Generally....
I recommend doing 1 project in isolation, then listening or measuring for results. I don't always do this, and occasionally get overzealous. Zeal can cause problems. If you can manage to do 1 item or area this will allow you to assess if there was a benefit from that particular operation. Also, if you accidentally CAUSE a problem it's much easier to find - aka reversing a diode can be a big oops and has been done previously
.
So, I think that diodes are a good 1st step and I did hear a minor improvement when I did this in isolation in my Jolida 302b. I think that any better quality diode is worth the effort. Diodes are relatively inexpensive, even the good stuff, so I suggest using them.
I never replaced C0 or C9 (Per your schematic). I didn't have C7 or C12 installed (as in your schematic), so I added a bypass filter capacitor(s) in this location. I didn't accomplish this task in isolation, so I can't convey the individual impact.
Resistors... Once upon a time I did a comparison between Eagle Wirewound, Mills Wirewound and... Ohmite ( I think ). They all had about the same voltage and current ratings and were all about the same physical size. I installed them behind a tweeter and listened. There was VERY little difference between them. Maybe the Mills resistor sounded better, and maybe the Eagle resistor sounded "worse", but... they were all very close. I decided to use the Mills resistor because I didn't want to memorize the color coded rings on the Eagle resistors, and I didn't want my customers making a mistake with the color coded rings. So... I used Mills.
More on Resistors.... several years after my initial test I did a comparison between the larger Mills MRA-12 (12 watt) and MRA-5 (5 watt) resistors. This swap was mildly audible and I immediately switched to using the smaller 5 watt resistors in all of my kits. The impact wasn't huge, but I thought it was mildly audible. The smaller wire-wound Mills resistor sounded better - at least to my ears. I never performed this comparison with a blindfolded friend, so it is simply my opinion.
In my opinion...
"The single most important, valuable component that any hifi enthusiast can and should modify in their hifi system are the coupling capacitors in their tube amps/circuits !" Dave Ellis . Nothing will approach the value that upgraded coupling caps will have in a tube circuit (tube, preamp, or power amp). When I first did this in my Anthem CD player I thought it was broken, then.... oh my... the music
.
So... the reason I sold my Jolida 302b was its El34 appetite. My 302b would eat a Svetlana "C" logo tube about ever 3-4 months, and it just became tedious. My accountant really wasn't happy with the cost of replacing vacuum tubes
. The AVA Ultimate 70 sounded better in the midrange and highs, and was vastly more reliable (Tube rectifier
). So.... my 302b was sold.
IF your Jolida 502 seems reliable and you are willing to live with it... the $ on coupling caps C1, C3, and C4 could be mild... Perhaps this might include a Sonicap Gen I or Auricap. Or you could go Mundorf Silver & Oil or perhaps Jupiter. I never have tried the Jupiter HT capacitors, but they sure seem appealing. The Mundorf Silver/Oil are darn fine capacitors. The Sonicap Platinum capacitors are becoming rare and expensive. if you decide on the better capacitors, the cost will be about $200 for just the capacitors. This is kinda painful, but if the amplifier is a keeper then it's money well spent - in my opinion.
http://www.humblehomemadehifi.com/Cap.html /// Also, 1% tolerance is fine.
Hopefully this helps
