Audio Note Kit2 / 12W SE 6550tube amplifier

[assendor]

Hello you guy's,

My lack of time for this Audio hobby seems to continue, err.....
To take some shortcuts I want to buy this used Audio Note Kit2 amplifier.

Some information on what to tweak and improve on this amplifier would be very much
desirable.
It is quite obvious that it is not perfect at this price, so some tweaks is most certainly a MUST.

Since Audio Note have a rather good reputation of making useful equipment it is interesting,
another question is how interesting the price tag on their stuff is.

Now Obviously they have other price ranges than EXTREME high too,
these amplifier kits is an example of this.
Still it must be looked at as a relatively high price range, or at least to most DIY'ers.

When an offer of some used equipment pops up, and available nearby, then this opportunity
to check out the quality of this Audio Note Kit2 compared to my other Frankenstein stuff, is JUST a MUST.
It would be interesting to hear if I am only tumbling around in complete darkness with the stuff I have done so far.

I will implement all the tweaks Tony have though me, with caps an such, at a reasonable price.
MKV/Teflons/BG's, Some places at least.
Of course yeo's various Common mode type of filters IS A MUST.

One direct question, There are Three amplification steps in this amp, Pre/Driver/Output.
This is quite much, or at least to some it is? Romantizing the sound too a degree of too much Tube coloring??
I don't see why not cutting Preamp section, or changing whatever in there to get it the way that is more
in the direction of Simplicity.
Does anyone have an opinion about this?

Input stage 6SN7 Tube probably is to my knowledge adding some 2'nd harmonic contents.
SRPP driver stage with the 12AX7 adds 3'rd harmonics.
Output stage is SE 2'nd harmonics adding stage, much more than previous stages, due to the power, this is right isn't it?

There are some feedback here, is this really needed?
I know it is needed to get good measuring results, but for our ears I do think it is a pest.

OK, anyone have any input?

Hope I don't get tricked with this one, like with the Layfette Fullrange's I bought without getting.. ERR:.

-Jonas

[ian444]

Hi Jonas,

If the tranny's are good, then that is the most important thing in my opinion, from my limited experience. After that, valves and passive components are relatively cheap.

Ian.

[yeo]

hi jonas

try to get the schematic. post it then we could discuss further.

generally, 2 stages amp are "truer" to the music while 3 stage amps do "romanticize" the music to a certain degree.

of course, just a general statement.

yeo

[assendor]

Hello Ian,

Let's hope the output trans are of proper type then..
Cheap, yes transformers are a high cost in Tube amps, especially when we want really really good ones.
If the price is nice, quality good, then the shipping cost is terrible, or it's impossible to pay in any reasonable way..

Hello yeo, here's the Schematics I can find.

Kit2 Audio Circuit Schematic:
http://www.audionotekits.com/PDF/Schematics/Kit2_AudioCircuit.pdf

Kit2 Power Supplies Schematic:
http://www.audionotekits.com/PDF/Schematics/Kit2_PowerSupplies.pdf

I'm interested in the R21/C14 Looks like a Global feedback in the Power amplifier section?
We don't like Global Feedback now DO we, could be interesting to try with and without.

When it comes to stages in the Amplifier, it looks quite easy to disconnect the Preamp part, and only use the
Driver/output stage. Then the "romanticize" Effect could be avoided, or changed to another type of Preamp.

BTW:
I've red about a guy that had some trouble with the Bias current trough the output transformer/Output tube,
when the tubes gets older, or worn.
I guess this must vary with different brand tubes.
Anyway the current increased so much that finally the R28 got it solder points melted. He claimed the current to get
above 200mA.
He also had to change the power transformer and C21 since the voltage increased above 50v, and
then blown the cap... ??happening again. Strange?
He put in a Fuse of 125mA to stop this from happening again.
These things must be happening because of something going on in the Grids, doesn't it?

Wouldn't it be better to put in a CCS here then?
OR is this impossible because of the bias system with the Grid #2 supply.
I would assume the Grid #2 supply is directly involved with the Bias current..?

To some degree my mind is a little bit starting to understand some of the Triode SE part pretty well,
but when it comes to these Pentode things ,it gets a little complicated to understand what happens especially when
something goes wrong and does not operate as planned.

If everything goes as planned now, I get the amp at Wednesday Evening, or Thursday morning..
From 0.7W with Frankenstein upset, to 12W Properly made... interesting.

-Jonas

[vt4c]

Hello Jonas,
  Seems like the B+ to the plate of 6550 is a bit on the high side based on the cathode resistor value.
and the PSU circuit....65mA - 70mA is what I would run the 6550 at.

  It looks like the 6550 will be over-driven based on what I understand from the mentioned schematic.
I might be wrong about this but a second opinion is most welcome.

[assendor]

Hello vt4c,

thanks for the important input, Yes that seems to be explaining why this other guy had that type of problems.

I've red several places that increased Bias current on many tubes gives a higher pleasure of "magic sound".
More 2'nd harmonics maybe?? More I don't know???
Maybe this tempts the designers to Run it a little HOT, so that it sounds as magic as possible, risking a sooner failure
of especially the cheaper types of tubes? Or all types of tubes for what I know.

Is this something that happens to a Tube when it start to fail, that it conducts more?
Is this more likely to happen on a Pentode?
It does cross my mind to have red about shorted Tubes, is this a completely gone one?
Sorry if these questions seem a little strange....

BTW, Off topic:
the speakers the Guy I'm getting this Audio Note Kit2 from has, are these:
Audio Vector M3:
http://www.audioreview.com/mfr/audio-vector/floorstanding-speakers/m3-signature/PRD_124484_1594crx.aspx

He claims this is the best amplifier he has ever owned, and he's had a quite a lot, HE SAY anyway.
No matter what he means, or what anyone would think or say, I then have an amplifier that can be used
as a reference to other stuff I like to listen to.

A stupid Guess would be that Charlize2 Could be a REAL competitor to this amp on those M3 speakers he has.
Especially a tweaked one, well I know it would give any amp troubles if the other things matched.

-Jonas

[ian444]

Hi Jonas,

I'm sure Charlize will give it a run for its money. Not saying one will be any better than the other because only your ears will tell you that. I'm still trying to get one of my tube amps to sound as good as a generic TA2020 and its a lot of work, but getting closer and closer, maybe its just my ears, maybe they just like the TA2020 sound, I don't know. Various chip amps including gain clones, and traditional SS amps simply do not do it for my ears. If I give up on one of my valve amps I will sell it and buy a Charlize immediately. Something inside me says a valve amp will do it, but the road is long, and hopefully rewarding, whichever way it leads me. But keep in mind I started with cheap tube amps and am paying the penalty or reaping the rewards from learning, or a bit of both...

That amp you are looking at certainly looks like it is carrying the necessary weight in iron.

Running the tubes close to their max power dissipation is risky business, from what I have read, different brands of the same type tube have different limits. If you look at the Simple SE by Tubelab, he (George) seems to prefer the sound when running higher B+ with lower cathode current (to keep it under the max power dissipation). Don't worry too much about triode/pentode/ultralinear operation. The best mode seems to depend on the valve type and circuit it is in, pentode mode works fine in one of my amps, another works better in ultralinear. But you know me, I don't know much so keep that in mind. The grid #2 supply you mention is how they run it in pentode mode, it has nothing to do with bias of the control grid. The bias is controlled by the voltage drop across the cathode resistor R28/29, which might result in something around +20 volts (give or take a few) at the cathode. Then the control grid is referenced to ground through the 220K resistor so the bias on the control grid is -20 volts (give or take a few) compared to the cathode.

I like this website for valve reference, well-written but not too much depth, biased towards guitar amps but with references for hifi where applicable  http://www.valvewizard.co.uk/ The 3 stages Pre/Driver/Output in your amp, you will find here also i.e. triode gain stage/SRPP/single-ended with writings about advantages/disadvantages, various notes, load lines etc.

Hope it works out great Jonas.

Ian.

[assendor]

Hello Ian,

Thanks for Interesting reply and link. (I've been reading all there is on those pages at Valvewizard before, Good stuff)
Still I do not have the perfect overview of all the things, Strange+? HAHA.
So the grid2 supply is to get it in Pentode mode, interesting. So it is in practical circuits just biased like a regular Triode circuit then?
   Pentode, This grid2, what Hmm,, the signal has to pass trough all the 3 grids, and I have to read more about it to understand more of this I guess.
   OK, I understand more, still it feels like some tests is needed to get the complete detailed picture of how they operate.

It's always something new to learn, been playing quite a "lot"(relatively) with Triode Tubes, Pentode can be connected as a triode, (Isn't this Correct?? )But here this is not the case then, This I understand now.
So the Grid2 supply in this Audio Note Kit2 amp really has nothing to do with the Bias current, This is good to know.

Another thing I find to be interesting reading about the Pentode Circuit:
""They produce predominantly odd-order harmonics"", a triode predominantly 2'nd harmonic.
A SRPP circuit also produce predominantly odd-order harmonics.

Quite cool, Here we have a Triode Buffer/Preamp at the input, then a Preamp/Driver circuit in SRPP, then a Pentode on the output.
Very high input impedance on the triode input, then also high input impedance on the SRPP, and finally rather high input impedance of the Pentode output stage.. Hope I got this right.

What also is interesting then, is that all the amplifying stages in this design is Reversing phase, and to get it right again on the output, cause there is 3-gainstages, the Primary of the output transformer is fed with B+ on lower side.

   Enough Iron, You might be right. It is supposed to be heavy to be such a low power amp.
Se amps are often heavy, but maybe this is Heavier than others?

TA2020, Have heard a lot about that Chip, and also some Gain clone amplifier.
I guess it is like with most of things, it depends on what it's used for if it fits perfectly or not.
In many cases the SE Tube amps are not as suitable for the usage some people try to use them for.

I've heard that to get a Tube amp to sound really really good, well it's a lot of work.
Now this could be the truth, or at least this is till now my experience too.
You probably do know about my experience with the Cheap Tube amp I had, it caught Fire when it eventually was
performing rather perfectly.
Now I hope it is less easy to get a fire going with this, since this is a much lower voltage amplifier.
When closing in on a 1000volts DC, it is a little mezzy when something fails.

Increasing the Voltage and Decreasing the Current, maybe this is an approach that could be better in some cases.
Total dissipation increases dramatically with raised voltage thought, P=U2/R  ((I=U/R)), so this has to be taken into consideration when we want to do some changes like that.

Good Luck to you finding a solution with your tube amps.

-Jonas

[yeo]

hi jonas

grid2, if tied to b+, makes the pentode run in pentode mode.

grid2, if tied to plate via a resistor (usually 100 ohm), makes the pentode run in triode mode.

try it and see which you like.

i tried on my simple el84 and clearly prefer the triode mode by a large margin.

going from pentode to triode mode, you lose quite a lot of power but gain in sonics.

yeo

[assendor]

Hello yeo,

it is funny how much fun it's possible to get out of the simplest electronic device..
Thanks for explaining that trick with the Triode connected. Had heard/read it before, but had forgotten about it.
IT is of course something that has to be tried out.
There is no risk of something going completely wrong doing this?? I guess not since the tube also has Datasheets
when to be used in triode mode.

BTW::
Now this is not an attempt to be hopelessly difficult, but of course some investigation about the difference between
Pentode in triode mode, and a "pure" triode had to be done. (Maybe I regret trying to find more about this now.. )

http://www.t-linespeakers.org/tubes/triode-trick.html

I guess everything in this world can be done more complicated. It is another story if we desire simplifying things.

What in the world is all that stuff he's doing to make a Pentode act like a "pure" triode???
There has to be a reason why he has bothered doing all that investigation on the subject, I mean before implementing all those components, instead of just adding one simple Resistor from Grid2 to Plate.

Now this guy's comment in the end is this:
Have fun discovering what a triode connected tetrode/pentode really sounds like.

SO I guess there is no way around, not trying this tweak too....

ERR:
I had some problems understanding this though:
Note that if you are using true, three grid EL34s that the suppressor is best connected to about -5V or so. Just voltage divide off your bias supply. Biasing that grid slightly negative reduces its electron interception to essentially zero.

Another thing about running it in Triode, 1/3rd of Pentode power to be expected, some claims...
12/3= 4Watt, Still tremendous amounts of power for me, how wonderful.

This post here makes me wonder once again, DO I COMPLICATE THINGS TO much and/or to often?=?
Well, Yes I do.

HAHAHA, How wonderful!!!

-Jonas

[ian444]

Hi Jonas,

If you try triode mode, in a lot of amps SE or PP, you can disconnect the negative feedback, which on this amp passes through R21/C14 and R22/C13.

Ian.

[assendor]

Hello Ian,

Interesting.... Thanks for that INFO:
SO the Negative Feedback has to do with the Pentode mode amplification factor??
I understand the Gain is, or can be rather high on Pentode's, compared to Triodes.
You could get a "runaway" situation without the Feedback then? I'm guessing a lot here. (Like always)
(Hmrf, Better to Guess a lot, then try to prove it, or try to un-prove it, than not to think/mean anything about everything, or nothing).

One other thing is that Pentode's suppose to have a more "transistor like" Distortion character, or coloring of sound.
Maybe this distortion is so Undesirable that they JUST have to add NEGATIVE feedback to it??? HA HA HA!!!!

The negative feedback actually irritated me, I don't like Negative feedback, who does?? HA HA.
Just wondered why it was there.

But hey, yeo, You are having a pretty good time with your Simple EL84, this doesn't have any feedback
while in Pentode mode, does it?
I guess this needs some more investigation, LOTS more, Too much of course.
Maybe the Feedback is only needed if really pushing the amp in Pentode mode, or to get proper measuring results,
or to decrease output impedance, or a combination...? 

-Jonas

[yeo]

hi jonas

i have never bothered playing with negative feedback. if you play with negative feedback, you need lots of gain which is then "reduced" since it's --- negative feedback.

this is why a pentode gain stage is often used in the classic push pull tube amps of the 50s. usually a pentode gain stage (like ef86), followed by a phase splitter, then output tubes and some feedback.

the feedback lowers output impedance as well as improves linearity.

but if your design is linear, power supply is solid, tubes are linear, components are good (heck, does anyone use non-linear output trans?), then i guess you don't need all this negative feedback stuff.

which is why if you look at my simple el84 amp, there's no feedback at all. don't intend to go there, don't intend to try! 

yeo

[assendor]

Thanks yeo,

Then it will be tested with original Feedback, No Feedback, Less Feedback, Then Triode Connected with No Feedback.
Finally, Hopefully later with this Strange and more complex Triode Connection with all those Diode and capacitor stuff
in there.

If there is a too high amplification when removing the Negative Feedback, then it's not so obvious what to do.
Maybe change the input Buffer Tube to a lower gain type.
Tempting is it to change it to Two 1626 VT-137.
Or maybe when running the tubes in Triode mode, they loose a lot of gain already..

We'll see.

-Jonas

[assendor]

Hello all,

I've got my Amp, well almost that is. It's mine, and it's in within reach. A friend of mine has to bring it to me,
and that will be tomorrow..
He's listening to it now, and has done so for a couple a days. (He loves it, compared to his expensive Commercial garbage)

Now I have to ask something here..
The input buffer TUBE is ONE 6SN7 type tube, coupled in regular Triode mode, as you can see on the Schematic.
Gain here should be something around 15 or so.

Now the next stage, /Preamp/Driver stage is a SRPP stage.. With some 
ECC83/12AX7, they have a Gain of about a 100.
Now this amplification stage does most likely not have a gain of 100, but how much do they have ??
I probably could find out reading and trying to calculate with the values there, but it's so much easier to ask someone else..

   OK, reason for asking.. Yes when removing all the feedback, how about the gain?
Better to have the output just about right, when the output signal of my source (Monica DAC) is 100%.

It's to be understood that the Gain reduces when connected in Triode mode, so it's important not to do
too much with the gain before this connection is in place.

   - A pair of the reissued Genalex Gold Lion KT88, instead of the 6550 has been ordered.
Couldn't really decide if it would been a better choice to take the Winged "C" (SED) KT88.
EAT KT88 Diamond is not an option, as this is simply too expensive.

- For the buffer the plan is to use Russian version of the 6SN7-GT 6H8C tube, metal base with "T" plates.
Another option for the 6SN7 is to change it to the Russian tube 6N6P, it's just irritating the socket isn't the same..

- The 12AX7 is harder to decide... Telefunken, Siemens, RCA, Mullard, RFT, Or simply rewire just a little bit and replace it with a 6N2P. It's not always that high price tag is equal to success... Us DIY'er know this pretty well...
These 6N2P tubes are suppose to satisfy many ears pretty well.

BTW,
The hype about good, or super good quality Electrolytic caps, can really Electrolytics be better in Power supply
than Proper Polypropylene(PP) or even Paper In oil capacitors in the power supply section of a Tube amp??
It is interesting, or tempting to change all the caps, all over to Paper in oil, PETP, Polypropylene combinations.
Now some has to be bypassed with Teflon Capacitors as well. Or smaller value Paper in oil caps.

Shouldn't this possibly be better than any Electrolyte? Or is the BG's an example of something that can't be outperformed by anything else?

OH yes, I am aware of the problems involved, or potential problems with changing the values of the capacitors
to a higher value, when a rectifier tube is used.
Especially this is, or can be critical for the capacitors connected directly after the Tube.
Was this due to increased startup/Inrush current possibly over-loading and breaking down the rectifier Tube?
Think it was...

Thanks for any input, whatever it must be.

-Jonas