Fun with Feedback

What people are working on at the moment
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Paul Barker
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#226 Re: Fun with Feedback

Post by Paul Barker »

LOL
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RhythMick
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#227 Re: Fun with Feedback

Post by RhythMick »

Cressy Snr wrote: Fri Jan 06, 2023 10:24 pm Ooh look! Pentodes in the parlour (good name for an arty band) you know, just to see how it sounds with the Mission 773 floor standers like:
73E179F8-08DE-4414-A353-DF10215E41E7.jpeg
Nice build. Late query. I like the circular grill valve bases, where are they from?
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Cressy Snr
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#228 Re: Fun with Feedback

Post by Cressy Snr »

RhythMick wrote: Wed Jan 18, 2023 8:09 amNice build. Late query. I like the circular grill valve bases, where are they from?
They are PC fan grilles. I bought them from Maplins a long time ago (2008)
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Cressy Snr
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#229 Re: Fun with Feedback

Post by Cressy Snr »

Been having big fun over the duration of this week. :)
The EL34 SEP sounded great - when it worked. However it always seemed to be living on the edge of LF instability. So it was back to the drawing board and the audio circuit was taken to bits. Revisiting the filter frequencies, I started from the input stage and set the inter-stage coupling cap/grid resistor -3dB points so that they increased by a factor of 10, stage-to-stage.

So stage 1-2= 0.7Hz, stage 2-3 =7Hz. The bypass capacitor/cathode resistor combination on the second stage -3dB point was left at 1 point something Hz. The output stage bypass arrangements were set to 21Hz ( a factor of around 13x over the second stage bypass freq) which has an additional benefit of giving the cheap Hammond output transformers an easier time, in the bass region, further cleaning up the presentation.

Over the course of this week, the amp has been rebuilt with the new maths and it is now stable. I checked the stability with:

no inputs connected
inputs connected
speakers connected
speakers disconnected
low frequency single pulse injection

In all cases, it remained free from either HF, RF or LF oscillations. No frequency compensation was needed on the input stage. The only compensation it required was the cap across the feedback resistor which on it's own, cleaned up the 10KHz, square wave response.

Oh and it now sports 807s running in pentode mode, as below:
807SEP.jpeg
807SEP.jpeg (106.97 KiB) Viewed 7844 times
I used the book values for screen voltage, anode voltage at 300V and bias at -12.5V from the 1st column of the Brimar data sheet for single valve pentode class A, taking the specified precautions to avoid RF oscillations from the anode. Did a belt and braces check with an AM radio near the valves and it was all clean.
The design gives a nice 6.5WPC.
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Mike H
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#230 Re: Fun with Feedback

Post by Mike H »

:thumbleft:
 
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Paul Barker
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#231 Re: Fun with Feedback

Post by Paul Barker »

Like
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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Cressy Snr
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#232 Re: Fun with Feedback

Post by Cressy Snr »

For those interested - circuit below:
Screenshot 2023-01-21 at 09.22.21.png
Screenshot 2023-01-21 at 09.22.21.png (59.5 KiB) Viewed 7694 times
Good value for money.
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Cressy Snr
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#233 Re: Fun with Feedback

Post by Cressy Snr »

Now I've got single ended pentodes with global feedback reasonably worked out, I would like to do a steampunk influenced visual design, based on the above circuit - something like this:
3a044ccc058014acbb9027da92f6f182.jpg
3a044ccc058014acbb9027da92f6f182.jpg (62.52 KiB) Viewed 7673 times
Last edited by Cressy Snr on Sat Jan 21, 2023 10:20 am, edited 1 time in total.
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Ray P
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#234 Re: Fun with Feedback

Post by Ray P »

The image is a bit small Steve, it's difficult to see any detail?
Sorry, I couldn't resist!
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Cressy Snr
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#235 Re: Fun with Feedback

Post by Cressy Snr »

Ray P wrote: Sat Jan 21, 2023 10:06 am The image is a bit small Steve, it's difficult to see any detail?
Yes, It's a bit crap isn't it.
never mind, I've put up another image above to give a better idea.
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Ray P
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#236 Re: Fun with Feedback

Post by Ray P »

Cressy Snr wrote: Sat Jan 21, 2023 10:21 am ...I've put up another image above to give a better idea.
Yeah, I saw that one when I tried to find a better version of the original earlier - I like.

Don't forget you can get parts 3D printed too, there are a lot of filament colours available, for example,

https://www.amazon.co.uk/s?k=Esun+store ... nb_sb_noss
Sorry, I couldn't resist!
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Cressy Snr
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#237 Re: Fun with Feedback

Post by Cressy Snr »

Cheers Ray.
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Cressy Snr
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#238 Re: Fun with Feedback

Post by Cressy Snr »

Change of appearance on the way to Steampunkery. BlackGate "Heart Of Muse" also makes a welcome return from its bubble-wrap... Yessir!
IMG_0126.jpeg
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IslandPink
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#239 Re: Fun with Feedback

Post by IslandPink »

I bask in its glory !
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Cressy Snr
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#240 Re: Fun with Feedback

Post by Cressy Snr »

IslandPink wrote: Wed Jan 25, 2023 8:01 pm I bask in its glory !
Muttley-picture.jpeg
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