Remember These?

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Mike H
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#106 Re: Remember These?

Post by Mike H »

Nick wrote: Sun Dec 02, 2018 6:43 pm
Is this untidy enough to qualify as a breadboard?
Its a start :-)
I can tell you've been thinking about this.
That's the bit that breadboards are meant to avoid :-)

No time to worry, fret and talk yourself out of trying the idea, just see if it works, and if it doesn't nothing is lost.
You need to nail a couple of extra bits to the edges, that'll make it more properly untidy. :D
 
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#107 Re: Remember These?

Post by Cressy Snr »

HT on the output stage, with the 960mA load is 159V (blue meter):
Image
That is a good sign :)
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#108 Re: Remember These?

Post by Cressy Snr »

Mike H wrote: Mon Dec 03, 2018 5:10 pm
Nick wrote: Sun Dec 02, 2018 6:43 pm
Is this untidy enough to qualify as a breadboard?
Its a start :-)
I can tell you've been thinking about this.
That's the bit that breadboards are meant to avoid :-)

No time to worry, fret and talk yourself out of trying the idea, just see if it works, and if it doesn't nothing is lost.
You need to nail a couple of extra bits to the edges, that'll make it more properly untidy. :D
Allright allright :mrgreen:
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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pre65
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#109 Re: Remember These?

Post by pre65 »

Somewhere I have got some (at least 4) 2.5R (from memory) potentiometers. Don't know the wattage but they must be about 1.75" diameter.

If they might be of interest I'll try and find the buggers and confirm the details.

I'll donate them to the project. :)

PS it would look better with a coat of white paint on the wood. :lol:
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Mike H
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#110 Re: Remember These?

Post by Mike H »

Gloss or satin? :lol:
 
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pre65
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#111 Re: Remember These?

Post by pre65 »

Mike H wrote: Mon Dec 03, 2018 5:20 pm Gloss or satin? :lol:
Whatever is in the paint cupboard. :wink:
The only thing necessary for the triumph of evil is for good men to do nothing.

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#112 Re: Remember These?

Post by pre65 »

What wattage 2.4R resistors are you using Steve ?

I should have a bag of 2.4R which were ex MOD from Chatham Dockyard, some were close tolerance.
The only thing necessary for the triumph of evil is for good men to do nothing.

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#113 Re: Remember These?

Post by Cressy Snr »

pre65 wrote: Mon Dec 03, 2018 5:41 pm What wattage 2.4R resistors are you using Steve ?

I should have a bag of 2.4R which were ex MOD from Chatham Dockyard, some were close tolerance.
They are 3W.
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#114 Re: Remember These?

Post by pre65 »

Ah, these are 2.4R Welwyn 5%, but they only look like 1/2w or possibly 1w.

I've got several hundred of the perishers.
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Mike H
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#115 Re: Remember These?

Post by Mike H »

2.4R ? A bit unusual?
 
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#116 Re: Remember These?

Post by Cressy Snr »

Been doing some calculations. Its the first time I've tried doing the proper algebra and stuff, in order to try and better understand what is going on in this amplifier, So be gentle with me.

Gain of the output stage works out to be 0.5, which is -6dB, so you lose half the signal because of the inefficiency of driving an 8 ohm load from a 29R source resistance. To generate 1 Watt into an 8 ohm load, requires 8V pk-pk, which means that to get one watt out of the output stage requires 16V pk-pk of drive, due to the -6dB issue. With the 12V of 13E1 bias I have at the moment, I should, in theory, get 24V pk-pk

On the face of it then, the 5687 cascade should be OK (maybe)

Gain of the unbypassed 5687 first stage works out at 10, which equals 20dB

Gain of bypassed 5687 second stage works out at 13, which equals 22.3dB

so the total voltage gain of the amp is 130, which equals 42.3dB

If we allocate say 14dB for the closed loop gain of the amp, that leaves 28dB for feedback.

14dB of gain after feedback means a closed loop voltage gain of 5, so 2Vrms from a cd player will give 10Vrms output, which is 28.28 V pk-pk, capability at the output cathode, so 14V of bias is needed at the 13E1 cathodes. 16V would be even better.
I don't have that at the 'mo, so the current through the 13E1 needs to go up a bit (probably nearer to 600mA) in order to avoid premature clipping.

Given a following wind, I should get around 1.75WPC out of it, into 8ohm speakers, but IF and only if, the 28dB of feedback is enough to keep the bass under control. 2W is probably not possible; not into 8 Ohms at any rate. I'll have to build a 32 ohm speaker.

Of course, as this is the first try at calculating these things, all my calcs and reasoning could be faulty. The answer does look reasonable enough though. If it is I now know what to expect from the project and why.
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Nick
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#117 Re: Remember These?

Post by Nick »

I don't actually know how that will work out, The only way I can see the output impedance being lowered is by driving the grid of the cathode follower harder than if it was a simple CF with a big load resistor. To get 1v into the output, if its 8R, then thats 125ma, if the gm is 35ma/v then that would require 3.6v, so that would put the gain at 0.28 instead of the 0.5 you calculate (you may be right).

Me I would either get a spice model and sim it, or just build it and see. I would start without feedback, then add feedback to give the gain you want, then measure the output impedance and see how it goes.
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#118 Re: Remember These?

Post by Cressy Snr »

Looking again, the first calculation I did is incorrect.

I used the wrong value for the plate resistance ie. 29R (ra reduced by u+1) rather than the true plate resistance of 130R. Plugging the right value into the gain equation gives a gain of 0.25; so the gain is halved again, which means 32V pk-pk of drive to get back to 1W (16V of bias on the 13E1)

I'll build with the 5687 cascade and tweek the feedback so that I'm just getting away with it. It may be that my arbitary 28dB of feedback is way too much. Wolfgang and Ray are driving 15 Ohm speakers so their feedback requirements are not like mine, so it'll have to be suck it and see, aided by sig gen, scope and ears.

I can see now how 8 Ohms would have been impossible with the single 6C33C. The 13E1 will maybe, just about do it, but it'll be at pretty hairy (600mA) current levels. :shock:

Aren't breadboards fun :)
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Nick
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#119 Re: Remember These?

Post by Nick »

I used the wrong value for the plate resistance ie. 29R (ra reduced by u+1) rather than the true plate resistance of 130R. Plugging the right value into the gain equation gives a gain of 0.25; so the gain is halved again, which means 32V pk-pk of drive to get back to 1W (16V of bias on the 13E1)
Yep, 29R is the resistance at the cathode not the anode. We both got about the same value anyway for the gain. Shows how decoupled the grid and cathode is going to be in this "cathode follower" with such a low value load.
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#120 Re: Remember These?

Post by Cressy Snr »

Did a little more messing with the current programming resistor today and there appears to be no way with the HT I have that I can get enough cathode voltage to allow even 1W worth of swing into an 8 Ohm load wihout either exceeding the dissipation limit of the valve or the CCS chip. 12V centre (24V pk-pk) is what I'll have to accept. It doesn't matter what the driver will do if the cathode is restricted in how far it can go in response.

I'll take it to its conclusion though. Be daft not to now. Then I'll build a KT66 driven 13E1 job with an OPT. Easy 10WPC :lol:
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