I am interested in experimenting with choke-coupled low-wattage (12xx7) amplifier stages using the various power transformers I have lying around, but I know it is potentially destructive to have an unconnected secondary in a tube amp. My understanding is that shorting the secondary will saturate the core and paralleling the windings seems like a bad idea. Short of physically removing the unused winding, is there any way to do this safely?
Series connection makes sense - thanks for the prompt reply.
I thought leaving the unused secondary unconnected could damage the transformer/tube. Have I been misinformed?
I thought leaving the unused secondary unconnected could damage the transformer/tube. Have I been misinformed?
I was curious about the same thing, so I took my inductance meter and a filament transformer and measured things. Nothing surprising, but the primary measured ~0.2H, the secondary measured much less, shorting the secondary killed the inductance of the primary, as did paralleling them. Connecting them in series mostly added resistance. I'd leave the secondary out of it, and heat shrink the leads to be on the safe side.
yup - in case where we are talking about output xformer
unloaded tube + signal , possible arcing
Unfortunately a power transformer is unlikely to have nearly enough inductance to load any tube in the 12xx7 family. I use 100H - 270H chokes with tubes having an rp of <2K typically (5K worst case).
If you must use the on-hand parts, put the secondary in series, making sure to get phase right so inductance adds. Line power transformers have very high inductance for the turns because there's no core gap, but that also means they saturate fast. A low voltage secondary winding can offer a small increase in inductance but with little parasitic capacitance, (much higher SRF than the primary) handy at high frequencies. Of course all this is wildly variable depending on what you have.
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Thanks to everyone for the conversation. I'm need an amplification stage with a healthy voltage swing and just a tiny bit of power gain. Rather than burn 300v in a plate resistor, I thought a LC coupled stage would be appropriate. I've successfully pressed random power transformers into service as inductors in a number of solid state designs, so I thought I might be able to do the same with a preamp tube. I know how to plot a load line with a resistor or a transformer, but I'm shooting in the dark with chokes of unknown inductance.
yes.....and then theres the matter of winding phasing....depending on the polarity of the secondary relative to the primary when used in series you can get more inductance or less inducatance
Right, I was wondering if there was any way, without an inductance meter, that you could easily tell whether they were connected in series with the right polarity.
Hi,
That all makes sense, for a power supply choke. For interstage
choke coupling I'd expect all mains transformers to be very poor.
rgds, sreten.
A transformer has to be able to work offload so leaving the secondary open circuit is fine.
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