Please help identify this tube - Shuguang 6922? - diyAudio
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Old 23rd June 2009, 12:14 AM   #1
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Default Please help identify this tube - Shuguang 6922?

This tube was sold to me on eBay as a Shuguang 6922. However, using it in my valve-buffered chip amp, it produced an unexplained positive bias. Some kind folks over in the chip amp forum suggested that I ask here for help identifying the tube, which is not clearly labelled. The original markings have mostly worn off. On one line, I can make out the characters "6N". On the next, "722".

My original discussion about my amp build took place here http://www.diyaudio.com/forums/showt...postid=1860120 .

I was always slightly suspicious of this tube - Shuguang ones go extremely cheaply on eBay. The seller was in Taiwan. They did have 100% positive feedback.

Really appreciate any help you can give me
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Old 23rd June 2009, 12:15 AM   #2
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Old 23rd June 2009, 12:43 AM   #3
kheper is offline kheper  United States
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http://enshuguangtubes.chsee.com/old...tcomClass=1520

There are 8 pages of power and signal tubes which are listed on shuguang's site.

The 6922 is not listed.

6DJ8, ECC88 and 6N23P are subs for the 6922. However, none of these tubes are listed as being in current production on shuguang's site. Shuguang probably makes alot of tubes which they do not list on their site, so it is possible that you do have a 6922. However, I think it likely that you have another (non-6922) twin triode.

You could get a NOS or current production 6922, 6DJ8, ECC88, or 6N23P, then test the behavior of the known to be 6922 with the tube you have now.
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Old 23rd June 2009, 12:53 AM   #4
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It's probably a 6N1.

Same thing is true for Russian tubes, ebay sellers often describe the 6N1P as an ECC88/6DJ8/6922 equivalent, which it is NOT.
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Old 23rd June 2009, 01:03 AM   #5
kheper is offline kheper  United States
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The 6N1 is not listed in current production, but it was produced before, and it is billed as a sub for the 6922 and the 6DJ8. 6N1 is very likely the tube that gaplessophile has.

http://analogmetric.com/forum/viewthread.php?tid=233

Note: the page says the 6N1 is "Compatible with: 6922 and 6DJ8.", not specified as an exact sub. As always, buyer beware.
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Old 23rd June 2009, 05:12 AM   #6
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Well that explains it. The seller did also described the tube as a 6N1. If they are not in fact interchangeable, then this explains the problem. Looks like I need a new tube.

Thanks heaps for your help
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Old 23rd June 2009, 04:21 PM   #7
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Quote:
Originally posted by gaplessophile
If they are not in fact interchangeable, then this explains the problem. Looks like I need a new tube.
Whoa, time out. Why do you think you need a new tube? This is for one of those valve buffered gainclones, right? A 6N1 will make a perfectly satisfactory cathode follower, especially since it's only driving the input of a chip. Just make sure your filament transformer can deliver the extra heater current.

EDIT: as for the "positive bias" you mention, it sounds like you wired it wrong.
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Old 23rd June 2009, 04:55 PM   #8
cm is offline cm  Singapore
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The Shuguang 6N11 is equivalent to the 6922. To check if you have the 6N1 or 6N11, measure the heater current and you will be able to confirm. I cannot remember the exact current, but the 6N1 is higher than the 6N11. The 6N1 has similar current as the 6N1P, and the 6N11 has similar current to the 6922.

I have used a lot of 6N11. It is a robust tube, but I prefer the sounds of the NOS 6922 or equivalents. It is not a bad sounding tube. Just a matter of taste. And it is a highly regarded tube in China.

6N1 sounds slightly better in tube gainclone from my experience. Again, just a matter of taste.
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Old 23rd June 2009, 11:02 PM   #9
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Oh, that's good news then! Unfortunately though, I really cannot figure out this positive bias issue. I'm convinced I haven't made a mistake with the wiring, since I've checked it again and again, and received some help over at this thread in the chip amp forums Valve buffered IGC - volume pot causes DC offset, crackling.

To quickly sum up the problem, the grid and cathode both have negative voltages on them (below the 0V ground). My power supply is balanced to within 0.1 V (about +-35V). The problem is the same on both channels, so it's unlikely that a stray wire could be causing the problem.

I am using the same power transformer for the valve and the chip amp circuitry, but I do have separate rectifier bridges and of course capacitors for each channel of each stage (4 rectifier bridges total).

The following is copied from a post of mine in the other thread:

"To try to further isolate the problem, I disconnected the valve circuit from the chip amp circuit - cutting off from the 3u3 capacitor onward. I also cut off everything from the grid pin of the valve. Under these conditions, cathode voltage was approximately -25 V, and grid voltage was approximately -12 V.

I then connected a 30k resistor from the grid to ground. Grid and cathode voltages were now between -2 V and -3 V."



I very closely followed this design here http://www.customanalogue.com/diytub...lone/index.htm.
Power supply and wiring diagrams are here http://www.customanalogue.com/diytubegainclone/ps.htm.

My heater filament supply seems to be providing enough current. It's a DC regulated supply which puts out 5.95V. When the tube is plugged in, this voltage does not drop, suggesting that there is enough current available.

Would be really grateful if anyone could spot what I'm doing wrong!
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Old 24th June 2009, 01:10 AM   #10
kheper is offline kheper  United States
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I can't figure out what you say that the problem really is.

If the amp is functional, does it hum? If so, there may be a ground problem.

Try shorting the inputs to see if the "positive bias" goes away.
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