Modifying the Xiang Sheng 708B [56K!!!!]

[Chops]

Another update...

I've only had this amp back up and running for a few days now and I've already done another mod to it... Kinda.

I had a couple of Solen 51uF caps sitting around that I used in a loudspeaker project a short while back. They were used to absorb any turn-on thump from an amp to a pair of compression horn drivers (as they were connected directly to the amp and were crossed over via a digital x-over). However, there was never any turn-on thump, but just in case there was.

I decided to try them as bypass caps on my Mundorf's. I just have them connected will alligator clips for the moment until I can figure out how to install them. I think it may be time to think about getting a new chassis to transfer everything over to as I'm growing out of the current 708B chassis.

Initail impressions are that these Solen caps remove a bit of grain from the upper midrange and treble and also thicken up the bass just a tad bit. This is more the way a tube amp is supposed to sound. I also notice that there's more inner detail to be had now as well a little more spaciousness. BTW, these Solen 51uF caps are even larger than the Mundorf's!! What is it with me and huge caps?!?!

However, this is also listening through my Sony MDR-7506's which can tend to be a bit bright and a shade thin in the midrange, dare I say a little harsh even. Maybe I should look into getting another pair of better cans again, but nothing overly expensive this time around, and something easier to power as well.

Anyway, as usual, I have pictures! Imagine that! LOL So here we go...

[jrhill]

Hello All,
This is my first post here and in the middle of modding a friends 708B and thought I'd add a bit of measured info for others to see.

In Oz, we have 240v, 50Hz and this gives high volts (B+) of +215 on pcb, heaters of 7.3v ac, and the current thru the input valve (6N3, etc) is about 2.7mA, and 10.5 mA thru the 6N11s.

Reducing the B+ to 195v via p/s resistors (1k2 for the 1k0 and 2k2 for the 2k0) and also added 1R0 in heater line=6.3vac.

Some resistor changes are 34.5kR for 33k, 230R for 220R (on 6N11) and the rest of the resistors were simply changed to good quality Beyschlag M/oxides.

All the other upgrades completed too, including replacing the o/p wire and the crappy socket.

Perhaps this may be useful.

... jh

[glitch39]

@jrHill - this is useful

A lot of tube amp enthusiasts also have a tendency to roll tubes and tinker a bit. The 708B is very simple, has lots of space to work with and parts are relatively reasonable. By that I mean the knowledge learned from the tweaking is invaluable.

My 708B has been a reliable component for a while now.

[Chops]

Originally Posted by jrhill

Hello All,
This is my first post here and in the middle of modding a friends 708B and thought I'd add a bit of measured info for others to see.

In Oz, we have 240v, 50Hz and this gives high volts (B+) of +215 on pcb, heaters of 7.3v ac, and the current thru the input valve (6N3, etc) is about 2.7mA, and 10.5 mA thru the 6N11s.

Reducing the B+ to 195v via p/s resistors (1k2 for the 1k0 and 2k2 for the 2k0) and also added 1R0 in heater line=6.3vac.

Some resistor changes are 34.5kR for 33k, 230R for 220R (on 6N11) and the rest of the resistors were simply changed to good quality Beyschlag M/oxides.

All the other upgrades completed too, including replacing the o/p wire and the crappy socket.

Perhaps this may be useful.

... jh

I think most of us hear in the states have done this mod as well considering we are 120V/60Hz, 10V higher than the 708B p/s. As for the rest of the resistors, every single one in my amp has been replaced.

[jrhill]

Yeah, following on from your work and others in this thread is like getting first hand instructions and made the upgrade quite straightforward - many thanks to you and all the others for writing up your work ... jh

[Chops]

Originally Posted by jrhill

Yeah, following on from your work and others in this thread is like getting first hand instructions and made the upgrade quite straightforward - many thanks to you and all the others for writing up your work ... jh

I'm glad to hear that we were able to help in some way. It is much easier to perform mods to something when you have good instructions and plenty of good pictures to go by. I certainly wouldn't have been able to do any of this stuff without Derek's (dcheming) help throughout it all. You ask that guy a question and he tells you everything you need to know in great detail!

As if you couldn't tell, I'm trying to bring this thread back to life. I am getting back into headphones again and just bought a pair of HD600's (as seen in my Sig below). In fact, years ago, I started with a pair of Sennheiser HD545's. A couple months later I moved up to HD580's, then a couple months later the HD600's. That was over 5 years ago! Then a little over a year ago, I got into headphones again and bought the AKG K701's. And now I'm going back to the HD600's. Go figure... I think I might be mental! LOL

[Cloth Ears]

Originally Posted by jrhill

Hello All,
This is my first post here and in the middle of modding a friends 708B and thought I'd add a bit of measured info for others to see.

Hi Head-fi'ers,

mine is the 708B that James is modding (also my first post, over from DIYaudio & AudioKarma). I'm going along tomorrow to have a listen and discuss, so I'll take the camera and hopefully have a couple of shots to document the progress.

[Chops]

Originally Posted by Cloth Ears

Hi Head-fi'ers,

mine is the 708B that James is modding (also my first post, over from DIYaudio & AudioKarma). I'm going along tomorrow to have a listen and discuss, so I'll take the camera and hopefully have a couple of shots to document the progress.

Welcome Cloth Ears! I knew that name sounded familair. I know you from DIY Audio. Looking forward to seeing some pics and hearing what you have to say about the new mods/sound.

Originally Posted by Chops

Maybe I should look into getting another pair of better cans again, but nothing overly expensive this time around, and something easier to power as well.

Well, so much for keeping within my new headphone requirements. I just ordered a pair of HD600's yesterday! LOL

[Cloth Ears]

Originally Posted by Chops

Well, so much for keeping within my new headphone requirements. I just ordered a pair of HD600's yesterday! LOL

Ain't that always the way...?

I read somewhere on this site (before I'd become a real person and signed up) that someone wasn't sure what the voltage to the audio board should be. So I asked at the source, and received this response:

Originally Posted by xshn

Dear sir: 708 transformer output alternating voltage is 215V, arrives at the enlargement part d C operation voltage should about 190V.

[Chops]

Originally Posted by Cloth Ears

Ain't that always the way...?

I read somewhere on this site (before I'd become a real person and signed up) that someone wasn't sure what the voltage to the audio board should be. So I asked at the source, and received this response:

Yeah, you'll have to insert a resistor in the heater curcuit to lower the voltage. Mine was 7.58v and after I installed a 1 ohm - 10 watt resistor, it dropped down to 6.63v. That's much better and much closer to the 6.30v that it should be.