Niobium - Valve pre for SS Power Amp

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little eddy
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Niobium - Valve pre for SS Power Amp

Post by little eddy » Sat Apr 07, 2018 11:48 am

Now that my LM3886 SS power amp is nearing completion , it's time to start thinking of a suitable partnering valve pre.

I am considering converting the Foverture5 SS amp to an F4 due to it's reported neutral character, so the aim is to achieve a good tonal balance by using a valve pre with a combination of conventional and SS support circuitry.

Based on the use of my existing 41MP valves, will call this project Niobium. Talking to Nick, he has confirmed these would be a suitable alternative to the 76 he has previously used with the F4.

So initially posting a draft schematic for comments/advice, and when firmed up I can start to acquire the remaining bits.
18-03-29 Niobium Mono PSU Amp Schematic 41MP IT Shunt.jpg
I have a 300-0-300 tx and wishing to employ a choke filter, the total dc current is limited to 78mA for a 100mA choke allowing for the ac current element.

Rectification is proposed using the double damping diode 6BY5G.

Although I have a pair of the MJ zener regulators, will initially try valve regulation to provide a degree of valve coloration, (this power supply is on reflection very similar to that employed in my AN DAC).

The one thing I am interested in would be the optimum grounding scheme, so the attached tries to show how it might be strung together.

The target is to have a prototype constructed for Owston.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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pre65
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Re: Niobium - Valve pre for SS Power Amp

Post by pre65 » Sat Apr 07, 2018 12:28 pm

After what Nick said about active pre amps (maybe on here, not sure) I'd think of a passive switchbox with pot and possibly a valve buffer. Just saying.
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Nick
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Re: Niobium - Valve pre for SS Power Amp

Post by Nick » Sat Apr 07, 2018 12:36 pm

pre65 wrote:
Sat Apr 07, 2018 12:28 pm
After what Nick said about active pre amps (maybe on here, not sure) I'd think of a passive switchbox with pot and possibly a valve buffer. Just saying.
That is a valve buffer.
Resistance isn't futile it's V / I.

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pre65
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Re: Niobium - Valve pre for SS Power Amp

Post by pre65 » Sat Apr 07, 2018 12:48 pm

Nick wrote:
Sat Apr 07, 2018 12:36 pm
pre65 wrote:
Sat Apr 07, 2018 12:28 pm
After what Nick said about active pre amps (maybe on here, not sure) I'd think of a passive switchbox with pot and possibly a valve buffer. Just saying.
That is a valve buffer.
Just goes to show what I know. :oops: :oops:
Life is what happens to you while you're busy making other plans. John Lennon

G-Popz THE easy listening connoisseur. (Philip)

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

Post by little eddy » Sun Apr 15, 2018 2:44 pm

Have most parts now so will start next week when the choke arrives. Using cheap plastic chopping boards for the 2 decks (poor mans Teflon).

The other layout option, perhaps favored, would be to swap the positions of the vr tubes and 41MPs having all of the AC and power towards the front. Signal, output txs and amplification at the back. Just seems a shame having the expensive triodes obscured by the vr valves. Weight would also be more evenly distributed and the output txs would be well away from the power tx and choke.

It's going to be a lot heavier than I though a pre-amp would be. I need to think again about how I will mount the top board off the bottom one. Initially was thinking of using screwed bar but would have little rigidity and lateral strength. Might order some alu angle and perforated sheet and build a frame.
Attachments
Layout top small.jpg
Layout bottom small.jpg
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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IslandPink
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Re: Niobium - Valve pre for SS Power Amp

Post by IslandPink » Sat Apr 21, 2018 7:00 pm

Nice thinking - good to see things being re-purposed !
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little eddy
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Re: Niobium - Valve pre for SS Power Amp

Post by little eddy » Fri Apr 27, 2018 12:19 pm

Have just read that I should not earth the filter choke frame as it introduces noise.

On a similar note, should I earth the power transformer body/frame as it is isolated from the rest of the amplifier with me using my plastic cutting boards as the chassis base/top plates?

I have connected the transformer screen to the chassis earth point.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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

Post by little eddy » Thu May 03, 2018 12:30 pm

Slow progress but psu now tested and 230V @ 80mA so bang in line with PSUD.

On with the CSS and vr tube network, and can then complete the chassis construction and then on to the gain stage.
Niobium PSU small.jpg
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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

Post by little eddy » Sun May 06, 2018 4:20 pm

The cutting, filing, drilling and tapping is taking for ever but getting an idea now of how it might turn out. Hope it sounds OK after all of the effort.
Frame from side small.jpg
Frame from back small.jpg
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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

Post by little eddy » Thu May 24, 2018 9:41 pm

Help please from those with experience of using VR tubes.

I have a shared B+ of around 232V that feeds both channels.

Each channel has a IXYS 10M45 set at 40mA and then I have a 10k dummy load resistor in parallel to a series pair of VR tubes to give a regulated 200V, the theory being that the dummy load will take 20mA to simulate the 41MP valves, the remaining 20mA going through the VR string.

One side powered up fine with both 90 and 108V VR tubes igniting, the voltage settling at 202V and roughly 20mA through the VR tubes.

The other side, neither of the VR tubes ignited and the voltage rose to just under B+ with just 1V across the CCS.

I had some spare VR tubes so tried a few. On one occasion, one of the 90V VR tubes ignited but not the 108V one, yet the voltage seemed to be regulated at 200V with circa 20mA through the VR tubes. I would have thought with only one VR tube igniting, there would be no regulation!

I swapped the tubes between channels and the 'fault' moved to the other channel.

It has probably something to do with the ignition voltage. The 90C1 has an Vig max 125V and the 108C1 a minimum ignition voltage of 133V. It suggests a higher voltage is needed for lower ambient light. So methinks I need a higher B+ to ensure ignition. To test I put two 90C1 VR tubes in series and they both ignited.

Some thoughts please.

Is the 'issue' a result of using the dummy resistors of the size I have chosen? Would altering the resistance up or down help? Am I likely to have the same issue when the 41MP valves are fitted as I don't really want them to be seeing 232V?

My current train of though is what can I do to increase B+. Can't really do anything regarding the tx and choke resistance. I have 2 100R resistors in the supply legs from the tx to the rectifier, and then I have 2 RC filters each with 100R resistors. The spec sheet for the 6BY5G says effective plate supply impedance (each plate) 100R. So if my tx winding is 65R, does this mean I could get away with 35R series resistors?

Option 2 is to put slightly more capacitance in front of the choke.

I will have to increase the size of the CSS heatsink if I do this.

The other option is to chose a lower supply voltage of say 180V but Va will be lower than that. I would try to maintain the Ia of 20mA through the 41MP, but Va would be circa 20V lower than I run them in my Rocky amp.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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Nick
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Re: Niobium - Valve pre for SS Power Amp

Post by Nick » Fri May 25, 2018 9:02 am

as I don't really want them to be seeing 232V?
Well, if they are running at 200v, I don't see how that would be any different. They will have to see the ignition voltage of the VR tube whatever you do. I bet it will be fine with the valve instead of the resistor, the valve will not pull any current at start-up, so that will allow the voltage to get high enough to ignite the tubes.
Resistance isn't futile it's V / I.

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Re: Niobium - Valve pre for SS Power Amp

Post by izzy wizzy » Fri May 25, 2018 10:25 am

Following on from waht Nick said. The way to see if they will strike is to set the CCS to 20mA and test without the dummy load. VR tubes all vary and so you want them to strike under worst case without having to select; the top of the strike voltage range.

cheers,

Stephen

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Nick
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Re: Niobium - Valve pre for SS Power Amp

Post by Nick » Fri May 25, 2018 12:08 pm

I would just try it without the resistor. You can't have the CCS set to a current the VR can't handle, it needs to cope with a failed valve drawing no current.
Resistance isn't futile it's V / I.

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Re: Niobium - Valve pre for SS Power Amp

Post by izzy wizzy » Fri May 25, 2018 12:53 pm

Another thought would be to just use 90C1 in both positions and move on with the project. In the grand scheme of things, a slightly lower voltage won't mean a thing IMO. At 200V, your running at maximum where typically the anode voltage according to datasheet is 150V.

cheers,

Stephen

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Nick
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Re: Niobium - Valve pre for SS Power Amp

Post by Nick » Fri May 25, 2018 1:39 pm

Agreed, in general misplaced precision and valve don't go together.
Resistance isn't futile it's V / I.

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