Dave; Thanks for the explanation. No way I could explain it like that.
I'm saving it to my archives for future perusal.
Larry
I'm saving it to my archives for future perusal.
Larry
tech is working on my fisher 800c to fix it but a few questions?
- Thread starter tonyg88
- Start date
I am aware that the original 330k resistors ran the output tubes very hard, and I'm also aware of the need to change the resistors in tandem with the change in coupling cap values in order to maintain the original time constant. However, darned if I don't recall reading something here on AK (Drew, perhaps?) that said that it also ran the driver tubes a bit easier.
Maybe I had it reversed in my memory.
In an output stage, if no output transformer were used, then continuing to reduce the speaker load (numerically in value) would constitute a greater and greater load on the output stage, until there was a simply a dead short presented to it. This makes the output stage work harder and harder until it is ultimately overloaded, and represents the exact scenario that output transformerless vacuum tube designs (OTL) and modern day solid state amplifiers must contend with. Fortunately in this regard however, most vacuum tube output stages do employ an output transformer, and with multiple secondary impedance taps on it as well. Therefore, as the speaker load is numerically reduced in value, we can simply use a lower impedance tap, which will then keep the same relative reflected load presented back to the output stage.
In the case with grid return resistors however, there is no intervening transformer we can use to compensate for a numerically lower load. As the grid return resistors are reduced in value, the effective load that the driver stage must operate into has been increased making it work harder, just as with the OTL and SS designs mentioned above.
One reason the grid return resistors were set so high in the first place, was to extract as much gain from the design as possible. All possible gain is needed to compensate for the gain loss produced when NFB is applied, and to reduce the number of stages needed to produce the overall gain requirements. All else being equal, keeping the value of the "following stage grid return resistor" as high as possible then allows a given stage to produce a higher working gain level, relative to a lower value following stage return resistance being used.
Another reason for using such a high value grid return resistor, is that it meant that a smaller value coupling cap could be used to achieve a target time constant for the coupling circuit. Caps of smaller value are typically smaller in size, so using a higher value grid return resistor aids in the space considerations of the design as well. Back in the day when these units were produced, components were generally much larger than an equivalent component is today, so space considerations were as much of a design issue back then as they are with today's microelectronics.
In this case however, because the stage preceding the output stage is a cathodyne phase inverter, the effect of reducing the value of the output stage grid return resistors to 220K produces little effect on the effective gain of the driver stage, while its peak output capabilities remain well within that necessary of driving 7591 class output tubes to full power output.
Dave
In the case with grid return resistors however, there is no intervening transformer we can use to compensate for a numerically lower load. As the grid return resistors are reduced in value, the effective load that the driver stage must operate into has been increased making it work harder, just as with the OTL and SS designs mentioned above.
One reason the grid return resistors were set so high in the first place, was to extract as much gain from the design as possible. All possible gain is needed to compensate for the gain loss produced when NFB is applied, and to reduce the number of stages needed to produce the overall gain requirements. All else being equal, keeping the value of the "following stage grid return resistor" as high as possible then allows a given stage to produce a higher working gain level, relative to a lower value following stage return resistance being used.
Another reason for using such a high value grid return resistor, is that it meant that a smaller value coupling cap could be used to achieve a target time constant for the coupling circuit. Caps of smaller value are typically smaller in size, so using a higher value grid return resistor aids in the space considerations of the design as well. Back in the day when these units were produced, components were generally much larger than an equivalent component is today, so space considerations were as much of a design issue back then as they are with today's microelectronics.
In this case however, because the stage preceding the output stage is a cathodyne phase inverter, the effect of reducing the value of the output stage grid return resistors to 220K produces little effect on the effective gain of the driver stage, while its peak output capabilities remain well within that necessary of driving 7591 class output tubes to full power output.
Dave
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Got it! Thanks, Dave!
Humm, I have just recapped a Fisher and replaced all the coupler caps with the same value as what was on the published schemetic. And was going to check all the grid resistors and replace them with the 220 ohm resistors. So after reading this it seem to indicate that I should have replaced all those caps with higher values?
And when you indicate higher values do you mean instead of say a .047 uf at 400 volts I should replace them with a .090 uf at 400 volts?
This is getting a bit frustrating!
And when you indicate higher values do you mean instead of say a .047 uf at 400 volts I should replace them with a .090 uf at 400 volts?
This is getting a bit frustrating!
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I am not very educated on all this, but am trying to learn as much as possible. I was reading about your recommendations of the resistor for the cathode bias and read this. If you put a resistor to establish cathode bias it states taht it introduces dynamic and harmonic distortion and then talks about putting a shunt capacitor to overcome the problem. Would this apply, or does the Fisher fall into the catorgory of the exceptions at the bottum of the explanation?
Establishing cathode bias
To establish cathode bias, a resistor is placed between the emitting element, or cathode and the negative return of the "B" or HT supply. Current drawn through this resistor by tube conduction places the cathode slightly more positive than the negative return. The grid input is returned directly to the negative supply, causing it to be negative with respect to the cathode. Thus, changes in tube conduction are automatically compensated by changes in bias.
This scheme inherently introduces dynamic, even harmonic distortion. As the input signal becomes more positive, cathode current increases, increasing bias and reducing gain. As the input signal becomes more negative, cathode current decreases, decreasing bias and increasing gain. The result is a plate signal with positive excursions greater than the negative input and negative excursions smaller than the positive input. It must be borne in mind that the input and output signals are ideally exactly out of phase. Since cathode bias is normally employed at audio or very low radio frequencies, issues such as transit time and interelectrode capacitance may be disregarded and the ideal assumed.
[edit] Overcoming problems
To overcome this problem, the bias resistor is typically shunted by a capacitor. In general, the capacitor value is selected such that the time constant of the capacitor and bias resistor is an order of magnitude greater than the period of the lowest frequency to be amplified. The capacitor thus acts as a dynamic battery, and makes the bias constant through input signal swings.
In some designs, the degeneratve feedback inherent in cathode bias may be desirable. In this case, two carefully designed successive stages may be employed, such that the distortion introduced by the first stage is exactly cancelled by that introduced in the second. This technique is not recommended, as the design considerations become very complex. Other degenerative feedback techniques are easier to design, and should be used.
An exception to the general rule may be made in the case of "push-pull", or balanced circuits. A pair of tubes, driven by identical signals 180 degrees out of phase, may share a common unbypassed cathode resistor. Slight differences in tube conduction are then dynamically balanced by bias variations that tend to reduce distortion. This technique is useful in the input circuits of balanced line receivers or push-pull power output circuits.
Establishing cathode bias
To establish cathode bias, a resistor is placed between the emitting element, or cathode and the negative return of the "B" or HT supply. Current drawn through this resistor by tube conduction places the cathode slightly more positive than the negative return. The grid input is returned directly to the negative supply, causing it to be negative with respect to the cathode. Thus, changes in tube conduction are automatically compensated by changes in bias.
This scheme inherently introduces dynamic, even harmonic distortion. As the input signal becomes more positive, cathode current increases, increasing bias and reducing gain. As the input signal becomes more negative, cathode current decreases, decreasing bias and increasing gain. The result is a plate signal with positive excursions greater than the negative input and negative excursions smaller than the positive input. It must be borne in mind that the input and output signals are ideally exactly out of phase. Since cathode bias is normally employed at audio or very low radio frequencies, issues such as transit time and interelectrode capacitance may be disregarded and the ideal assumed.
[edit] Overcoming problems
To overcome this problem, the bias resistor is typically shunted by a capacitor. In general, the capacitor value is selected such that the time constant of the capacitor and bias resistor is an order of magnitude greater than the period of the lowest frequency to be amplified. The capacitor thus acts as a dynamic battery, and makes the bias constant through input signal swings.
In some designs, the degeneratve feedback inherent in cathode bias may be desirable. In this case, two carefully designed successive stages may be employed, such that the distortion introduced by the first stage is exactly cancelled by that introduced in the second. This technique is not recommended, as the design considerations become very complex. Other degenerative feedback techniques are easier to design, and should be used.
An exception to the general rule may be made in the case of "push-pull", or balanced circuits. A pair of tubes, driven by identical signals 180 degrees out of phase, may share a common unbypassed cathode resistor. Slight differences in tube conduction are then dynamically balanced by bias variations that tend to reduce distortion. This technique is useful in the input circuits of balanced line receivers or push-pull power output circuits.
If the grid return resistors of the output stage are reduced in value, it is desirable to maintain (nearly) the same RC time constant that was obtained from the original circuit values. This will then maintain the LF stability margin of the original design. Therefore, if the original 330K resistors are reduced to 220K (for example), then the original .047 uF coupling caps should then ideally be increased to .07 uF. Many folks use .1 uF, which is about a 40% increase over the theoretical value required to maintain the RC time constant of the original circuit. As long as the LF stability of the circuit is unaffected, then the use of .1 uF couplers is fine in this example.
LF stability can best be checked by pulsing the input of the amplifier from a 1.5 volt battery (watch the volume!), and watch the woofer cone as the amplifier is pulsed with the battery. The cone's movement should be sharp and precisely follow the pulse event. On the other hand, if the cone takes time to settle down after each pulse (similar to a bad shock on a car taking time to settle down), then LF stability has been compromised, and the size of the coupling caps installed should be checked to ensure their value is appropriate for the grid return resistors installed.
Dave
LF stability can best be checked by pulsing the input of the amplifier from a 1.5 volt battery (watch the volume!), and watch the woofer cone as the amplifier is pulsed with the battery. The cone's movement should be sharp and precisely follow the pulse event. On the other hand, if the cone takes time to settle down after each pulse (similar to a bad shock on a car taking time to settle down), then LF stability has been compromised, and the size of the coupling caps installed should be checked to ensure their value is appropriate for the grid return resistors installed.
Dave
Hi Ken -- I'm not sure if I understand your question correctly, but may I assume it relates to points #1&3 of my post 19?
If so, then:
As your description explained, in a cathode biased amplifier, the control grid is made more negative than the cathode by referencing the grid to ground, and then elevating the cathode to some positive potential above ground by allowing current flow through the tube to produce a voltage drop across a resistance connected between the cathode and ground. Since ground is the most negative point of the circuit in this case, and the point to which the grid is referenced to, then any positive voltage on the cathode -- relative to ground -- is the same thing as the grid being negative in potential relative to the cathode's potential.
However, the typical Fisher receivers in question here do not employ cathode bias, but rather, use fixed bias for their push-pull output stages. In this scheme, the cathodes are grounded, and the grids are connected to a negative voltage source of appropriate value below ground potential. While the basic effect is the same as with cathode bias (in that the grids operate at a negative potential with respect to the cathode), the bias voltage does not change with changing current flow through the tube, and so the bias is said to be "fixed".
The grid return resistors discussed are the resistors that connect the control grids to either ground (in cathode biased amplifiers), or the negative voltage source (in fixed bias amplifiers). There are limits as to how large these resistors can be in value, and that is the discussion of point #1 in post 19.
The small 10 ohm resistors discussed in point #3 of that post are much to small to supply any effective bias for the tubes. Yes, they are a cathode resistor of sorts, but only big enough to provide a usable reading to determine current flow -- not establish any working bias for the tube. Therefore, the effects of their inclusion in the cathode circuit of the output tubes may completely be discarded from a performance point of view.
I hope this helps answer your questions!
Dave
If so, then:
As your description explained, in a cathode biased amplifier, the control grid is made more negative than the cathode by referencing the grid to ground, and then elevating the cathode to some positive potential above ground by allowing current flow through the tube to produce a voltage drop across a resistance connected between the cathode and ground. Since ground is the most negative point of the circuit in this case, and the point to which the grid is referenced to, then any positive voltage on the cathode -- relative to ground -- is the same thing as the grid being negative in potential relative to the cathode's potential.
However, the typical Fisher receivers in question here do not employ cathode bias, but rather, use fixed bias for their push-pull output stages. In this scheme, the cathodes are grounded, and the grids are connected to a negative voltage source of appropriate value below ground potential. While the basic effect is the same as with cathode bias (in that the grids operate at a negative potential with respect to the cathode), the bias voltage does not change with changing current flow through the tube, and so the bias is said to be "fixed".
The grid return resistors discussed are the resistors that connect the control grids to either ground (in cathode biased amplifiers), or the negative voltage source (in fixed bias amplifiers). There are limits as to how large these resistors can be in value, and that is the discussion of point #1 in post 19.
The small 10 ohm resistors discussed in point #3 of that post are much to small to supply any effective bias for the tubes. Yes, they are a cathode resistor of sorts, but only big enough to provide a usable reading to determine current flow -- not establish any working bias for the tube. Therefore, the effects of their inclusion in the cathode circuit of the output tubes may completely be discarded from a performance point of view.
I hope this helps answer your questions!
Dave
tonyg88
Super Member
I got my reciever back after being worked on by ascernman and he did it amazing job! I first would like to thank everyone for giving him the info that was needed to do the mods to make it sound great! It does sound alot better then when I first got it... Hard to say which mod made it sound the best but also the output tubes are now westinghouse originals. I was using EH before.
30ma to a 7591 is pretty much loafing along. Kinda like running an engine with it in the best area of the powerband (Torque). You get performance AND better Fuel Economy than it would out of the powerband.
The Westies should do well there for a fair amount of time. But you also have to consider what their previous use was. You never know unless you physically know the history on them (ie: they were your dad's and he babied them the whole time). But taking into consideration that most of the 7591 amps/receivers made ran them at the ragged edge, they have some miles on them. Me personally, I'd keep a closer eye on them than most other tubes. But they should do good for a couple years provided you don't abuse them.
Larry
Glad you got it back and are enjoying it now! Larry summed up the tube life question perfectly. I know you will enjoy your Fisher for a long time to come.....
Dave
Dave
Glad you're back in action Tony. As I was working on the case for my receiver today, I started wondering whatever happened to that dude who almost fried his unit a while back. Glad you got it into good hands and it's now performing up to spec. Enjoy!!!! 
Hi Dave!
So we are using going to use 182K ohm resisters for our grids (from Mr. McShane I think)
Are those .1uF Russian caps OK to use for our coupling caps with this value?
(I can't believe I am saying these things!
)I hope everything is GREAT with you!!!!!!!! :thmbsp:
Phyllis
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