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The point of an 845, 211, 805 amp?
The point of an 845, 211, 805 amp?
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Old 15th July 2019, 10:55 AM   #221
dhaen is online now dhaen  Europe
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Have not noticed any microphonics on 845s. It took me a while to find 6SN7s that were not microphonic. Ended up buying a pair of NOS. My power chassis is separate.
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Old 15th July 2019, 03:20 PM   #222
Bigun is offline Bigun  Canada
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The point of an 845, 211, 805 amp?
How about 6SL7's - any thoughts on microphonics ? I am thinking about a 6SL7 directly coupled to grid of the 2A3 (as driver for the 845) since I would have plenty of B+ to play with. I do have a 12AX7LPS bought for this purpose but somehow, using that little pimple of a valve when there's an 845 on the chassis just doesn't feel right anymore.
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Old 15th July 2019, 04:31 PM   #223
grovergardner is offline grovergardner  United States
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Quote:
Originally Posted by Bigun View Post
How about 6SL7's - any thoughts on microphonics ? I am thinking about a 6SL7 directly coupled to grid of the 2A3 (as driver for the 845) since I would have plenty of B+ to play with. I do have a 12AX7LPS bought for this purpose but somehow, using that little pimple of a valve when there's an 845 on the chassis just doesn't feel right anymore.
The Sylvania 6SL7WGT, brown base with short black plates, is the least microphonic in my experience, also the best-sounding 6SL7 IMO.
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Old 15th July 2019, 07:04 PM   #224
dhaen is online now dhaen  Europe
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If there's any DC coupling to be done, I'd recommend it to the grid of the 845. That way you can drive it into A2 so get more power for a lot less dissipation. Transformer coupling also opens up this possibility but is more expensive.

I remember when I first mooted - on this forum - that I was doing "grid current" there were shrieks of horror and predictions of early failures. Fortunately sanity has prevailed and nobody bats an eyelid about A2 now.

In an earlier post on this or another thread, I said I'd been running this set of 845s for about 10 years. Well I've since checked the dates and it's closer to 13 years. My spare set is still on the shelf.
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Old 15th July 2019, 07:42 PM   #225
Bigun is offline Bigun  Canada
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The point of an 845, 211, 805 amp?
I have a Hammond 126B interstage transformer in my "junk box" that will allow me to drive the grid of the 845 from the plate of the driver. I don't intend to push much for A2, there's a thread I started on that topic already and it is better served with a different operating point and topology than I am now considering. What i am now considering is more of an A1 dominated amp, Sakuma inspired if you will.

I also envisage direct coupling of the input tube to the grid of the 2A3 given that there is no shortage of B+ on-hand, or I can use another 126B interstage as I have a pair of them.
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Last edited by Bigun; 15th July 2019 at 07:44 PM.
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Old 15th July 2019, 08:07 PM   #226
dhaen is online now dhaen  Europe
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You have your ideas set out and I respect that. Be true to what you want , it will be great おかげさまで。
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Old 15th July 2019, 08:48 PM   #227
Bigun is offline Bigun  Canada
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The point of an 845, 211, 805 amp?
I certainly have some ideas set out but they are not set in stone - this was supposed to be a quick and simple project to use up some SV811-10's from a friend who no longer wanted his amplifier - it sounded like a great opportunity to dismantle his amplifier, which was a complex A1+A2 amp built on two chassis and to use the parts to make something simple myself, to put 'my stamp' on it. This was distracting me from building my 2A3 Loftin-White but what the heck, free parts, how can I resist. But this SV811-10 isn't a simple tube to use as I have now since discovered. There aren't many designs out there for this tube and it is no longer in production. Initially looking to avoid high B+ and re-use a power trafo it only made sense if operated in A2 but there didn't seem a path forward to make a simple A2 amplifier. So I moved my goal to an A1 amplifier and I started to study Sakuma's designs, offering A1 operation with DHT driving DHT in simple cathode biassed designs. I thought why not an SV811 driving an SV811 - but they like to be run hot to keep the getter activated and that adds a complication when you need to run your driver as hot as your output combined with voltage limitations on interstage transformers etc. Short story - it got complicated again. So... I started to realized that I might prefer to switch to the 845 and go back and look at Sakuma's designs, cathode biassed, lots of iron, not always that conventional but sure look like fun. Does any of this make sense, am I the only weird guy here ?
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Last edited by Bigun; 15th July 2019 at 09:07 PM.
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