Looking for constructive criticism on my first design

[gingertube]

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

[nazaroo]

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.



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



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

[revintage]

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?

[nazaroo]

Originally Posted by revintage

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.

[revintage]

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?

[nazaroo]

Originally Posted by revintage

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:

[pebmrb]

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..
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.

[revintage]

Hey N,
Take a deep search into Google about bifilars. You can also find valuable info at :: Index to learn.

[pebmrb]

Originally Posted by Wavebourn

You may split it, like 27k at the bottom, 6k2 at the top.

The result will be almost like CCS load.

Thanks for the tip.I will try it.

[nazaroo]

Originally Posted by revintage

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: