|Author:||John Hannon (registered user: 395 posts )|
|Date:||Mon, Jun 25th, 2012 @ 10:57 ( . )|
Yeah, I'm really digging this tube. I picked it based on size and price mostly--the chassis had all these 7-pin sized holes, and I knew I wanted a pentode as the second stage, having had success with a 6SJ7 in that position in my last build. I wasn't expecting much given how much I loved the 6SJ7 and how tiny the 5654 turned out to be. But while the each contributes a different flavor to the sound (AFAICT--they *are* in different circuits), I'm not sure which I like more. Different flavors, both quite tasty.
I'd love to build a test amp where I could swap in a variety of small signal pentodes and compare how they sound driven. Maybe that's what I should do with the prototyping rig I started to build last year, before I said f**k it and started "live prototyping" point-to-point in the intended chassis.
FWIW, my process for figuring out bias points for preamp pentodes has devolved into something like this:
-look at data sheet, draw a load line to come up with a Ra value based on guesstimated voltage and aiming for a bit below the knee (more or less)
-sim in ltspice, find value for Rg2 that gives a good ratio of plate to screen voltage (usually screen slightly lower than plate)
-in LTSpice, mess with the cathode resistor until it looks like I'm getting good gain and maintaining reasonable voltages on plate/screen
-build, then tweak Rk and Rg2 by ear with ye olde resistor substitution box (my sub box is pure comedy, a HeathKit from probably the late 60's early 70's--huge carbon comps that are nowhere near the indicated value--in this case my sim told me 2K would probably be a good value for Rk. When I tweaked with the sub box I arrived at 1.5k [indicated]. I unhooked the sub box and measured it. 2K, almost exactly.)
This process gives me a bias point with pretty low gain for a pentode. I get some of it back using the bootstrapped CF. I think lower gain is probably a better way to use these tubes though--in both cases my amps with pentodes in the preamp have gone into combos, and I've had no issues with microphonics.
What does it mean that FireFox thinks 'guesstimated' is spelled correctly, but 'pentode' is not? Then again FireFox thinks 'FireFox' is a misspelling...
--* New prototype amp!
6/25/2012 @ 11:02--John Hannon
6/25/2012 @ 14:40--MasonAtom
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