Can you have too much heater current headroom?

[Jamier]

I’m building an amp that requires 4A heater current to power 4 tubes (1A/ tube).I have found a transformer that will deliver 3.15V-0-3.15V (CT) @6 amps, which will work, but I’m wondering if you can have too much current capability in a transformer.

Jamie

[Paul Joppa]

The only negative (besides size and weight) is that the voltage may be slightly higher than nominal. Usually that's not a problem with heaters, but if it is, a small series resistor will fix you up.

[Jamier]

P.J., Thankyou for your reply. On the same subject (this transformer), how can I reduce the HV secondary? The Trans has a 180V / 300mA secondary and I need 170V/300ma. Adding copper to the primary or removing copper from the secondary make sense in theory but it is probably real difficult, or impossible to do. I thought of using a voltage divider but I think it might reduce the current (am I right?) I have also considered just going with the 180V and seeing what happens. The PS is SS full wave bridge rectified (UF4007s or 1A schottkeys) and is a simple CRC. B+ is around 210V. I've used the Schottkeys before on a similar amp and while they are an improvement maybe the 4007s will give me a little more V drop(?) if I use the transformer as is.

Jamie

[Paul Birkeland]

You can use a bucking transformer or a bucking winding to bring the HV primary voltage down.  If you have a 180V transformer that has a 6.3V heater winding, you can wire it in series and out of phase with the primary winding to bring it down to 173V (but you won't be able to use the 6.3V winding as a heater winding anymore!).  For your example, a 10V low current power transformer could be used to buck the winding down to where you need it.

[Jamier]

I only want to reduce the voltage on the HV secondary. I need the heater winding @ 6.3V. Can I place a Bucking Winding on the HV secondary side ? However, if I did Buck the primary side I would end up with 5.95V on the heaters. Would this be too low? As long as the current is still available I think this would be OK(?)

Jamie

[Deke609]

Jamie - why not play around with the PS filter config to get B+ down 10V? If you know the current draw of the tubes at your operating point, couldn't you estimate the R required to drop the 10V?  Then you could adjust the existing R value in your CRC filter, or even add another RC stage, or just add a dropping resistor. You could use PSUD to model it.


cheers, Derek

[Paul Birkeland]

Yes, you just buck the secondary side.

[Jamier]

PB, thank you for that info.

Derek, the PS is very basic and the OT center tap connects at the junction of the rectifier and the first cap, so there is not a resistor to adjust before this. I could redesign the PS, but even with PSUD there is still some trial and error ( or experience that I don’t have yet). And to be honest, the amps are so quiet now that anything I could design might be a compromise. So, the Buck winding makes the most sense for me ( or calling Edcor and finding out how much a custom Trans would cost).
But,honestly,the buck winding appeals to the DIY in me.

Jamie

[Deke609]

Apologies Jamie - What I wrote above is a bit confusing. I had in mind dropping the excess B+ voltage after rectification, but mistakenly referred to dropping 10V which suggested a pre-rectification voltage adjustment.  Post-rectification, my understanding is that the unwanted additional 10V rms from the secondaries would actually become approx. 10 * 1.414, or 14V. I thought that this could be dropped with an appropriate upsizing of the resistor in the CRC, or by adding a stage or just a dropping resistor at the tail end of the filter.


But I get not wanting to muck with a well-functioning PS circuit and wanting to try the buck approach for the diy fun of it. 


cheers, Derek

[Jamier]

Derek,

     Your dialogue was clear, not confusing at all. In a lot of PS designs the OT connection would come after a resistor or choke which could be adjusted. This circuit is incredibly simple and in its stock form has only about a dozen or so components. I built the first one(s) from some orphaned boards someone gave me, and was so impressed that I decided to try a P2P version to see if I could build an amp from scratch( or semi-scratch in this case). I’m trying to make small steps toward my goal of being able to scratch build more complicated circuits. This way I have a working model to compare my results to. I’m using different Transformer specs so I can use a variant tube which is more common than the orignal tube the amp was designed for. Thanks for your input, it may still come in handy down the road.

Jamie

[Jamier]

PB,

     When I find an appropriate transformer (10V/ .1A or thereabouts), I’m assuming that I should connect it’s primary and then connect the secondary with the HV secondary of the main transformer, in series but out of phase, just as you described with the 6.3V heater winding scenario?
Can you suggest a good candidate for this Bucking trans? My wall voltage runs around 117V.

Jamie

[Paul Birkeland]

Here's something that would work:

https://www.newark.com/triad-magnetics/fs20-600/bobbins-transformer/dp/44F3333

When you start needing something like a 10V/100mA power tranformer, you'll have to settle for something much larger than what you actually need.