Specifications to look for in a high fidelity vacuum tube amplifier

Just wanted to share something written to a fellow audiophile looking for a good tube amp.

There is a lot of BS floating around regarding amplification. Furthermore, reviews having descriptions ('Warm', 'Smooth', 'Harsh', etc.) which are immeasurable/unquantifiable further confuse customers. It is best to start with an understanding of the definition of High Fidelity.

HiFi basically means faithful reproduction. The requirements for a "High Quality" (I say high quality as there is no such thing as perfect equipment, and no such thing as true fidelity in
electronic reproduction of sound. Human beings have tried their best to understand sound, and create electronics for HiFi but IMHO we're still learning) tube amplifier should be:

a) Low Noise & Distortion up to the maximum output level. These would be freq
components not present in the original signal. Harmonics of original frequencies and sum and difference tones of diff freq intermodulation is not desired. (This means high S/N ratio and low THD figures in the spec sheet.)
b) Good frequency response across audible frequency range. A perfect flat line is preferable (but seldom achieved) from about 30Hz to 20kHz.
c) Negligible phase shift throughout audible frequency range. Signals travelling through components arrive at the output with different time delays result in distorted sound.
d) Excellent transient response and power reserve to match. Anything below 4 microseconds (60s EL84 push-pull amps) should be considered quite good. ('Slew Rate' is the modern definition found in some spec sheets. If the manufacturer leaves this out you've got to perhaps wonder why?)
e) Output resistance should be less than impedance of the loudspeaker. This results in good reproduction of transients.
f) Low or negligible levels of hum and noise. Of course, there could be other requirements which you may require, such as the power to drive the speakers which you are using, inputs to match your source, etc.

The construction of a vacuum tube amp is very important. From what I see on the net these days, a lot of people seem to be building amps from just looking at old circuits!! They pay little or no attention to avoiding pitfalls in the placement of components.

Construction requirements (as set down by Philips and others in the 50s):
a) Spacing of valves (there must be AT LEAST one tube diameter between valves)
b) Audio inputs should be placed as far from the mains transformer and power supply components in the chassis as possible
c) Tube heater wires should be reasonably stiff (not flopping about all over the place), be twisted together (assuming its 6.3Volts AC as usual), and placed as close to the chassis as possible, being kept away from grid and anode connections
d) Transformers should be at right angles to each other so as not to induce unwanted currents in each other. This means output transformers at 90 degrees to each other, and a power transformer LAYED DOWN on its side, all as far from each other as possible. This is not 'pretty' or symmetrical but it is a hallmark of GOOD ENGINEERING.
e) Grid and anode leads must never run in parallel with the heater leads
f) Input, volume, tone controls and grids of input to preamp stages should be screened, these screened leads and all grid and anode leads should be kept short
g) Earthing points of each valve should be connected to a common point on the chassis
h) Coupling capacitors should be of the highest generic quality available

Having said all that, I can understand why some designers dislike tone controls - Bass and Treble, as the flat freq. response curve they worked so hard to get would be altered. However, IMHO the condition of your room, and the fact that playing at low volumes often necessitates some boost of lows and highs (the LOUDNESS button seen on 60s and 70s sets) to remain 'nice' sounding, justifies having bass and treble knobs, just to play around with, and a bypass button to go for minimalist, direct sound.

It may sound contradictory but the basic requirements for good engineering is not affected by my personal desire for the flexibility of having tone controls. I've got Baxandall's original Wireless World issue where his tone control circuit was first published so its also sentimental!

Anyway Audiophilia is a MADNESS, but I hope my humble advice helps you judge
the engineering which has gone into some amps, and the shameful lack of engineering in certain products. There is NO VOODOO in what I have typed here. Everything can be found in old Mullard/RCA books and Philips Technical Library book "Electronic Valves for Audio Frequency Amplifiers" by E. Rodenhuis, published in 1954.

Evidently a lot of 'Audio Gurus' neglect to mention these basics and keep going on
about the COMPONENTS. There is no point in having super high class components if the basic circuit topology and construction is bad. No adding of Black Gates, VCaps, Auricaps, Kiwame resistors, etc. will help! On the other hand a simple but well-built amp using the best quality off-the-shelf components can smoke any BS amp, anytime, IMHO.

You're right scudracer.
I have a Leak Stereo 20 made in the late 50's I believe and it's pretty basic.
Mine's still waiting for the original preamp (Point One Stereo).
Heard a similar unit at my friends place and was mighty pleased with his very basic setup.

The teleman

Leak Stereo 20. My first ever HiFi amp. Still have it, with the Point One Stereo preamp. The circuit is basically from the Mullard tube circuits book but some values have been subtly changed. This, coupled to a pair of 1977 vintage Bose 301s produced very satisfying sounds using a Sony 1-bit CD deck.

Actually these days a good, well-designed solid state Class A amp can put most tube amps to shame. We simulated an Eico HF-81 amp in PSPICE and a sinewave pumped through was twisted up rather badly. With solid state there's no worries with expensive output transformers. I must get the Eico HF-81 fired up one of these days and compare to Leak Stereo 20 and Sansui AU-70.

Will post report here but sadly I don't have 'Golden Ears' or perhaps am a skeptic, hence can't often tell much difference between equipment.

scudracer wrote:Leak Stereo 20. My first ever HiFi amp. Still have it, with the Point One Stereo preamp. The circuit is basically from the Mullard tube circuits book but some values have been subtly changed. This, coupled to a pair of 1977 vintage Bose 301s produced very satisfying sounds using a Sony 1-bit CD deck.

Actually these days a good, well-designed solid state Class A amp can put most tube amps to shame. We simulated an Eico HF-81 amp in PSPICE and a sinewave pumped through was twisted up rather badly. With solid state there's no worries with expensive output transformers. I must get the Eico HF-81 fired up one of these days and compare to Leak Stereo 20 and Sansui AU-70.

Will post report here but sadly I don't have 'Golden Ears' or perhaps am a skeptic, hence can't often tell much difference between equipment.

Hey scudracer, I would appreciate it if you could tell me whether anyone's wanting to part with their their Point One Stereo Preamp.I'm very much a retro kinda guy and would very much like to get my Stereo 20 up and running.
teleman51
012-3980810

Well, after all these years, a Point One would need repairs to the potentiometers (dirty?) unless it's been kept in pristine condition. Why not consider something else? Don't be too afraid to use any good modern pre-amp. I'll keep an eye open though I've not come across a Point One in real life since getting mine back in 1994.

Hi Teleman51,

What speaker you drive with Leak 20? The famed LS3/5a? Just interested to know.

BULK METAL FOIL RESISTORS

Without prejudice, many people are recommending components based on how they 'sound'. I recently bought a pair of output transformers which 'sounded' good. When I got home a friend and I tested it with a function gen and scope. The frequency response curve was like Mount Kinabalu! Aaargh!

Don't fall for components because someone tells you 'it sounds good'. Basic technical characteristics of the components must be 'GOOD' before you even consider a listening test, otherwise, it would be a waste of time, wouldn't it?

For resistors I've found that Vishay "Bulk Metal Foil" ones are the best as these "Bulk Metal Foil" resistors are constructed in such a way as to minimize stray inductances and capacitances beyond the capabilities of metal film resistors. I believe that the long term stability, low TCR and PCR (changes in resistance due to temperature and power), consistent frequency response, and low current noise of the foil resistors are the primary benefits for specifying foil resistors in audio applications.

Read this Vishay tech paper - Audio Noise Reduction Through the Use of
Bulk Metal Foil Resistors — "Hear the Difference"

http://www.vishay.com/docs/49414/vse-an00.pdf

It is my sincere hope that the audiophile community will insist on proven components rather than accepting products which are claimed to 'sound good' but offer no measurable/quantifiable proof of their performance.

Guys,
This sounds like an interesting forum. It's good to see a healthy level of mutual respect here. Let's maintain this level of decorum and always watch for a healthy balance of sharing knowledge against imposing your views on someone else.

Hi-Fi 4 Sale are looking for some more contributors here who can add to the body of knowledge that we have to make it interesting & enjoyable for everyone, newbies and veterans alike.

Thanks! I don't want to seem elitist or anti-subjectivist. My humble apologies if I've ruffled anyone's feathers. It's just that I think we should be sort of 'mythbusters' in a way, because it's very sad to see people being cheated or being fed lies. The people who fall for lies are usually not rich, and struggle to buy equipment which may or may not be properly built in the first place. Let's share info here and help each other make a better choice.

"Better Choices through Research" - Hey that's a good slogan eh?

CH wrote:Hi Teleman51,

What speaker you drive with Leak 20? The famed LS3/5a? Just interested to know.

I only have the Leak Stereo 20 power unit and as such am on the lookout for the matching preamp that originally came with this model.
As the prices on ebay and the fierce bidding are too much for me to bear, I may just have a tube premp built for me.
As my personal economy is not too good, I'm planning to go the passive preamp way if I can't afford a tube premp.You can also call me El Cheapo if you like .Passives are simpler to build and may be a way out for me.
As for speakers I am using, I now use a pair of Rogers Ls4a2 with the drivers replced with a pair from Banridge given to me.Those poly type cones on the original driver can't really hack it in our Malaysian weather.Or maybe they would if they were in air con all the time .
To me they sound okay although a little laid back.
I'm a retro 70's music kind of guy ,so a more forward type of sound may do me right.

scudracer wrote:Well, after all these years, a Point One would need repairs to the potentiometers (dirty?) unless it's been kept in pristine condition. Why not consider something else? Don't be too afraid to use any good modern pre-amp. I'll keep an eye open though I've not come across a Point One in real life since getting mine back in 1994.

Thanks for your personal interest scudracer.
I also appreciate the sound advivce on getting a S/state preamp.May well go on this.
Always loved the sound of tubes since I got my 1st tube guitar amp.You can see from my avatar that I love guitars and as such come in from a different direction.
I always have a healthy dose of respect for you hifi guys.But not always able to pay the prices out there.
Again it's about the music.
Good news: Just found my old Luxman TT model PD272 in the storeroom and turned it on . Still works!!!!!!.Time to start searching for my old vinyls from the70's.Will keep you posted.

teleman51

Teleman51 - If you like tube pre-amps, why not give this a shot. No tone controls but that just means less chances for phase distortion. Apparently derived from a Marantz design.

http://octave-electronics.com/Kits/m7i.shtml
http://sites.thestar.com.my/audio/story.asp?file=/1997/8/07preamp

The review states that the tube heater voltages are DC so this preamp should be SUPER QUIET and beat even the Point One. I'm not sure if you can use those Vishay Bulk Metal Foil Resistors (they may not the right power ratings), but try your best. The Octave brand caps should suffice but you may want to knock yourself out with big name VCaps/Multicaps.

The preamp needs to be super quiet. Any noise or distortion here will just be amplified. Just my 2 sen.

CLEANING UP YOUR AC

By the way, Teleman51, if you're going to build the M7i, please use a Schaffner IEC filter. It will give you clean AC in, free from RFI. It will cost a lot cheaper than all those fancy power cables (where does it stop? Wall wiring, house to power station?). RM3-5 for China copies but I'm sure you can get the real thing (Schaffner is made in Thailand) for a good price from Farnell Components in PJ. Just use any generic computer IEC mains cable.

FN9244 is the latest part number.

http://www.schaffner.com/image/FN9244_1200x847px37.jpg

scudracer,
Thanks for all the info on the various options on the matters discussed.I will go down to Octave next week and check out that kit.
Also for IEC filter mains, I will take up your recommendation on it. Have a friend who's son works for a musical instruments dealer in Tmn Maluri.Spoke to him about using his skills in putting it all together for me.He's more eager than I am.
Will decide on the chassis later after I've checked out what's in Octave.
By the way, Can the Leak Stereo 20's power supply be modded to also use the Schaffner?
Just wondering about that as the original 3pin power (female) cord did not come with the power amp when I bought it just over 2 years ago.
What's your recommendation?
Thanks again scudracer. I will try try to include some pics of my power unit in my next post just to give you an idea of the condition.

teleman51

Hi Teleman. The Schaffner IEC filter is just my low-buck, anti-subjectivist engineering answer to the absurdly priced power cables I've recently seen. I don't want to condemn that part of the hobby but since I can't afford it , have to find alternatives which make sense and have published, measurable/quantifiable results.

Googling "Mains RFI filter" I found this article which you may enjoy - http://www.tnt-audio.com/clinica/filter.html

Actually the ham radio community prefers these things as their rigs pick up lots of noise from various sources. Light switches, motorcycles passing by, etc. all result in audible noises coming out.

Hope your friends son and you will have fun putting it together. Get a good soldering iron, tinned properly on the tip. http://www.expertvillage.com/video/4890_electronic-circuit-tinning-iron.htm and google "tinning soldering iron" for more info.

The Leak Stereo 20 to me had no problems as I remember it. Just browse www.Schaffner.com and you'll find tons of filtering solutions, even PCB mounted ones. Far more important to make the pre-amp super quiet especially at the initial input stages of the signal as any noise there will get amplified. Personally I wouldn't mess around too much with the Stereo 20 itself.

Use good gold plated pin sockets for the tubes. Good gold plated RCA input sockets (Made-In-China Neutriks at Farnell are about RM6 each). All those old carbon resistors might be bad now (some of mine just cracked in two!). The caps may very well be goners so best replace what you can too. Treat the GZ34 rectifier tube like gold cos that's what you're gonna be paying for to get a replacement these days!

Test a few mains transformers for the M7i to make sure they don't hum. It can be very irritating. Perhaps one of those resin-impregnated, potted/encapsulated transformers would be fine. I'm sure that with a careful build you'll have a knockout pre-amp which can rival the big name guys. Just put it in a nice, well-finished Ayon Audio style black box with some funky polished knobs and label it "TELEMAN TM7i"!

Hi scudracer,
Tried finding the Shaffner IEC connector online browsing Farnell online catalog, but could't see it there.Will call their number tomorrow to find out more.
Man, I myself just realised that there was a real world alternative to those expensive power cables out there,not that I'm implying that they don't really work or are not worth the ringgit we have to cough up.It's just that sometimes the trends make us believe that if we don't have one of these, then we're a distance away from sonic nirvana.
I always ended up feeling this way.
Back in the 70's(Thats way way back, before you were born,scud) to own an AKAI or Kenwood stereo system(the complete rack systems) was the epitome of having reached there.I had a cheap Superscope receiver amp (designed by Marantz and made in Taiwan),A cheap Dual T/Table,a pair of Wharfedales made from a kit and I felt real real good about my system.
Played lots of records on those and to me sounded good,at least back then.

Now, over 30 years later, the hunt for that sound (analogue,that is) has bitten me and the fever has started again.

I now use a very modest system with an old Yamaha CD player,a Rotel integrated amp and a pair of modified Rogers LS4a2 speakers.It's keeping me happy while I'm at home.
So now I'm on another journey of discovery, left off after getting married 25 years ago.I'm real appreciative of guys like you who make this hobby seem accessible to almost anyone with a heart for it.
You go past all the "snake oil", voodoo and myths that seem to float around these days.
Thanks,scudracer,

Will write again soon.

teleman51

Actually I honestly am a skeptic for things unquantifiable/unmeasurable. I tried listening but couldn't detect the difference. Maybe my ears are messed up and not 'Golden'.

My guess is, we should investigate and make sure the stuff we use is good, from an engineering perspective before moving on to the listening test. If not, we're wasting our time. Former McIntosh Acoustic Research Director Roger Russell has some funny views on this: http://www.roger-russell.com/truth/truth.htm

I was born in 77 and briefly experienced horrible 2 channel (RTM1 and RTM2) black and white TV (we couldn't afford colour till 1983). House always had radio (WW2 vintage BC348, HRO, HRO Navy rack-mount, AR88, Yaesu FT-101 and others).

Records eh? The temptation is there (have got a few old records - never been played by me). Vinyl is making a big comeback and Marantz has a superb top end model apparently made for them by ClearAudio Electronics GmbH, Germany. The pro audio guys like Technics/Pioneer/Denon stuff. Kenwood made a turntable rebadged for Bose in 80s, I believe. However the disadvantages of a mechanical system, dirt, etc. are still present so I'll stick to CDs.

Actually I too, am back in this so-called HiFi world having left it in 1996 or so. As a skeptic, like Scully to the Mulders who want to believe, in the HiFi World, I must apologise if I offend anyone, but the Truth Is Always Out There. Albeit buried under tons of snake oil, voodoo, myths and legends.

I know the sound you mean, the texture. The NOS capacitors and tubes are responsible for the vintage sound. These have got scarce and expansive. Circuit designs are more less constant.

Vinyl is your best bet but it is getting $$$. Truth is vintage will be gone.

70es was era of rock & roll, it is a pity that the recording is mediocre.