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In Reply to: RE: Hey Mike, how about this idea? posted by keto on April 27, 2009 at 16:50:05
Because if you are not, may I suggest the 112A. It's a better sounding DHT and it's easier to work with as far as load goes. You can bias the tube so that the plate resistance will fall between 3k5 and 4k making either the 10K or 15K version of the B7 an ideal load.
I used the 112A as the last (line stage) of my last preamp. Had a pair of S&B TX-102s TVCs in front and loaded it with a BCP-14. Very sweet, liquid tube. Gets you 95% of the way to a 26 without the filament hum hassles.
FWIW, the 112A replaced a 76 output stage that was C4S loaded. The OPT in both cases was a Peerless 15095.
The only potential downside is the loss of gain. Mu of the 112A is around 8.5. Still, if that works in your system, I think it's a better choice than the 76. Globe 112As (recommended) are still pretty reasonable on eBay.
Maybe it's just me, but in my experience, the 112A is about 110% of a 26 :-)
The lack of filament hum hassle is icing on the cake.
I think it depends on just how much aggravation you need in your life. ;-)
Sounds really interesting. What did you do for the 5V 250mA filaments?
Then I was using current-regulated DC. Basically the Ronan Reg with whatever number of stages were required to deliver 5VDC at the required current. I *think* I started with a 10V 1A filament tranny. Filaments were dead quiet.
Today I would scrap the SS regulators in favor of filament chokes with a cap input only if I needed it to get the required voltage. Make sure the choke is the last element before the filaments. I want to stress that I'd make that change not because I think current regulated filaments are "bad", but because my design philosophy has shifted toward simplicity and as few active components as necessary. But that's just me.
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