Another MoFo

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Nick
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#61 Re: Another MoFo

Post by Nick »

No, I meant make a 120ma CCS using the 317 or mosfet and then use that to provide the filament bias current.
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Ray P
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#62 Re: Another MoFo

Post by Ray P »

pre65 wrote: Sun Dec 27, 2020 9:14 pm Ray, re the numbers not making sense. As well as the filament voltage/current there is the cathode bias voltage and current to take account of.
You're right Phil. I had completely taken my eye off the 20mA at the 2P29L anode :oops:

That makes sense now, with the schematic with the filament bias resistor I just adjust the Andrew DHT supply so there's 2.2V across the filament and the rest takes care of itself.
Last edited by Ray P on Mon Dec 28, 2020 3:03 pm, edited 1 time in total.
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#63 Re: Another MoFo

Post by pre65 »

Well done Ray.

It took a while for filament bias to penetrate one of my two brain cells, and because my valve (on the 833a driver circuit) is transformer coupled I also need to use a cap with the filament resistor. Only found that out last night.

I'm watching with interest as my MoFo could benefit from a gain stage. :)
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Ray P
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#64 Re: Another MoFo

Post by Ray P »

Cheers Phil

I've just placed an order for the parts to build a MoFo to test the bandwidth limiting.
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#65 Re: Another MoFo

Post by Ray P »

If a more powerful version of the MoFo is of interest, Andrea Ciuffoli's developments might be of interest;

http://audiodesignguide.com/Inpol/index1.html

He uses the 100mH Hammond 195T5 capable of handling 5A.

He includes a valve-based voltage gain stage and offers the files to get PCBs fabricated (or they're available to purchase from Jims Audio) - might be of interest to some here (Phil?).

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#66 Re: Another MoFo

Post by izzy wizzy »

pre65 wrote: Mon Dec 28, 2020 12:19 pm Well done Ray.

It took a while for filament bias to penetrate one of my two brain cells, and because my valve (on the 833a driver circuit) is transformer coupled I also need to use a cap with the filament resistor. Only found that out last night.

I'm watching with interest as my MoFo could benefit from a gain stage. :)
This confused me. My understanding was that filament bias enabled the removal of the cap bypass on the filament resistor at the cost of a beefier and hotter cathode resistor as it was a much lower value. Especially applicable for a transformer loaded stage.
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#67 Re: Another MoFo

Post by pre65 »

The "problem" with filament bias happens when the valve is transformer coupled.

I read this piece and it explains why.

https://vinylsavor.blogspot.com/search?q=filament+bias

It could, of course, mean I have totally misunderstood the article.
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#68 Re: Another MoFo

Post by Nick »

There is nothing magic about transformer coupling or for that matter choke loading and fill bias. It's just as the link says the cathode AC impedance is my multiplied by gain and is seen at the anode. So will form a LR filter. It's just a matter of the numbers.
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#69 Re: Another MoFo

Post by Ray P »

I received all the parts for the test (bandwidth limited) MoFo build today so I'll aim to get it up and running soon, hopefully over the coming weekend.
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#70 Re: Another MoFo

Post by pre65 »

Ray P wrote: Mon Jan 04, 2021 9:38 pm I received all the parts for the test (bandwidth limited) MoFo build today so I'll aim to get it up and running soon, hopefully over the coming weekend.
Looking forward to a picture, or two.
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#71 Re: Another MoFo

Post by Ray P »

DIY chores today so time a bit limited but I have managed to recycle a heatsink as the 'chassis' for my test MoFo build - just caught the last of the decent daylight.

Image
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#72 Re: Another MoFo

Post by pre65 »

Is the "yellow thing" your inductor ? If so it's a tad smaller than the MOT (microwave oven transformer) like wot you sold me. :wink: :)
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#73 Re: Another MoFo

Post by Ray P »

pre65 wrote: Sat Jan 09, 2021 4:59 pm Is the "yellow thing" your inductor ? If so it's a tad smaller than the MOT (microwave oven transformer) like wot you sold me. :wink: :)
Yep, but you don't seem to be paying attention Mr. Frogg! The aim is to build a bandwidth limited MoFo for a full-range driver, smaller inductor is part of rolling off the bottom-end at around 180Hz
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#74 Re: Another MoFo

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Ray P wrote: Sat Jan 09, 2021 6:43 pm
pre65 wrote: Sat Jan 09, 2021 4:59 pm Is the "yellow thing" your inductor ? If so it's a tad smaller than the MOT (microwave oven transformer) like wot you sold me. :wink: :)
Yep, but you don't seem to be paying attention Mr. Frogg! The aim is to build a bandwidth limited MoFo for a full-range driver, smaller inductor is part of rolling off the bottom-end at around 180Hz
Yes Ray, I was paying attention, but trying to inject a smattering of humour. I obviously failed miserably. :(
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#75 Re: Another MoFo

Post by Ray P »

Finally took delivery of the missing MoFo parts today so, hopefully, I'll get some music out of the test MoFo this weekend. If that works out I'll knock out cheap baffle for the full-range speaker unit and try to measure the FR (and have a listen).
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