Remember These?

What people are working on at the moment
Cressy Snr
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Re: Remember These?

Post by Cressy Snr » Sat Jan 05, 2019 2:15 pm

Image
1KHz square wave.
“ What a load of cobblers! ”.....Said Dougal.

Cressy Snr
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Re: Remember These?

Post by Cressy Snr » Sat Jan 05, 2019 2:17 pm

Image
10KHz squarewave.
“ What a load of cobblers! ”.....Said Dougal.

Cressy Snr
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Re: Remember These?

Post by Cressy Snr » Sat Jan 05, 2019 2:19 pm

Image
100KHz squarewave
“ What a load of cobblers! ”.....Said Dougal.

Cressy Snr
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Re: Remember These?

Post by Cressy Snr » Sat Jan 05, 2019 2:21 pm

Image
100KHz sinewave.
“ What a load of cobblers! ”.....Said Dougal.

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IslandPink
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Re: Remember These?

Post by IslandPink » Sat Jan 05, 2019 2:59 pm

Phwoar !
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

Cressy Snr
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Re: Remember These?

Post by Cressy Snr » Sat Jan 05, 2019 4:21 pm

Revised Schematic:
Image

Coupling cap to output stage has been changed to a 0.47uF Jupiter moulded paper capacitor.
It's gradually evolving from the 'bitza' I built out of what I could find at the time.
The lower value cap puts the -3dB point of the output stage at 4.5Hz and has lowered the filter time constant, to give a faster recovery from overload.
Could be a bit faster but it's a ruddy sight better than it was.
“ What a load of cobblers! ”.....Said Dougal.

izzy wizzy
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Re: Remember These?

Post by izzy wizzy » Sat Jan 05, 2019 4:39 pm

Sorry Steve, but I got another question. Didn't notice before but you have the grid resistor for the 13E1 after the 10k. Is that right? If so why? If not, I'm assuming you have a 10k grid stopper and that makes a bit more sense.

Great set of scope pics! You must be pleased.

Cheers,
Stephen

Cressy Snr
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Re: Remember These?

Post by Cressy Snr » Sat Jan 05, 2019 4:50 pm

No its a grid stopper. I usually leave them off diagrams, but as Ray is building an amp with this valve, I thought I'd better put it in.
One of these days I'll put a schematic up without errors on it, but I wouldn't hold your breath. :roll:

Aye, I'm well pleased with the way it 'scopes out.
“ What a load of cobblers! ”.....Said Dougal.

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Re: Remember These?

Post by Cressy Snr » Sat Jan 05, 2019 4:59 pm

One more time!
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“ What a load of cobblers! ”.....Said Dougal.

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Nick
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Re: Remember These?

Post by Nick » Sat Jan 05, 2019 5:03 pm

What would be interesting is if you have a dual trace scope to see the 1kHz output and the grid signal at the same time.
Resistance isn't futile it's V / I.

Cressy Snr
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Re: Remember These?

Post by Cressy Snr » Sat Jan 05, 2019 5:20 pm

Nick wrote:
Sat Jan 05, 2019 5:03 pm
What would be interesting is if you have a dual trace scope to see the 1kHz output and the grid signal at the same time.
I've got dual trace, but only one probe.
That might be good to look at when I bring the board to darkest Halifax.
“ What a load of cobblers! ”.....Said Dougal.

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Re: Remember These?

Post by izzy wizzy » Sat Jan 05, 2019 6:58 pm

I imagine it might seem I'm coming late to the party but I have been following from the start. I asked about the 2nd stage bias coz I only looked at 2 x 220 for the driver and 1 x 220 for the input. Guilty of not doing calcs as Mark pointed out, so it wasn't really much lower current given the voltages posted on the schematic.

So why was that? Given both run of same HT, same valve, one with twice the cathode resistance for bias yet both running near the same current and so roughly same anode voltage.

It obviously works well and sounds great so am I missing something obvious here?

Cheers,
Stephen

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Re: Remember These?

Post by Cressy Snr » Sun Jan 06, 2019 10:17 am

I've no idea what was going on with that.
Both 5687s are now identically biased and produce near as dammit the same voltages in each half.
They are a bit thrashed, so I'll probably treat the proper build to a new pair.
“ What a load of cobblers! ”.....Said Dougal.

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Mike H
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Re: Remember These?

Post by Mike H » Sun Jan 06, 2019 10:59 pm

Image Watching.

Another interesting thing that could be done with a dual trace scope is to look at the input signal and the feedback signal on the cathode of the first stage. The difference should be not a lot. In fact the difference will be, what the signal level has to be, if there was no NFB, i.e. open-loop gain.

Say the open-loop is 1,000, and you wanted a closed loop of 20: to get an output of 1V requires 0.05V in.

But if you look at what's actually between grid and cathode, it will be more like 0.001V, which multiplied by 1,000 gives 1V out.

This is why peeps are saying the first stage doesn't need a large o/p swing, cos it's net input is very small.
 
I never make the same mistake twice. Instead I prefer to fill my day with a variety of different mistakes

Cressy Snr
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Re: Remember These?

Post by Cressy Snr » Mon Jan 07, 2019 10:41 pm

Sounds interesting Mike. No doubt Nick'll put it through the wringer and we'll report back warts 'n all before the end of the month. I'm just tying up a few loose ends with the front room remodelling, so I'll PM Nick about a suitable time sometime this week.

I've tied up one loose end with the room today and installed a shelf just above the equipment bench. This shelf now houses the TV, cable modem and AppleTV,,which means I've been able to reinstate the analogue front end (JBE/Mayware/Audio Technica turntable and NVA Phono1.

I'm listening to the new Van Morrison double vinyl, "The Prophet Speaks" a Xmas pressie from our Ant, via the OTL. Van the Man is in fine voice and the band is in the room. Lovely! :D
“ What a load of cobblers! ”.....Said Dougal.

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