Hi folks,
I'm a bit of a newbie to the headphones & amp world, but I've a bit of a love (or fetish) for valves. Was thinking of getting a Little Dot, but kept seeing bad news about them having design problems. The 708B seemed to get a fair bit of much better news/reviews in this respect. Was also looking for an amp that had valve rectification, which this one has.
Having done a bit of reading around this and other forums, I've purchased a 708B from "on9mart" via ebay for £72 including airmail. It arrived very well packaged in about 4 or 5 working days, got the tracking ID from the seller. In fact, the parcel was so ornately wrapped, it was a shame to rip it open!
Bear with me, I'm going to give a quick run down of the mods I've made...
So, first impressions, are that it's not at all a bad bit of kit. It's well put together, and the inside is neat and tidy. Straight out of the box, I first powered it up using a Variac set to 220VAC (instead of my usual 237VAC), and checked the heather voltages were roughly 6.3V, which they were. So the first mod was definitely going to be sorting those out, as they went around 7.0-7.2V when running on 240.
Anyway, it even then after about 4 hours burnin, it sounded better than the headphone output my Dads old QED amp or another old Technics amp. The major critiscm was an amount of AC hum in the headphones that was not loud but a little distracting in quieter music. BTW, I'm using a pair of Senn 555's,and a Technics CD SL-PG370A CD player.
Mod 1
SAFETY FIRST!
Being a bit electrically minded, I didn't fancy the idea of those really large PSU caps staying charged up. So, I've fitted a 1Mohm bleeder under each, as they're not quite in parallel with each other, owing to them being in a CRC filter network. I originally had 3W 68Kohm, but these were getting warm, and I don't like the idea of slow cooking caps, 'cus it shortens their lifetime. The 1Mohms don't get much above gently warm.
I've also added a wire from the earth pin on the mains IEC socket to the chassis. It's a bit naughty not to have that!
Mod 2
Had to be to get it to run on UK 240VAC, so that's sorting out the heater and HT voltages.
First off, heaters. A bit of trial and error found that I needed about 1/2ohm in series with the rectifier/frontpanel valves. Not having anything suitable to hand, a length of about 20CM stranded equipment wire was giving the correct voltage drop. This is neatly folded up in some heatshrink tubing under the PSU board (which I've raised slightly on it's mounting bolts).
The second heater circuit for the 3 valves on the amp board. I've had to change this again, as since I originally modified it, I've installed a different pre-amp valve, which changed the voltage again. Now it's 0.47 and 0.5 in series, both 5W. I had a 1ohm but that dropped the voltage too much.
So, now on an average day with about 237+/-7VAC on the line, I get about 6.1 to 6.35V on all the heaters.
Mod 3
Folks have already mentioned in the thread about the 2 high wattage resistors on the PSU board getting cooked. I've basically made roughly the same resistances up by paralleling some 3W bits together. In doing this, I've increased the resistances slightly, which helps the AC hum filtering in the CRC network. It's also had the useful effect of dropping the B+ rail back to about 185VDC now I'm running it on higher UK mains voltage.
While the PSU board was out, I replaced the 1N4007 diodes with Vishay SF4007's, as mentioned elsewhere on the forum. Didn't notice anything obvious when these were replaced though.
Mod 4
This isn't critical, but I replaced the phono sockets. The stock ones were ok, but they seems about 0.5mm too thin in diameter. So, these have been replaced with some new gold finished ones. I replaced all 4, even though I don't plan to use the pre-amp out.
Mod 5
Again, not critical, but I replaced the 2off 47u electrolytics on the amp board, only because the top of one looked a bit dented.
Mod 5
This is probably the biggest and most dramatic improvement I've made to date, having read up with folks' experience of tube rolling. I've replaced the Chinese 6N3 with a 1987 NOS GE JAN 5670W. Wow. Immediately the AC hum is almost none existant - the M6 motorway is 1.5miles away, and at 8pm with the window open, that's louder than the hum. This valve has really brought the sound of the amp to life. Also shortened the spring clip to suit the lower height of the 5670W.
Mod 6
Replaced the potentiometer with an ALPS 100KA. I have access to a PCB machine at work which scratches copper from copper clad board. So, I knocked up a simple 2-sided board to interface the ALPS PCB pins to the phono wires.
The ALPS needs a new hole drilling for the locating peg, and the stock knob doesn't fit. I still need to sort a new knob out
for it, of course!
Having read other reports of changing this, I was expecting more improvement than I got. The old pot was fairly well balanced, but it did suffer from not much graduation in volume when set at about 0 to 25%. This ALPS is definitely a lot better for this.
At this point, I thought I'd lost the plot. When I first powered up, I got a deafening hum out of my Senn's, which were quickly unplugged. It turns out that somehow the ALPS pot creates an earth loop, where the stock pot didn't. This was fixed by adding qty=10 of 100R 1/2W in parallel, wired between the earth pin on the mains socket and the earth commons on the isolated phono in's.
Mod 7
As I'm not exactly a millionaire, I was going to replace the WIMA 220nF interstage coupling caps with Musicaps. For some reason, I couldn't find any reasonably priced ones in the UK (compared to the US prices). So, I've dropped a pair of SONIQS SAX220nF/630V in. Had these on a component tester at work. These were both bang on 223nF and measure an ESR of about 1/2ohm at 100hz, 1khz and 10hz.
These too have made an improvement, the sound is certainly a little clearer than with the WIMA's.
Mod 8
My CD player seems to have a LOUD output level. So I've put a 560K resistor in series with the phono in socket, which has given a lot more play on the volume control. Maybe I should have dropped a 560KA ALPS pot in there instead or had a go at reducing the amplifier gain.
In Conclusion
The amp was £72, I've spent about another £40 in bits.
That's probably as far as I'm going with the mods. It sounds excellent (to me anyway), it reproduces a wide range of live music recordings exceptionally well. Listening to the BBC CD of "Last night of the proms" is a new experience, compared with the old transistor amps at home. CD's like Norah Jones, and classical recordings and the such like have come to life, I can now hear the musicians shuffling around, not just the music
My advice, go for it, and do a bit of tweaking - especially replacing the 6n3 and sorting the heaters if you're not on 110V or 220V!
Cheers,
Chris.
) 