holy single-ended output, batman!

A portion of a thread that started in the lab but i think belongs here.

the complete thread is here:

http://www.groupdiy.com/index.php?topic=10180

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NewYorkDave:

I just ordered four samples from the XSM series. I'll let ya know what I think when I get them.

I want to try the XSM10K/600 in a transformer-coupled version of my "one-bottle" preamp, replacing the plate resistor and coupling cap.


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funkydiplomat:

NYDAVE!!

here's a quick one for you... this is probably more drawing board material. If the thread gets out of hand, I'll start a new one over there....

When you replace the plate resistor and coupling cap in your design and send the B+ through the primary of the output tranny, does the DC resistance of the transformer become the plate resistor more or less? I've never done an output like this... only with a buffer stage AC coupled to a transformer. Mostly because it's a little bit cheaper. big-ass output iron that specs DC through the primary always seemed on the expensive side to me. Until edcor offered this... If it works out, I may try an amp like this.


joe

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NewYorkDave:

As a first approximation, ignoring hopefully "minor" parasitics: at AC, the plate sees the primary inductance and the reflected (stepped-up) impedance from the secondary in parallel--and this is in series with the copper resistance. At DC, it's just the copper resistance. Since the small copper resistance drops a lot less voltage than a plate resistor, the transformer is not a drop-in replacement. You have to redesign the output stage for the new DC conditions.

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funkydiplomat:

i see. can you make up the difference with a resistor? is it common for the resistor to be on the b+ side or the plate side of the primary? or would you re-design completely? A lower plate resistance value would make a lower output-Z at the expense of voltage gain for the stage, correct? just trying to make sure I'm on the right track here. You were using voltage feedback in your 'one-bottle' design, i think. you would still tap into the same place (between the plate and the tranny, instead of the plate and the plate load resistor) for the feedback? Occasionally I see feedback coming from odd places on output transformers, it may be from a power output stages I'm thinking of, though.


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The RCA tube pre uses a gapped output transformer.

My question is how much inductance does the edcor have at different current ratings and will it be enought. I have a set of transformers for an RCA build. The Lundahl 1680 is rated at 5ma I don't know the inductace. If I understand correctly the current rating is set by the gap and the bigger the gap the lower the inductace.

The cool thing with outputs like this is you can get a lot of output drive and the gain can be very high because the inductive plate load kind of acts like a constant current. The higher the inductance the better.

Because of the lower resistance of the windings compared to a plate resistor one can reduce the B+ voltage.

I wonder Lets see 48V DC to 70V DC like from a phantom supply and a tube that can work well at this voltage???????????? maybe like what CJ built?

The Edcor is rated at 50mA AC+DC in the primary. This is for the XSM series.

The specs they provide on their web page have always been way too vague to be useful. For $12 each, I figured I'd just buy some samples and test them for myself.