audio-talk

Still no real progress.Probably more when winter sets in.

For some reason my mind is having problems with the big Hammond mains transformer. In some ways I'd like to go back to the Antek toroids, but 500-0-500 instead of 370-0-370.

I'll do some sims in PSU designer and see what comes up.

Another thing, with the 6D22s damper diodes I use, they have been 6.3v AC heated. Do the experts think that DC heating (@ 1.9A each) could be worthwhile ?

On this build I'm going to try for the least possible noise.

[quote]Another thing, with the 6D22s damper diodes I use, they have been 6.3v AC heated. Do the experts think that DC heating (@ 1.9A each) could be worthwhile ?
/quote]

No, silly idea.

That told you!

Mike H wrote: ↑Sun Jul 16, 2017 2:36 pm That told you!

Certainly did.

Must admit, with AC heating (on the GK-71 amp) I tried a virtual centre tap (with two resistors) but could tell no difference hum wise.

I'll keep with AC heating then.

Id have thought ac heating on a gk71 would have been unbearable!!!

andrew Ivimey wrote: ↑Sun Jul 16, 2017 3:29 pm Id have thought ac heating on a gk71 would have been unbearable!!!

Sorry, my bad.

Still talking about 6D22s damper diodes.(in the power supply).

Aha!

Have a think about the situation. A very small induced AC from the heaters compared to the whopping great AC waveform the diodes are rectifying which is then smoothed by L & C or whatever.

Hence Nick's comment.

cheers,

STephen

Thanks Stephen,your explanation helps silly people understand.

I've managed to get two 130uf @ 1900v polypropylene capacitors for the power supply.

£18 each inc postage.

I think I'm almost there with parts for the rebuild.

At the moment I heat the GU-48 (833a) filaments with a 12v SMPS (30A) and two RC stages to get it down to 10v.

I'm looking for other ways to heat the filament, and wondered if using two LTC3780 modules fed from the 12v @30A SMPS , and set for 5A each might do.

The safe continuous current from each module is 7A, so 5A is within that.

Is the LTC3780 a suitable chip ?

Ebay modules here,

https://www.ebay.co.uk/itm/New-10A-130W ... Sw70BbV9-4

I think you have problems paralleling the two up. This might be a better choice

https://www.ebay.co.uk/itm/800W-30A-DC- ... 2714482436

Not wanting to throw a spanner in the works. I've been reading a lot about filament supplies, AC vs DC with a little experience blah blah. What I've concluded, not the rule but my opinion, the goodness about AC heating allows signal ac across the filament. OK, the signal gets modulated by the ac heating, not so good. Regulated dc voltage across the fil sucks the life out as ac signal can't be developed as much across the fil. Cap at end of dc fils does similar. So last element being an inductor isolates the fil from the filter c so allowing ac sig across the fil. Current reg similar. Coleman reg isolates fil from regulation effects by ccs one end gyrator the other allowing ac sig on fil. Andrew's reg different but same result. Now smps I'm less sure about now as that must either be a reg across the fil or a filter cap and maybe therefore the smps signature imprinted on the fil. Hope that makes some sense.

Cheers,
Stephen

I too have thought about this and so dropper resistors may be the answer.

However in this case, would have to be 0.2Ω, which is not a lot.