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#1
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Hi folks,
What is the reason the Fisher 30-A uses (very) different valued grid resistors for the power tubes? - R9 = 2K2 for V2 - R4 = 33K for V3 https://www.flickr.com/photos/derekv...n/photostream/ A later (or earlier?) 30-A version has no grid resistors, also no balance circuit for the phase inverter. |
#2
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On 2020-03-01 00:56, G.Re wrote:
Hi folks, What is the reason the Fisher 30-A uses (very) different valued grid resistors for the power tubes? - R9 = 2K2 for V2 - R4 = 33K for V3 https://www.flickr.com/photos/derekv...n/photostream/ A later (or earlier?) 30-A version has no grid resistors,Â* also no balance circuit for the phase inverter. screen grid? probably did a 'cost reduction' version, and different resistor values (in your case) as a hard-set "balance" of sort. Also see if the output transformer is actually driving the screens in 'ultra-linear' mode or not. Makes all the difference in the world for screen circuit if it's ultra-linear... which may have NO screen resistors - lots of things dependent on one another in design, screen current limit, screen volt limit, whether or not it's ultra-linear and where the taps are, supply voltage, bias, yotta yotta. -- (aka 'Bombastic Bob' in case you wondered) 'Feeling with my fingers, and thinking with my brain' - me 'your story is so touching, but it sounds just like a lie' "Straighten up and fly right" |
#3
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![]() "Big Bad Bob" wrote in message ... On 2020-03-01 00:56, G.Re wrote: Hi folks, What is the reason the Fisher 30-A uses (very) different valued grid resistors for the power tubes? - R9 = 2K2 for V2 - R4 = 33K for V3 https://www.flickr.com/photos/derekv...n/photostream/ A later (or earlier?) 30-A version has no grid resistors, also no balance circuit for the phase inverter. * Typo, R4 must be R11 screen grid? * No, R9(2K2) & R11(33K) are control grid resistors, see the schematic. The schematic linked above is not accurate, here's a better one ... http://www.fisherconsoles.com/non%20...030-a%20sm.pdf ... but for the question it does not make a difference. probably did a 'cost reduction' version, and different resistor values (in your case) as a hard-set "balance" of sort. * That seems unlikely since there's a balance pot (R2). Also see if the output transformer is actually driving the screens in 'ultra-linear' mode or not. Makes all the difference in the world for screen circuit if it's ultra-linear... which may have NO screen resistors - lots of things dependent on one another in design, screen current limit, screen volt limit, whether or not it's ultra-linear and where the taps are, supply voltage, bias, yotta yotta. * No ultra linear mode here, see the shematic. -- (aka 'Bombastic Bob' in case you wondered) 'Feeling with my fingers, and thinking with my brain' - me 'your story is so touching, but it sounds just like a lie' "Straighten up and fly right" |
#4
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G.Re wrote:
Hi folks, What is the reason the Fisher 30-A uses (very) different valued grid resistors for the power tubes? - R9 = 2K2 for V2 - R4 = 33K for V3 https://www.flickr.com/photos/derekv...n/photostream/ A later (or earlier?) 30-A version has no grid resistors, also no balance circuit for the phase inverter. When the EL84 start pulling grid current the difference of impedance of the cathode side and the anode side of the phase splitter comes into play. To balance the grid current the anode side grid resistor is of lower value than the cathode side resistor. That is careful design. The engineers probably found out that, for consumer grade equipment, the distortion due to this effect are negligable compared to the the distortion due to overload and canceled the resistors to save some cents. Kind regards, Eike |
#5
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Eike Lantzsch, ZP6CGE wrote:
G.Re wrote: Hi folks, What is the reason the Fisher 30-A uses (very) different valued grid resistors for the power tubes? - R9 = 2K2 for V2 - R4 = 33K for V3 https://www.flickr.com/photos/derekv...n/photostream/ A later (or earlier?) 30-A version has no grid resistors, also no balance circuit for the phase inverter. When the EL84 start pulling grid current the difference of impedance of the cathode side and the anode side of the phase splitter comes into play. To balance the grid current the anode side grid resistor is of lower value than the cathode side resistor. That is careful design. The engineers probably found out that, for consumer grade equipment, the distortion due to this effect are negligable compared to the the distortion due to overload and canceled the resistors to save some cents. Kind regards, Eike not good etiquette to answer to ones own posting ... anyway in the 70ies, when still an adolecent, I built an amplifier of very similar design with ECC83 and EL84. I also put the two unequal grid resistors in. Only difference with my amp was that each EL84 has its own cathode resistor and cap. This amplifier still does service here after almost 50 years but after 2x restoration of course. Kind regards, Eike |
#6
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On 2020-03-03 01:08, G.Re wrote:
"Big Bad Bob" wrote in message ... On 2020-03-01 00:56, G.Re wrote: * No, R9(2K2) & R11(33K) are control grid resistors, see the schematic. The schematic linked above is not accurate, here's a better one ... http://www.fisherconsoles.com/non%20...030-a%20sm.pdf ... but for the question it does not make a difference. oh I get it. yeah flickr links are most likely "scripty" and I don't do them (as in 'must I allow script and lower my security JUST to view this?') I also noticed a fixed screen voltage supply with NO series resistors Grid series resistors like those are typically there to limit grid current in an AB2 situation. However, different values MIGHT introduce non-linearity when it starts to clip. I have to wonder why they would do this. The balancer pot would allow you to adjust for different gain factors on the tubes as well as imprecise resistor values in the cathode and anode circuits. So no indicator as to why they did different series resistors on the control grids... |
#7
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On 2020-03-03 02:18, Eike Lantzsch, ZP6CGE wrote:
When the EL84 start pulling grid current the difference of impedance of the cathode side and the anode side of the phase splitter comes into play. To balance the grid current the anode side grid resistor is of lower value than the cathode side resistor. That is careful design. The engineers probably found out that, for consumer grade equipment, the distortion due to this effect are negligable compared to the the distortion due to overload and canceled the resistors to save some cents. interesting. normally if you care enough to add the series resistors on the output tubes, you'd have both halves of a 12AU7 driving them so that it isolates grid current issues from the splitter circuit. But yeah that's one more tube in the design, and extra $$. For a guitar amp, I'd do it this way (with the extra 12AU7) becaue a 'totem pole' type of splitter does really weird things at the overload point. if you WANT that sound, go with it. Otherwise, the somewhat cleaner "final stage distortion" of the splitter - 12AU7 - power tube combination would be better. Also a series resistor on the splitter's grid would be needed. Then you'll have very nice predictable behavior at the clipping point, and WAY beyond that. aside from that, the different resistor values on the grid resistors is a bit silly, probably why they were just removed, later. "Muntzed" -- (aka 'Bombastic Bob' in case you wondered) 'Feeling with my fingers, and thinking with my brain' - me 'your story is so touching, but it sounds just like a lie' "Straighten up and fly right" |
#8
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![]() "Eike Lantzsch, ZP6CGE" wrote in message ... G.Re wrote: Hi folks, What is the reason the Fisher 30-A uses (very) different valued grid resistors for the power tubes? - R9 = 2K2 for V2 - R4 = 33K for V3 https://www.flickr.com/photos/derekv...n/photostream/ A later (or earlier?) 30-A version has no grid resistors, also no balance circuit for the phase inverter. When the EL84 start pulling grid current the difference of impedance of the cathode side and the anode side of the phase splitter comes into play. To balance the grid current the anode side grid resistor is of lower value than the cathode side resistor. That is careful design. The engineers probably found out that, for consumer grade equipment, the distortion due to this effect are negligable compared to the the distortion due to overload and canceled the resistors to save some cents. Kind regards, Eike * Thanks Eike, that makes sense. I had a closer look at both the early & late version schematics. The earlier version is dated 1958-xx, it shows no EL84 control grid resistors nor balance pot. The later version is dated 1959-10, it shows (different) control grid resistors plus balance pot. Both mentioned dates are the documention printer's (H.W. Sams) date. Thats no proof of Fisher's 30-A versions issue order but it's likely documentation issue and 30-A issue are in the same order. So, probably the change was to gain amplifier performance. Best regards, Gio. |
#9
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![]() "Big Bad Bob" wrote in message ... On 2020-03-03 01:08, G.Re wrote: "Big Bad Bob" wrote in message ... On 2020-03-01 00:56, G.Re wrote: * No, R9(2K2) & R11(33K) are control grid resistors, see the schematic. The schematic linked above is not accurate, here's a better one ... http://www.fisherconsoles.com/non%20...030-a%20sm.pdf ... but for the question it does not make a difference. oh I get it. yeah flickr links are most likely "scripty" and I don't do them (as in 'must I allow script and lower my security JUST to view this?') I also noticed a fixed screen voltage supply with NO series resistors * The earlier version of this amp has 68Ω screen stoppers, no idea why they left them out in this (later) version. Grid series resistors like those are typically there to limit grid current in an AB2 situation. However, different values MIGHT introduce non-linearity when it starts to clip. I have to wonder why they would do this. The balancer pot would allow you to adjust for different gain factors on the tubes as well as imprecise resistor values in the cathode and anode circuits. So no indicator as to why they did different series resistors on the control grids... |
#10
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Big Bad Bob wrote:
On 2020-03-03 02:18, Eike Lantzsch, ZP6CGE wrote: When the EL84 start pulling grid current the difference of impedance of the cathode side and the anode side of the phase splitter comes into play. To balance the grid current the anode side grid resistor is of lower value than the cathode side resistor. That is careful design. The engineers probably found out that, for consumer grade equipment, the distortion due to this effect are negligable compared to the the distortion due to overload and canceled the resistors to save some cents. interesting. normally if you care enough to add the series resistors on the output tubes, you'd have both halves of a 12AU7 driving them so that it isolates grid current issues from the splitter circuit. But yeah that's one more tube in the design, and extra $$. Yes that would be a better design. For more costly amps with more oomph it was applied - such amps equipped with EL34 or KT88 and the likes. But for the more economic ones with around 15W output and EL84 tubes the engineers usually didn't do that. But the latter were good amps anyway. For a guitar amp, I'd do it this way (with the extra 12AU7) becaue a 'totem pole' type of splitter does really weird things at the overload point. if you WANT that sound, go with it. Otherwise, the somewhat cleaner "final stage distortion" of the splitter - 12AU7 - power tube combination would be better. Also a series resistor on the splitter's grid would be needed. Then you'll have very nice predictable behavior at the clipping point, and WAY beyond that. aside from that, the different resistor values on the grid resistors is a bit silly, probably why they were just removed, later. "Muntzed" Kind regards, Eike |
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