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SET amps and biasing 300B's; expert advice needed

Another tube biasing thread, sorry for that. I'm driving my Audion SET amp with a pair of EAT 300B's and am perfectly happy with them. Yet, this hobby being the never-ending money squandering activity it is, I felt the itch to try a couple of Shuguang black bottles, used - but alledgedly with far less than 100 hours - and therefore offered cheap enough to give them a go. What held me from it was the drag of having to re-bias every time I would change tubes. The dealer tried to convince me, not of importance with SET amps as long as I use matched pairs. That flies in the face of what I've been reading so far. At the very least, I would think, the tubes would have to show similar specs (in this case, it would seem both abide by the original WE 300B specs). Tube guys, please advise, was he just trying to find a dummy to get rid of the tubes or is there (some) truth in this?
karelfd
5 responses Add your response
brf
1,715 posts
 
I am not familar with your amp, but if it has a bias pot, it is there for a reason. When ever I change tubes I FIRST turn th bias pot way down, insert the new tubes, warm up for a couple of minutes, turn up the bias pot to the recommended setting, and recheck the bias after a couple of hours of playing time.
karelfd OP
476 posts
 
Thanks Brf, the amp unfortunately does not feature any easily accessible biasing help and there is not a word in the very short manual about self-biasing, so I presume the latter is not the case.
It is the blanket statement of the dealer with regard to SET amps that I find disturbing and I think I may interpret your answer as a clear negation of it.
swampwalker's avatar
swampwalker
5,138 posts
 
From what I can gather from a quick web search, the current crop of Audion integrated/amps are auto-bias of some sort.
"The Audion 300B single-ended amplifiers employ pure Class-A circuitry from the input to the output, with two voltage gain stages culminating in a high dynamic driver circuit driving a Super Linear (1) output stage that operates with auto-bias."

It's stated explicitly at the end of the manual:
" No other voltage adjustments will be necessary but the valves should be replaced as matched pairs."

I'd check w Audion tech support to be sure. The amps either have a bias pot somewhere or have some sort of auto-biasing functionality.
karelfd OP
476 posts
 
Well, I had to drag it out of the dealer, but the tubes on offer were actually bought in Hong Kong by the first owner. According to information I found on the internet, products for the Chinese domestic market can deviate by up to 40% of original specs. Who knows what other qualities may be different? Another case of "what looks too good to be true probably is", and one dealer I am not going to spend any money with. Case closed.

Swampwalker, thanks for the information on Audion. Actually my amp is a German product that goes by the full brand-name "Audion-by-Fischer" (and looks remarkably like the Audion integrated with separate power section). Anyway, I will not solve the biasing conundrum unless I ask Mr. Fischer himself.
swampwalker's avatar
swampwalker
5,138 posts
 
Kerlfd- NP. If it looks "remarkably like" and has similar name, I would not be surprised to find out that there is a "common ancestor". None of my business and I think you probably made a good call on the tubes, but it wouldn't hurt to contact Audion UK and see if the products are related and if they can answer your biasing question. 'Nuf said. Good luck.
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