In the process of rebuilding my old 300B amplifier, I decided to make a new filament power supply. It all came up around the components I had at hand, so it could’ve been improved but that meant extra cost:
I have a pair of custom-made JMS transformers with multi-taps secondaries. This helps me tweaking the right output voltage. Anyhow, any 15V transformer would do. Perhaps you want 14V to ensure you don’t dissipate too much on the filament regulators (e.g. Rod Coleman regulators)
I used my flexible LT supply PCB which allowed me to build this in less than 1 hour. I also used some existing chokes made in the UK by “Spirit” which are ok for this purpose. The Lundahls are in use, so can’t reuse them:
I used some SOT-128-2 schottky rectifiers but any other should work as well. Resistors are wirewound and the CMR choke is what I had in stock as well. A simple 15mH/3A should do fine.
The output measured well at 9V with a 6R load which dragged 1.5A. A bit more than the 300B but should be a good indication of performance. Also ripple level is good at 2.5mVrms. The rest will be cleaned up by the regulator itself 🙂
Hi Ale,
Your raw DC supply seems to include a bleeder resistor, R5, that never seems to show up in Rod Coleman’s raw DC supplies. The inclusion of a bleeder has been something I’ve thought about, as I always include one in high voltage supplies, so could you please comment about that?
Also, is the LED simply an indicator that the unit is *on*? And, how did you pick the choke?
Is your board available for purchase, and if so does it include the SMD Shottkys soldered in? Price? Shipping to USA?
Thanks and Best Regards,
Robert Chambers
Corinth, Vermont USA
c
Hello Robert,
I believe that the reason that Rod Coleman’s Raw DC Supply does not include a bleeder resistor is that it is capacitor-input DC supply as opposed to a choke-input DC Supply. At startup, the Coleman Filament Regulator is slowly feeding current into the high impedance of the cold DHT filament. With a capacitor-input supply at startup, the raw DC voltage supplied to the regulator is only slightly higher during this warmup phase. However, with a choke-input DC supply at startup, there is not enough current flowing through the choke so the raw DC voltage will soar to 1.4142 * Vrms of the transformer secondary instead of the roughly 0.9 * Vrms of the transformer secondary during normal operation after the DHT filament has warmed up. Adding a properly sized bleeder resistor to the choke-input DC supply keeps enough current flowing through the choke to keep the raw DC voltage at roughly 0.9 * Vrms of the transformer secondary.
Best regards,
Paul
Hi Robert and Paul
As Paul explained, it’s a choke-input supply hence you want the minimum load current to prevent the output voltage from raising. Nevertheless is a good practice to overrate cap maximum voltage in case of any issues. I also do the test of removing the load and monitoring the potential “ringing” of the supply with an oscilloscope in DC-mode. I previously checked with PSUD2, however is always good to double check that the supply has a nice response to step changes in the load and no signs of ringing.
There is a nice chapter in Morgan Jones’ Valve Amplifiers Fourth Edition which covers well this topic. In particular, this supply is not a pure choke-input, it’s rather an “intermediate-mode” PS. It uses the voltage “tunning” capacitor C2 in my board.
Hi Robert
I have a few made for myself only. I’m not convinced in offering these as I may end up with tons of questions regarding the supply design, which I don’t have the time to support am afraid. Also don’t have the time to pull together a BOM. If you’re still interested in some for your own work (and you know well what you’re doing with the PS), then drop me an email.
Thanks
Ale
Hello Paul and Ale,
I thank you both for your replies…it’s helping me to think about how to make this more of a cookbook approach to sizing a bleeder resistor for a choke input supply.
But being an (old) ME, not EE, I am challenged and slow as you may gauge by the time it took for me to reply to your posts. 🙂 I keep thinking about this and re-reading your replies, I still cannot come up with the cookbook recipe, although I think Paul has explained it well…I will keep on trying.
Very Best and Thanks,
Robert.