Transformer versus RC coupling experiemnt

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Paul Barker
Posts: 59
Joined: Sun Feb 20, 2005 10:20 pm
Location: UK

Transformer versus RC coupling experiemnt

Post by Paul Barker »

The PX25 amp as heard at the recent Yorkshire Boatanchor event had the first stage RC coupled. The valve is ML4, Holco resisto load 14k 1uF Mundorf Silver Gold cap, grid leak on px25 100k holco. Power supply for the amp is -r-c, driver stage takes power straight from same shunt regulated B+ as output stage. Marginal amount of hum but no distortion from this approach.

So to implement IT coupling I have to drop HT to driver, rather than use the resistors,which were previously anode loads and decoupling with a cap, and in keeping with Steves principles, I gathered all the small choakes I could to provide sufficient dc resistance and used an 0d3 for the volt drop.

so to recap the benefit of -r-c power supply is maintained, there is now no resistor on the anode of the ml4 or on the grid of the px25 so we have eliminated resistor sound issues, the coupling cap though the best I could afford is also now a non issue.

The result is that the soundstage has stepped forward to the level of the drivers. In my room the soundstage is always behind the speakers. Now the voices step forward. There is more clarity and improved base further.

These are just my own fairly ordinary IT's on ei120 layered manyfold and insulated after each pass of the winder, 1:1 something like 150h capable of 50ma (I wound them for the CF 211).

I suppose to sum it up in very few words, a film has been removed which was slightly obscuring and pushing back away from me the sounds.

Paul
dave slagle
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Post by dave slagle »

COOL!

when you say many layers, do you mean alternating layers are primary/secondary for a large number of primary and secondary sections?

Two other things to try just for kicks... (not that i think they will sound better mind you)

place a bridging cap across the transformer. (grab 6 bitters and 6 caps and spend an evening with it... invite a friend and make it 12 bitters)

i have the LTspice file for this setup if you are interested.

parallel the windings of the IT and call it a plate choke and use the mundorf to cap couple it to the next stage.

dave
Paul Barker
Posts: 59
Joined: Sun Feb 20, 2005 10:20 pm
Location: UK

Post by Paul Barker »

Yes I was considering that, but didnt bother to try it because when I tried the bridging cap in other transformers it didn't sound any better.

Unlike you guys I don't write notes about what I've wound so I don't know what these trransformers consist of, but at a guess 6 primary 6 secondary a few passes to each layer but insulation on every pass. Takes a long time to do as I'm sure you know. They are gapped for 50mA but I'm only using 10mA. They sound better than bifilar.

Actually I have a plan to use another 5k ri valve at 5mA instead of this ml4 2k8 so I'll regap them for greater inductance. I'll have to wind something else anyway because I want to step up 1:2 so I can keep the number of stages down. So much experientation to do before a final conclusion, will there ever be one?
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