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Looking for constructive criticism on my first design

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pebmrb,
I see above that it has been suggested that you get a better output transformer. You may like to do that eventually but I suggest you get it all going with the Hammond first.
I had some correspondence about these transformers with the Hammond Engineers. It was pointed out that they were not intended for HiFi use, their limitation being low primary inductance which would limit bass response. It was also pointed out that HIFi performance could be obtained from them by driving them from a low impedance, that means from a filamentary triode (like you are doing) or a triode strapped pentode. You may be pleasantly surprised by the performance you get from it. Once it is all up and running then you can contemplate getting something better (more expensive) if you feel it is required.
My discussions with the Hammond Engineers were about driving the 125ESE with a triode strapped EL34 and they stated that that combination would give HiFi performance.
Your proposed circuit should do even better.
Cheers,
Ian
 
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Here's a few details to help if you wish to use your OT as per my suggestion:

For the interstage, these resistors will provide ground reference,
and hence cathode bias, provided they follow recommendations for the RS242.
This is in case your interstage secondary doesn't have a c.t.

283799d1337759575-looking-constructive-criticism-my-first-design-rs242-interstage.jpg


Here's a more detailed sketch of one half of an output section:

283800d1337759575-looking-constructive-criticism-my-first-design-rs242-outputstage.jpg


The 750 ohm load resistor only needs to be 2-5 watts.
 

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Hey N,
Do you really know how a bifilar like 126C would work here?
Have you really calculated the Ri of the so called CCS?

I'll put it this way:

I'm 100% certain that the xformer will sound better
WITHOUT DC flowing through it, than with.
Although unnecessary, the extra iron is now a bonus.
You could improve the performance further,
by adjusting the now unneeded gap.
But that is beyond amateur-class modification skills.

The CCS stack works well enough to make any ordinary Output transformer
sound audibly better than a Macintosh.
Try it.
Just get the circuit adjusted properly, and reasonably balanced.
 
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So you didn´t know:
126C or any bifilar can only be used SE with both primary and secondary referenced to ground.
Ri of the so called CCS is in the ballpark of 3k, plus the 750 added an this is not acceptable. If you move the lower end of the 0,1u to the output tubes anode(below the 750) you get a bootstrapping effect and a significantly higher Ri. Drawing error?
 
So you didn´t know:
126C or any bifilar can only be used SE with both primary and secondary referenced to ground.
Why do you think this?
Is his OT an autotransformer?

Your grammar is unclear.

Do you mean
"All bifilars can only be used SE, and only then with both Primary and Secondary grounded.",
In his original diagram, the Primary is not grounded except (AC) through the PS cap..
If he is misusing his transformer, perhaps you'd better tell him.

...or do you mean,
"All bifilars, when used SE, can only be used with both P and S grounded."
Please clarify.
If its not an autotransformer, I don't see a problem in my circuit.


If you move the lower end of the 0,1u to the output tubes anode(below the 750) you get a bootstrapping effect and a significantly higher Ri. Drawing error?

Yep. Sorry about that. Up late.
Here's the corrected version:
283811d1337769618-looking-constructive-criticism-my-first-design-rs242-outputstage.jpg

 

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Wow.Lots of brainstorming while I sleep.I like it!
nazaroo:thanks for the interesting ideas,unfortunately getting a second pair of these RS242s won't be in my budget any time soon,as I have already pushed the WAF to the max on this project.:D.
gingertube:I chose this OPT based on cost(obviously),it's multi impedance abilitites,and this article:
Hammond 125ESE

I put this amp on the scope and found a good,fairly flat frequency response from about 50hz to 20khz.The OPT was definately the weak link and was rolled off about 3db at 20 hz,which I found tolerable considering my full range fostex in BLHs barely go below 40hz.This was done at 2v p-p input,so under substantial power.
Note:my test equipment is a bit old and mediocre at best,so I don't exactly have a precision measurement.
But the one thing I did see on the scope that I wasn't too sure about was below about 60 hz,with 2v p-p input,a stair step or "ledge" on the trailing edge of the sine wave.It would go away by backing off on signal voltage or stepping up frequency.This was caused by the OPT,as the sinewave was perfect at the plate of the output tubes.Can anyone tell me what was causing this?surely I am not saturating the core,as these things are meant for a lot more current than I am putting to em?
Other than this one issue,the amp looks good on scope as far as I can tell.
 
Hey N,
Take a deep search into Google about bifilars. You can also find valuable info at :: Index to learn.

I think you are a bit confused:

Almost all output transformers are bifilar, so that DC for instance in Push-Pull styles are mostly cancelled out.
SE transformers are also bifilar, the purpose being mainly close coupling between windings.

In SE (single ended) transformers, the only design difference of significance here,
is that there is a physical gap in the core and larger core mass to prevent early saturation due to an imbalanced DC current.

The 'bifilar' characteristic, meaning paired wiring is used for close coupling has nothing to do with suitability for grounded or ungrounded use.

The circuit diagram for this transformer shows nothing unusual,
or unsuitable for my circuit:

An externally hosted image should be here but it was not working when we last tested it.
 
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Here's a good quote about the real difference between SE and PP transformers.

It illustrates that it is hard to use a PP for an SE, without careful planning.

However, you can certainly go the other way,
when you have no DC current issues, as in my circuit
(although a PP tranny is more efficient):

In SE, the DC current through the primary must be half of the peak signal current for the primary, or the transformer will forever be running into saturation on one polarity of signal peak.

The amount of DC current offset that a transformer can withstand is composed of two parts - that part that offsets the iron magnetically, and that part that creates a magnetic field in air gaps. The magnetic "goodness" of iron is tens of thousands of times (literally; that's a representative number for iron's permeability, not something I made up as a figure of speech) better than air. But iron saturates and air doesn't.


Mother Nature has rigged things so that iron is expensive and air is almost free. So the trick in designing a P-P transformer is to design one with a minimal air gap because the currents sum to zero, or very nearly. That lets us get the maximum signal swing with the least iron. Air gaps only make things worse for P-P transformers because they lower the magnetic qualities and that forces us to use big amounts of iron and more turns (pounds/kg) of copper to get the performance we want, and that costs money.


As a side benefit, in P-P mode, you are pulling the transformer core (in a magnetic field sense) from zero/balanced toward positive saturation on one polarity of the input signal, and toward negative saturation on the other polarity of signal. This lets you use the full positive-to-negative swing of the magnetic field in the iron for transforming signal.


All this taken together means that P-P transformers are the cheapest way to build an OT for a specific power output and frequency range. The price you pay to get this is that you must use two (or paired) output devices and the circuit must be a bit more complicated to drive the push-pull nature of the output swing. Generally, you save so much on the transformer cost that the extra parts are paid for, and you get the deluxe benefit of much higher power output per pound of transformer. This paragraph pretty much puts in a nutshell why most tube amps are P-P.


SE operation however, requires that there be a standing current of half the peak output current in the transformer all the time, on average. With only one output device to make the transformer primary current swing up and down, the iron must be "biased" to one half of its saturation magnetic field, and can swing only up to max and down to zero. Comparatively, the swing is one quarter of the available swing of the same amount of iron used in P-P because you only get to use one polarity of the M-field in the iron, and then only half of that because you have to do positive and negative swings inside the half you can use.


Just that requirement means you have to use at least four times as much iron in an SE transformer than you would in a P-P. But it gets worse. Relying on just the iron to hold you in the middle of the magnetic field operation is not very usable. The only good way to ensure you won't saturate is to add an air gap to give you some saturation immunity. Adding the air gap means that the magnetic field has a much bigger range of swing before saturation makes things go to pot; but it also lowers the good magnetic properties of the volume of iron that the magnetic field is in by that factor of tens of thousands for the portion of the magnetic path that the air is in. Add in air gap, you lose primary inductance, and have to make the iron core bigger again to get back the low frequency response that losing primary inductance cost you.


Overall, a good rule of thumb is that an SE OT needs to be about ten times the mass of a P-P for the same output power for normal operation.


...
The point here is not that SE transformers must be operated in SE mode
(i.e., can only be operated that way),
but rather that to operate in SE mode, you have to use an SE transformer,
because to operate in SE mode, you have to keep the iron half-saturated (magnetized in one direction) by DC current,
just like with a tube in Class A mode you have to pick an operating point that allows swing,
while keeping the current flow always flowing, and only in one direction.
 
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From the Hammond web site:
Key Features (126 Series):

Designed for driver use ONLY in single ended tube output amplifiers. These units replace coupling capacitor circuits for improved sound.
Closed style with minimum 8" long leads.
All models use bifilar wound windings for exceptional coupling and bandwidth.
Minimum frequency response 20 Hz. - 20 Khz (+/- 1db max. ref. 1 Khz.).
The core is gapped to support the specified D.C. bias current.
Maximum working voltage - 400 VDC
Units are Hi-Pot tested at 1,000 VDC
Core uses Hi grade silicon laminations (29M6).
Units weight - 3 lbs. 2 oz.
Are we suggesting they are OK for a push pull arrangement? (Is this going off topic?) And why would the transformer designer emphasize "ONLY".
 
From the Hammond web site:
Key Features (126 Series):

Designed for driver use ONLY in single ended tube output amplifiers. ...
All models use bifilar wound windings for exceptional coupling and bandwidth.
Core uses Hi grade silicon laminations (29M6).
Units weight - 3 lbs. 2 oz.


Are we suggesting they are OK for a push pull arrangement?
(Is this going off topic?)
And why would the transformer designer emphasize "ONLY".

Yoo hoo...
Look at my circuit again.

Yes its "Push-Pull",
but its also MINUS the standard PP-feature,
namely theres NO DC-current flowing through the primary,
not in either, nor in both directions. None at all.
So this circuit isn't "push-pull" in the normal sense.

Think again.

click on pic to enlarge brain capacity:
283777d1337748392-looking-constructive-criticism-my-first-design-rs242-schematic2.jpg


(1) Nazaroo is usually right on the key point.

(2) In case of questions refer to rule 1.

 
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I think you are a bit confused:

Almost all output transformers are bifilar, so that DC for instance in Push-Pull styles are mostly cancelled out.
SE transformers are also bifilar, the purpose being mainly close coupling between windings.
.....snip.....

An externally hosted image should be here but it was not working when we last tested it.


I was under the impression from construction diagrams that OPTs were wound in interleaved segments, not bifilar wound.

A 1:1 transformer (such as the Hammond 126 series) can be wound bifilar, and 2:1 or 1:2 can be wound trifilar and then two segments placed in series but the results is not bifilar. Beyond that the windings need be wound in segmented layers.

By definition, one can not wind 1600 turns of primary bifilar with say 50 turns of secondary. One could wind 50 turns of primary bifilar with 50 turns of secondary, and wind the rest in segments, but it is impossible to wind 1600 turns bifilar to 50 turns.
 
Hey Gimp,
You´re so right OPTs are normally wound interleaved, not bifilar, for minimum coupling capacitance. A bifilar is wound 1:1 with the secondary and primary windings parallall to each other. This means a transformer like 126C will be coupled with something like 20n.

For those willing to learn I still recommend Daves forum.
 
Revintage:
I did briefly consider direct couple to the output tube,but I was more comfortable with the 6SN7.I'm sure it would be an option,though,but may require something else as driver?not sure.
While we are on the topic.I have a question about the operating points for the 6SN7.I have the load lines for the two halves of the 6SN7 running very close together,with the cathode follower slightly steeper.I was wondering if it would be desirable for me to make efforts to get these lines directly on on top of one another,or if it is more a matter of personal preference of operating points?
 
Here's a few details to help if you wish to use your OT as per my suggestion:

For the interstage, these resistors will provide ground reference,
and hence cathode bias, provided they follow recommendations for the RS242.
This is in case your interstage secondary doesn't have a c.t.

Now it looks better. One more thing: don't you think that 100K and 0.1 uF are odd near that upper tubes? Why don't you short and remove them?
 
Now it looks better. One more thing: don't you think that 100K and 0.1 uF are odd near that upper tubes? Why don't you short and remove them?

Ahh ok.

Actually, you are probably right again:
The output as I have configured it
may sound as good using some simple SRPP,
as it would with a Mu-stage.
I have to confess I haven't played with SRPPs much,
especially on output stages.

But no worries. I doubt a couple of polyprops in CSS
are going to add much distortion here,
in comparison to the gains over a typical Output stage / transformer config.
 
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