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Low Voltage Valve/Tube Buffers/Hybrids/Amps

Printed From: Graham Slee at Hifi System Components
Category: And the rest
Forum Name: Amplification
Forum Description: Share your interests or views on amplifiers, preamps, etc
URL: https://www.hifisystemcomponents.com/forum/forum_posts.asp?TID=2785
Printed Date: 22 Oct 2019 at 11:33pm
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com


Topic: Low Voltage Valve/Tube Buffers/Hybrids/Amps
Posted By: Graham Slee
Subject: Low Voltage Valve/Tube Buffers/Hybrids/Amps
Date Posted: 26 Jan 2016 at 8:16am
How do valves/tubes work at the low voltages output by wall-wart power supplies?

The greatest fad ever in Audiophile land (or is it audiophoolery?) seems to be anything containing a valve--even better if it's on show.

Lots of devices are on the market which use a valve or multiple tubes which are low voltage DC powered. Hang-on! My understanding of valves/tubes was that they were optimised to work on HT--high tension--high voltage!

Are these special low voltage valves? Just how can the electrons move from cathode to anode/plate at 12 volts or thereabouts? It turns out that these valves are your ordinary olde worlde valves meant for HT but operated at ridiculously low voltages. So how's it done?

I've been doing my research and I found it quite interesting. It came about because I have a customer using a Solo Ultra-Linear and he was using a low voltage valve buffer between his CD player and the headphone amp. He was toying with the idea of sending his Solo back because it sounded so bad!!

Lucky for me this customer was open to suggestion--many aren't--and he did as I suggested. He put the valve buffer to one side and just used the Solo direct from his CD player. After a few days "burn-in" he was happy as Larry. He's since returned the valve buffer to the shop instead.

So what made it sound so bad? I mean, the buffer he was using is "highly respected" in audiophile circles!

Could the audiophiles be wrong? The reviews glow brighter than the tubes! Just what is it which makes these low voltage valve devices so appealing?

All I can say is that solid-state must have become so unlistenable that these low voltage tube devices are filling the vacuum (pardon the pun). However, when you look under the bonnet of most of these devices you see they are solid-state, and the tube came along for the ride. The tube simply can't drive sufficient current so it requires an output transistor, opamp or MOSFET to do that job.

Taking a look at valve data we see there's a graph of plate (anode) current versus voltage. The origin like most graphs is zero and the curves show their linear portion is at high voltages, and their non-linear portions are at low supply voltages. So these devices will feature high distortion.

Digging further into my research I found an article which delved into how it's done. Reading the sub-heading "Linear amplifiers under starved conditions" I started to feel better. Why? Because I thought I'd missed a trick, but the sub-heading was telling me my suspiscions were right--that these circuits are audiophoolery.

What's more, you usually expect high input impedances with valve inputs. The technical article went on to explain that because of the high grid current required at such low DC supply voltages, the grid bias resistor has to be quite small indeed. It talks about 22k and that's in parallel with the grid impedance which is also quite low. These devices will place a heavy load on your sources (unless the buffer is buffered by some solid-state circuitry).

Now, because the valve is operating way outside its optimum voltage its linearity is basically crap! However, the richness the 3% or so distortion the article cites might be desirable in some weird way, but it completely veils the micro-detail people like me have strived to preserve.

It could be argued (and I'm sure it will) that speakers distort more than 3%. However, even on speakers the differences between hearing micro-detail and not hearing it is noticeable. Speaker distortion is no excuse.

So how did these devices come about? The answer is in guitar and musical instrument effects. Effects are great in music, but you really have to ask whether you want a musical effect added to the recorded music you're playing?

If you do, then I reckon you're bored. If you're bored you'll obviously play around to make things more exciting or interesting--that's what usually happens with humans.

Why would you be bored? If a solid-state device isn't delivering the fidelity it ought to, then that's good reason, and it says a lot about today's hifi products if people buy and use these valve devices to brighten up their lives.

The technical article can be read here: http://www.valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf

It should be noted that the author did not use an audio analyser to obtain the distortion figures and as such where it says "no distortion" it doesn't mean there isn't any.

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Not simple enough for Google-Bot to understand...



Replies:
Posted By: morris_minor
Date Posted: 26 Jan 2016 at 8:39am
This should be a blog post, Graham!

To my eternal shame (Embarrassed) I was once the proud owner of a "tube input buffer" from a "well -regarded" manufacturer. It went between a universal disc player and amp to tame (?) a tendency to digital stridency. (Yes, this was well before the Majestic was around). With typical audiophoolery expectation bias I found it did a great job! Until that is I moved the system and took it out (for a reason I don't remember) and found I didn't miss a thing. . . Confused

I sold it (for a profit) on a well known auction site. Thumbs Up 

Lesson learnt!  Shocked LOL


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Bob

Majestic DAC/pre-amp
Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V
Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50
2 x Proprius + Spatia/Spatia Links


Posted By: Graham Slee
Date Posted: 26 Jan 2016 at 10:06am
I keep forgetting how to blog, Bob.

Having taught myself valve basics from REME training manuals many years ago (we'll call them valves because we're British and it's a waste of time trying to get noticed in the States on Google for the keyword tubes), I always fancied the idea of designing a valve "offering".

The problem is power supplies. To sell world-wide through dealers a manufacturer must comply with all kinds of national safety laws, and it is the easy option, plus it's saner from a costing point of view, to use a ready-made off the shelf pre-approved power supply.

There are some really good ones that do 48 volts, which is near as damn it the same voltage as the 50V Leclanché cell the BBC used for reserve power for valved equipment. Performance would be much better running on 48 volts.

The problem is the heaters which run at 6 - 6.3 volts. These were run by smaller batteries if mains derived power wasn't available.

Imagine the situation today: you'd need a 48 volt power supply and a smaller power supply plugged into the back of a decent valve preamp! Who's going to go for that?

There are however ballast tubes which could be employed to drop the 48 volts down to the voltage required for the heater circuit. These are basically light bulbs: http://frank.pocnet.net/sheets/141/b/B6.pdf

Simply stated, valve cicuits were very wasteful of energy. Oil and coal was cheap and global warming wasn't an issue like it is today. Losing part of the 48 volts to get the voltage down for valve heaters generates heat as any lossy device would do, and heat is wasted energy. Heater circuits take 0.6A usually and at 6 volts, if you have 8 valves you'd just wire the heaters in series and run them off 48 volts. But say a design only needed 4 valves, you have to drop 24 volts, and at 0.6A that's 14.4 watts... more than a PSU1 uses on standby.

A big power resistor would do the job. It would get hot, but so do valves. If everybody used valved products today the people of the Lake District would probably be flooded out permanently.

So now we see that using a 12 volt wall-wart makes great business sense. But the problem is the linearity when operating valves at starvation voltage. The result is distortion - it is totally mad!

Will valves eventually get a bad reputation because of this way of usage? And if so, will that result in valve manufacturers giving up? And then it wouldn't be viable to design a proper valve preamp. Such questions are important.

And where do I get the valves from - at reasonable cost? To say they're so popular the prices don't reflect that. What used to get made for pence is now inflated to several pounds. We obviously have to buy Chinese valves - provided they're not inflating their prices too.

And what about the quality? You only need to ask Leo about Chinese power transistors... they were nothing of the sort! It cost a certain musical instrument amplifier company its reputation and was a contributing factor in its closure.

Perhaps a Slee valve preamp is just a fanciful idea?


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Not simple enough for Google-Bot to understand...


Posted By: morris_minor
Date Posted: 26 Jan 2016 at 3:54pm
Originally posted by Graham Slee Graham Slee wrote:

Perhaps a Slee valve preamp is just a fanciful idea?
On a par with porcine aviators, maybe?

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Bob

Majestic DAC/pre-amp
Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V
Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50
2 x Proprius + Spatia/Spatia Links


Posted By: Fatmangolf
Date Posted: 26 Jan 2016 at 6:34pm
"Porcine aviators" would bring a new meaning to a tailspin. I must say I don't even like starved valves in effects and guitar amps. You get a fuzzy buzz and not the twang leading to crunch of a properly supplied preamp valve.

It's best not to look at the power requirements of a valve amp. My Deluxe Reverb sounds great but warms the room up for 22 watts of power, I've got a single ended one that events worse inefficiency although it does sound great.



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Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.


Posted By: Graham Slee
Date Posted: 27 Jan 2016 at 8:06am
Well, you could say that they're actually flying in virtual space!

I spent too much time yesterday looking at possible plate current and grid voltage using a 12AX7 or ECC83 than I should, for a 48 volt supply.

But with around 120 kilohms of plate resistance it would have to be a hybrid operating into an opamp to get the output impedance low enough to drive anything.

I'd be looking for a voltage gain of 2.5 (+8dB) with hopefully near to 0.1% THD. What's that you say? Porcine aviators??



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Not simple enough for Google-Bot to understand...


Posted By: oldagetraveller
Date Posted: 27 Jan 2016 at 12:10pm
I remember, as a youngster, growing up in the 1950's, we had a valve radio powered by an accumulator. My father used to take the discharged accumulator to a local hardware shop, swap it for a freshly charged one and pay a small fee. Glass casing(?), you could see the internals. How did these work with valves? The accumulators were only low voltage and current.
I searched and found this.  http://vintageradio.info/cell.html


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Peter

P T- LPT/RB300/G1042, Pink Triangle Tarantella/Nima/Ortofon 2M Black, SL1210II, Naim CD5, NAC112, NAP150, Flatcap2, Proac SC1, GS SoloUL,GS Accession , Senn HD250 & HD540.


Posted By: Graham Slee
Date Posted: 27 Jan 2016 at 1:24pm
My Junior's teacher once brought in some batteries for us to experiment with. One was a 90V HT battery!!!

I accidentally shorted it with some thin bare wire which instantly glowed and burnt in two. It also discharged it...  Luckily I avoided the cane.


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Not simple enough for Google-Bot to understand...


Posted By: BAK
Date Posted: 27 Jan 2016 at 3:17pm
Originally posted by Graham Slee Graham Slee wrote:

My Junior's teacher once brought in some batteries for us to experiment with. One was a 90V HT battery!!


Have I got my A B C s mixed up?

Isn't that where we got "B+" to mean the high voltage power supply?
 ... 90 volt HT battery was called the "B" battery, as I recall.

An "A" cell was the grid bias battery?

A "C" cell was the filament ? 


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Bruce
AT-14SA, Pickering XV-15/625, Technics SL-1600MK2, Reflex M, Lautus, Technics SH-8066, Dynaco ST120a, Eminence Beta 8A in custom cabs;; Using Majestic DAC
Enjoy Life Your Way!


Posted By: DaveG
Date Posted: 27 Jan 2016 at 7:01pm
Originally posted by Graham Slee Graham Slee wrote:

..  Luckily I avoided the cane.


Ha! if it happened these days it's more likely Injurylawyersrus would be encouraging the parents to sue the skool for scaring little Graham Tongue


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Dave

Michell Gorbe + HR PSU -> Cadenza Bronze -> SME V -> Elevator -> Accession -> Proprius -> B&W CM6 s2 | Cusat 50 & Spatia cables ->


Posted By: Fatmangolf
Date Posted: 27 Jan 2016 at 7:55pm
Indeed! If we could just get all the injury lawyers to visit each other's offices, fall over and sue each other... perpetual inflation.

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Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.


Posted By: Graham Slee
Date Posted: 27 Jan 2016 at 11:53pm
Originally posted by BAK BAK wrote:

Originally posted by Graham Slee Graham Slee wrote:

My Junior's teacher once brought in some batteries for us to experiment with. One was a 90V HT battery!!


Have I got my A B C s mixed up?

Isn't that where we got "B+" to mean the high voltage power supply?
 ... 90 volt HT battery was called the "B" battery, as I recall.

An "A" cell was the grid bias battery?

A "C" cell was the filament ? 


Sounds right to me.


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Not simple enough for Google-Bot to understand...


Posted By: Graham Slee
Date Posted: 28 Jan 2016 at 12:11am
US Patent 5450034 looks interesting and reached its 20 years last September, so it will be turning up in all sorts of valve hybrids soon (if not already).

http://www.google.co.uk/patents/US5450034

It's called a Reflected Plate Amplifier and is very similar to the two transistor stage used in many a transistor preamp circuit.

It suggests the use of one half of a twin triode, but the other half couldn't be used for the second channel (because of crosstalk) and would be wasted.

It gave me the idea to try making a differential input stage which would use both halves and traditional negative feedback could be applied in a more predictable manner (I do so hate wasting time and components on "what if's").

At least the invention could make use of the same plate voltage used in a class-a 90V HT supply circuit using just 45 volts for its supply, as there is no conventional plate (anode) load. Therefore the valve's designed characteristics would be met.

I will try and draw up the basic circuit and present it here, and if it succeeds you never know, the stage could be offered as a kit...

We'll see.


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Not simple enough for Google-Bot to understand...


Posted By: Graham Slee
Date Posted: 28 Jan 2016 at 2:05am
Just an idea...


It may work Clown



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Not simple enough for Google-Bot to understand...


Posted By: Graham Slee
Date Posted: 28 Jan 2016 at 11:38pm
Anyway, I ordered a couple of valves from RS earlier and will give it a try and report my findings.


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Not simple enough for Google-Bot to understand...



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