Fun with Feedback

[Paul Barker]

LOL

[RhythMick]

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?

[Cressy Snr]

Nice 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)

[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:

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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.

[Mike H]

[Paul Barker]

Like

[Cressy Snr]

For those interested - circuit below:

Screenshot 2023-01-21 at 09.22.21.png (59.5 KiB) Viewed 10883 times

Good value for money.

[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 (62.52 KiB) Viewed 10862 times

[Ray P]

The image is a bit small Steve, it's difficult to see any detail?

[Cressy Snr]

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.

[Ray P]

...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

[Cressy Snr]

Cheers Ray.

[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 (106.35 KiB) Viewed 10689 times

[IslandPink]

I bask in its glory !

[Cressy Snr]

I bask in its glory !

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