The Tubegeek
OK, I just played my first record with this new phono preamp, but I didn't get it working the way I showed it below. The absolute voltage reference of the gas reg and the (slightly) mismatched sections of the 6N1P didn't allow enough leeway for the DC voltages to settle correctly. So I punted.
As you can see, I had to put cathode bypass electrolytics back for the 6SL7. I sure hope they break in :)
I'm not listening too closely, the air conditioner is making some noise (that's actually a GOOD thing, trust me) but so far I am not really impressed. I've got David & Igor Oistrakh on DG playing, a nifty record, and it sounds OK.
I'll keep it playing for a while and see what we get. Maybe I'll put in some film bypasses on those cathode caps, but that never seems to do much.
Any suggestions?
-j
posted by J Epstein @ 7/27/2005 07:17:00 PM
8 Comments:
why not try a gruntier o/p valve. The Atwood design has a powerful, low Z o/p stage. Try say 6c45 or triode wired pentode d3A or similar. Have been using battery grid bias in phono stages for years now but not in the grid itself so must try that too. cheers.
Well, the phono stage is only loaded by a few inches of twisted pair and a 100K volume pot so grunt doesn't seem necessary. The gain is OK, so that'd basically be adding a stage for little reason.
-j
How's about moving the battery from the grid to the cathode? Or, how about swapping the tubes, i.e. input = 'SL7, output = 6N1P? Probably need to rebias.
John: I built this after hearing glowing reports about the Artemis Labs PH-1 which John Atwood designed. So I thought I'd try his front-end, with 6N1P swries-grid-bias, just for fun. Wanted to try something new.
I've done a 6SL7-input phono (with battery cathode bias) and it was mushy and boring. Which does NOT describe this phono!
-j
I think I agree with Stephen when he says you need a gruntier o/p stage. After all the original uses a 12AX7 where you have the 6SL7 followed by a choke loaded 5687. Either you add another tube or some other sort of driver. High voltage mosfets with current sources in the source make pretty nice source followers and might be just the ticket.
Or if thats not the ticket then you need to think about regulated power supplies.
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@ etf.04 manfred demonstrated the sound of different biasing strategies. fixed bias with the battery in the signal path yielded a grey-ish, uninvolving sound. think of the battery as a cap... so i'm not surprised if you're kinda underwhelmed with the results of your phonobaby. why not go with LED bias? i was pretty impressed when manfred loaded the bias LED with additional current and forced it into a more linear operating point.
Christian, thanks, I had forgotten about that method. In the past - just using a bare LED, no additional current - I thought THAT method sounded greyish. I remember hearing about that method - one which Steve bench has discussed too, and thinking it sounded promising. Anytime Manfred and Steve agree on something, I expect it's worth a try.
BTW, the 6N1P is breaking in pretty nicely, I actually am in no hurry to revise it just yet.
-j
-j
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