Hello All,
I just worked on a Silvertone radio. When I got done with it, I noticed some distortion after it was on a short time. Studying the circuit I found some drifted resistors and replaced them. But nothing else seemed to differ from the schematic. But it didn't cure the distortion problem.
A study of the schematic found a cathode bias resistor but no capacitor across it, like I have seen in other sets. When I added the capacitor, there was a tremendous difference in performance. The distortion was completely gone. It sounds great now. Before it sounded awful. How could the designer have left out this very important component?
Then I looked on the internet to see if there was such a thing as cathode bias with no capacitor and found this:

http://en.wikipedia.org/wiki/Cathode_bias

This page actually mentions the problem, and the capacitor as a solution. I hope that the usual 20 microfarad capacitor was the correct value for the Silvertone 4777.

Maybe someone knows why they could have left out the capacitor.

Thanks,

Dave

As I said in my other reply, resistors that drift as you mention will affect performance little, if at all.

Regarding the distortion, this sounds more like a gassy tube, especially how it occurs after the set has been on for a while. While the capacitor in the cathode circuit of the output tube may improve audio quality, millions of sets have been produced without this capacitor, and all function well without distortion, and I doubt that this set would have sold with a distortion fault.

T

Thomas,
Several output tubes were tried and none of them are gassy at all. The cathode capacitor removed the distortion.

Thanks,

Dave
:As I said in my other reply, resistors that drift as you mention will affect performance little, if at all.
:
:Regarding the distortion, this sounds more like a gassy tube, especially how it occurs after the set has been on for a while. While the capacitor in the cathode circuit of the output tube may improve audio quality, millions of sets have been produced without this capacitor, and all function well without distortion, and I doubt that this set would have sold with a distortion fault.
:
:T
:

Interesting. I really don't know what to say. Have these tubes been tried in other radios as well?

I do know, however, that millions of output circuits like the one found in your radio have been produced, and they all perform without distortion.

...It sounds like the cathode bypass cap is covering up some other problem, but I could be very wrong.

T.

Thomas,
Yes, they were in other radios. They didn't distort in the other radios. I borrowed them for this radio. Now they don't distort in any of the radios I have here.
Whatever. If this capacitor works then, that's fine.

Thanks,

Dave
:Interesting. I really don't know what to say. Have these tubes been tried in other radios as well?
:
:I do know, however, that millions of output circuits like the one found in your radio have been produced, and they all perform without distortion.
:
:...It sounds like the cathode bypass cap is covering up some other problem, but I could be very wrong.
:
:T.
:

Thomas,
Yes, they were in other radios. They didn't distort in the other radios. I borrowed them for this radio. Now they don't distort in any of the radios I have here.
Whatever. If this capacitor works then, that's fine.

Thanks,

Dave
:Interesting. I really don't know what to say. Have these tubes been tried in other radios as well?
:
:I do know, however, that millions of output circuits like the one found in your radio have been produced, and they all perform without distortion.
:
:...It sounds like the cathode bypass cap is covering up some other problem, but I could be very wrong.
:
:T.
:

Hi Dave:
I am no expert on this.. and..I'm sure you are noticing some sort of "distortion" ....but I agree with Thomas, in that you are probably not hearing the (even order harmonic) distortion that is characteristically inherent in all cathode bias designs without a by-pass capacitor.

Yes, you will always hear a louder and somewhat "improved" sound when you add a cap to the cathode resistor because the capacitor simply acts as a "dynamic battery" so-to-speak ....and therefore it acts to cancel-out any variations to the bias voltage that is otherwise created by plate current changes in the cathode resistor voltage drop.

In other words, without a cap the plate current changes cannot help but pass through the cathode resistor thereby varying the voltage drop across it.

Those changes in current through the cathode resistor work to vary the voltage drop across it and hence the grid bias.

So as the grid signal tries to go positive with input signal swings... that increase causes the tube to "turn-on" more... drawing more current. But that increased current through the cathode resistor increases the voltage drop across it thereby creating a more negative bias level.

"Negative feedback" is what it is called.

But this "feedback" classified as "distortion" is rarely ever heard as any form of disturbing-type of distortion that you seem to be referring to.

When a capacitor is placed across the cathode resistor it acts to counter the changes in the voltage drop across that cathode resistor much as a battery would do there.

So ..Yes the elimination of the negative-feedback by adding a capacitor may now increase the volume a bit for sure, and it may also sound a bit " better"
however... I don't believe that the distortion you refer to as objectionable is the same as the "even order harmonic" distortion.

So perhaps the capacitor is, in fact, causing an improvement of sorts ...but if you are hearing any truly "objectionable" distortion without the cap.. I believe you have some other problem.

If the circuit was designed without that cap.. I can guarantee that it should sound very good without any "objectionable distortion" without the by-pass cap.

Hello Peter,
OK.

Thanks,

Dave
:Hi Dave:
:I am no expert on this.. and..I'm sure you are noticing some sort of "distortion" ....but I agree with Thomas, in that you are probably not hearing the (even order harmonic) distortion that is characteristically inherent in all cathode bias designs without a by-pass capacitor.
:
:Yes, you will always hear a louder and somewhat "improved" sound when you add a cap to the cathode resistor because the capacitor simply acts as a "dynamic battery" so-to-speak ....and therefore it acts to cancel-out any variations to the bias voltage that is otherwise created by plate current changes in the cathode resistor voltage drop.
:
:In other words, without a cap the plate current changes cannot help but pass through the cathode resistor thereby varying the voltage drop across it.
:
:Those changes in current through the cathode resistor work to vary the voltage drop across it and hence the grid bias.
:
:So as the grid signal tries to go positive with input signal swings... that increase causes the tube to "turn-on" more... drawing more current. But that increased current through the cathode resistor increases the voltage drop across it thereby creating a more negative bias level.
:
: "Negative feedback" is what it is called.
:
:But this "feedback" classified as "distortion" is rarely ever heard as any form of disturbing-type of distortion that you seem to be referring to.
:
:When a capacitor is placed across the cathode resistor it acts to counter the changes in the voltage drop across that cathode resistor much as a battery would do there.
:
:So ..Yes the elimination of the negative-feedback by adding a capacitor may now increase the volume a bit for sure, and it may also sound a bit " better"
:however... I don't believe that the distortion you refer to as objectionable is the same as the "even order harmonic" distortion.
:
:So perhaps the capacitor is, in fact, causing an improvement of sorts ...but if you are hearing any truly "objectionable" distortion without the cap.. I believe you have some other problem.
:
:If the circuit was designed without that cap.. I can guarantee that it should sound very good without any "objectionable distortion" without the by-pass cap.
:
:
:

Hi Again Dave:
Have you measured the cathode resistor and the voltages on it and the grid?

The tube chart indicates that a 6V6 with 250v on the plate like yours has.. should be drawing about 45ma with no input signal ( quiescence ).
You have a 250 ohm cathode resistor. Ohms law indicates that 250 ohms @ 45ma should have about 11-12 volts drop across it.

With no signal your cathode should be about +12v with the grid read about -12V.

If you are reading much higher than that or if it starts drifting high... then perhaps the tube is going bad.
Check all the voltage readings and resistances around that 6V6.

I assume the cap feeding the grid is new?

The grid should read 0 volts to ground. Running without a cathode bypass introduces negative feedback. It does not cause distortion.

:Hi Again Dave:
:Have you measured the cathode resistor and the voltages on it and the grid?
:
:The tube chart indicates that a 6V6 with 250v on the plate like yours has.. should be drawing about 45ma with no input signal ( quiescence ).
:You have a 250 ohm cathode resistor. Ohms law indicates that 250 ohms @ 45ma should have about 11-12 volts drop across it.
:
:With no signal your cathode should be about +12v with the grid read about -12V.
:
:If you are reading much higher than that or if it starts drifting high... then perhaps the tube is going bad.
:Check all the voltage readings and resistances around that 6V6.
:
:I assume the cap feeding the grid is new?
:
:
:

:The grid should read 0 volts to ground. Running without a cathode bypass introduces negative feedback. It does not cause distortion.
:
Yes that's correct... maybe I said it wrong above?
It is zero volts w/respect to ground yes.
But -12v with respect to the cathode... right?

btw technically speaking... negative feedback is in and of itself a form of distortion isn't it?
...because it effectively alters the output-shape of the incoming signal on the grid. Anything that alters the shape of the incoming audio signal is distorting it.

Negative feedback can reduce the amount of clipping distortion.

Negative feedback in itself isn't distortion. What it usually does is clarify and reduce harmonic distortion. That is, distortion of frequencies, where some are more pronounced than others. By sending a negative of the output back to the input, the excesses and inadequacies are somewhat nullified.

T.

Hi Thom:
Again I'll stress that I am no expert to say the least...lol.
... but I think I was referring to the simple sense of the word distortion.

More specifically ....to define "distortion" as anything in a circuit/amplifier that modifies the ORIGINAL signal in ways that change the shape.

For instance if you start with a simple pure input signal of a single pure-clean, (single frequency) 400Hz sine-wave ...and if the circuit or amp changes the shape into something other than a sine-wave.. then it has been distorted. ....Right?

Even a tone control for instance can (purposely) "distort" or modify the shape of a signal to please one's ears... no?

I think that this seems to be referred to as "Linear distortion"?
.. and Thomas, what you are talking about may be "non-linear" or harmonic-type distortion where negative feedback works to nullify or suppress or reduce the effects of such "harmonic" modification (distortion).

This following excerpt from an article on the subject of DISTORTION & NEGATIVE FEEDBACK helps explain it:

"There are linear and non-linear forms of distortion.

Linear distortions affect the amplitude and phase of audio signals but don't show up on harmonic distortion analyzers as added frequency components that weren't there in the first place. Tone controls are a good example of circuits with linear distortion.

Non-linear distortions are those which add new frequency components to the original signal, either as harmonic multiples of the original frequencies or as sidebands resulting from their nonlinear interaction between the original frequencies. Non-linearities are often deliberately created in musical instruments themselves but they are unwanted in music reproduction.

We will be talking about nonlinear distortions.

We use negative feedback in audio amplifiers to stabilize the gain, increase the bandwidth, lower the output impedance and lower the nonlinear distortion. "
REF:( http://www.6moons.com/industryfeatures/distortion/distortion.html )

Peter,
This is getting quite interesting. I looked in some of my books and there are a few ways of eliminating the C battery. The three I now know about are Fixed Bias (Cathode connected to power supply Negative), Cathode Bias (resistor connects cathode to power supply negative) and cathode bias with bypass capacitor. I am not sure that these are the only ways to eliminate the C battery.
In the Silvertone it didn't hurt anything to add the capacitor and the sound is fantastic now. Why it wasn't so good without the capacitor is not an issue anymore because it sounds so good with it.
This should be a different and interesting thread about the different types of bias connections.

Dave
:Hi Thom:
:Again I'll stress that I am no expert to say the least...lol.
:... but I think I was referring to the simple sense of the word distortion.
:
:More specifically ....to define "distortion" as anything in a circuit/amplifier that modifies the ORIGINAL signal in ways that change the shape.
:
: For instance if you start with a simple pure input signal of a single pure-clean, (single frequency) 400Hz sine-wave ...and if the circuit or amp changes the shape into something other than a sine-wave.. then it has been distorted. ....Right?
:
:Even a tone control for instance can (purposely) "distort" or modify the shape of a signal to please one's ears... no?
:
:I think that this seems to be referred to as "Linear distortion"?
:.. and Thomas, what you are talking about may be "non-linear" or harmonic-type distortion where negative feedback works to nullify or suppress or reduce the effects of such "harmonic" modification (distortion).
:
:This following excerpt from an article on the subject of DISTORTION & NEGATIVE FEEDBACK helps explain it:
:
:"There are linear and non-linear forms of distortion.
:
:Linear distortions affect the amplitude and phase of audio signals but don't show up on harmonic distortion analyzers as added frequency components that weren't there in the first place. Tone controls are a good example of circuits with linear distortion.
:
:Non-linear distortions are those which add new frequency components to the original signal, either as harmonic multiples of the original frequencies or as sidebands resulting from their nonlinear interaction between the original frequencies. Non-linearities are often deliberately created in musical instruments themselves but they are unwanted in music reproduction.
:
:We will be talking about nonlinear distortions.
:
:We use negative feedback in audio amplifiers to stabilize the gain, increase the bandwidth, lower the output impedance and lower the nonlinear distortion. "
:REF:( http://www.6moons.com/industryfeatures/distortion/distortion.html )
:
:
:
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