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Xiansheng 708B headphone tube amp - Page 2

post #16 of 413
Quote:
Originally Posted by titoyd
I'm living in france and here we use 230 volts like some europeane countries here too.the 708B it wrote that it 's 220-240 Volts so there should not be the problems...The 6922 tubes also should not be the problem...!but seem the r-core sending and working too much to drive all the tubes (5 tubes) don't your guy thing so?
I had a look in some old electronics books I have around and it would seem that a smoking transformer usually means the input voltage is too high. This seems a little strange to me because where I live in New Zealand we use 230V, 50HZ much like France, and the supply may vary between 220V and 240V depending on the location. This would make me think that if voltage was the problem I would have a smoking transformer too. I wonder if it is possible that maybe the actual voltage on your wall socket might be above 240V? I don't know about France, but in Australia, South Africa and some parts of New Zealand 250V is not unheard of. If that was the problem, a power conditioner might help although they can be expensive.
post #17 of 413
Quote:
Originally Posted by drarthurwells
My 708B is dead silent and runs cool.

Hum is usually tube related - some tubes are noisier than others even though otherwise good.
In my case I'm pretty sure it's not the tubes, I've tried a few different ones but the hum is the same. It also doesn't increase when you turn the volume up (so it's getting in after the signal has been amplified) and mostly isn't really that noticable, it's only when the music is totally silent that I can just pick it. I might be wrong but I think it is a 50Hz hum, and the power frequency here is 50Hz which makes me suspect it's the power supply circuitry. I know the power input is clean, I have it plugged into a power smoothing box with RFI filtration. Next time I open it up I might try to find if my suspicion is right about the transformer and the signal wire being too close.
post #18 of 413
Quote:
Originally Posted by mirumu
In my case I'm pretty sure it's not the tubes, I've tried a few different ones but the hum is the same. It also doesn't increase when you turn the volume up (so it's getting in after the signal has been amplified) and mostly isn't really that noticable, it's only when the music is totally silent that I can just pick it. I might be wrong but I think it is a 50Hz hum, and the power frequency here is 50Hz which makes me suspect it's the power supply circuitry. I know the power input is clean, I have it plugged into a power smoothing box with RFI filtration. Next time I open it up I might try to find if my suspicion is right about the transformer and the signal wire being too close.
Have you tried different head phones?Or your RCA cables that conect from the CD player If it's the right signal..Some cable use only one direction signal flow...So i have been over a month for wong signal and didn't notice about that and all that time i have hum!but since i checking the cables directions well!it's.

Today I have received my 708B back from china!+ A Shengya CD-10 (470 $ included shipping UPS 7 days arrived from china If your guys would like to buy some try to contact the guy name acount on ebay's HOTPRODUCTSTORE he's very kind and serius person Ask for the best price that he could offer,Don't worry about he will stole your money it will not happen I bought alots of time with him aready.It you would like to be sure tell him it's from me "mr,Dech TT" introduce about this store for you he will be very please to serve you as well.Sorry not the advertise but once i have found this person, someone goods and cheapest price for such a good products like this i prefer to present if your guy interesting too.He's Not writing too much because he's not know too much english but he know what you want so you will not miss what you will get.Also the 708B i didn't bought from him but he replace it for me for free!Cool guy and i just send the machine back to him that's.Well I still not turn that 708B on yet because i afraid to burn it again so i will wait till i found the right answer or solved my problems first.I have a Fr'eind's Father he know about this electric and do like a hobbie for his retry...so I will give it to him the one that burnt to take alook...May be it will solved after all so I will keep in touch to tell your guys all.
For the moment I just have fun with Shengya CD-10...Can make me forget a problem for a whiles...So! Sweet sound.
post #19 of 413
Quote:
Originally Posted by drarthurwells
Thanks for your valuable input Hirsch.

Do you know what the front window tube 6N1 (or 6N1P) does?

Is it a "preamp" part of the input gain for the drivers?

How about the 6N3 (or 6N3P) located in front of the two 6DJ8 drivers? Is this 6N3 also a part of the input gain that feeds the drivers?
The tube in the window, and the front tube on the side are the gain tubes. 6DJ8's are the outputs. There is a separate one of each for each channel.

Quote:

Is switching from the 6Z4 to a 6X4 simply a matter of re-soldering some socket pins? No other circuit changes required?
Yep. This is easier said than done, as the socket attaches to pads on the board, IIRC. However, it was a pretty simple matter to bend the affected pins upward, and run wires to the correct pads for the 6X4 pinout. No other circuit changes were needed. However, the Mullard 6X4 is simply too tall for the case, so I had to change it back to stock to close the cover.
post #20 of 413
Quote:
Originally Posted by Hirsch
The tube in the window, and the front tube on the side are the gain tubes. 6DJ8's are the outputs. There is a separate one of each for each channel.
Ah, thank you for that information, I wondered if maybe the 6N1 and 6N3 were being used that way since from their appearance they are twin triodes capable of handling two signals. I would assume then that these two gain tubes would most likely have the largest effect on the sound?
post #21 of 413
Thread Starter 
Quote:
Originally Posted by mirumu
Ah, thank you for that information, I wondered if maybe the 6N1 and 6N3 were being used that way since from their appearance they are twin triodes capable of handling two signals. I would assume then that these two gain tubes would most likely have the largest effect on the sound?
Yes.

Big impact on sound.

Use a 6N1P EB (make sure it is a EB military tube and not the modern Russian 6N1P) in place of the 6N1.

This alone only will improve the sound some.

As drivers, use two RCA 6DJ8 stamped UK (which I have for $10 each shipped anywhere) or two Russian 6N23P EB (make sure it is a EB military tube and not the modern Russian 6N23P).

Use a 6N3P EB (make sure it is a EB military tube and not the modern Russian 6N3P) in place of the 6N3. As the one in front of the two drivers.

Those old Russian military tubes are great - the modern ones suck.
post #22 of 413
I've just bought this amp from someone. After some listening, I've realised that there's a hissing sound that comes on in the right channel every minute or so. I'm wonder if any of us have this same problem as well?

cheers!
Robfow
post #23 of 413
Quote:
Originally Posted by Robfow
I've just bought this amp from someone. After some listening, I've realised that there's a hissing sound that comes on in the right channel every minute or so. I'm wonder if any of us have this same problem as well?

cheers!
Robfow
This is likely to be tube noise. To diagnose:

1) Exchange the gain tubes (the one in the window and the one closest to the front on the side). If the noise changes sides, you need to replace the noisy tube. (Note, check the tubes to make sure that the ones you're exchanging are the same tube type, just to be sure).

2) If Step 1 didn't find the tube, exchange the output tubes and listen again.

If the sound switches sides on one of these steps, you've located a tube to change. If the sound stays put, then there is likely to be another problem that's going to be harder to find.
post #24 of 413
Quote:
Originally Posted by Hirsch
This is likely to be tube noise. To diagnose:

1) Exchange the gain tubes (the one in the window and the one closest to the front on the side). If the noise changes sides, you need to replace the noisy tube. (Note, check the tubes to make sure that the ones you're exchanging are the same tube type, just to be sure).

2) If Step 1 didn't find the tube, exchange the output tubes and listen again.

If the sound switches sides on one of these steps, you've located a tube to change. If the sound stays put, then there is likely to be another problem that's going to be harder to find.
Somehow after swapping the tubes around, the noise seems to be gone already. Might be the connection that caused it? Keeping my fingers crossed that it won't come back.

Thanks!
post #25 of 413
Quote:
Originally Posted by Robfow
Somehow after swapping the tubes around, the noise seems to be gone already. Might be the connection that caused it? Keeping my fingers crossed that it won't come back.

Thanks!
Yep. A tube making a poor socket connection can be noisy. One way to clean tube pins is to put the tube in and out of a socket a couple of times. You may never hear that noise again.

If it comes back, you can try cleaning the pins with something like Caig DeOxit. I do that, and then follow with a silver treatment (I use Quicksilver AVExtreme). That seems to clean up any issues caused by poor contact that I've run into. No real need to do that if it's already sounding good (although I do it anyway).

Note: silver treatments are highly conductive, so if you use them on tube pins, you've got to use great care not to accidentally create an electrical connection between two pins, thereby causing a short.
post #26 of 413
The noise came back. I opened up the casing and took the tubes in and out of the sockets a few times. Think it is alright again now.
post #27 of 413
yeah the 708B is a value for money tube amp.. it actually has very good synergy with grado phones.. It sound even better then the stock Cayin HA-1A except the soundstage.. After i rolled the tubes in the Cayin with Mullard CDV4004, CV4003, 2 x JJ EL84... It supress the 708B way ahead with even wider soundstage, better instruments seperation, bass impact...
post #28 of 413
I just though I'd add an update regarding the hum I was experiencing with the 708B. I've been trying out some Sennheiser HD595s and I can't hear the hum at all, in fact with these headphones I can't even hear any background hiss until the volume knob reaches 12 o'clock. This makes me think that it was my Shure E500s exposing the hum and that possibly it is just inaudible on larger headphones. I would not think the impedance difference is a significant factor since the E500s are 32 ohm and these HD595s are 50 ohm.

Also, I don't know if anyone else with one of these amps was worried about the 6Z4 having no direct substitutes, it's quite a hard hard tube to find online. After much searching I tracked down two places selling it, Audiotriodes in France and Diyclub.biz in Hong Kong. I ordered a bunch of them from Audiotriodes myself and they arrived with no problems. It kind of goes without saying but I'm not associated with either of these places, just passing it on incase it helps someone find some spares of this seemingly hard to find tube.
post #29 of 413
A hongkonger builds it, am I right? sounds like very good for the price. by how much tubes could affect the sound traits?
post #30 of 413
Quote:
Originally Posted by ymngzhou
A hongkonger builds it, am I right? sounds like very good for the price. by how much tubes could affect the sound traits?
Xiang Sheng is a company in China but they seem to have many internet dealers in Hong Kong.

In my experience the tubes have quite a noticeable effect on the sound, especially the 6N3 and the 6N1.
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