Acrosound High Power Williamson - HELP!!!!
In an attempt to further my DIY "career" ;), I've breadboarded the Acrosound "High Power Ultra-Linear Williamson" according to the following schematic:
I have made the following changes:
Please bear in mind that this is my first attempt to build an amp solely from a published schematic rather than a magazine article (I've built several that way).
With all that out of the way, I'm having a devil of a time trying to get this thing to behave! There are two main problems I'm having.
First, I suspect the voltage at that 175 uf cap (the one directly after the choke) is WAY too high. I'm reading 510V, and I'm guessing the value should be somewhere around 450V. This is even after running the whole works off a variac (my mains voltage runs about 122V). I realize most of this is because of poor transformer regulation - I'm presenting a 160-175 mA load to a transformer designed to take 465 mA. Any ideas on how to tame this beast? Would a couple of 50 ohm power resistors in series with the rectifier plates help?
Second, I can't seem to get the output stage to stabilize. Tubes are "matched", but even when I crank the plate voltage down to 450V, -60V is still required on the grids just to keep the thing from motorboating, howling and drawing excessive current. What to do?
Re: Acrosound High Power Williamson - HELP!!!!
You are flat SOL! :( Williamson style topology works ONLY when the O/P "iron" is creme de la creme. As you have found out, mundane stuff (like Hammond) gets you eaten alive by the phase shifts.
Either you buy superior O/P trafos or you change topology. IMO, switching to Mullard style circuitry is your best option. CCS loaded 'SN7 sections as the voltage gain devices and 2X 12AT7s as the LTPs will give you a good gain structure and an excellent net HD spectrum. Don't forget to use a 10M45S as the LTP tail load, instead of a resistor. 12AT7 section voltage amplifiers and ECC99 LTPs are another workable small signal combo.
Have you tried running it open loop?
Running this thing open loop was going to be my next try.
As for using Hammond iron, I've always had decent success with it, considering its limitations. I built Rick Spencer's beginner's amp (that little 12V6/12SL7 job from a 2001 Audio Express article) and was extremely pleased with it. I've also breadboarded Still's 70W amp from the 1999 Glass Audio project book. This also worked, although it seemed to sound a bit more harsh.
Both of these ran in ultralinear mode, both used global feedback, and both used Hammond iron. Neither suffered from any instability problems (its worth noting the 70W job ran 550V on the plates, and the tubes were running at pretty close to their total dissipation limits).
My main focus is to build something around 50W that's SIMPLE, cool :cool:, reliable and relatively inexpensive - and maybe to learn a little in the process. I sort of figured I could worry about acquiring "boutique" iron later on.
Re: Re: Acrosound High Power Williamson - HELP!!!!
For sure, even the original Williamson using the original Partridge OPT showed quite doubtful LF stability by itself - this has been pointed out by several people who should know what they are talking about (f.e, Morgan Jones, Claus Byrith, et al).
From own experience, you can get a Williamson unconditionally stable even with comparatively cheap Hammond iron, but I wholeheartily would agree that this is not a task for beginners, and "better" iron will help, indeed.
On the other hand, I doubt that more than say, 10% of any and all commercial designs and DIY amps featuring more than 2 stages plus a gNFB path would qualify for being "unconditionally stable" ;)
Do not exceed the maximum value of the capacitor-input filter for your particular rectifier tube. This value is stated in the data sheet.
The original Williamson used a 20H choke between the HV applied to the O/P transformer and the pre-amplifier stages. This normally avoids motor-boating (poor supply regulation).
See the rectifier tube data sheet if it is possible to increase the size of the 4H choke. The DTNW amp used 10H.
A choke-first filter is best. But you would have to increase the output voltage of your supply transformer. Unless you are happy with a bit less power.
Try removing the 47pF (47MMF) in the feedback loop. One of my DTNW clones oscillated because of such a cap.
Re: Acrosound High Power Williamson - HELP!!!!
Next, get the level of gNFB down. The original Williamson required these uber special OPTs since the original design used insane amounts of gNFB. No well designed hollow state rig requires that much gNFB these days. A big part of the problem was that back when the Williamson was designed, speaker design was very much a "black art", and lots of vintage speeks did wierd things at certain frequencies. The amps needed a lot of damping to tame those beasts. Modern speeks are designed better, since the advent of the Theil design methodology. Hollow state amps start sounding like solid state amps at ~12db(v) of gNFB. Once you get up to the 26db(v) that some Williamsons used, well, might as well go to Circuit City and pick up a SS amp: lots less frustration, and you won't notice the difference anyway.
Less gNFB means less stability troubles, and it'll sound better too.
If you're having problems getting the finals to stabilize, then you may have gassy finals. If this possibility is ruled out, then look for high frequency oscillations. If it's trying to make RF, that will cause this problem. You'll need to look for that with an o'scope, or perhaps a shortwave xcvr operated nearby. If it's making RF, you can pick it up on the SW. From what I'm seeing here, I suspect you have problems at both ends.
Second the open loop, then add gNFB slowly and make sure it is actually Nfb and not positive. The Hammond 1650R is nice Iron, and it has better HF response than the TO330 in any case.
Still, Eli's Mullard topology suggestion is a good one.
I can't really add anything useful to the advice that you've already received, but I learned a lot from reading it - thanks, everyone.
I hope you sort it out. Good luck.
Whew doggies, Jethro!
It seems I've opened a can o' worms here. Much of what you guys are saying seems to make good sense.
I've had to suspend work on the amp this evening, as tomorrow is the first day of classes at my university. I'll probably get back to work on it tomorrow night and I'll definitely try out your suggestions - especially opening the NFB loop.
Thank you guys for all your help!
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