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MagneQuest/Peerless Forum Welcome! Need support, you got it. Or share your ideas and experiences. |
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MagneQuest/Peerless Forum Welcome! Need support, you got it. Or share your ideas and experiences. |
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In Reply to: The 100k is going !! ;-) posted by Bas Horneman on May 13, 2004 at 16:57:17:
I'm using the 6SN7 (mu=20) at the moment, with just the "grid-choke-lite" and find that almost all CD recordings will drive the amps into clipping, so my next experiment will be with the 27 (mu=9). If you're so inclined, you could probably put a volume pot between input and grid and a selector switch before that and call it an "integrated".While quality of resolution is progressing in my amp, there are still lots of mysteries.
If you look at the shared 2A3 cathodes...
1. In terms of function, you can put a 375R/50W or nice 375R/20W Mills resistor, with whatever bypass cap is typical (2200uF/100V?), and achieve a performance similar to the CCS. An unbypassed resistor may add a "swimming" sound to the amp.
2. There are three elements of particular interest for refining the sound: balancing filament humm; balancing DC current between primary "half"-windings; and AC symmetry. I tried a 100R pot between separate filaments and a CCS and though it got pretty quiet, the pot seemed to add something unpleasant to the sound. Also, the idea was to balance DC between halves, but the value was too high I think and I lost some of the neat noise rejection of the PP output, turning the pot through what must have been multiple nodes. A fixed pair of 25R resistors and shared filaments has been the best solution, sonically, to balance filament humm. This is part of Lynn Olson's solution, it would seem. Gary Pimm uses 11 ohm resistors in his PP47. Of course you've got to have a well-behaved filament transformer. My next experiment (weeks away) will be to use the 25 ohm resistor pair, and apply the 50R Vishay trimmer between one 2A3 plate and its OPT "half"-winding; first, switching the pair of Valve Arts with a zero setting, to find out which way they want to go, and then CAREFULLY trimming-in a little series resistance. I'm expecting that just a couple ohms will do the job, but am prepared to find that, yet again, I'm mistaken. The Valve Arts, BTW will sound spectacular if the circuit is functioning well. AC symmetry, so far, seems like something you "enjoy" in the components themselves.
Your circuit seems to have a lot of high-figure-of-merit elements, but it might be worth it to start with only the essentials, about half of what's on your schematic, get the amp quiet and sounding good, and then start complicating things. All paths do lead to good sound, but that way you can first eliminate basic inefficiencies, and then refine each truly necessary element, one by one. My B+ filter consists of two 1N4007s, a BFFC-150, and a 470uF/450V cap. Yikes, you will say...
I found a 6N1P sounded lean and clean as an input and the 6C45 really dynamic in a really great way; both running at 10mA and 150Vp. That said, I'm fascinated with the fixed volume idea, from my limited experience with the 6SN7 and grid-choke-lite, and so have slated the 27, 26 and 12B4A for the next round.
So the 100R is from J Lessard? That's a good one. You know the OB3, OC3 and OD3 have the same pinout? With two octal sockets you can range from OB3+OB3=180V to OD3+OD3=300V. Also, you might want to draw 20mA through the VR, equal to the ECC99's draw -- this idea, like 9/10ths of what is in my amp, from the Bottlehead brain trust.
Again, looking forward to reading about your adventures... --keto
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Follow Ups
- Re: The 100k is going !! ;-) - keto 08:32:45 05/14/04 (1)
- Re: The 100k is going !! ;-) - Bas Horneman 08:58:59 05/14/04 (0)
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