Snail Phono Pre

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little eddy
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#196 Re: Snail Phono Pre

Post by little eddy »

So the enclosure bottom replaced with temporary sections to give access to the underside of the phono.

While I was at it had taken a couple of pics of the SUTs. Does anyone recognise who they might be made by?
Attachments
Temp base support small.jpg
SUTs top small.jpg
SUTs bottom small.jpg
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little eddy
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#197 Re: Snail Phono Pre

Post by little eddy »

Sorry but I need some help with the series regulator I inherited from the original builder as attached.

To allow me to increase the valve operating points, I need to find an additional 15V.

I had thought the output voltage was determined by the Zener, and 3 way voltage divider so Vout would be 91/115*(115+47+100)=207

I wrongly assumed that if I reduced the bottom resistor to 100k, this would give me 91/100*(100*47+100) = 225V

But when I do this, the voltage at the grid of the lower valve falls to 83V so the regulated voltage effectively remains unchanged at 83/100*247=205V!

What am I wrongly assuming?
Do I need to increase the size of the upper 2 resistors and leave the bottom one alone?
Or is there something fundamentally wrong with how it was designed/built?

Thanks in advance.
11-11-19 Snail PSU Schematic.jpg
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Nick
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#198 Re: Snail Phono Pre

Post by Nick »

I would agree with you, reducing the 115 to 100 should have increased the regulated voltage. The voltage across the 115 will be more like 90 than 91 as the grid will be -ve WRT the cathode, but I still agree with your maths other than that. I would check the other voltages just to make sure its working as you expect anyway. Have you tried it off load to see if there is any difference?
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little eddy
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#199 Re: Snail Phono Pre

Post by little eddy »

Nick.

Did some testing with the full valve complement drawing around 30m, voltages as attached. Very strange the high voltage between the two 6N2P triodes.

So as you suggested, tried on no-load (fine), and then gradually added valves. The regulator seemed to work fine @ 225V up to around 24mA, but then the output voltage reduced at 27mA, and fell further at 30mA.
19-11-16 Snail PSU Schematic 225V.jpg
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Nick
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#200 Re: Snail Phono Pre

Post by Nick »

Not sure if those two triodes are in a good op point. The upper ones grid will be a 130v, the lower one and 89v (assuming 220 on the output). The upper ones cathode will then be around 131v, which means you only have 40v or so across the lower triode. The measured voltage on the upper triode indicate that its shut off with its grid at 124v and its cathode at 200v. Maybe increase the 100k resistor to the upper triodes grid to balance them out a bit more.
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Mike H
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#201 Re: Snail Phono Pre

Post by Mike H »

Yep, run out of "headroom". I agree need to get equal anode Volts on each triode.
 
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Nick
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#202 Re: Snail Phono Pre

Post by Nick »

Mike H wrote: Wed Nov 06, 2019 7:33 pm Yep, run out of "headroom". I agree need to get equal anode Volts on each triode.
Personally I would think a single pentode would be better for a 200v regulator with a 90v reference than a cascode. There is just not that much voltage left to spare.
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IslandPink
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#203 Re: Snail Phono Pre

Post by IslandPink »

That's normally how these things are done, isn't it ?
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little eddy
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#204 Re: Snail Phono Pre

Post by little eddy »

I'm going to make a series of measurements so I can better understand how the regulator valve conditions are affected as I increase the load current at 225V output.

Is that the 'load' at 470k larger than would be expected and is this 'compressing' the voltage remaining for the two triodes.

Update tomorrow.
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Nick
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#205 Re: Snail Phono Pre

Post by Nick »

IslandPink wrote: Wed Nov 06, 2019 10:14 pm That's normally how these things are done, isn't it ?
Yep, the cascode is in effect making a pentode (well at least a tetrode, four terminal device) out of the two triodes.
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Nick
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#206 Re: Snail Phono Pre

Post by Nick »

little eddy wrote: Wed Nov 06, 2019 11:27 pm I'm going to make a series of measurements so I can better understand how the regulator valve conditions are affected as I increase the load current at 225V output.

Is that the 'load' at 470k larger than would be expected and is this 'compressing' the voltage remaining for the two triodes.

Update tomorrow.
The problem is that you have 90v at the bottom, and the top voltage is restricted by the 220v you want out, so the shunt valve grid has to be less than 220v. You have to then share that 130v remaining between the two triodes. It doesn't give either of them much scope for moving, but in theory their operating point is set by the potential divider chain on the grids. But i think if you move it from its balanced point, you push one of the valves into positive grid, grid current then gets added and the divider voltages are all over the place.

It would be simple enough to rebuild that valve base around a ef86 (for example).
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little eddy
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#207 Re: Snail Phono Pre

Post by little eddy »

I was going to post last night but thought I'd investigate the current design further, even if it is just for me learning a bit more.

Coincidentally I had been looking at an EF86 pentode regulator in MJ. I had drafted the attached, but inevitably have the following questions:

Yes I can with a bit of effort change to a B9A pentode but with the current wiring arrangement it would be much easier if the 100k load resistor and 180k g2 bias resistor were taken from the regulated side of the EL84. I have seen this elsewhere but would there be any significant detriment to the operation?

I am assuming that grid stoppers are precautionary and there is no detriment using large 4k7 in this application, (currently have 1k5 on the EL84)?

There is a 470nF cap across the Zener. Some designs use much larger caps such as 22uF?

Would the use of a lower voltage zener be beneficial and if so what voltage? Some designs I have seen use 85V so not a lot of difference.

Is the 470nF cap on the output beneficial?

On the EL84, g2 is connected to the anode. Quite a few designs I have seen have a resistor in this leg between 150R and 1k0. Would this be recommended and if so, what would the rating need to be in this application? Or is this valve dependant and the EL84 doesn't benefit?
19-06-11 Snail PSU Schematic 225V EF86.jpg
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Nick
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#208 Re: Snail Phono Pre

Post by Nick »

Yes, you can drive g2 from the regulated side (recalculate the devider for the lower voltage), no you can't drive the anode from the regulated side, consider the voltage across the load resistor would only ever be the bias on the shunt valve, Yes, the 4k7 are grid stoppers, worth doing for pentode. Unless you have a scope and electronic load that you can use to drive the supply with a square wave, they are more than precaution as you won't know if its stables, 1k5 or 4k5 probably not a big difference. Work out the grid capacitance and see what the HF roll off will be to decide if the value is something to worry about, 22uf would be better across the zener for noise, but work out the current through the cathode and how long it would take to change the 22uf then decide if that’s a problem waiting that long for the supply to come up to voltage. Probably not. If you use a lower voltage zener, you will be reducing the percentage of the output voltage that is used to compare, so reduce the loop gain, so reduce the regulation and increase the output impedance. May matter, or may not. The g2 resistor you are describing is partly a grid stopper but also it ensures the g2 voltage is always below the anode voltage.
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little eddy
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#209 Re: Snail Phono Pre

Post by little eddy »

Thanks Nick.

When I look at the MJ design, he has the EF86 g2 at 20V for Zener/Vk/Vg3 of 85V. Seems awfully low - is this correct? Assuming it is, I will target 26V, similarly 65V below the Zener voltage.

In terms of the EL84 g2, 220R/1W seems to be about the average?

Mike.
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little eddy
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#210 Re: Snail Phono Pre

Post by little eddy »

So testing the cascode, as I increase the load current from 9 to 24mA, the only measurements that really alters is Vg2 of the EL84 and Va of the top 6N2P (tied together by the 1k5 grid stopper).

They start at 187 and gradually fall to 170V. Vk of the 6N2P is steady at 137V up to then so all OK with Va of falling from 51 down to 33V.

Va of the bottom 6N2P is a stead 45V.

At 29mA, Vg2 of the EL84 and Va of the top 6N2P drops to 168V but Vk shoots up to 165V.

I am guessing that the loss of voltage across the top 6N2P is driven by the reduction of the EL84 g2, which for some reason drops as the current it is conducting increases????

Or if g2 is just a reflection of the 6N2P anode voltage, what is driving down the top triode anode voltage as I increase the load current as I can't see the logic?
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