And now I will post about the tweaks and mods that I have performed to my 708B in the order that I did them in. Of course, I wouldn't have done any of this if it weren't for
dcheming and his excellent research and testing through all of this. So here it goes.
First, a couple of tweaks...
Power Delivery:
First off was upgrading the nasty, tiny stock power cord. Luckily, I had a nice 3' long, 3 conductor, 14awg multi-strand power cord laying around unused, so it was a natural choice to go with.
Tube Rolling:
The stock Chinese tubes that come in this amp are not impressive at all. They have a rather thin midrange, are pretty bassless, have a brittle top end, and like to run out of steam early on and distort before the music even begins to get loud.
So for the first run of tube rolling, I first went with
drarthurwells' recommendations of replacing the stock Chinese tubes with a NOS Russian Military 6N3P EB driver tube and a pair of matched NOS RCA 6DJ8 output tubes, but then switchd out the Russian 6N3P with a NOS GE JAN 5670.
These tubes really brought the stock 708B up to speed. No more distortion, a bit more bass, a fuller midrange and a tamed top end.
Upgrading and adding parts:
Next in line is my first batch of mods. This includes replacing the volume pot, coupling caps, output caps and adding a bleeder resistor to the PS filter caps.
1) I pulled the stock WIMA 0.22uF - 250V coupling caps in favor of a pair of Hovland 0.22uF - 400V Musicaps.
2) I removed the stock 150uF - 200V electrolytic output caps and upgraded to a pair of huge, monstrous Mundorf 100+100uF - 450V M-Lytic HV electrolytics. These are basically TWO 100uF capacitors in one body with a single (-) terminal and two (+) terminals. With the two (+) terminals wired in parallel, each one of these caps equals 200uF.
3) I replaced the stock cheap carbon volume pot with a quality ALPS 100k Blue Velvet. I won't go into the installation details and/or the "why" since dcheming has already done so in his first post.
4) I added a 68k - 2W resistor to just one of the PS 330uF - 450V filter caps. Since these two caps are wired in parallel along with the two 22uF - 400V filter bypass caps on the audio board, this single 68k resistor completely bleeds off the stored power in ALL four of these caps within a couple of minutes after shutting down the amp. This allows for a safe environment when working on the amp.
NOTE: THE OUTPUT CAPACITORS WILL NOT BE DISCHARGED FROM THE INSTALLED PS RESISTOR(S), AND THEY WILL PACK QUITE A PUNCH IF YOU SHORT THEM OUT, SO PLEASE BE VERY CAREFUL WHEN WORKING ON THIS AMP.
Refers to #'s 1 & 2
Refers to # 3
Refers to # 4
And one overall shot of the first batch of mods completed.
