500K, then input impedance =  83K.

 

[Grainger49]

Yes, the fixed 100k ohm resistor would be seen as the input impedance.

However... in the case of the pot and a fixed resistor the input impedance is the series part of the pot (unless it is at maximum) and the parallel combination of the fixed resistor with the remaining part of the pot.

That would be a reason for R1 to be a very high value.  It would make very little difference and the grid leak value will remain the value of the pot.