Sonic improvement would indeed focus on the tubes and coupling cap. That make sense. One would not include in refurbishing swapping out the output transformer; that would be like building something new, not a mere refurbish/upgrade. Other tweaking might include making the amp more compatible with certain tube variants.
Rebuilding Vintage Tube Amps for Best Sound
Rebuilding vintage tube amps and receivers for best sound requires a few rules to be followed. Replace all electrolytic capacitors. Power tube HV capacitors should use highest capacitance that will fit. For voltage doublers I use two 1000uF 250 volt capacitors. Solid state use 470uF 450 or 500 volt depending upon voltage. Tube rectifiers present an issue as 47uF is maximum capacitance that can be used. Place a 50 ohm resistor on 47uF high voltage to another 270uF capacitor. Push Pull should have less than 8 volts AC ripple and Single Ended less than 2 volts AC ripple. The other HV capacitors can be the original value. I would use Audio Grade cathode bypass caps and poly type capacitors on power tubes with cathode bypass caps. 22Uf is sufficient for each tube or pair of tubes and 47uF if one cap used on four tubes.
Coupling capacitors are very important for best sound. I would only use paper in oil type capacitors. The Russian K40Y-9 are best IMO. These caps are getting somewhat scarce in 400 to 500 volts. Any tube with less then 200 volts can use 200 volts rated. K40Y-9 except power rube grids that need 400 volts rated. Also, more common .033uF can replace 047uF. Above 200 volts on the tube use 400 volt rated.
If the amp uses diodes for HV, use Cree 600 colt rated nr voltage doubles or 1200 volt rated on full0wave CT or bridge rectifiers. Cree have zero switching noise like a tube. If amp power is over 15 watts per channel use a small heat sink on Cree rectifier.
Tubes make a large difference in sound quality, Per my experience I have a few suggestions. GE 6BQ5 or 7189A for voltages over 350 volts. Although 6BQ5 are rated 300 volts none have failed at under 350 volts plate. Gold Lion KT-88. These outperform vintage Tung Sol 6550 and can sub for 6550. 6L6 use RCA. 6BM8 use Siemens. EL34 use Svetlana as Mullard are way too expensive in NOS unless you have a deep wallet then use Mullard. 6AN8 use Sylvania. 12AX7 use JJ E83CC. This frame grid tube is better than $200 each NOS Mullatd 12AX7 long plate. Sounds same as Mullard but extends farther into the listening room.12AU7 use CBS/Hytron 12AU7 or 5814A.
Finally check carbon resistors for proper resistance.
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@jimbennet I've had a number of such caps develop a slight amount of electrical leakage; IOW exhibit a bit of output DC Voltage. That can throw off the bias point in an output section, which could damage valuable parts like the power transformer if not caught. I don't recommend using them. There are better parts out there anyway. |
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