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 Post subject: 6sn7 gtb v 6h8c
PostPosted: 02 Jan 2019, 21:53 
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Joined: 13 Jan 2018, 21:33
Posts: 232
Location: australia
My amp project at viewtopic.php?f=9&t=6153&start=40#p54218 uses GE 6sn7 gtb to drive the 300b. I bought a spare set but are actually 6h8c, not sure of the brand - has a pentagon symbol above the number and looks unused. information suggests their specifications should be the same but i note the 6h8c i have are slightly taller than the 6sn7. With the 6sn7’s in place the amp is fairly quiet - some small amount of hum I can live with. with the russian tubes in place the hum is quite loud and therefore unplayable. Any ideas what is causing this would be helpful. it’s a shame because through the hum the underlying sound seems good.

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 Post subject: Re: 6sn7 gtb v 6h8c
PostPosted: 03 Jan 2019, 11:43 
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Joined: 06 Apr 2009, 10:08
Posts: 1596
Location: US Pacific Northwest
Ostensibly, the 6H8C is a direct equivalent for a 6SN7. Keeping in mind that the "H" and "C" are Cyrillic characters and not Latin characters. The "H" being equivalent to N (i.e. dual triode) and the "C" being equivalent to S (i.e. large glass envelope; Octal or special type). So the proper Latin designation of the "Russian 6SN7" should be 6N8S.

That being said, there are equivalents and there are "equivalents". I have run into this mostly commonly with the big dual rectifier tubes coming out of China (5U4, 5U4GB, 5R4, 5AU4, 5Z3, 5V4). These are all different tubes with different specifications, but the new production Sino tubes all appear to be 5U4GB tubes (based on the measured I-V curves). You may be seeing something similar here.

In any event, no 6SN7 tube should be giving you hum problems in an audio amplifier. The 6SN7 is probably the quietest (lowest noise) audio tube ever produced. And the low gain of the tube should not be exersabiating any hum pickup. Personally I have used NOS 6N8S tubes in place of 6SN7s without issue. So the real problem is determining what tube you really have. Keeping in mind that an unscrupulous dealer may also be foisting off factory rejects as good tubes. There are unfortunately a lot of sellers (especially on eBay) that will do this.

The main issue is that you could spend a lot of time on trying to determine "what" they actually are and "why" they don't work in your amp. But the cost and effort required to answer those two questions will likely swamp the cost of a couple of new 6SN7s from some reputable supplier. Depending on what test equipment you have, you could be some simple checks of µ and rp to see if they are in the neighborhood of the 6SN7. You could also put one in a CCTA configuration and introduce a hum field around the envelope and see if they have a significantly different hum pickup than a standard 6SN7. However these are academic exercises; they will not make these tubes usable in this amplifier.

My advice is order a couple of JJ or new production Tung-Sol 6SN7s from Tube Depot or Antique Electronic Supply and put these odd tubes on the shelf. You can play with them at your leisure if you want; but I wouldn't waste any more resources on them. If you can get the seller to refund your money, i'd do it. Otherwise, just chalk it up to "one of those things".

This is all just my opinion of course. YMMV.

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 Post subject: Re: 6sn7 gtb v 6h8c
PostPosted: 03 Jan 2019, 16:17 
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Joined: 13 Jan 2018, 21:33
Posts: 232
Location: australia
Thanks for quick reply Matt. The ebay seller is approachable and has exchanged these once already but if anything these tubes are worse than the former. I have minimal test equipment so a new set is the most practical approach.

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 Post subject: Re: 6sn7 gtb v 6h8c
PostPosted: 03 Jan 2019, 16:29 
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Joined: 02 Mar 2009, 12:41
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Location: Vänersborg, Sweden
And as a complement, the Russian 6N9S is an equivalent to 6SL7.

If price is a matter, some 6V-tubes also comes in 12V versions. Got 12SN7, 12SJ7 and 12SC7.

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 Post subject: Re: 6sn7 gtb v 6h8c
PostPosted: 05 Jan 2019, 09:41 
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Joined: 13 Jan 2018, 21:33
Posts: 232
Location: australia
Magnus, thanks. I’ve asked the supplier to check whether they are actually 6h8c. i have my doubts as the gain seems much higher than my 6ns7’s. Without the hum i think i would like the sound. i have asked tubesandmore for a quote on the 6ns7. with our weak aus dollar and $12 dollar shipping the price is not too bad. about the same as the local supplier about 40 kms away.

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 Post subject: Re: 6sn7 gtb v 6h8c
PostPosted: 05 Jan 2019, 09:47 
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Joined: 13 Jan 2018, 21:33
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Location: australia
The manufacturer of this tube is probably novosibirsk based on the pentagon symbol.

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 Post subject: Re: 6sn7 gtb v 6h8c
PostPosted: 20 Jan 2019, 06:04 
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Joined: 13 Jan 2018, 21:33
Posts: 232
Location: australia
JJs arrived from thetubestore. All is well. The old second hand GEs made in USA I have been using hold up remarkably well and possibly slightly quieter with respect to hum which i will have to track the source of eventually.

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