Niobium - Valve pre for SS Power Amp

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little eddy
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#16 Re: Niobium - Valve pre for SS Power Amp

Post by little eddy »

Thanks guys. That makes it simpler.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
little eddy
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#17 Re: Niobium - Valve pre for SS Power Amp

Post by little eddy »

There she glows!
IMG_20180525_200710996 small.jpg
So relief with 184V on both channels. Now 42V across CSS so bigger heat sinks ordered but on with the rest of the build.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
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Dave the bass
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#18 Re: Niobium - Valve pre for SS Power Amp

Post by Dave the bass »

Phwwwwwwwwwwwwoooooooooooaaaaaaaaaarrrrrrrrrr! Like!
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IslandPink
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#19 Re: Niobium - Valve pre for SS Power Amp

Post by IslandPink »

This is a really nice project !
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little eddy
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#20 Re: Niobium - Valve pre for SS Power Amp

Post by little eddy »

Beneath gain section small.jpg
Gain stage now mostly constructed
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
little eddy
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#21 Re: Niobium - Valve pre for SS Power Amp

Post by little eddy »

So wired up Andrew's DC heater boards and the 41MPs.

On no-load they were set to 4.0V on the assumption that the centre tap resistors would draw enough current for them to regulate fully.

With the loads now attached, both are regulating at 3.8V. 95% of voltage or I assume 90% heater power but will this affect the valve operation/sound?
All valves from front 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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Nick
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#22 Re: Niobium - Valve pre for SS Power Amp

Post by Nick »

Whats the voltage on the output of the board?
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little eddy
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#23 Re: Niobium - Valve pre for SS Power Amp

Post by little eddy »

Voltage at each board output is nominally the same - 3.8 to 3.85V with centre tap taken to 0V.

41MP valve heater is quoted at 4V, 1A, (separate board for to each valve).

It's not that I can't adjust the potential divider on the regulator, just a bit of a dock yard job to get the regulator boards out. So would leave at 3.8V if it's unlikely to make any difference.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
little eddy
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#24 Re: Niobium - Valve pre for SS Power Amp

Post by little eddy »

OK so an afternoon in the garage and almost there.

Heater voltages adjusted to 4.0V.

I then powered up but not quite at the operating point I envisaged. The two 3.1V LEDs are giving me less than 5.8V so am running at a higher current than planned. Va 166 and Ia between 23 and 24mA. Photo is showing a shunt current through the VR tube string of 15mA and total CCS current is 38 or 39mA.
powered up small.jpg
Do people think I am driving the 41MPs a bit too hard at this current as was aiming for between 20 and 22mA?

Will try a couple of other blue LEDs I have supposedly 3.6V but likely to drop Ia below my targeted minimum of 20mA. May end up reverting to RC combo!
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
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izzy wizzy
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#25 Re: Niobium - Valve pre for SS Power Amp

Post by izzy wizzy »

If you were aiming for 20-22mA and got 22-24mA, that's a result in my book unless you're flirting with the max dissapation? It's within 10% which IMHO is close enough for valve work.

Get it going and fiddle with it after that :)

cheers,

Stephen
little eddy
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#26 Re: Niobium - Valve pre for SS Power Amp

Post by little eddy »

Yes it was dissipation I was concerned about.

Datasheet attached although it doesn't have anything specific about a dissipation limit.

Maximum Va is 200V and it says 'Anode current for 200 Anode volts' - 24mA. So does this mean a maximum dissipation of 4.8W?

If normal Va is 150, using the recommended Rk of 320R, that would mean around 16mA for Vk of 5.1V, so dissipation would be down at 2.4W.

I'm running at 166*.024 = 4W so suppose should be fine at 83%, (if my assumption on the maximum dissipation is correct).
41MP Datasheet.pdf
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TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
little eddy
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#27 Re: Niobium - Valve pre for SS Power Amp

Post by little eddy »

So the 41st element lives!
On test small.jpg
Letting it soak a while before bringing inside and trying on my home system. Test a few more voltages and temperatures but all initially looking OK.

There were a couple of pops on the output when the VR tubes ignited. I found an article that suggested bypassing one of the series VR tubes with a 200 - 500k resistor. This might stagger ignition if that helps, but might also limit the peak voltage on switch-on if one would ignite before the other. Anyone any experience? I'll try and find the article and post. I think it also suggested that the 90V VR tube may need above 105V each to ensure ignition, so in theory B+ may rise to 210V or higher before being pulled down. Bypassing one of the tubes might limit this to 195V.

Do I get a prize for the most cluttered bench?
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
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#28 Re: Niobium - Valve pre for SS Power Amp

Post by IslandPink »

No, you don't get a prize unfortunately - I've seen Nick's bench !
Great work by the way.
The VR tubes will pop, I think Steve C had a fix for this maybe by muting or delaying something else. I just live with it !
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little eddy
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#29 Re: Niobium - Valve pre for SS Power Amp

Post by little eddy »

In the following link, it suggests that the 90C1 may take 125V to ignite. No wonder it takes so long for them to kick-in. Once one of the series tube ignites, I'm presuming the voltage over the 2nd tube will jump by up to 35V and will therefore also strike shortly afterwards. To the visible eye, they ignite at the same time. http://www.tetrode.co.uk/vrtubes.html

I couldn't find the old guide/manual I previously found, but one author said something similar "To aid starting when doing series strings, you can connect a 1.0M resistor between the plate(s) of the tube(s) below the uppermost VR tube and the DC supply. This will supply the necessary starting voltage so that they all start up, but it pulls very little current when they're going so that nothing poofs and degrades the voltage regulation." I think I will do some measurements and try to see if it works as a very simple mod.

The use of the older OB3 has a lower strike threshold so I may keep my eyes open on slease-bay. And might look great with the double diode and triode valves.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
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izzy wizzy
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#30 Re: Niobium - Valve pre for SS Power Amp

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