apparently amplifier tests conducted by the military

Some interesting tidbits in that file...

"Practically all Williamson amplifiers tested showed a tendency to "ring" at about two or three cycles. This tendency was eliminated in all cases by changing the values of the coupling capacitance in the grid circuit of the output tubes from 0.25 mfd to about 0.02 mfd. The low-frequency response was not in any case seriously affected by this change"

Don't know if I've ever looked for ringing at such a low frequency. I wonder if it's an anomaly of the power supply...

I've not run the timing constants, posted on March 18, 2015 at 10:23:42

Ralph
Manufacturer

Posts: 4772
Location: Minnesota
Joined: April 24, 2002

-but in general if the power supply has timing constants that are higher than that of the amplifier circuit, there will be some instability in the amp (LF resonance) as a result.

These days we are more likely to fix that with bigger filter caps, but actually cutting off a little higher with a smaller coupling cap is the better move IMO as the cap will have less effect otherwise on the sound- its easier to build a good small cap than a good big cap.

"its easier to build a good small cap", posted on March 18, 2015 at 10:35:30
Triode_Kingdom Audiophile Posts: 10047 Location: Central Texas Joined: September 24, 2006	"its easier to build a good small cap than a good big cap" And in addition, smaller coupling caps can produce a much shorter overload recovery time. I'm always careful not to use caps that are larger than necessary for this purpose.

What TK said~nT, posted on March 18, 2015 at 10:48:14
Cleantimestream Audiophile Posts: 7551 Location: Kentucky Joined: June 30, 2005	~! The Mind has No Firewall~ U.S. Army War College.

"Low frequency ringing"..., posted on March 18, 2015 at 11:18:21

Steve O
Audiophile

Posts: 12376
Location: SE MI
Joined: September 6, 2001

...I've noticed this phenomenon in a number of vintage amp designs, notably the various Williamson implementations. It manifests as an intermittent to continuous low frequency "bouncing" of the output at a couple of Hz with an amplitude of maybe 1/2V P-P. Easily noticible as woofer pumping. It can be induced by power line perturbations, large signal transients or even just physically pushing on the woofer which injects a low frequency signal into the output that the feedback circuit attempts to correct. While the PS may indeed be a factor, I was not able to significantly reduce the ringing by adding practically large amounts of decoupling capacitance betw stages. The cure for the designs I've worked with involved modifying the time constants of the coupling of two cascaded stages within the global feedback loop.

Typically, the ratio of time constants of successive stages in the unstable vintage designs was about 5:1. A rule of thumb even back then was that the ratio should be a minimum of 10:1. I've found that 20:1 to 25:1 to be most effective w/o significant affect on LF response or distortion. Some stable designs of the era achieved the staggering of time constants by using large coupling caps at g1 of the finals and small caps at g1 of the drivers. I like to reverse this with relatively small caps at the finals and large caps at the drivers. This seems to reduce the effects of output stage blocking under heavy drive conditions.

RE: "Low frequency ringing"..., posted on March 18, 2015 at 11:52:41

Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000

It may not be possible to use small caps. at the I/Ps of the "finals", due to grid to ground resistance limits. This is especially true, when "fixed" bias is employed.

As for blocking, I recall reading where 1 fellow was going to implement "PowerDrive" using ZVN0545As, even though positive grid current was not permitted. A rather high valued grid stopper prevented significant g1 current from flowing.

Eli D.

PowerDrive

Small is relative..., posted on March 18, 2015 at 15:13:27

Steve O
Audiophile

Posts: 12376
Location: SE MI
Joined: September 6, 2001

...Ex: Heath W-3AM Original circuit uses 0.01uF/470K at input to driver (6SN7) for fc~34Hz and 0.25uF/100K at input to finals (5881 w/1K "stoppers") for fc~6.4Hz. Ratio of tc~5. It bounced at LF. Revised circuit uses 0.47uF/470K at input to driver for fc~0.72Hz and 0.1uf/100K/1K at finals for fc~16Hz. Ratio of tc~22. No bounce and nearly identical LF distortion @-3dB (~10W) of max rating with 20Hz THD easily meeting original spec of ~0.3%.

Interestingly, the revised values are exactly those of the Craftsmen C500A, an inherently stable triode Williamson design and they're similar to Dave Gilespie's stability enhancing revisions to the Heath W4, an ultralinear Williamson using 6SN7s and 5881s but with a lower quality OPT than the W3.

RE: "Low frequency ringing"..., posted on March 18, 2015 at 16:43:08
lovetube Dealer Posts: 3008 Location: Melbourne Australia Joined: June 8, 2003	Interesting Steve. i might try this my self as I have one EL34 PP that do the same thing's in the LF . it sound great but do have some woofers pumping not much but you can see it. thanks for the post LT

Morgan Jones is rather adamant about using Voltage Regs on B+ for that reason, posted on March 18, 2015 at 16:57:03
richardl Audiophile Posts: 3555 Joined: September 5, 2002	He said in Valve Amplifiers that it was a waste of power to have the amp slowly oscillating at 0.5Hz or so and firmed up the power supply to remove it. He said that most amps that he measured did oscillate at very low freq. It was a surprise to me.

RE: Morgan Jones is rather adamant about using Voltage Regs on B+ for that reason, posted on March 18, 2015 at 17:20:03

Steve O
Audiophile

Posts: 12376
Location: SE MI
Joined: September 6, 2001

While regulation is desirable, it's not practical in vintage designs. In new/DIY designs, regulation may be possible but it adds expense and size, esp at higher power levels. Real world implementation of regulation typically involves solid state devices that in my experience are not as reliable as the tube circuits they supply....kinda brute force too. I prefer circuit/component solutions if possible. As always YMMV.

RE: Morgan Jones is rather adamant about using Voltage Regs on B+ for that reason, posted on March 18, 2015 at 17:34:32
PakProtector Audiophile Posts: 12364 Joined: May 14, 2002	that and I have yet to need more than 3 stages, including the linestage. usual practice is two stages, plus a linestage. just no need for even a 5-20 style 3-stage. cheers, Douglas Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

RE: apparently amplifier tests conducted by the military, posted on March 18, 2015 at 18:49:54

Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001

The ringing is caused by the large amount of global negative feedback, together with inadequate attention to the phase response and stability margin. To be fair, this was the early days when feedback was still new to most people. The extreme form of this instability is called "motorboating". :^)

The real problem is that the output transformer is inside the loop, and its inductance is a strong function of signal frequency and level. I have always guessed that this is the reason the Dynaco Stereo 70 has some feedback from the output tube plates, reducing from the transformer secondary while maintaining large amounts of total feedback.

RE: Small is relative..., posted on March 18, 2015 at 19:06:58
Triode_Kingdom Audiophile Posts: 10047 Location: Central Texas Joined: September 24, 2006	"0.1uf/100K/1K at finals for fc~16Hz" This seems preferable to me also, but it does raise the issue of low frequency phase shift. I'm skeptical as to whether such shifts are audible, but many DIY audiophiles do whatever they can to minimize the effect. There's definitely a conflict of interest in resolving the problem.

RE: Morgan Jones is rather adamant about using Voltage Regs on B+ for that reason, posted on March 18, 2015 at 19:12:28
Caucasian Blackplate Industry Professional Posts: 8313 Location: Seattle Joined: June 18, 2004	It's not all that practical in new designs either, unless you stay all class A, which sort of defeats the need in the first place. The demands placed on such a regulator in a class ab amp are pretty nuts, but at the same time, a choke input filter will get you close.

LF phase shift..., posted on March 18, 2015 at 20:21:18
Steve O Audiophile Posts: 12376 Location: SE MI Joined: September 6, 2001	...can be significantly reduced thru judicious use of negative feedback. See attached plot from D.T.N Williamson's article in Wireless World. I'd also predict LF "ringing" @ 1-2Hz based on this plot. The circuit this plot is derived from has a tc ratio of about 1:1, much less than the 10:1 rule of thumb.

RE: Morgan Jones is rather adamant about using Voltage Regs on B+ for that reason, posted on March 18, 2015 at 20:37:32
Steve O Audiophile Posts: 12376 Location: SE MI Joined: September 6, 2001	I think a good case could be made that an active regulator is a feedback amp that has to operate down to DC with great stability compared to an audio amp that only has to run down to 10Hz or so. Regulating an amp's supply is sorta like cascading two amp systems.

RE: LF phase shift..., posted on March 18, 2015 at 21:45:12
Triode_Kingdom Audiophile Posts: 10047 Location: Central Texas Joined: September 24, 2006	I love the way it goes through a 180 deg shift at about 2 Hz. I can practically hear the OPT screaming for mercy... :)

RE: apparently amplifier tests conducted by the military, posted on March 18, 2015 at 23:40:42
Michael Samra Dealer Posts: 36118 Location: saginaw michigan Joined: January 30, 2005	Some of that ringing can be corrected by changing the feedback bypass caps in value and I have even corrected it by using better filter caps in the power supply.When you lower the impedance of the power supply by using better caps,it has an effect on the higher frequency sq waves. "For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

I don't disagree, posted on March 19, 2015 at 10:16:25
richardl Audiophile Posts: 3555 Joined: September 5, 2002	I threw it out there because I do not regulate B+ at all times and was wondering what happened when you do. I have only regulated B+ on drivers not power tubes.

My current "Unit Under Test" doesn't do this, posted on March 19, 2015 at 10:29:23
Triode_Kingdom Audiophile Posts: 10047 Location: Central Texas Joined: September 24, 2006	That does sound like a plausible explanation. I checked the HK A500 that happens to be on my bench at the moment, but was unable to detect any sign of this phenomenon. Maybe the relatively low Z of the voltage doubler and the bandwidth of the OPTs are factors. I'd like to find an amplifier that does this so I can confirm for myself that the cause lies with NFB functionality.

RE: "Low frequency ringing"..., posted on March 19, 2015 at 14:29:20
Michael Samra Dealer Posts: 36118 Location: saginaw michigan Joined: January 30, 2005	Steve The W4s are probably one of the worst offenders in that regard because of the iron. "For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

RE: My current "Unit Under Test" doesn't do this, posted on March 19, 2015 at 15:18:44
Paul Joppa Industry Professional Posts: 7296 Location: Seattle, WA Joined: April 23, 2001	It's been long time, I might be wrong, but IIRC Williamson himself mentioned this in the first article.

RE: "Low frequency ringing"..., posted on March 20, 2015 at 03:54:29
DrRick Audiophile Posts: 44 Location: On the (formerly) beautiful Jersey Shore Joined: January 28, 2005	Sadly, this is true for the W4. Peace, DrRick

RE: apparently amplifier tests conducted by the military, posted on March 20, 2015 at 04:05:56

DrRick
Audiophile

Posts: 44
Location: On the (formerly) beautiful Jersey Shore
Joined: January 28, 2005

Has everyone gotten a copy of thus report? It was done in the early-mid 50's in an effort to buy some small amount of hardware (for Navy sonar, I think) off-the-shelf.
I requested it in another life when I clearance high enough to request a lot of cool stuff, although I had no "need to know" for this report. I was working on Star Wars related sonar stuff. I think the only reason I got this report was because it was 30 years or so old at the time and no one had ever bothered to lower or de-classify it.
There was a lot of joking about me finding anything tube-related to request!

It really is the way to construct and author such a report. The proposed uses for the equipment were very clear if one were to read between the lines back then, thus the comments on stability, ringing and bandwith.
Peace,
DrRick

RE: apparently amplifier tests conducted by the military, posted on March 20, 2015 at 04:08:14
DrRick Audiophile Posts: 44 Location: On the (formerly) beautiful Jersey Shore Joined: January 28, 2005	I just looked at the link. That's the report I remember. Back then I was amazed to find it existed. Peace, DrRick

time constants, posted on March 20, 2015 at 07:09:04
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	Steve, If following the 10:1 rule of thumb, this means that the time constant of the PS to the input and driver stages should be 10 times that of the PS to the output stage, or what? Probably this is obvious to the discussants, but not to me. However, this is a great thread for learning from. Thanks.

RE: time constants, posted on March 20, 2015 at 10:47:19

Steve O
Audiophile

Posts: 12376
Location: SE MI
Joined: September 6, 2001

The time constant rule of thumb applies to interstage coupling when those stages are contained within an overall feedback loop. The basic idea behind this is to stagger the RC time constants (or, effectively the Fc) of cascaded gain stages to avoid stacking of phase shift at or near one frequency. IMO, the power supply is not directly involved in this although a high impedance supply may incite the ringing of an inherently unstable design due to susceptibility to outside variation at low frequencies.

I'll look for a schmatic I can edit into here where the components of interest can be highlighted for clarity.

RE: time constants, posted on March 20, 2015 at 11:10:21
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	Thanks, Steve. Glad I asked the question, because it appears I was on the wrong track to understanding this. Someone else mentioned "motor-boating", which I think can be a consequence of PS filtering. That set me on the wrong path.

RE: "its easier to build a good small cap", posted on March 20, 2015 at 13:30:38
DrRick Audiophile Posts: 44 Location: On the (formerly) beautiful Jersey Shore Joined: January 28, 2005	+1 Peace, DrRick

RE: time constants, posted on March 20, 2015 at 20:04:48
Steve O Audiophile Posts: 12376 Location: SE MI Joined: September 6, 2001	See attached image for time constants under discussion. In this example (Heath W-3AM) the time constant ratio is ~ 5:1. An interesting aside is that when considered at the base units, the unit of the RC product is the second (s).

RE: time constants, posted on March 20, 2015 at 22:02:54
Lew Audiophile Posts: 10911 Location: Bethesda, Maryland Joined: December 11, 2000	Thanks for going to all that trouble to illustrate the principle. I get it now.

RE: My current "Unit Under Test" doesn't do this, posted on March 21, 2015 at 13:34:54
Triode_Kingdom Audiophile Posts: 10047 Location: Central Texas Joined: September 24, 2006	I just made another measurement in preparation for stowing this project away temporarily. The output section of the A500 has only 13dB NFB applied. That's considerably less than many amplifiers, and maybe the reason the effect being discussed doesn't appear in this model.

"[...] unless you stay all class A, which sort of defeats the need in the first place ", posted on March 22, 2015 at 05:41:34
jorisdallaire Audiophile Posts: 356 Location: Quebec city Joined: July 26, 2012	Hello, Can you elaborate on this? My undertsanding might be wrong, but I always thought a class A amp was where there was the greatest benefits to have a regulated B+. Since in class A the PS is in the signal loop, the lower Z it has, the better. Of course, as Steve posted below, the regulator's error amp is an other amp that follows the supplied stage's signal demand. Thakns, Joris

RE: "[...] unless you stay all class A, which sort of defeats the need in the first place ", posted on March 23, 2015 at 11:14:03

Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004

In a class A amp, provided the output transformer has sufficient inductance, you aren't going to see huge deviations in power supply demand.

If you have an AB push-pull amp, when you leave class A and move towards class B, the current draw from the power supply changes drastically. (Regulation helps here)

This isn't to say that a fully regulated class A amp is a bad thing, it's actually easier to do and will sound different, but it's not solving the obvious issue that class AB amps have.

Thanks for the clear explanation! (nt), posted on March 24, 2015 at 10:54:07
jorisdallaire Audiophile Posts: 356 Location: Quebec city Joined: July 26, 2012

Tube DIY Asylum

apparently amplifier tests conducted by the military

98.117.63.211