Tír Na HiFi

Ok Guys,
Hello :)

I was doing some research into valves as i was nearly swapping my amp for valve WAD from woody - who is a gentleman in all aspect and a real pleasure to deal with ! (and very knowledgeable) - and i realized theres so much into valves, and theres not too much on the web for , say, valve "noobs".

So i though, how about a general topic on the board where ppl can ask simple questions, discuss various valves, amps, maybe even set-ups and provide helpful hand ?

I might start with some simple questions, hopefully itll kick off from here.
Any answers from people experienced in the field would be greatly appreciated.

some stuff i would like someone to explain / confirm / bust ?

1. Single Ended sounds different to Push Pull.
Accordingly to a person from Icon Audio (whom i contacted recently):

A Push Pull amplifier has two valves per channel and in simple terms is that one valve handle the incoming signal and the other on of the pair handle the outgoing signal, whereas in a Single Ended amplifier it has one valve per channel doing both jobs. As such a single ended amplifier does take more power and also gives less power output. Apart from that they do sound quite different, many people say that the single ended sound is "tight" whereas a push pull amplifier is more of a rounded sound. Most people go for push pull amplifiers to be honest.

Is there any other way to describe the difference ? (lets put aside the fact its best to listen to both)

2. For entry level (starter) system/amp - is it recommended to go SE or PP ? or is it just subject to personal opinion ?

3. A lot of times theres a talk about speaker sensitivity - what does it mean ? And how valves come into that ? Is it better to match "W" output per channel on valve to speakers or it doesn't matter anyway ? a lot of times people say lower output amp were driving bigger speakers with no problem....

4. power - since SE require more and give out less, is that an issue at all ? like, electricity bills, is there anything required (power surges etc) or is it simply tech-talk and it doesn't really matter as long as you take care of your amp ?

5. for entry level - whats the most important part of tech spec of valve amp ? say i have speakers already, and a source - what should i be looking for especially in spec of the amp and why ?

6. the heavier the better - true ? (that might be a bit silly but...)


so there.
I dont mind learning something technical about speakers and amps - hopefully thread will kick off from here and we'll discuss some amps later on as well !

Great idea for a thread.
Can I ask the following for starters.

What are 'square getters' 'rib plate' 'Declined Square getters-mottled glass' '1 hole each side of the plate. Test Strong on my tester better than 95% against a new el84' 'triple-mica black plates' 'triple-mica black plates ' 'dual-mica tall plates' 'triple-mica grey plate ' 'clear tops'

thats enough for the moment

I'll take a few of these:

1. PP/SE - I'll come back to this!


2. Start off with PP or SE. The choice here depends very much on the speakers you have. For most people, PP will be whats needed as these are far more likely to sound good with "normal" speakers. Most SE amps are low powered so need horns.

3. Speakers sensitivty is rated in terms of dB/w/m (ignore the watts rating on them, thats just how much max power the drivers can handle before melting down). So a speaker rated as 88dB (typical bookshelf) will need a greater power amp than say a horn rated at 96dB. Straight off what would happen if you took the same amp, hooked it up to both speakers and measured the dB at 1 meter from the speaker, with a signal level that represented 1W? Well, the bookshleves would give you 88dB but the horns would give you 96dB (much louder for the same input power). So what does that really mean? It means that to use low powered amps, eg SE 300B at 5W you need to use sensitive horns to get a decent volume. Now its all actually a bit more complicated than that too, as this ignores other factors like the damping factor of the amp etc etc, so you may find that some speakers will work OK with lower powered amps. Often though what happens is that the bass sounds uncontrolled and monotonic, and sometimes the treble becomes hard and glassy on dense passages.

4. Watts when talking about an amp means the amount of power that amp can send to the speakers before it reaches a critical level of distortion. Remember watts = voltage x current. So its the amount of voltage (ie the "loudness") by the current (the ability to drive the cones). So You can have a valve amp that uses electricity to power the valves, waste much as heat etc etc, maybe consume 500w but only deliver 7W to the speaker. Likewise you could have a digital chipamp that consumes 30W and delivers 27W to the speakers. So some amps use and waste more electricity than others - important to some and not to other people. Also, the valves do wear out - not so much the smaller signal valves, but very much so the bigger power valves, eg KT88, 845 and so on - so its gonna cost you every so often. Also, valve amps tend to be heavier than solid state amps (more transformers), give out more heat so positioning in a rack may be more of an issue.

5. You should be looking for an amp that is powerful enough to drive teh speakers you have.

6. Heavier the better? Maybe since it usually means that there are bigger tranformers, chokes etc in there. Not always though.

Other Tony stuff:

The getters etc etc - all that stuff is to do with the way the valves are constructed. Internet lore has grown up that some particular construction of valve is better than others (kinda similar to the way people used to say that toyotas assembled in japan were superior to the ones assembled in england). Of course teh $$ goes up very quickly for these valves. Clear top, mica discs, anode plate shape, blackburn stamp etc etc... all the same.

Testing on a tuber tester:
In the valve days, many companies made machines to test the state of valves. Kind of like a digital multimeter today. you would look up a manual, set the switches to the appropriate values for the valve in question, plug in the valve and get various readings off it. I think anode current and transconductance were the main values that told you what your valve was like. So when you hear of matched sets, these fellas stick the valves in, get a reading and then put together 2 or 4 that give very close readings. The idea here is that a matched set perform almost identically, so in a PP amp (of which more later) the signal would be stitched back together very well with almost no distortion or interruption.


Hows that for now?

I need more time to explain PP vs SE.......

well I've learned loads.
Thanks Fran.

SE and PP...


OK, so imagine a very simplified sound wave: its like a simple sine wave:


So, in an SE amp, the single valve (or multiples in parallel) will amplify (i.e. make bigger) both the crescent and the trough of the sine wave. In other words, in the pic above, the valve will amplify all of the wave both above and below the line. That's fairly wasteful of energy because the zero point of the amp (i.e. the zero signal-being-passed point) must sit a pretty high level above ground. In other words, it needs to sit at a fairly high voltage and bias point - there is a fairly high amount of current flowing all the time through the valve. This single valve handles the entire sound wave - hence single ended, SE.

In the PP amp the arrangement is different. In this case, a pair of valves is used, tied together. What happens is that one valve handles the top have of the sound wave (above the line in the pic above) and the other handles the bottom half (below the line). This means that only half the current is needed to achieve the same power output. Now what really happens is that this facility is used to get much more power out of the amp - typically nearly twice what you would get from the same valve in SE mode.


So why wouldn't everyone always want PP? Well, there are a number of design considerations. One is that you will need to acurately split the signal into 2 halves and then somehow combine it back together again. Typically, the signal is split using another valve (usually a dual triode like a 12au7, 12AT7 or 12AX7, or single triodes like 6SN7, 6SL7) and its recombined in the output transformers. So you can imagine that if this splitting and recombining is not done perfectly, then the signal is distorted. This little pic shows what I mean in exaggerated form:


But there's another thing. What happens if one of the pair of valves used in the push-pull pair works worse or less efficiently than the other? What you end up with is that one half of the sine wave is reproduced perfectly while the other is not. Also add to that the fact that you have more devices that the signal can pass through...... more chances for something to affect the signal..... well you get the idea. In fact it is quiet the testament to the circuit and valve and transformer designers that the thing works not just OK, but that most PP amps sound wonderful. That is also why you will often see reference to needing a matched pair of valves - efficiencies of both valves are the same, so there is less chance of any distortion creeping in.

So in many cases in an ideal world you would have an amp that operates in SE mode, could give you loads of power, but didn't suck up too much power. Unfortunately that doesn;t really exist. AFAIK, the 845 is what comes close, about 40W of power per valve, but it sucks a lot of power to do that, maybe only 10-15% efficient.

I should also mention "ultralinear" - this is a slightly different wiring arrangement in the output transformers that results in slightly lower distortion values. Other things that are a topic for later are pentodes and triodes - some of you probably have amps that you can switch between these 2 modes.


Lastly a few pics of valves:

grey plate (the anode is grey) and you can see the square looking wire right up at the top of the valve - that is the getter:


This one has the standard mirrored top:



whereas due to different internal structure, this one is "clear top"


BTW, that "getter" and the mirror coating is a part of the manufacture. When they make a valve, they seal the valve under high vacuum. But its never good enough, so what they do is add in a little bit of special material, that absorbs the oxygen that may be present still in the valve, and also that may leak in as the valve ages. When they finish making the valve, they heat that material, it flashes off and is deposited near whereever the getter is - often at the top of the valve (getter - it "gets" oxygen).

If you are buying valves and the inside looks cloudy, or that mirrored surface looks faded or worn away, its a sign that oxygen is or has creeped in - and most likely the valve is not long for this world. See the pic below for an example:



OK now I'm off for a lie down in a darkened quiet room! I'm sure youse are already asleep!

eh, before I go, here are a few youtube videos showing manufacture of valves by mullard etc:

http://www.youtube.com/watch?v=HveUY7eivfI

http://www.youtube.com/watch?v=V4rZLNodQ0s

http://www.youtube.com/watch?v=PrLSLxb5Jqg

http://www.youtube.com/watch?v=d-828JdmWqA

Thanks Fran, Very comprehensive and informative as usual. Even as a bad student if I dont understand it all today its there to come back to when required. You have filled in a lot of blanks already that no searches to date have managed to do. Like any teacher you deserve a 1 month winter break and sure take 3 months off in the summer. (I hope no teacher I know reads this)

Well done Fran.You have a teacher's gift in simplifying the incomprehensible for us mere mortals....I know, I was that teacher........once upon a time :-)

Excellent descriptions Fran....thank you for that!

excellent tutorial fran. i may have always known what to look for with valves-but never understood the Why part until now. and that, my freind, why you've always been my geek-head-o-rama audio god.

-b