Improving the Fisher 400

Hi Dave

Could you inform me the caps values (there in uF or pF?) onto the last drawing(control section).

Thank you!!!
 
Hi Luis --

The cathode caps are in uF, as are the coupling caps at the plate of each tube. The cap connected across the terminals of the Loudness Switch (which I forgot to label as .022) is also in uF. All of the remaining caps are in pF values.

Thanks again!

Dave
 
ok Dave

Another question: the coupling cap in paralell with the 220 k resistor after bypass switch for L channel is 33pF and the R channel is 34pF? am I correct?

Pls let me know!!

Thank you!!!
 
Luis -- They are different values for each channel to compensate for the different lengths of shielded cable used in the original build. While the left channel is in fact 33 pF, the right channel is 39 pF -- not 34 as you are indicating.

Dave
 
I'm very grateful for this thread and Sony6060's power supply upgrade thread, as well as Eduarsan's schematic update.

This collaboration is an example of why I've contributed not only my knowledge, but money to AK.
My feeling is that if I contribute what I know to educate others, I'll receive the same type of education in return. This thread (actually all four if we consider EFB in the Fisher SA-100 clone and in the Fisher SA-100 as well) shows that to the finest degree.

Eduarsan, can I impose on you to put dotted-line borders or some other type of flag for future readers around the schematic changes so we have a way to know where to look for the changes from stock in at least one version?

I know I'm going to print it out later and now I know where the changes are, but later I may not and those who stumble across this later also won't know where to look.
Links to the relevant threads:
Power Supply: http://www.audiokarma.org/forums/showthread.php?t=500753&highlight=power+supply+update

EFB in the Fisher 400: http://www.audiokarma.org/forums/showthread.php?t=509090&highlight=power+supply+update

I performed a minor change to my upgraded power supply. Schematic shows the 10uF capacitor connected to the high voltage after the choke and ground side connected to top (positive) of lower 680uF cap. I changed that 10uF capacitor negative side connection from 680uF cap to ground.

Note that the small 10uF capacitance is only acceptable with a poly type capacitor with regard to ripple current. If an electrolytic type capacitor is substituted, increase the capacitor capacitance to 47uF @ 450 VDC. A small 10uF electrolytic capacitor does not handle ripple current near as well as a 47uF capacitor or any poly type. Thanks.

I will review your CAD design print. Very cool work.
 
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Well this is a first. This is the first time I have seen someone actually do the work required to bypass the tone controls. Usually a "minimalist" will just eliminate the controls with the expected disastrous results.

Well done Dave.
 
Can I take a liberty on Dave's PI design with regard to components? I assume ok.

Should the plate and cathode resistor on the cathodyne side be one watt?

Also, after testing a few electrolytic capacitors I notice sonic differences even in a cathode bypass position. I recommend 22uF Elna Silmic II in that position for likely best sonics. I may be 'picking hairs', but a hand full adds up.
 
All input is appreciated! In the end, the cathode bypass caps solved far more problems that they created, so in they went. Parts recommendations are always welcomed. The benefits of their inclusion are that:

1. They increase the negative voltage feedback, and

2. Eliminated the negative current feedback, which

3. Lowers the output impedance -- which is always a good thing when driving capacitive loads like shielded cable and tone stacks.

4. They also minimize performance differences between different tubes and their individual sections, and

5. Actually allows the unit to now meet its published Aux input sensitivity of 280 mv to produce full power output.

1 watt components certainly wouldn't hurt in the PI section. The existing plate resistor dissipates just under .20 watt for a healthy 60+% operating safety margin, and the cathode components have even a slightly greater margin of safety. Still for best temperature stability, 1 watt components would be even better.

Dave
 
Dear all

@Sony6060 the PS supply is added!! so let me know is ok.

I´m working on the control section modification.

Regards!!!


Luis
 
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Sony; Considering the 800-C, 500-C and the 400 are very similar, what would be the chances of success incorporating the P.S. mods to an 800-C. And what values would need to be changed if any? I'm thinking of putting (eventually by the end of the year) the EFBII, Buffer Circuit, into my 800-C. It's already modded with the IBAM, .1uf caps, and 200K grid resistors, 100ohm screen resistors, 10oHm cathode resistors, Full recap, and 7591EH's. Small signal tubes are a mix of Tele's, Mullard's, RCA's and JAN Sylvania's. Tuner Tubes are all GE. (for some reason I like the GE 6AU6, 6BE6 and 6BA6. fisher 6HR6. My hearing isn't good enough that i can tell sonic differences between tubes and caps so I tend to ones that work, no boutique stuff. The only exception to that was the K40Y russian caps as they wer in when i got the unit.

Thanks
Larry
 
Sony and eduarsan1;

I just noticed something on the Filter caps layout There are 2 57A's. The 57B is paralleled with another 30uf cap. and named 57D. 57C is 680uf. Wouldn't it be better to just put a 60uf cap(or 68uf) single in that position and call it 57D?? Then the 2 57A's would be 57A & B with 57C being the 680uf and 57D being a 60 or 68uf?

Could you use a 33uf for the 30's, a 68 single for the DUAL 30's, and a 680 for the Voltage Doubler? Or am I blowing smoke out of my ass again?

Larry
 
Larry thank you for the corrections.

I´ve made the corrections in the caps tags for the 680uF and the 30uF as well. The reason to use four 30uF caps is due to are coming in one single can and there are the values.

If is possible to use 33uF and 68uF to replace the 30uF? Off course you can.

For my side I don´t want to replace the original caps, my idea is keep them and put the new caps in the bottom, I´ll use 33uF caps, the others still the same as sony says.

I´ve attached the corrected drawings.

Regards
 

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Sony and eduarsan1;

I just noticed something on the Filter caps layout There are 2 57A's. The 57B is paralleled with another 30uf cap. and named 57D. 57C is 680uf. Wouldn't it be better to just put a 60uf cap(or 68uf) single in that position and call it 57D?? Then the 2 57A's would be 57A & B with 57C being the 680uf and 57D being a 60 or 68uf?

Could you use a 33uf for the 30's, a 68 single for the DUAL 30's, and a 680 for the Voltage Doubler? Or am I blowing smoke out of my ass again?

Larry

No smoke at all. Like Dave said, no question or input is less than welcomed. That is a sign of a good engineer with priorities (good engineering).

My restoration used can type caps from Antique Electronic Supply called CE brand to keep the factory stock appearance. The capacitor values I used that were cans with four 30uF sections. Those cans were not too tall for the cabinet. A 33uF and 68uF is fine business. In fact, capacitors varied from -10% to as much as +50% of advertised capacitance during the 1960s. New capacitors usually have much tighter tolerances now.
 
Epilog

The 400 has been all buttoned back up and is at bat in the current rotation in my listening room. This has been a very involved project, starting with simply trying to install EFB(tm) into it to help verify a universal EFB design I am working to launch. But, we all know that projects don't always go as planned.

For me personally, this project has taken a unit I had little affection for, and made it near and dear to my musical heart. It wasn't just because the sound didn't captivate me, but with the ugly knobs, a level control that had no clue of what the meaning of balance is, and lousy FM in Atlanta, there was just no motivation to make anything of it. Frankly, choosing it as a candidate to try out elements of my universal EFB design on lead to all the other modifications, that along with cleaning it up and installing a decent level control, has given me a Fisher I love to look at and listen to as well now -- and I've always wanted one. I've heard plenty, and worked on even more, but this one is a first for me.

For all the changes and modifications, because they are quietly hidden away and electrically installed into the original 400 circuit, you still look on this unit as being a Fisher 400, which I am most pleased about. I absolutely did not want this thing to become a shell of its former self, where the chassis is just a vehicle for an entirely new design. Rather, I wanted it to be the best it could be for what it was designed to be, and I believe that has been accomplished. Besides being much more usable and versatile in today's audio environment, the fact that it now has a superb line level control section, and equally superb power amplifier section, produces those listening sessions where you just shake your head in amazement at how crazy good it sounds. It remains quite cool temperature wise for a piece of vacuum tube equipment, even after many hours of operation, and all with the output tubes idling at 21 ma per tube.

Some final modification thoughts and observations include:

1. My line voltage here runs about 122 vac, but the power supply in my unit must be an over achiever, as the DC heater supply was delivering 28.0 vdc to the small signal tube heater strings. Even backing the line voltage down to a more normal 117 vac still produced nearly 27 volts to these heaters, which is still 7+% high, so an adjustment was in order. Adding a 10 ohm 2 watt resistor in series with the heaters brought that voltage down to 25.1 vdc, or darn near perfect. The addition of the resistor begged for an additional 1000 uF cap to be placed on the output side of this resistor, which reduced the ripple voltage at the heaters down to about 40 mv, for a very quiet DC heater supply.

2. Regarding ripple on the main B+, my unit with stock filter caps produced a measured ripple voltage of 5.3 volts peak at the main B+ supply point. Others have indicated the level here is much higher that this, but I believe that figure was produced from simulation, which I have no information on. In any event, the figure is reasonably low, no doubt aided by the reduced current draw that EFB affords the output stage to operate at, with the EFB screen grid regulator reducing the ripple voltage at the screens by about 40 times, for a peak ripple voltage of just .13 volts peak at that point. In any event, it is very hard to tell if the unit is on by simply listening up close to the soundboard of my Cornwalls: There just isn't any noise to be heard.

3. I had previously provided a pic of a 10 kHz square wave, as produced by the modified control section with the tone control circuits engaged with the display being achieved with the controls landing within 1/2 hour of a 12:00 noon setting. What I had totally forgotten is that many years ago when I first got this unit, I had electrically tweaked the tone controls themselves for the best on-center flat response at the time. Removing those tweaks had the controls off by as much as requiring a 2:00 setting for some of the controls for the best flat response, in any event, producing no better a display than I had posted earlier. The tweaks simply let my particular controls produce that display with the controls "closer to home". At any rate, this suggests that the tone control bypass option would be a particularly beneficial modification to install, correcting not only the less-than-flat response of the basic tone control circuit, but also potentially significant on-center response error from the controls themselves as well.

4. In Aux mode, the RF front end and first IF amplifier stage have their B+ disabled, allowing those tubes to cool down considerably. However, the remainder of the IF strip remains "hot" and produces considerable heat. If you live in an area where you do not use the FM regularly, you might consider simply removing the FM and multiplex tubes, as when they are all operating, they pull significant current -- about 60 ma off station -- which all told produces as much heat as three of the output tubes in the modified design. Also, it is not good to let tubes operate for long period without any B+ applied, so these are just some considerations to be aware of. In my unit the heater winding supplying the tuner section ran higher than the audio heater winding did -- with the audio winding providing 6.60 vac, and the tuner winding providing 6.75 vac (on a 122 vac line), which is getting up there. If you remove the tubes, the tuner winding also powers the heater of the Stereo Beacon tube and festoon panel lamps, which would make the voltage to them even higher. So in that case, a dropping resistor might be in order, or in order no matter what. I'm just putting the issue out there so you can be aware of it, and deal with it as is appropriate for you. Do note that in the earlier version of the 400, the opposite is true: In Aux mode, the IF strip is shut down, while the RF front end is kept alive.

Finally, since the circuit Luis is producing has had numerous additions added to it by other talented AKers, I am providing all the original schematics of my modifications, as developed in this and the EFB thread, here in this post. These schematics have been updated as well, but the updates do not represent any significant circuit or component changes. Rather, there are informational changes on all of them that speaks to accuracy of the finished project. The information also helps clarify insertion of these circuits into the original Fisher product. A final pic of the underside is also provided to show the look of the finished project.

Since each schematic represents a "module" then as it were, they collectively offer a number a excellent upgrades in performance, versatility, and extension of tube life, that would likely be appropriate for most of the Fisher Stereo receivers, and many of their integrated amplifier offerings as well.

Happy listening!

Dave
 

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I've been following this thread with interest.

Tonight I had the opportunity to visit Dave and to see and hear his modified 400 in his listening room.

Dave asked me for my honest opinion.

I told him that I had restored a number of Fishers, including the 500 and 800, as well as a 400. Although some were great cosmetic specimens, I decided not to keep them. To my ears they sounded a bit "dark" and as if "something were missing."

The real (subjective) test for me is to listen to a piece for hours after burn-in. If there is no listener fatigue, it's a winner, but if there is (and I give it a number of tests on different days to be sure), then it goes back onto the bench or out the door.

I had done all of the "conventional wisdom" upgrades such as replacing caps, making the bias circuit adjustable, etc. But nothing I did overcame the original objections.

Well, back to the listening test. Dave's 400 sounded like no other Fisher I had ever heard. It was detailed, musical, and lacked absolutely nothing. When he told me that it contained the original ceramic coupling caps, I was completely blown away. This is a great amp!

About 1/2 hour later, Dave asked me to feel the power XMFR and it was barely warm at all.

I have several amps with vintage (expensive) output glass, and the thought of using the EFB circuit to improve performance and extend the life of the tubes is exciting indeed.

GREAT JOB, DAVE!!

- BuzzK
 
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