DHT filament heating with SMPS + CCS

[Fuling]

Hi

Over the last 2-3 years I´ve been collecting RCA 808 tubes in order to build a good looking class A2 SE DHT amp.
I´ve already written a lot about this project in other threads on this forum so let´s get straight to the topic: Heatings those hungry filaments in a hum-free and cost effective way.

The filaments draws 7,5V 4A each and AC heating is out of the question since µ=49. I have already built a pair of old-fashioned LCLC-filtered power supplies but I´m not satisfied with them so let´s forget about them.

I recently bought a 12V 10A SMPS on Ebay in order to warm up a bunch of IDH tubes in a future OTL project. As this kind of PSUs can be had almost for free from various chinese manufacturers I also bought two 12V 8,5A units from another seller.

The plan is to use one SMPS per channel to feed the filaments of those 808s through adjustable mosfet-based constant current sources.
RC-filters or voltage regulators would perhaps be easier ways to drop the voltage from 12V (actually 10,8V as the power supplies are adjustable +-10%) to 7,5V, but according to several sources constant current is better than constant voltage in this context.

A couple of questions:
1: Should I expect trouble with noise and RF interference?

2: What will happen if I decide to use cathode feedback in the output stages? I assume that the SMPSs are insulated from input to output, but does that mean that it would be safe to let the output swing, say, +-40Vp WRT ground 20Hz-20kHz?

Here´s the SMPS in question:
http://cgi.ebay.com/ws/eBayISAPI.dll...MEWN:IT&ih=020

[astouffer]

Everything sounds fine to me. Noise can easily be cleared up with chokes and make sure the frames of those supplies are grounded. Insulation shouldn't be a problem because the primary of the switching transformer has a few hundred volts on it already.

[Gerrit Boers]

I performed some measurements on SMPSs for heating a 300B. Maybe they are of interest to you:

DC heater supply part 3, the final insult

I tested just a couple of SMPSs, but it seems that an SMPS is only really clean if the output is grounded. This is fine if the cathode is connected to ground but not with autobias.

I have already built a pair of old-fashioned LCLC-filtered power supplies but I´m not satisfied with them so let´s forget about them.

Why were you not satisfied? (just out of curiosity)

[Fuling]

Thanks for your replies.

If necessary I´ll ditch the cathode feedback, I´m not sure it would work properly in a class A2 circuit anyway.
Would common mode between the SMPSs and the CCSs improve anything?

My LCLC supplies works OK electrically but construction-wise they are a mess. I could rebuild them of course, which is my plan B if the SMPS - CCS idea fails.