I've gone back to my trusty Shanling PH-100 for the time being, and will work on the 708B as and when I can.
In answer to the above, yes the vol pot greatly affects gain and is pretty poor.
My 708B stopped working a month or so ago. I noticed that the output phono connectors were badly corroded! (gold plated?) When I opened it up, the 6z4 had dropped through. 2 of the soldered terminals were broken. I just re-soldered them, removed the wire tube clips(the cause), and fitted quality phonos. All working again!
Upon reading this brilliant thread, I was inspired!
Here are the mods that I have made so far.
1-Changed the 6n3 for a russian 6n3p- more gain, clarity, detail.
2-Changed input wiring to ofc/ silver plated- again, more clarity, detail, much improved treble.
3-Changed 2 rear valves to russian 6n23p- at first very bright, but after 2 hours running became very smooth, with typical old school valve sound. Fab mid range and treble.
4-Changed vol pot to Audio Note(100k), ofc/ silver wired output, replaced output resistors with Kiwame. Now sounds very smooth, detailed, holographic. Treble is all there, but clean. There was some distortion in mids/highs prior to this part of the upgrade, now totally gone. There was a big drop in gain with the Audio Note pot, but now more realistic. The old pot only hit about 8 o'clock to give high volume. I kept the original knob by using next size up drill bit to enlarge the hole size. Had to file the front panel hole to fit. It works great and all looks original.
The amp just sounds amazing. I am using a high end Alchemist DAC with Audioquest Diamond and a pair of stunning Beyer DT990 600ohm. Each upgrade has been very easy to hear. I am looking at changing the output caps next.
I'm looking at buying some caps for my 708B, namely some to replace the Wimas (C104,C204), and I've found these:
Would these work okay? I understand that they are PIO and not metalised mylar as per the Wimas, but not sure whether they will be good to use?
Can anyone confirm that the values for R107 and 207 are 1/2W 1kohm resistors and the headphone output resistors are 10 ohm 2W?
On page 1 the input resistors are shown as 1/2 and 1 watt (don't know which one is correct), and page 1 also shows 2 W and 1 W for the output resistors. I just want to make sure I have the right values before I order.
Just finished modding my 708B. Did a recap and swapped all the resistors for Kiwame/Vishay Dralorics etc.
I have two problems with it now though, which is annoying. The first issue is I bought a cheapo version of a Neutrik locking headphone socket, and it's crap. The socket allows the 6.35mm plug to wobble about and it loses connection. The second issue is that I have a low frequency humming on the right channel. It's not volume dependant, and starts almost as soon as the amp is powered on. It's intrusively loud, so I need to have a fiddle to see what the issue might be.
I removed the preamp output caps (to make room on the PCB) and resistors R9 and R10, but I'm pretty sure that's nothing to do with the hum as it's only coming through one channel.
@ vrao81 - all the resistors you mention in your post I used 2W versions of in my build.
That really has nothing to do with the 708B as the only things that remained was the chassis and power supply section. All of the important parts that make it an amplifier was completely gone. All you have there is a preamp, that's it. It's pretty much pointless in this thread.
And on that note, here's my old 708B amp that I just pulled out of mothballs today. Hooked it up, fired it up and... Still works! Still sounds pretty darn good as well, even with these rather demanding headphones of mine.