For anyone running a K-502 with a FR driver

[Jeb-D.]

Run it in pentode mode, remove the NFB and slap a Zobel on the driver.

Do it...Do it.

[kevinkr]

You might want to tell those of us ignorant of the K-502 just what it is?? Your post is just a bit too obscure...

I design and build all of my own gear so if this a kit or something a point of reference might help.

[planet10]

a low budget PP amp based on a triode/pentode similar to 6BM8 -- there are a couple other variants available as well

http://www.tubesandmore.com/scripts/...FXP?item=K-502



dave

[Jeb-D.]

My bad Kevin. Yes, it's a kit that's dirt cheap. I usually build my own designs as well, but once when I was unemployed and couldn't afford to build one of my own, I built one of these.

That which I mentioned above is for those running low Q full range drivers with a k-502 kit amp. Try it, you won't be sorry.

[kevinkr]

Cool...
I tend to be very focused on making my own stuff, kit looks pretty decent. I think the tip might apply to amplifiers with a similar topology and full range drivers like the ones you are using. Tell us a bit more about this setup. I'm always curious and it may serve as a good starting point for recommendations to people new to the hobby..

[planet10]

What it sounds like to me is that you are taking advantage of a pentode being a natural current source... by putting in variable feedback you could literally have a "tuning" knob.

Without feedback the amp will be more sensitive to any shortcomings in the power supply (or alternatively, an opportunity to see where the power supply needs improving. Regulating the screens would also be a potentially big improvement.

dave

[Jeb-D.]

Experimenting was done with a FE167E driver in a BR box. With a little bit of tweaking the combo (K-502 + FE167E) can produce excellent results for the beginner or person on a very tight budget.

This amp paired with these speakers sounded awful initially. I've heard the amp sound good with other speakers so this made me start tweaking with different configurations. Both on the amplifier and by adding correctional circuitry on the driver. My observations may or may not only apply to this particular application, but experimentation lead me to believe the following:

Pentode output stages usually get a bad sonic reputation because of their high output impedance. It is not because of the nature of their curves as most people would think.
This is due to one of the following circumstances:
-Open loop Pentode amp driving a dynamic loudspeaker that has a rising impedance (which is all of them that don't have a Zobel) can produce a harsh sound. This starts the "Pentodes sound harsh" statement.
-Some designers choose to apply negative feedback to lower the impedance. This can often (but not always) lead to a more sterile, lifeless sound. This starts the "Pentodes sound like transistors" statement. Yes, their curves do resemble a transistors, but how often do we hear a good, linear, transistor with no NFB?

Most designer’s triode strap the tube or run in UL. This usually produces greatly improved sonic results, but there is no denying that it would be nice to have the extra power that Pentode puts out.

So, adding a Zobel to the driver greatly improved the harsh upper midrange. Removing the NFB brought the low frequencies up to a level that was perceivably equal (ok, maybe a little bit more, I lika da bass) to the "shouty" vocal range. But the bass is tight, not boomy and all smeared together like you would think. There is a slight perceivable dip between 150-ish to 400Hz, probably do to baffle-step. While the low frequencies are being compensated for, the lower mid range isn't. Removing NFB also made the instruments much more vibrant and gave them a greater presence, so now the vocals don't totally dominate as before. The attack sounds right on, especially percussion and piano. The amp speaker combo now sounds 100x better than initially. Dare I say, that for this amp, the sound of Pentode even spanks Triode strapped! While I am totally pleased with the sound, it wouldn't be a hobby if I stopped there.

Plans for the future of this project:
-As Dave pointed out, power supply hum did go up a little. It is still not audible from listening position. But, I still plan to reduce it.
- Take advantage of the 50Hz impedance peak. Build a larger box that's tuned lower. I may be able to lower the f3 by 10Hz or more.
- Once the box is tuned to be theoretically flat, add an active baffle-step-compensation circuit that is appropriately tuned to the width of the baffle.

Other thoughts:
- Pentodes for Magnepans? Mmmm... Nice flat impedance curve, and can supply the power.

Nelson Pass also did some experimenting with using high Z out amps with single drivers:
http://www.passdiy.com/articles.htm
Current Source Amplifiers and Sensitive / Full-Range Drivers:

[gofar99]

Good info, I likewise removed the NFB on mine. FYI the K-12 is essentially the same as the K-502. A web search will turn up lots of mods for these amps.

[Jeb-D.]

Good info, I likewise removed the NFB on mine. FYI the K-12 is essentially the same as the K-502. A web search will turn up lots of mods for these amps.

Yeah, I've actually read your article on diyaudioprojects before. I plan on doing the high speed rectifier + snubber mod too. The coupling capacitors have already been upgraded and the input capacitors deleted.

[gofar99]

Hi Jeb, The PS is one of the weak links in the K series amps. The new K-8 has a much larger ps trannie and it ought to help. A few diyers have changed the ps to a choke input using Triad C-40XS chokes from Allied Electronics (cheap parts) with good results. Another fix is to beef up the ground and power connections on the board. A few have gone so far as to star ground the amp. I have not gone that route so I rally can't say how much improvement it makes. If you are interested in such mods, let me know and I'll look up their link.

IMHO, the best "easy" mods were the coupling caps. Simple not too expensive and made a big difference in mine. The Hammond outputs and general cleaning up of the B+ were next. Part of the reasons for switching to the Hammonds was that my main speakers are 4 ohm and the original trannies were only 8. Every little bit helps when you only have about 5 watts.