|Author:||MasonAtom (registered user: 973 posts )|
|Date:||Mon, Jun 25th, 2012 @ 14:40 ( . )|
On 06/25/2012 @ 10:57, John Hannon wrote :
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...
John, FYI, I've measured about 80V peak to peak gain with a similar setup on a 5654 (not bootstrapped, but a 39k load with 1.8k cathode + 180k screen resistor). I originally started with like 220k on the plate but ran into microphonics, which were largely cut down by lowering the gain.
--* New prototype amp!
6/25/2012 @ 16:03--John Hannon
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