Snail Phono Pre

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Nick
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Re: Snail Phono Pre

Post by Nick » Thu Nov 07, 2019 2:43 pm

By g2 on the el84 I assume you mean g1 on pin2? The voltage on the lower triode anode is fixed by the upper cathode voltage which is set by the voltage on its grid.
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Nick
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Re: Snail Phono Pre

Post by Nick » Thu Nov 07, 2019 2:48 pm

The el84 grid is driven by the triode anode unless there is grid current. Convert the voltages you are measuring to currents across the resistors and it may make more sense. Does the el84 anode voltage remain steady?
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Re: Snail Phono Pre

Post by little eddy » Thu Nov 07, 2019 6:46 pm

Nick wrote:
Thu Nov 07, 2019 2:43 pm
By g2 on the el84 I assume you mean g1 on pin2?
Yes your assumption is correct.
Nick wrote:
Thu Nov 07, 2019 2:43 pm
The voltage on the lower triode anode is fixed by the upper cathode voltage which is set by the voltage on its grid.
The upper triode grid voltage remains fixed at around 134V, as per the voltage divider on the output, but it is the cathode voltage that shoots up when Iload is above 24mA, I am guessing because the top triode shuts off due to insufficient Va.

So as I increase the load current, Vk of the upper triode remains constant at 137V while it conducts, so it must be the falling anode voltage on the upper triode that triggers it to stop conducting, somewhere below 33V.
Nick wrote:
Thu Nov 07, 2019 2:43 pm
Does the el84 anode voltage remain steady?
yes around 330V.
Nick wrote:
Thu Nov 07, 2019 2:43 pm
The el84 grid is driven by the triode anode unless there is grid current.?
The issue, at least at 29mA is that the upper triode stopping to conduct as Va falls below 33V, Vgk = -3V up until it stops conducting. Above 24mA, the EL84 is still regulating but I assume only driven by the lower triode, and the regulator's performance will be somewhat reduced.

But what I don't understand is: because the output remains relatively constant at 225V, (even after the upper triode stops working), why is Va of the upper triode falling (or being constrained) as the load current increases?

I can't see how the load current would affect the cascode error circuit as this is 'driven' by voltage is it not! So is it the EL84 that is increasing its Vgk and driving Vg1 down as its current increases?

And if I knew, I might have a chance of understanding what I might be able to do, or if there is any way this type of circuit could regulate at 225V, or even 235V and with a load current of 34mA, 10mA higher than the highest good operating condition.

God I feel thick as you've probable told me the answer somewhere.

I have an Excel table of the valve conditions with increasing current, but difficult to understand when pasted as looses the columns.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Mike H
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Re: Snail Phono Pre

Post by Mike H » Fri Nov 08, 2019 5:42 pm

I can't see how the load current would affect the cascode error circuit as this is 'driven' by voltage is it not! So is it the EL84 that is increasing its Vgk and driving Vg1 down as its current increases?
Looking at it again, try moving the + end of the 470k to the input Voltage side (Hammond choke), aka EL84 anode, and reducing its value to say 100k. I think where it is, it is starving the 6N2P of current, which gets worse as the output Voltage drops.

NB: I also like the using a pentode idea.
 
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Re: Snail Phono Pre

Post by Nick » Fri Nov 08, 2019 6:07 pm

I can't see how the load current would affect the cascode error circuit as this is 'driven' by voltage is it not
As the current through the el84 increases, the el84 grid and cathode will get closer to each other. As l they get closer the current through the 470k resistor will reduce (it has to, its driven by the voltage across it) so the current through the cascade will reduce. Remember valves are transconductance devices, the voltage on the grid controls the current through the valve, so its false to say they are only voltage devices.
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Re: Snail Phono Pre

Post by little eddy » Fri Nov 08, 2019 6:17 pm

Will do. From what Nick said about my proposed pentode design, the load resistor should come off the anode side of the EL84 anyway, so probably an improvement over the originators design. Just a bugger to do.

Running short term with the pre-amp ECC82s removed to limit the load.

Sounds great this afternoon with what I've done.

I am trying to accumulate a selection of ECC83 valves so will try something else than the Mullard I63.

Currently building a BOM and looking out for some EF86s, and also a spare EL84 so can just see if that's behaving how it should.
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Re: Snail Phono Pre

Post by Mike H » Fri Nov 08, 2019 9:02 pm

little eddy wrote:
Fri Nov 08, 2019 6:17 pm
Just a bugger to do.
Do know what you mean. Think it would be worth it tho. EF86 is a useful thing. :D
 
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Re: Snail Phono Pre

Post by little eddy » Sat Nov 09, 2019 4:43 pm

Well spotted Mike. I couldn't understand why the EL84 Vg was so large when looking at it's triode characteristics, should have been a volt of so at the most for Va 100V and 35mA.

Left the load at 470k (as per a Rozenblit circuit) and I think all is OK now with voltages as attachment :D . 41V across the bottom triode and 84V on the top one. Vg for all three parts circa -1V.

Added the EL84 g2 resistor and cap across the Zener while I was at it. Also reduced the load voltage slightly to get to 100V across the EL84.

So no need to switch to the EF86, but will get some anyway.
19-11-09 Snail PSU Schematic 217V.jpg
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Re: Snail Phono Pre

Post by IslandPink » Sat Nov 09, 2019 5:12 pm

Ah, very good. I seem to remember that years ago when I built my first series-reg supply for the main amp, I had a problem like that.
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Re: Snail Phono Pre

Post by little eddy » Sat Nov 09, 2019 5:35 pm

Why might the designer have added the 100k and 220k resistors to the cathodes of the bottom and top triodes in the regulator?

Looking at my Rozenblit book, with a single triode regulator, there is no resistor from the ECC83 cathode to the regulated output.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Nick
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Re: Snail Phono Pre

Post by Nick » Sat Nov 09, 2019 9:26 pm

Inject a bit more current maybe?
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Re: Snail Phono Pre

Post by little eddy » Sun Nov 10, 2019 1:37 am

I'm inclined to remove them if not fully justified in my quest for simplification.

As per the original amplifier stage design, although perhaps well intended, they seem excessive and less may hopefully be more!
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Re: Snail Phono Pre

Post by little eddy » Mon Nov 11, 2019 4:07 pm

So getting psyched up for trying to do a square wave test.

Trying out my cheap Chinese signal generator and oscilloscope. Is the attached image showing a good enough waveform to do this test, shown at 950Hz?
Square Wave Small (2).jpg
The Inverse RIAA card has a selector switch for -40 and -60dB, so I sould use -60dB with me having 1:10 SUTs?
Inverse RIAA Small.jpg
So what should be the input voltage? is it a matter of say 1V peak to peak (phono stage output) divided by the product of the 2 gain stages, i.e. 1/100/40=0.25mV, then /10 for the SUT is 0.025mV?

Then multiplied by 60dB for the RIAA card, (*1000) is 25mV

I am thinking of using a potentiometer or potential divider to step down the 5V signal generator output, so 200:1.

If using -40dB my input should be 250mV?

If OK, what frequency is recommended for the inverse RIAA test?
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Re: Snail Phono Pre

Post by little eddy » Wed Nov 13, 2019 4:40 pm

So a bit of tinkering and the inverse RIAA is sort of working.

Initially there was a bit of 'undershoot so have reduced the series RIAA resistor from the theoretical 284k to 269k, resulting in:
Inverse RIAA 269k small.jpg
My question is how much more can this be improved, is this reasonable, and I guess how far should I allow myself to deviate from the theoretical RIAA values?

Will give this a listen tonight but guess may have to adjust the TT for the increased HF through the RIAA.

Will continue reducing tomorrow to see if there is a point at which there is overshoot then I know I have gone too far.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Re: Snail Phono Pre

Post by little eddy » Mon Dec 09, 2019 12:36 am

So finally in a position to box this up tomorrow after doing a lot more than I had envisaged or planned. Learnt a lot and found three faults with the amp:
As we know, voltage regulator not working (original builder's fault)
Channels swapped over at the line stage (original builder's fault)
Incorrect RIAA resistors fitted (I'd missed out a 1k resistor in my 1st rebuild)

But also done a lot more:
Film cap power supply filter added to 1st phono stage
Increased supply voltage to enable higher current/bias for both phono stages
Rewired line stage so volume control is now on the input rather than output (you all suggested I did this years ago!)
Takman/Elna components added throughout
Tube rolling ending up with Telefunken ribbed ECC83s

I couldn't manage to increase the currents/bias of the line stage due to the increased current and transformer/choke resistances reducing the voltage across the regulator below the recommended 100V.

The other thing I didn't manage was the inverse RIAA/square wave tests as per Nick's suggestion, held back by my cheap signal generator and scope. I know I've been told that you can't tune the RIAA characteristics by ear but what I ended up doing was switching between my digital source 'direct' and then via the inverse RIAA and phono amp in series, and gradually adjusting R1 of the RIAA to achieve the same balance. Lo and behold, I ended up with damn near the exact same value as per my original calculations. I then adjusted the tonearm to keep a neutral balance throughout.

So very happy with how it all sounds and can now spend more time listening rather than tinkering.

I don't think it's worth doing anything else with this amp as there are a number of fairly major limitations such as the fitted SUTs and limited voltage/current of the original power supply. But plenty else to be getting on with, a valve tester being my next challenge. Then looking at an ultrasonic record cleaner as well as tweaks/completion of my 41MP line stage and MoFo power amps.
Attachments
19-11-25 Snail Reworked ECC82 small.jpg
19-11-24 Topside rewired small.jpg
19-11-24 Beneath rewired small.jpg
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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