Discussion:
Fisher 30-A amp
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G.Re
2020-03-01 08:56:10 UTC
Permalink
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/derekva/7211152916/in/photostream/

A later (or earlier?) 30-A version has no grid resistors, also no balance
circuit for the phase inverter.
Big Bad Bob
2020-03-01 22:14:18 UTC
Permalink
Post by G.Re
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/derekva/7211152916/in/photostream/
A later (or earlier?) 30-A version has no grid resistors,  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"
G.Re
2020-03-03 09:08:42 UTC
Permalink
Post by G.Re
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/derekva/7211152916/in/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%20console%20manuals/fisher%2030-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"
Big Bad Bob
2020-03-05 12:12:30 UTC
Permalink
Post by G.Re
* 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%20console%20manuals/fisher%2030-a%20sm.pdf
Post by G.Re
... 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...
G.Re
2020-03-05 14:19:07 UTC
Permalink
Post by G.Re
* 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%20console%20manuals/fisher%2030-a%20sm.pdf
Post by G.Re
... 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...
Eike Lantzsch, ZP6CGE
2020-03-03 10:18:35 UTC
Permalink
Post by G.Re
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/derekva/7211152916/in/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
Eike Lantzsch, ZP6CGE
2020-03-03 10:30:00 UTC
Permalink
Post by Eike Lantzsch, ZP6CGE
Post by G.Re
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/derekva/7211152916/in/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
Big Bad Bob
2020-03-05 12:21:11 UTC
Permalink
Post by Eike Lantzsch, ZP6CGE
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"
Eike Lantzsch, ZP6CGE
2020-03-08 10:39:11 UTC
Permalink
Post by Big Bad Bob
Post by Eike Lantzsch, ZP6CGE
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.
Post by Big Bad Bob
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

G.Re
2020-03-05 12:50:31 UTC
Permalink
Post by G.Re
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/derekva/7211152916/in/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.
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