Detail Fisher TA-600 Rebuild
I was asked to share the rebuild.
The Fisher TA-600 receiver is built like a tank. The power transformer appears to be 400-500 VA and the audio transformers are generous for a pair of 7189 (6BQ5 with higher ratings).
The power supply makes use of two rectifier tubes. A GZ34 for the amplifier and a EZ80 for the receiver section. The power supplies are unusual with 150 ohms resistor installed after the rectifier and before the 60uF main filter capacitor. The receiver uses 47 ohms resistance in-between the rectifier and first filter capacitor. Although 60uF performs, I prefer better filtering of the high voltage. Per PSUII power supply designer, 11 volts of AC ripple exists. The high voltage is at 410 volts.
I rebuilt the power supply using a 10uF @ 370 VAC motor run type filter cap at the rectifier output. To fit in the location, the capacitor is not a can type and only one inch in diameter. To maintain a high voltage output of approximately 400 volts, a series resistor of 250 ohms @ 25 watts was placed at the capacitor negative lead to ground. Without the series resistor, the voltage would soar to approximately 450 volts. The output is now a more conservative 390 volts. The 10uF capacitor is a poly type to handle the higher ripple current. Using a 10uF electrolytic in this location may overheat a small value capacitor.
Next, room was found at the under chassis can capacitor that is held with a clamp. The can capacitor was replaced with two 47uF & two 22uF capacitors rated at 450 volts DC. Original was 30uF-30uF-10uF-10uF. The left over space allowed space for a 1HY choke with a DCR of 50 ohms for a new CLC filter.
The two above chassis can capacitors were replaced with CE manufacture 40uF-40uF-20uF-20uF rated at 525 volts DC. I parallel two 40uF sections for 80uF effective capacitance for the high voltage CLC filter of 10uF-1HY-80uF. The rest of the can capacitor sections were used for the other existing circuits. The AC ripple is now approximately 3 volts with the new filter design.
I have never been a fan of the 7199 for a preamp/phase splitter. Purchasing 6GH8 to 7199 socket adapters, a pair of Amperex 6BL8 tubes were installed in the adapters. The 6BL8 shares same pin out with the 6GH8. The 7199 triode section has a gm of 40 and the 6BL8 of only 20gm. Enough gain is available as to not under drive the circuit.
Imagine a film being lifted from a window pane. You would not know the film was in place until removed. That is how the Amperex 6BL8 sound compared to the RCA 7199s.
I was fortunate to find my favorite 6BQ5 family of tubes in place being GE manufacture.
Rolling various tubes I would have believe my favorite RCA 12AX7 tall black plate the best performer. The Raytheon 12AX7 tall black plate won out in the preamp and tone circuits. The phono section may likely be best served with Brimar CV4035, however Westinghouse 12AX7 narrow short plate will serve well here for about $150 less per pair.
Four of the 12AX7s have cathode bypass capacitors. I replace the old capacitors with Elna Silmic II. I find the Silmic II to sound as good as the discontinued Black Gate capacitors. The 7189 cathode bypass was replaced with a Elna Silmic II of 220uF @ 100 volts DC. The well matched 7189s did not require individual bias. This is a call here for a single or dual capacitors serving each pair.
All coupling capacitors were changed to K40Y-9 paper/oil type. The capacitors are true foil, not metalized capacitors.
The rebuilt TA-600 is a beautiful sounding receiver. The new case is just beautiful. This TA-600 is a very clean example and a pleasure to upgrade.
