sony6060
Super Member
I like the Fisher 400. The audio is just stunning. Rolled a few tubes, sounds great. The amp section does not need any upgrades such as boutique coupling caps. If it sounds good, do not fix it.
To ensure the Fisher 400 performs, I rebuilt the power supply. 50 year old electrolytics are a risk. I also noticed the can type capacitor next to the transformer operated very hot. Although picking up heat from the transformer, still was overly hot due to excessive leakage current.
I used a software based power supply simulator and found the factory stock power supply produced over 20 volts of ripple on the high B+. Part is due to the unbalanced voltage doubler using unmatched 200uF & 100uF in series. Fisher did this due to the first cap (200uF) does supply 1/2 wave DC to lower voltage circuits. However, the current draw is minimal. Calculating a new design power supply using much higher proposed capacitance on a power supply simulator, the peak to average current from the power transformer only showed minimal difference. Excessive high peak & short duration capacitor charging current can lead to excessive transformer heating.
The new power supply uses CDE 680uF @ 400 volt capacitors in series in the doubler circuit for effective 340uF at the high B+. Some manufactures such as Dynaco ST70 makes use of a small choke after the first filter section for a C-L-C filter feeding the power tubes. This is called a capacitor-inductor-capacitor (CLC) filter. I designed a 340uF-1HY-10uF filter. The new filter provides less than .6 volts AC ripple on the B+ (high voltage) for the power tubes. I also prefer a polyproplyene cap in the last section for best possible sonics. I used ARC manufacture vs Solen manufacture that I do not favor. The non-polarized ARC 10uF capacitor is rated 330 volts AC as viewed in picture below. To conservatively convert the AC rating to DC I calculate 330 volts AC (x) 1.4= 462 volts DC rating. 462 volts DC is well within safe operating parameters in the Fisher circuit. The choke I used is a Triad C24X rated at 240ma and 50 ohms DCR. The Triad C24X is available at Mouser for $8.77. The choke is hi-pot tested to 1500 volts. The choke does not spec maximum DC volt rating, but should be well within parameters to operate at 420 volts DC plus being wound on a plastic bobbin provides for extra insulation. The choke drops only 7 volts B+ (high voltage) with the Fisher 400 current draw.
I also rebuilt the bias supply. In stock form the bias was only supplying -16.6 volts. I added two 4700uF caps & reduced the 1K resistor that feeds the last filter section 100uF capacitor to 680 ohms. I also replaced the stock 100uF capacitor with a new capacitor rated 100uF @ 35 volts DC. I prefer to retain the factory additional bias filter section some simply remove. The simulated AC ripple from the rebuilt bias supply is approximately 1 millivolt or well within low imposed hum parameters for the sensitive 7868 high gain power tube grids.
Now the 7868 tubes enjoy -19 volts bias and operate cooler plus less stress on the power & audio transformers too. The expensive NOS RCA black plate 7868s I use will last longer. Some prefer -20 to -21 volts bias, however at 19 volts bias & 410 volts plate including 370 volts screen voltage the plate curves indicate 30ma plate current. That equals 12.3 watts plate dissipation with a 19 watt rated plate. The rest of can type capacitors (2) were replaced with 4 section 30uF @ 475 volts can type capacitors available from Antique Radio Supply. On one replacement multisection can type capacitor I changed from the factory 40-40-40-20uF sections to 30uF each for no other reason than the capacitor values was available off the shelf. The upgraded PS doubler section provides more pure DC to this new capacitor. In other words less capacitance did not exceed the Fisher 400 factory stock AC ripple content to other circuits. The other can type capacitor I replaced that now 'lost' the 200uF doubler capacitor section for 680uF, I simply paralleled two 30uF sections for 60uF vs the factory designed 50uF. I also changed the rectifiers to two series UF4007s in each leg now providing a conservative 2000 PIV rating. UF4007 rectifiers are low noise type diodes. I also removed the original power B+ shunt capacitor for clipping spikes and replaced it with a .01uF @ 1KV ceramic cap. Also, this power supply modification must add a C90 thermistor in the 120 volt AC line to limit start up inrush current that would be excessive with 680uF capacitors installed.
The Fisher 400 with new power supply provides the same sonic signature as original with now a slightly tighter bass response, brighter overall with the slight darkness removed and more airy presentation in a blacker background. The difference is not a 'night & day' change, but is sonically noticeable. Also interesting is the treble control does not need to be fully maxed out now. The sonics are more crisp with the power supply upgrades. The new can capacitors operate cooler & the power transformer also operates cooler with the higher bias on the 7868 tubes. The 1-2 volt higher 7868 bias from factory specs negates the slightly higher peak to average current demand of the new 680uF main capacitors. I avoid drill holes in a collectable vintage chassis, plus removing the power transformer for new mounting screws would be difficult. I simply soldered the new choke to the grounding lug on the lug type solder terminals. Although installed a little askew, the choke is hidden under the chassis. A 200 watt soldering gun or pencil is needed for this task.
Enjoy your Fisher 400 with a superior power supply. The Fisher engineering team back then could only dream about the high performance capacitors available today.
To ensure the Fisher 400 performs, I rebuilt the power supply. 50 year old electrolytics are a risk. I also noticed the can type capacitor next to the transformer operated very hot. Although picking up heat from the transformer, still was overly hot due to excessive leakage current.
I used a software based power supply simulator and found the factory stock power supply produced over 20 volts of ripple on the high B+. Part is due to the unbalanced voltage doubler using unmatched 200uF & 100uF in series. Fisher did this due to the first cap (200uF) does supply 1/2 wave DC to lower voltage circuits. However, the current draw is minimal. Calculating a new design power supply using much higher proposed capacitance on a power supply simulator, the peak to average current from the power transformer only showed minimal difference. Excessive high peak & short duration capacitor charging current can lead to excessive transformer heating.
The new power supply uses CDE 680uF @ 400 volt capacitors in series in the doubler circuit for effective 340uF at the high B+. Some manufactures such as Dynaco ST70 makes use of a small choke after the first filter section for a C-L-C filter feeding the power tubes. This is called a capacitor-inductor-capacitor (CLC) filter. I designed a 340uF-1HY-10uF filter. The new filter provides less than .6 volts AC ripple on the B+ (high voltage) for the power tubes. I also prefer a polyproplyene cap in the last section for best possible sonics. I used ARC manufacture vs Solen manufacture that I do not favor. The non-polarized ARC 10uF capacitor is rated 330 volts AC as viewed in picture below. To conservatively convert the AC rating to DC I calculate 330 volts AC (x) 1.4= 462 volts DC rating. 462 volts DC is well within safe operating parameters in the Fisher circuit. The choke I used is a Triad C24X rated at 240ma and 50 ohms DCR. The Triad C24X is available at Mouser for $8.77. The choke is hi-pot tested to 1500 volts. The choke does not spec maximum DC volt rating, but should be well within parameters to operate at 420 volts DC plus being wound on a plastic bobbin provides for extra insulation. The choke drops only 7 volts B+ (high voltage) with the Fisher 400 current draw.
I also rebuilt the bias supply. In stock form the bias was only supplying -16.6 volts. I added two 4700uF caps & reduced the 1K resistor that feeds the last filter section 100uF capacitor to 680 ohms. I also replaced the stock 100uF capacitor with a new capacitor rated 100uF @ 35 volts DC. I prefer to retain the factory additional bias filter section some simply remove. The simulated AC ripple from the rebuilt bias supply is approximately 1 millivolt or well within low imposed hum parameters for the sensitive 7868 high gain power tube grids.
Now the 7868 tubes enjoy -19 volts bias and operate cooler plus less stress on the power & audio transformers too. The expensive NOS RCA black plate 7868s I use will last longer. Some prefer -20 to -21 volts bias, however at 19 volts bias & 410 volts plate including 370 volts screen voltage the plate curves indicate 30ma plate current. That equals 12.3 watts plate dissipation with a 19 watt rated plate. The rest of can type capacitors (2) were replaced with 4 section 30uF @ 475 volts can type capacitors available from Antique Radio Supply. On one replacement multisection can type capacitor I changed from the factory 40-40-40-20uF sections to 30uF each for no other reason than the capacitor values was available off the shelf. The upgraded PS doubler section provides more pure DC to this new capacitor. In other words less capacitance did not exceed the Fisher 400 factory stock AC ripple content to other circuits. The other can type capacitor I replaced that now 'lost' the 200uF doubler capacitor section for 680uF, I simply paralleled two 30uF sections for 60uF vs the factory designed 50uF. I also changed the rectifiers to two series UF4007s in each leg now providing a conservative 2000 PIV rating. UF4007 rectifiers are low noise type diodes. I also removed the original power B+ shunt capacitor for clipping spikes and replaced it with a .01uF @ 1KV ceramic cap. Also, this power supply modification must add a C90 thermistor in the 120 volt AC line to limit start up inrush current that would be excessive with 680uF capacitors installed.
The Fisher 400 with new power supply provides the same sonic signature as original with now a slightly tighter bass response, brighter overall with the slight darkness removed and more airy presentation in a blacker background. The difference is not a 'night & day' change, but is sonically noticeable. Also interesting is the treble control does not need to be fully maxed out now. The sonics are more crisp with the power supply upgrades. The new can capacitors operate cooler & the power transformer also operates cooler with the higher bias on the 7868 tubes. The 1-2 volt higher 7868 bias from factory specs negates the slightly higher peak to average current demand of the new 680uF main capacitors. I avoid drill holes in a collectable vintage chassis, plus removing the power transformer for new mounting screws would be difficult. I simply soldered the new choke to the grounding lug on the lug type solder terminals. Although installed a little askew, the choke is hidden under the chassis. A 200 watt soldering gun or pencil is needed for this task.
Enjoy your Fisher 400 with a superior power supply. The Fisher engineering team back then could only dream about the high performance capacitors available today.
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