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Generalization for rebuilding vintage PP amps

Posted by Neff on June 15, 2008 at 06:07:12:
As you stated replace all the capacitors and out of spec resistors. JJ makes a multi-section can type capacitor that are ok for that factory stock look. I would use Panasonic capacitors everywhere else if Solen polyproplyene type are too large for the PS sections. I use Black Gate for the power tube cathode bypass caps when Solen will not fit due to the large size. However, I do not like Black gate on small signal tubes. The Panasonic FC series are an inexpensive alternate & a very good performer for any cathode bypass application.

I would install a choke in-between the rectifier tube and first cap. A new cap will be required at the rectifier tube for a CLC or voltage will be much lower. I use Vishey and Mills resistors. The resistor type is less important when one side is connected to ground vs series connected like a plate resistor.

I like Russian K40Y-9 capacitors in PP amps and very much so at the PP power tube grids. A .22uF is a good value at the power tube grids. (If the amp was single-ended I would suggest the Russian FT-3). You may want to try a few different coupling capacitors if the sonics is less than the quality you wanted to achieve. Remember, the first 50-200+ hours may not sound super, give the capacitors time to break-in.

Finally, 'rolling' various tubes are very important. I like older RCA 6L6G and suspect the HF-20 uses about 375 volts plate. If much higher, the 6L6GC types must be used. Watch that 6L6 cathode bias voltage & do not operate the tubes at a low bias point causing the 6L6 plates to glow dull red.

I do not like any 12AU7, but the Brimar 13D5 is ok. If you have the money, drop-in Amperex 6085/E80CC and hear a serious increase in performance. Another alternate is rewiring the sockets for another tube type such as the unfortunately rare Raytheon 6CG7 USA manufactured black plate with center shield or try some 12BH7s. I do believe the sixty-five or so plate volts at the 12AU7 input tube is a little low for highest performance regarding tube viewing various plate curves for the most linear operation. But, if the sound is pleasing, leave it as is.

One problem recommending components is the end result may not be perfect. However, for the most part I am likely on target here.