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24.27.41.140
My apologies for banging this drum again. It's holding up my project, because the entire layout depends on the positioning of the power and output transformers. I've been studying this topic in more detail and searching everywhere for commercial examples that might point to an orientation known to work well. But in terms of the relationship between the transformers, as well as between the transformers and the output tubes, there seems to be no correlation whatsoever. The output transformers appear to be oriented every which way relative to output tubes, including beam power tubes, and there's absolutely no observable pattern to the positioning of the power transformer.
This issue is probably exemplified best by the Dynaco SCA-35, shown below. The two output transformers are positioned in what must be both the best and worst locations relative to the power transformer. The same is true of the relationship between the OPTs and the output tubes (although the latter in this case are pentodes, not beam power). The point is that when I search Scott, Fisher, Dynaco, HK and others, the orientations appear to be totally random. Cores pointing toward the output tubes and away. Power transformers both aligned with and 90 to the OPTs. And none of this seems to create noticeable differences in the specifications. The SCA-35 - as only one example - does not indicate any unusual numbers regarding either hum/noise or distortion.
I guess this leads to a critical question. Has anyone here seen a PP AB1 amplifier with hum that was induced between the power transformer and an OPT, or issues that could be traced to interaction between an OPT and output tubes? As time goes by, I'm becoming more and more skeptical about the real-world likelihood of this occurring, due primarily to the huge variety of layouts and orientations used by well-respected commercial manufacturers. Nevertheless, I'm putting out the call to anyone who has seen this issue in practice. I just want to explore all the available resources before committing to a layout and cutting the chassis.
Thanks for everyone's patience with my obsessiveness on this topic.
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Buy Chinese. Bury freedom.
Follow Ups:
Did not consider this on my first build. Huge Irons, and I had the power facing the same way as the output. Choke in the middle facing 90-degrees to both. hum was low, but as soon as I lit the power TX it started( long before the power tubes got warm and conducted ).
The new amps have power and choke ||, and the OPT 90 degrees to this.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Thanks for that info. Were the PT and OPTs all vertical mount above the chassis?
--------------------------
Buy Chinese. Bury freedom.
Yes the were all on the top. The OPT was a canned A441 Dynaco. That beast is no longer with me...the big Dynaco OPT's have all kinda sucked IMO.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
the archives need a reason why you think "the big Dynaco OPT's have all kinda sucked IMO" so please clarify. And please don't quote any gobbledeegook, just your test figures construction aural reasons etc. I and others would like to know.
Ptolomy Almagest
the atwood scan.... (page 1)
2
Edits: 04/06/21
Is the John Atwood list related to a particular test jig or amplifier (as the notes seem to indicate)?
Atwood clarifies in notes dated 12-29-90Test Set-up:
The transformer under test is driven by a push-pull pair of Tung-Sol 6550s with a regulated plate voltage of 475V and a regulated screen voltage adjusted to give 75ma idle current in each tube. The 6550s are driven by a very wide bandwidth and low distortion driver stage. There is no feedback across either the 6550s or of the output transformer. Measurements are made into a 4, 8 or 16 ohm resistive load. the Z p-p and ultra linear ratio are calculated from voltage measurements made on the primary when the secondary is adjusted to a fixed voltage @ 1KHz corresponding to a 100 mw output.
Thanks for that :-). I can now see that the results are from John's one electron website.So PP stage is operating in pentode mode, except for those UL listings.
The comments about testing with feedback are interesting, as there would be very little effective practical feedback happening at the -3dB ends in a normal amp.
Edits: 04/06/21
It's not clear from descriptions...to me at least...that UL taps were used when present. Also, FWIW, when I measure pri/sec impedances (turns ratio actually) of some of my samples of OPTs in Atwood's tests, I arrive at different values than Atwood reports. My meas usually closely parallel mfgr's specs. I think part of the difference is that Atwood assumes the 8Ω tap is truly 8Ω whereas I've found in many cases it's actually 9Ω.
"Atwood assumes the 8Ω tap is truly 8Ω whereas I've found in many cases it's actually 9Ω."
How would you know?
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Buy Chinese. Bury freedom.
With 4V present at 16 ohm tap, 3V will be present at 9 ohm tap and 2V at 4 ohm tap.
With 4V present at 16 ohm tap, 2.83V will be present at 8 ohm tap and 2V at 4 ohm tap.
Either apply nec voltage to pri to achieve 4V at 16 ohm tap and then meas others or apply 4V to 16 ohm tap and meas others. Generally you'll get either 4:3:2 relationship or 4:2.83:2 relationship. Assumes 16 ohm tap is indeed 16 ohms. Usually is.
There are a few families of OPT's that like the 9R tap. The Peerless 20-20 Plus comes to mind. Two 4R, CT secondaries. 1, 4, 9 and 16R are possible.Should have said, *some of the 20-20 Plus. Take for example the 5k/40 Watt models, S256Q and S258Q. The former is a 2, 4, 8 and 16 secondary pair, and the S258Q is a 1, 4, 9 and 16. Both 20-20 Plus family.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Edits: 04/08/21
unlike the other guy I know who's played around with a lot of transformers. :)
I see a few patterns from these tests I posted above. (and have a few questions)
1- the lower impedance transformers seem to measure well
2- dynaco seemed to compromise other parameters in the names of pretty square waves
3- peerless seemed to have compromised square wave performance for other parameters
A quick glance at this would tell me I'd prefer my dynaco Z565 to my peerless S265. That is not the case from listening.
Is the square wave performance an important measurement in evaluating a transformer?
Why are lower impedance transformers not popular given that they seem to do well in both HF extension and square wave measurements? typical recommendations I see always are more primary impedance for less distortion and a small sacrifice in power output.
A lot depends on how and what you enjoy
listening to, and what are the speakers?
I have built and serviced many tube amps over the
years-- most of them push-pull.
I like to hear clean, fast, accurate, on-time dynamics.
I like detail also, but will not sacrifice the above to get it.
I have found (not all for the same reasons,
but as a generality that happened in MOST cases),
that I liked tubes run at low plate dissipation levels,
and that loaded heavily with a very low-DCR, lower impedance
primary of the output transformer.
Understand that I will not listen to Negative Feedback-- it
destroys musician intent.. I have never had my hands on a tube
amplifier that was not drastically improved (musically) by
getting rid of its FBK loops.
Basically, I like an amp that acts like great Solid-State,
but has the best "tube" qualities also... and I don't
mean overly smooth or slow in timing by this-- or "nicer"
to listen to, What I mean is better rendition of music..
a sense of true presense.
This taste of mine necessitates that I pay attention to
measurements and note the theories and measurements that
are out there, but it also means that I will break any
rule anytime I can get better performance and reliability
by doing so.
Your questioning the theory that light (high-DCR, high
primary impedance) loading of an output tube that is run
according to design-center values, while it will give the
best measurements, might NOT sound the best, is VERY well
taken.
In fact, by-the-rules kinds of amps never sounded
excellent to me, but a few did sound just OK.
This observation can be applied both to old tube
equipment, and also to the very latest.
To sum up: run a tube near the bottom 1/3 of its plate
power curves, but don't go TOO far with this-- find out,
instead, what you can get away with. If you get too close
to the bottoms of those curves, you will get much less
low-distortion power.
So, the question is how much power delivers the best sound
from the speaker that you are using? What are the effects on
THAT SPEAKER when you make the tube more linear by going
closer to design-center values. How MUCH BETTER will it
sound if you run at LESS plate dissipation and LOWER
impedance (heavier loading) on the tube?
What does your speaker NEED?
Question everything they tell you. They're not wrong.
They are usually right--- about something-- . BUT there
is a better-- much better-- world out there to be had.
You'll have to find those op. and loading points by trial--
and you should also find out if some brands of tubes agree
with what you're doing, and maybe others don't. It is VERY
difficult to know how a given tube will behave when operated
outside of it's approved design-center values.
But you can find that out-- "try look?". It's your call.
-Dennis-
Just keep in mind the test rig John Atwood used for this testing. I talked to him about this one year at The Mill, and we agreed that driving the secondary with a set source impedance( a SS amp and series resistance equal to that of the prescribed loading) with a resistive load across the primary would have yielded much better results. So for an example OPT, a 5k to speaker, putting 2.5k across each half of the primary, and with the 16R tap driven, a 16R series resistor to effectively set the driving impedance( counting the SS amp as zero output Z ) would be the preferred way to accomplish that.
The in-circuit performance would be quite difficult to 'apples to apples' given the radically different loads applied vis a vis the tube characteristics set by their plate and g2 supply. Since there is no way I'd set up a tube circuit the same for a 3.2k load as I would a 10k load, it would be best to use another test procedure. Either tune the tube voltages for the load, or get rid of the tubes entirely. The latter is certainly a lot easier... :) and delivers compare-able results between the various OPT's tested.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
thanks!
The audio and power transformers in the picture are oriented 90 degrees (laminations) to the power transformer and that is correct.
The OPT is 90 degrees from both choke and power. Power and choke are the same orientation. I pulled the choke cover plate to confirm.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I think it depends on the Dynaco - as the A431 is pretty good. Z565 isn't bad either.
I have mk3's Z216 which perform quite well to very well compared to anything I've had. The original circuit design is another thing entirely.
Keroes and Hafler knew their shit and if he's comparing them to recently made nickel cobalt's that's a different story entirely. I don't like to see their names disparaged in any way, especially from someone who knows little in the first place.
BTW, thanks for the list of tranny's.
Ptolomy Almagest
When rigged for CFB, they developed a large low frequency peak with the addition of 3% plate to grid FB around the finals.
They also lacked the clarity/detail that smaller Dynaco outputs like the A431 and A470 delivered easily...let alone the Z565.
So then, what amps have you built around the A440/441 outputs?
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
if that's what you want to think then go right ahead and use your brain for your own opinionated deviant purposes.
Ptolomy Almagest
or if you even heard an amp built around them... :)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
You're worming your way and try to defend a position you created days ago and looked things up to make sure you knew what you were talking about over a stupid statement to begin with.
Let's recap shall we - "the big Dynaco OPT's have all kinda sucked IMO". I stepped in to infer you were a boob unless you could explain yourself. You now say, after days of reflection, they develop a large (!) low frequency peak when used a certain way. I know what I said.
I took offense to your original comment. And since Dave is dead now he can't defend himself and explain the technologies back in the 50's to you and others in which he created his amplifiers in the first place. He also cannot defend himself and explain the creation of Hi Fidelity and what it was like to be a very important part of bringing HF to the masses. Kinda like why most of us are here. If he was alive today he would marvel at the refinements.
To put it simply, I think you kinda suck for such a rash statement.
Ptolomy Almagest
And you have ever listened to one to know they're brilliant?
Beyond being unstable, they just don't sound all that good. The Chicago CFB outputs OTOH, have been universally brilliant.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
you evaded everything to defend yourself. I have 2 3's and 2 stereo 70's, and now you're on the INN list. Enjoy yourself.
Ptolomy Almagest
I evaded nothing.
And since you have never built an amp with an A440/441, and likely never even heard one( though you did neglect to answer that question), your opinion's value has just been defined... :)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
You never did say what you built with your A441's.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
It seems that you have covered your own concerns if you can find hum levels on the SCA-35.
Note that all because a signal in one winding can be picked up by another winding in another transformer, that may not be the total outcome when the pickup transformer has multiple windings or when the pickup level is dominated by other forms of hum or when feedback suppresses any hum.
Then there's the fact that testing on the bench with most of the windings open isn't even remotely representative of real-world results. Considering how loosely coupled these inductors are, I would expect the induced voltage to simply fall away under load.
--------------------------
Buy Chinese. Bury freedom.
Perhaps you should breadboard it then?
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