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Last week, Ron Oehlert was very kind in helping to diagnose a bad transformer heater supply. New tranny and I'm now getting very close.This is a KT88 SEUL amp. I have 6SJ7 drivers working very well - Fi Primer 6L6 style But not content to leave week enough alone, I am trying to dial in a 717A driver for a SEUL output.
Compared to the 6SJ7, the 717A is bright (different component values of course)- but running the 717A plenty hot. My question is what variables on the driver control the high frequency roll off.
B+=266, Rp=33k, Vp=160, Rg1=100k, Rg2=160K, Vg2=117, Cg2=1uf, Rk=680, Ck=220uf, Vk=3.4.
Rnfb=3.8K with R to ground of 200R.
Follow Ups:
This 2A3 schematic shows the same 717A operating conditions that Yamamoto use for their 717A/45 SET.http://www.audiohobbyist.com/images/7172A3/7172A3sch.jpg
The 45 is biased at around -50V so driving a KT88 should pose no problems.
The 6SJ7 can be a very good driver for pentodes, so I wouldn't necessarily assume that something more exotic will sound any better.
I've been looking at using the 6sj7 as a driver for a 300B amp, and I have been wondering about how it would compare to the 717A . To hear you say, difference were subtle helps me.I will reserve the 717a for a wierder old Globe triode/retro looking amp (and I think I will use the little 717a lookalike rectifier 7e8, to accompany.
Big Sunshine today.
I wonder how the Sun is in Tucson AZ?
-3db
A bright sound isn't necessarily a result of tilted frequency response. In fact, I'd expect virtually any tube in this application to have fr tremendously flat compared to other parts of the system. Attempting to fix a bright sound by bleeding off high frequency is, in my opinion, not the way to nirvana; it's a band-aid that attempts to cover a different problem. Better to fix the problem that to try to hide it.So what is the problem? I dunno. Could be the 717A is just not the tube for you in this application. You might try different operating points.
It could be you have some high frequency oscillations going on. Stopper resistors on the screen and/or grid might fix everything. Try a couple of k ohm on the screens right at the socket. A couple hundred ohms at the grids might also be necessary.
If those things don't get you where you want to be, then if I were in your position I'd look for a different tube. :)
A bright sound isn't necessarily a result of tilted frequency response.A classic example is when comparing copper v silver wire - the frequency plot looks identical, but they sure don't sound the same.
Thanks Dave. I have 1k on the input of the KT88 outputs. Are you suggesting some on the grid and screen of the input?
I remember Pete Millet noting the same issue and suggesting, in the audioxpress writeup of his E-linear amplifier, a small valued capacitor from pentode plate to ground to roll of the high frequencies . He was using a D3A pentode driver.Here's the schematic link from his site. The 1500uF/600V cap is what he used.
http://www.pmillett.com/file_downloads/KT88_SCH.PDF
correction, .. i meant 1500pF (not 1500uF), .. but the schematic's correct.
Well, I put in 820pf polystyrene, and it is much better. Now the differences between the 717A driver and the 6SJ7 are much more subtle and will take some more listening. There is an obvious gain difference. Maybe I'll have to adjust the NFB down just a bit - but don't want to lose control of the bass.Darn this is a fun hobby.
If you do ever feel like tinkering some more with your project, I thought I'd suggest you study that schematic I linked. It uses the UL tap of the OPT to supply the driver plate. It also uses regulated screens on the driver
You know, I've been looking at that off and on. My April 05 AudioXpress got lost somewhere so it awaits me rounding up another.
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