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#226 Re: Fun with Feedback

Posted: Mon Jan 16, 2023 7:18 am
by Paul Barker
LOL

#227 Re: Fun with Feedback

Posted: Wed Jan 18, 2023 8:09 am
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?

#228 Re: Fun with Feedback

Posted: Wed Jan 18, 2023 11:14 am
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)

#229 Re: Fun with Feedback

Posted: Thu Jan 19, 2023 8:11 pm
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 7326 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.

#230 Re: Fun with Feedback

Posted: Fri Jan 20, 2023 8:32 pm
by Mike H
:thumbleft:

#231 Re: Fun with Feedback

Posted: Fri Jan 20, 2023 9:39 pm
by Paul Barker
Like

#232 Re: Fun with Feedback

Posted: Sat Jan 21, 2023 9:38 am
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 7176 times
Good value for money.

#233 Re: Fun with Feedback

Posted: Sat Jan 21, 2023 9:59 am
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 7155 times

#234 Re: Fun with Feedback

Posted: Sat Jan 21, 2023 10:06 am
by Ray P
The image is a bit small Steve, it's difficult to see any detail?

#235 Re: Fun with Feedback

Posted: Sat Jan 21, 2023 10:21 am
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.

#236 Re: Fun with Feedback

Posted: Sat Jan 21, 2023 12:29 pm
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

#237 Re: Fun with Feedback

Posted: Sat Jan 21, 2023 7:19 pm
by Cressy Snr
Cheers Ray.

#238 Re: Fun with Feedback

Posted: Wed Jan 25, 2023 7:07 pm
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
IMG_0126.jpeg (106.35 KiB) Viewed 6982 times

#239 Re: Fun with Feedback

Posted: Wed Jan 25, 2023 8:01 pm
by IslandPink
I bask in its glory !

#240 Re: Fun with Feedback

Posted: Wed Jan 25, 2023 8:41 pm
by Cressy Snr
IslandPink wrote: Wed Jan 25, 2023 8:01 pm I bask in its glory !
Muttley-picture.jpeg
Muttley-picture.jpeg (14.02 KiB) Viewed 6956 times