Field Coil Speaker Test Bed?

[johnnyreece]

So, I have a ton of speakers. Lots of them have field coils. Sure, I can measure the coil and the primary on the OT to be sure they check out, but the secondary side of the OT and voice coil aren't so easy without desoldering connections (unless both the VC and the secondary are open, there will be a reading). I started thinking about making a simple test bed amp out of some stuff I have laying around. I modified an old basic amp version (if it looks familiar, I posted it up here long ago). Does this seem like it would be reasonable? It wouldn't be a very high voltage B+ and it's such low current, so the hope is it won't stress the FC too much. My plan is to probably just run music from my phone through it. This would also allow me to hear if something is rubbing or rattling before sending it to someone who maybe wouldn't be so understanding about receiving a dud. I appreciate any feedback (speaker pun!) you might have!

[maxkracht]

Looks reasonable enough to me, though i'm not sure if the speaker would sound a bit weak if the field coil is getting a lower than expected voltage. Probably easier ways to do it but I don't see why that wouldn't work

[johnnyreece]

Thanks for taking a look. It's kind of a peace of mind thing for me. It's one thing to know a speaker *should* work, but knowing it *actually* works would make me feel a lot better. I've had to eat the cost of a couple (plus the shipping) because they tested out fine on a meter, but wouldn't play right when hooked up. Some of these bad boys are heavy, and the shipping is not cheap.

[Astronomicum]

With only the screen and front end current running through the field coil, not sure if that is enough to generate a sufficient magnetic field. The few examples I have seen run all current through the FC.

[johnnyreece]

So, swapping B+ and B++ on the field coil? I know a lot of radios run the Screens off the first node.

[Astronomicum]

So, swapping B+ and B++ on the field coil? I know a lot of radios run the Screens off the first node.

No. Both the Plate and Screens, and the feed to the next node come off after the FC. I have attached one example of what I am referring to. At the voltages and outputs you will operate at, there should be no issue running the plates and screens at the same voltage for your 50L6.

A couple other observations:

You did not mention the VA rating of the PT you intend to use. You will need at least twice the current at the lower (60V) as is needed for the HT due to the doubler. In addition, I think you can expect much lower efficiency from the PT based on your intended primary scheme. I am thinking this because I had intended a similar scheme to get a lower secondary voltage and contacted the manufacturer to see if I could get the current I wanted. The answer I got was that I could expect only 25% of the PT rating and subsequently, abandoned the idea. If your secondary voltage is lower than expected, this could explain why.

Without disconnecting the VC to determine the VC impedance and to allow testing for the turns ratio of the OTs, you will not know what your reflected impedance will be on the 50L6 from your FC speakers. The 50L6 will be happiest at 2-3K based on the RCA tube data sheet. I have limited experience with the full range of possible OT turn ratios for FC speakers. Just throwing that out there for you to consider.

[johnnyreece]

Thank you for the detailed answer! I had done some math a while back on the PT, but it's been so long I've forgotten. It's a Triad N-68X, rated at 50VA. The current iteration of the amp is still alive, and I built it in 2015. I do realize this won't be an optimal setup for every OT (many of these speakers are for 6F6 or 6V6 based designs), but I'm not planning on running it particularly hard or long; just enough to see if a speaker functions. I even omitted the tone control, because this is not intended for any real listening purpose other than proof of function of a speaker.