audio-talk

Posted: **Sun May 17, 2020 12:35 pm**

I just looked at the spec sheet for the 12A.

You can read all sorts of things on the internet.

Posted: **Sun May 17, 2020 12:50 pm**

Nick wrote: ↑Sun May 17, 2020 12:35 pm I just looked at the spec sheet for the 12A.

You can read all sorts of things on the internet.

I have the RCA 112A data sheet, and yes there are all sorts of things on the internet, but not all can be believed.

Posted: **Sun May 17, 2020 3:32 pm**

pre65 wrote: ↑Sun May 17, 2020 12:50 pm I have the RCA 112A data sheet, and yes there are all sorts of things on the internet, but not all can be believed.

Exactly which is why I wondered why you posted

although I have read that slightly starving filaments may be an improvement.

Posted: **Sun May 17, 2020 3:35 pm**

Nick wrote: ↑Sun May 17, 2020 3:32 pm
pre65 wrote: ↑Sun May 17, 2020 12:50 pm I have the RCA 112A data sheet, and yes there are all sorts of things on the internet, but not all can be believed.
Exactly which is why I wondered why you posted

although I have read that slightly starving filaments may be an improvement.

But, by the same token, it does not mean it's untrue either.

Posted: **Sun May 17, 2020 4:05 pm**

pre65 wrote: ↑Sun May 17, 2020 3:35 pm
Nick wrote: ↑Sun May 17, 2020 3:32 pm
pre65 wrote: ↑Sun May 17, 2020 12:50 pm I have the RCA 112A data sheet, and yes there are all sorts of things on the internet, but not all can be believed.
Exactly which is why I wondered why you posted

although I have read that slightly starving filaments may be an improvement.

But, by the same token, it does not mean it's untrue either.

I did a search on Audio-talk for "starved filaments" and there have been discussions about the topic. Whether the views held then are still applicable I can't say.

Posted: **Sun May 17, 2020 8:39 pm**

I have a new batch of PCB, Phil, if you want to 'speriment?

Posted: **Sun May 17, 2020 8:58 pm**

Andrew wrote: ↑Sun May 17, 2020 8:39 pm
I have a new batch of PCB, Phil, if you want to 'speriment?

Can you remind me Andrew, do your boards have a trimmer for fine tuning the output ?

Posted: **Mon May 18, 2020 5:11 pm**

pre65 wrote: ↑Sun May 17, 2020 8:58 pm
Andrew wrote: ↑Sun May 17, 2020 8:39 pm
I have a new batch of PCB, Phil, if you want to 'speriment?

Can you remind me Andrew, do your boards have a trimmer for fine tuning the output ?

Yes, but I also keep saying trimmer pots are rubbish, and not to use them. Sadly, I am often ignored

However, if it's a prototype then I'll excuse you.

Posted: **Mon Dec 28, 2020 6:01 pm**

Does anyone recall what the voltage drop across these DHT filament supplies is?

I'm planning to use a couple of these modules in my MoFo preamp - need them to deliver 9.3V @ 120mA so need to size a suitable transformer, which I guess will be >9VAC so I'll need to upgrade the filter caps too.

Posted: **Mon Dec 28, 2020 6:10 pm**

Ray P wrote: ↑Mon Dec 28, 2020 6:01 pm Does anyone recall what the voltage drop across these DHT filament supplies is?

I'm planning to use a couple of these modules in my MoFo preamp - need them to deliver 9.3V @ 120mA so need to size a suitable transformer, which I guess will be >9VAC so I'll need to upgrade the filter caps too.

About 4v I think (from the peak voltage obviously)

9v will get you about 8.7v max.

Posted: **Mon Dec 28, 2020 6:18 pm**

I built one up for a 300b, for 5v out a few weeks ago, that needed 8.5v AC in before there was no dropping out on the scope on the low points of the input AC, 8.5v = 12v for no load, so that would put the drop at 7.5v, but that was with a 30va transformer feeding it.

Posted: **Mon Dec 28, 2020 6:32 pm**

Nick wrote: ↑Mon Dec 28, 2020 6:18 pm I built one up for a 300b, for 5v out a few weeks ago, that needed 8.5v AC in before there was no dropping out on the scope on the low points of the input AC, 8.5v = 12v for no load, so that would put the drop at 7.5v, but that was with a 30va transformer feeding it.

Yes I fell into the trap of underestimating the VA requirements of the transformer. Immediate buzz killed when I got my head around the sizing and replaced them.

Posted: **Mon Dec 28, 2020 6:55 pm**

I've got a 10VAC transformer to hand but it looks as though I might need to go up to 12VAC - either way I'll need to go up to 25V rating on the filter caps.

Posted: **Mon Dec 28, 2020 7:00 pm**

Ray P wrote: ↑Mon Dec 28, 2020 6:55 pm I've got a 10VAC transformer to hand but it looks as though I might need to go up to 12VAC - either way I'll need to go up to 25V rating on the filter caps.

What's the spec of the transformer?

Posted: **Mon Dec 28, 2020 9:18 pm**

RhythMick wrote: ↑Mon Dec 28, 2020 7:00 pm What's the spec of the transformer?

I don't remember the specifics off hand - it's down in the garage - but it's relatively meaty for this requirement...

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