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