Tube DIY Asylum

Re: End Of Zen schematic

205.175.225.5

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>>. Its a power supply choke and not a plate choke so there are some compromises (as always)in its use as a plate choke. Tom can straighten me out here.<<

One thing paul joppa has mentioned about plate chokes is the resonance point. Not that this should be the primary factor but another thing to consider when making the purchase decision. I've measured power supply chokes with resonances at 6khz and a couple at 4.5khz. One plate choke I had measured had a resonance around 3khz. IMO, it depends more on the stray capacitance of the choke rather than what the dealer brands it as (plate or power supply). The stray capacitance is all over the board for different makes of chokes and labelling chokes as "plate" or "power supply" doesn't make much difference. In all honesty I haven't measured the 157G's inductance curve and I've been using the BCP-15 for a few months now. I did use a 156G for the plate choke for quite awhile and the sound was about 90% of the quality of the BCP-15 in this circuit.

>>The rule of thumb for the choke I know of is preferably ~20H per 1K primary Z<<

I made my choke value decision based on the rp of the tube. A 30H choke would put the -1dB point at 20hz when used to load a 12B4. 60H would only give you -.5 to -.6 at 20hz. You have to realize that the plate choke inductance is in parallel with the tranny primary inductance when referenced to the tube. Most non air-gapped transformers provide craploads of inductance leaving the deciding factor on the plate choke. I think I measured around 560H on an all permalloy TFA-2004 at 20hz. I imagine the 125E is no slouch either.

Just keep in mind that even with a 9H choke in, this circuit still produced great sound. Going to higher choke values only optimized circuit performance rather that dramatically transforming the circuit sound. It still works and it still produces similar sound with "budget" parts. The parts were chosen for cost/performance ratio. When you plan for production, there's not much sense in spending 700% more for only a 5-10% increase. The customers can spend that money if they personally feel it's worth it. I fully encourage using better performing parts but I also encourage keeping it within reason and personal budget. You can build another project with the cost difference in a single part! There's a lot of fun and satisfaction in building a great performing amp for under $200. I could have easily spent $800 on this project by just using fancier iron. The question is: would it have been worth it?

Tom §.


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