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

[lordmozilla]

hey,

thanks for the response. I do have another question :

I'm guetting some small cracking on one channel. weird because its never happened since this morning and i haven't changed anythign last night when it was fine. the other channel actually does crack a tiny bit too. i'm gonna swap the tubes to see if they are the cause.

Cheers

edit 1 : its not the 6n1p

[dcheming]

Originally Posted by lordmozilla

hey,

thanks for the response. I do have another question :

I'm guetting some small cracking on one channel. weird because its never happened since this morning and i haven't changed anythign last night when it was fine. the other channel actually does crack a tiny bit too. i'm gonna swap the tubes to see if they are the cause.

Cheers

edit 1 : its not the 6n1p

Did you try swapping out the 6N3 also?

[lordmozilla]

no i'm just at work at the moment, i'll try again when i get bak home. Only had time ot swap the 6n1.

i'm kinda hoping its the 6x4 (or 6z4 i never know which one it is!)

[lordmozilla]

The noise problems has nothing to do with the tubes unless its the 6z4 which i can't test. Maybe it's the pot? see the question below :

What are the side effects of removing C101 and C201 ? I don't really understand

another question is i get some noise after 13-14 o'clock which only gets worst. Could this have something to do with the new alps pot? How can it be grounded? does that mean a connection from the screw on the outer case to the ground?

Someone mentioned grounding the pot to the phono output and grounding that to the case with 10x100ohm resistors in parralel. ? Should i just do that and use 1 x 10ohm resistor thats 5W ?

[zer061zer0]

Check the solder points to and out of the volume pot and the wires to the RCA

[aesthetik]

More Tube Rolling:
I decided to go out on my own this time and try some tubes that interested me. One of which others have tried on a "stock" 708B, but never on a modded one. This is the NOS GE JAN 5670. This one in particular is from 1968. The reason no one liked this tube is because it created too much hum or noise into the signal. Again, this was with all stock 708B amps. However, I put this 5670 tube in my amp and it is totally, completely and 100% dead silent, even at max volume and everywhere in between.

I also want to try some different output tubes as well. I've heard that the Amperex-Japan 6922/E88CC tubes were actually made by Matsu****a, which are very VERY good tubes.

Here they are...





Out with the NOS RCA 6DJ8 output tubes and NOS Russian Military 6N3P EB driver tube...


And in with the new (NOS) GE JAN 5670 & Amperex-Japan 6922/E88CC tubes.


As for the sound, I feel that they have a better "grip" on the music than the UK made RCA 6DJ8's and Russian 6N3P EB that drarthurwells recommended. They DO have a wider soundstage, the dynamics are more dynamic, bass seems to be a bit tighter and definately deeper, the midrange is full and natural, and the treble is still smooth, crisp and extended.

And a close-up of them in action.

Hi all,

Hope no 1 minds the bump.

I just got mine yesterday, paid about 110€ all together with shipping. May be a bit expensive for an American but in Europe it's a pretty good price.

OK, I'm just looking for a stupidly simple clarification on the tubes.

Mine has 5 tubes. 4 that are the same size as only what I can call a 12ax7 (I only know that from playing guitars for 22 years.)

Two are in a row of three and say 6n. (circuit board VE2 and VE3)

1 of that row of 3 says nothing on the tube. (VE1)

1 is the 1 in the view window. (also no number printed on it)

A 5th is a different type all together and is much thinner.(6z4)

Could someone clarify which to change and with what?

I know there is some difference preference sound wise, and I'd be for hearing all the different likes and dislikes, but I need it dumbed down to which and how many are to be changed and which can stay.

Thanks in advance.

[aesthetik]

ok, think I'm going to answer my own question here just want confirmation.

Found a clearer post in the thread.

The front tube of the three tubes inside the amp (not the front window, that's just for show as you said) is the gain tube and should have the biggest influence on the sound. For this you can use:
For the 6N3 ------> 6N3P, 5670, ECC42, 2C51 and 396A

The two behind that are the left and right channels:
For the 6N11 -----> 6DJ8, 6922, E88CC, ECC88, PCC88, E188CC, 7308, 7DJ8 and 6N23

Just want to be sure....

1. Window tube is just a dummy?

2. Smaller tube is rectifier, no need to change for sound quality?

3. Front tube can be replaced with 6N3P, 5670, ECC42, 2C51 and 396A?

4. and back 2 tubes with 6DJ8, 6922, E88CC, ECC88, PCC88, E188CC, 7308, 7DJ8 and 6N23?

5.wrap up, only the 3 in a row tubes need to be changed for sound quality?

[dcheming]

Sorry I missed this.

1. The tube in the window is indeed only for show and can be removed completely without affecting the functionality of the amp.

2. The taller, thinner tube is indeed the 6Z4 rectifier. I believe there are only a few different brand 6Z4 tubes made and they're all the same basic quality level. So even if you did want to roll this I don't think it'd be worth it. Spend your money on the other three tubes which will make a difference.

3 & 4. I've only replaced the two 6N11's with some 6DJ8's so I can't directly comment on this. You have to be a bit careful because even if the pinout is the same between two different types the heaters might draw too much current for the power transformer to handle or the operating points might be different enough between them to cause issues. So some tubes on that list are closer substitutes than others. One of the sources that I use for checking this is this site.

5. Pretty much. This is where I'd focus on to get the most bang for your buck.

Originally Posted by aesthetik

ok, think I'm going to answer my own question here just want confirmation.

Found a clearer post in the thread.




Just want to be sure....

1. Window tube is just a dummy?

2. Smaller tube is rectifier, no need to change for sound quality?

3. Front tube can be replaced with 6N3P, 5670, ECC42, 2C51 and 396A?

4. and back 2 tubes with 6DJ8, 6922, E88CC, ECC88, PCC88, E188CC, 7308, 7DJ8 and 6N23?

5.wrap up, only the 3 in a row tubes need to be changed for sound quality?

[Chops]

How about this... It's been almost exactly a year since I was last on here!

Shortly after my last post here, my computer got slammed hard with a virus, thus taking out my hard drives and my website that I was hosting. Then shortly after that, I decided it was time for a new job. So since then I have been working my tail off with this new job, am a manager in training and have much better pay and benefits.

Also since my last post, my amp has remained the same and has been sitting for the entire time unused. I also sold my AKG K701's! However, tonight I just decided to fire up the ole' 708B and use it with my trusty Sony MDR-7506's. Those 1500uF caps certainly did take away a good chunk of the bass, so if I keep listening to this amp again, I'm going to put those Mundorf caps back in.

In the meantime when not working, I have been focusing all of my energy, time and mostly money into my 2-channel/7.1 home theater system. I have been spending some mad cash on this system, especailly lately. As it is, I have a pair of speakers on order right now waiting to be shipped.

Anyway, I have a new website up. Actually it's more of a gallery than anything, but I'm no longer hosting it on my PC like before. Pics load much faster and the layout looks great. It's: Zenfolio | CharlesT Photography

I think I'm going to be sticking around again for a while. There's some really decent people here who I lost complete contact with over the past year. So there you have it!

Since I can't post without leaving at least one pic, here's a pic of the speakers I'm getting. They are the Onix X-Statik from AV123.com. They're a hybrid sealed/open baffle setup. I'll give more info in my next post!

[Chops]

Well I just finished uploading the 708B pictures to my site then reinserting them into this thread in all my various posts. I also reinstalled the Mundorf output caps into my amp. They really are much better sounding than the 1500uF caps I had in there.

I hope to keep this thread going as it is chalk full of great information. It's simply amazing how far you can take one of these amps. A true sleeper!