Can A valve amplifier be both Class A and Class B?

[elgee]

Hi,

I'm looking at a ready built Velleman K4040 with upgrades to KT88 & 6SN7 tubes.

It's predecessor, the K4000, is described by André Jute at:
http://audio-nirvana.fortunecity.net/pag_esp/k4040.htm

as producing, "95Wrms per channel in Class B, enough to drive a large disco into hearing impairment, and 15Wrms per channel in Class A, which is what the hi-fi purist really wants to know." I assume this also applies to the K4040

but being new to valve/tube technology I'm not sure how it can switch from Class B to A. Is it just a function of the volume control ie. a cut off point where it becomes Class B over a certain volume (I assume this equates to > or < than 15Watts). Or, is there some kind of switching mechanism that enables the user to switch between the two?

Has anybody got a simple explanation fit for a novice, please? (this is pretty representative of my progress in this fascinating new field of discovery - I'm enjoying every minute of it!!)

Thanks, Les G

 

[roline]

How about a PP amp class "A" switching to class "B" as a function of the KT88's bias point on the curve and driving them from the linear region where both tubes are conducting, class "A" until one tube is cut off and the other tube is on, therefore class "B". I find the ratio of 15w=class A to 95w=class B to be rather large. I would expect something closer to 15w=class A to 60w=class B... depends on the output transformer impedance and output tube bias point.

 

[M Gregg]

Hi,

I'm looking at a ready built Velleman K4040 with upgrades to KT88 & 6SN7 tubes.

It's predecessor, the K4000, is described by André Jute at:
http://audio-nirvana.fortunecity.net/pag_esp/k4040.htm

as producing, "95Wrms per channel in Class B, enough to drive a large disco into hearing impairment, and 15Wrms per channel in Class A, which is what the hi-fi purist really wants to know." I assume this also applies to the K4040

but being new to valve/tube technology I'm not sure how it can switch from Class B to A. Is it just a function of the volume control ie. a cut off point where it becomes Class B over a certain volume (I assume this equates to > or < than 15Watts). Or, is there some kind of switching mechanism that enables the user to switch between the two?

Has anybody got a simple explanation fit for a novice, please? (this is pretty representative of my progress in this fascinating new field of discovery - I'm enjoying every minute of it!!)

Thanks, Les G

Hi,

There is no switching mechanism...It’s just a case of how hard the tubes are driven ......it's doubtful if you will notice it change up until ear bending volume, depends how sensitive the speakers are!....To run other tubes with Velleman you sometimes need to raise the voltage for the fixed bias....you probably have about 50V and you can tube roll O/P tubes if you raise available voltage to about 100V...always remember to set to highest bias voltage so the tubes are running very low and turn up about half way equally and then up about three quarters across the set wait about 15 mins then bias to full level because they can drift as it warms up!..I know someone who just used a voltage doubler to increase the voltage on the bias.

Hope this helps

Regards
M. Gregg

 

[elgee]

Ah! Let me get my head around that! Are you saying that it does make sense or that it doesn't?

At which point do they cross over and how would I know?

Thanks for your patience.

 

[M Gregg]

Ah! Let me get my head around that! Are you saying that it does make sense or that it doesn't?

At which point do they cross over and how would I know?

Thanks for your patience.

You probably won't be able to tell the point with your ear... it will just go from low to loud! with the movement of the volume control...

Regards
M. Gregg

 

[elgee]

Thanks M. Gregg. I think I understood most of that. I'll have a scan through my beginners electronics books to follow the thread of your explanation.

Thanks also to roline. It'll probably take me a while to work through those points but I'm pretty sure I'll learn a lot along the way.

From what I gather, I might need to adjust the bias on the tubes to make sure they are working as efficiently as possible.

 

[elgee]

When it switches over, assuming that I push the volume up beyond the crossover level, will there be a noticeable difference in the sound quality, or is it mainly a jump in volume?

I know some poeple's ears are so finely tuned as to be able to tell even small differences but I'm not at that stage yet. I did make some changes to the the way I wired the speakers and also to their position, and did notice wider and deeper sound staging (I think that's the right expression - the orchestral parts were coming from what could be described as a proper stage width and depth).

Best wishes,

Les

 

[M Gregg]

Thanks M. Gregg. I think I understood most of that. I'll have a scan through my beginners electronics books to follow the thread of your explanation.

Thanks also to roline. It'll probably take me a while to work through those points but I'm pretty sure I'll learn a lot along the way.

From what I gather, I might need to adjust the bias on the tubes to make sure they are working as efficiently as possible.

It's very important not to just set them up and walk away....set up in stages...and check again after about an hour to fine tune..
They should be OK then. Probably check every few months. Also the cathode resistors on the old velleman were a pile of half watts in parallel if a tube flashes over it blows them to pieces.. so change to a 10 or 12W of equal value.

Regards
M Gregg

 

[M Gregg]

When it switches over, assuming that I push the volume up beyond the crossover level, will there be a noticeable difference in the sound quality, or is it mainly a jump in volume?

There is no jump in volume just steady increase...you won't notice any change...it will just sound better at medium volume...


Regards
M. Gregg

 

[elgee]

Thanks M. Gregg. Novices like me find these forums really helpful. It's good toknow that there are people out there prepared to help and encourage others to get into a new hobby like this.

Thanks again,

Les

 

[M Gregg]

Thanks M. Gregg. Novices like me find these forums really helpful. It's good toknow that there are people out there prepared to help and encourage others to get into a new hobby like this.

Thanks again,

Les

Just make sure you test before you touch anything and take a look if it has discharge resistors across the power supply caps..just to be safe..

Good luck!

Regards
M. Gregg

 

[M Gregg]

Just for interest,

Don't want to drone on...I guess you know how to set the bias...

Voltage across the cathode resistor divided by cathode resistance gives current. so you want the lowest current at the start..

So if you know what voltage you expect to get across the resistor for maximum current for the amp you can set to that in stages.

Regards
M. Gregg

 

[elgee]

Uhm! I must be honest and say that the only experience I have had of this is via a couple of article I Googled. I haven't got the amp yet and will have to do a lot more reading before I do anything - and seek the advice of experts such as yourself, of course.

I have ot go out just now, but I'll give you an update tomorrow as to whether I buy it or not, and I'll have to take it from there.

The one thing I am pretty sure of is that I want follow the valve idea through. It sounds fascinating and also worthwhile in terms of the quality of sound.

Thanks,

Les

 

[M Gregg]

Uhm! I must be honest and say that the only experience I have had of this is via a couple of article I Googled. I haven't got the amp yet and will have to do a lot more reading before I do anything - and seek the advice of experts such as yourself, of course.

I have ot go out just now, but I'll give you an update tomorrow as to whether I buy it or not, and I'll have to take it from there.

The one thing I am pretty sure of is that I want follow the valve idea through. It sounds fascinating and also worthwhile in terms of the quality of sound.

Thanks,

Les

What I would say is that I have listened to a Velleman stereo amp and as a first experience it would be great...as long as the amp is in good condition and has not been hacked at!

Very powerfull....great sound ...if you have info on setting the bias nice (very heavy) amp...LOL

You will probably need a decent meter to set the bias...

Regards
M. Gregg

 

[elgee]

Yes, the meter was the next step. I have a cheap one which doesn't do too much. I also want to measure theimpedence of my speaker setup - I've got two pairs of 8 ohms running in series which should give me 4 ohms but I'd like to cj=heck anyway.

Is there a reasonably priced meter that you would recommend? I'm also going to try my hand at a few DIY project, like a pre-amp, so a meter would be useful.

Les

 

[M Gregg]

Yes, the meter was the next step. I have a cheap one which doesn't do too much. I also want to measure theimpedence of my speaker setup - I've got two pairs of 8 ohms running in series which should give me 4 ohms but I'd like to cj=heck anyway.

Is there a reasonably priced meter that you would recommend? I'm also going to try my hand at a few DIY project, like a pre-amp, so a meter would be useful.

Les

I’m tending to dominate the thread so I will step back let others have a say..
Last comment, a digital meter is essential , don’t go cheap you are talking dangerous voltages.
Cheap meter –leads mean trouble.
You will need a passive pre or some kind of volume control ..the vellman is a power amp..no controls to my knowledge.
Some of the Maplin meters are OK..I use fluke but I use it as a main tool!<<job related.
Regards
M. Gregg

 

[elgee]

I was going to use the volume control on my cd player initially. Then make a simple ss preamp to fell my way into it. I won't be doing anything too hairy, but I know that these are high voltages and even tinkering needs knowledge.

Thanks for your advice.

Les

 

[M Gregg]

I was going to use the volume control on my cd player initially. Then make a simple ss preamp to fell my way into it. I won't be doing anything too hairy, but I know that these are high voltages and even tinkering needs knowledge.

Thanks for your advice.

Les

A passive is fine..volume control and selector you do not want any gain with this amp it will blow your head off...LOL

Catch you later!
M. Gregg

 

[elgee]

Ok. Thanks

Les

 

[boywonder]

Elgee: Class A or AB (or B-not really suited for hifi applications) has to do with the load line chosen for a particular valve/tube. In general, the load line is determined by the B+ voltage (anode voltage) on the tube and the idle (bias) current for a given output transformer (load).

As you may be aware, in a PP amp, class A operation means that both output tubes are conducting all of the time, and for class B, each tube only conducts for half of the input signal cycle (one tube handles the positive side of the waveform, the other tube the negative side); this means that the tubes are handing the signal off to each other many times per second, usually resulting in crossover distortion where the hand-off happens. Class AB operation means that neither tube is conducting for the entire wave cycle, but there is some overlap where the hand-off occurs. Think of a relay race where one runner is handing the baton to the next runner, there is usually a small period of time where they are both holding the baton; this makes the crossover transition smoother (less crossover distortion).

As the volume of an AB PP amp is increased, there is a point where the amp will transition from the class A load line to the class B load line. Where this happens is a function of the B+ voltage, the idle (bias) current, and the load the output transformer puts on the output tubes.

In general, when you increase the idle (bias) current and decrease the B+ voltage, and amp will remain in class A operation longer before transitioning to class B operation, at the expense of less total output power. This is called "running deeper in class A". Some amps are designed to operate only in class A by selection of the operating point, B+, idle current and output transformer.

The amp you mentioned sounds like it is biased to operate mostly in AB (transitions out of class A operation early). This implies a relatively higher B+ voltage and lower idle current.

For a more detailed explanation from an expert, see this:

loadmatch4-pp-beamtetrodes

Notice the two load lines on Fig 1; these are the class A and class B load lines for the chosen operating point. The transition occurs at point C.

Interestingly enough, the example is somewhat close to your amp above....the 6550 valve used in the example is the little brother of the KT88......