30 Watt Class A

[Yobleduwop]

Does anyone have a schem that comes under that title?
I was looking at Goodsell amps and he has a 4 el84 amp that is 30 watts of real class A
I have a schem from an old Mayfly amp is this class A or is it AB like most vox knock offs?

[Alex/Tubewonder]

Not knowing the supply voltages and transformer data is difficult to be 100% sure. Judging by the 220 ohm decoupled cathode resistor common to all 4 tubes I'd say it's class AB or even class B. By virtue of self-regulating cathode bias the voltage developed by idle cathode current on 220 ohm shared resistor would be high enough to push the tubes into almost cut-off i.e class B. It is not recommended to share cathode resistor on more than 2 tubes, 4 tubes is asking for very uneven current distribution between them unless the tubes are very well matched.

[Amp Kat]

Yeah your right Alex, and it kills me when sellers do this and promote it. The thing is it may be class A at quiescent but as soon as you turn the volume knob it's AB. The main thing when judging Class A is if the plate voltage is 250 volts or below and even at that can be driven into class AB at higher volumes so it's just way overhyped IMO.

[Yobleduwop]

Thanks heaps for you replys guys,
Has anyone looked into the Goodsell amps to see if theyre really class A? He claims them to be but as has been said before so do alot of people

[Bruce / Mission Amps]

Well, before jumping all over the guy... you fast draw guys ought to reread the schematic.
It looks to me like there is no value for the cathode resistor.
I read it as resistor #220 not a 220 ohm resistor.
And regardless, the idle current or the biasing of any tube (or tubes) is not the definition of it's class of operation.
I'd suspect that resistor is somewhere around 50 ohms to 68 ohms in order to run four cathode biased EL84s.

[Alex/Tubewonder]

Bruce, when you're right you're right

Of course, cathode bias by itself doesn't guarantee class A operation over full output power range. One needs to know supply voltage and transformer parameters to asses that. It may very well be that presented circuit does deliver full power in class A. B+ = 250V, Rk ca 100-120Ohm per tube pair, Rsg= 1.5k per pair and transformer Raa=4k will do it with Pout=20-22W. Pushing it with higher B+ to 300V and more will definitely make it a class AB.
This thread has high probability of developing into a class A vs AB discussion.

Have a nice Sunday

[Amp Kat]

Being that the voltages aren't present could be a wash although with the bypass cap would put it in A2. I rarely see 250 to 300 on the plates out there but yes it could be Bruce. Is this yours ? ha ha !

[Alex/Tubewonder]

Being that the voltages aren't present could be a wash although with the bypass cap would put it in A2.

This circuit won't do class A2, cathode decoupling cap or not. You need a PI capable to drive enough current into power tubes' control grids to force it into A2. With a post PI master volume it would need cathode followers between master volume pots and power tubes.

[Bruce / Mission Amps]

Ha ha... well... no. But I do have more artwork for this amp with the parts values.
I don't regularly critique other's work so suffice to say, R220 is just over 60 ohm,
the PT is 500VCT@350ma, the B+ is under 350vdc at the first B+ node and I do not believe this to be a true Class A amp design. It isn't Class A2 though.

I was mearly pointing out that the hand drawn artwork only had part numbers listed and I thought it was funny that the Class operation popped up.
Everyone's comments about Class operation seemed correct but I get a little tired out by all arguments over Class A vs Class AB, vs Class AB, vs Class B vs Class C.... blah blah blah... ha ha.

[KevinOConnor]

The main thing when judging Class A is if the plate voltage is 250 volts or below

Hey Guys!

The above statement is erroneous. You can have a class-A amp that operates at kilovolts and be true class-A.

Class-A is a bias condition with respect to the signal size. An AC-30 is a class-A amp up to 22W of output. After that, it switches class as Alex has explained.

There is a detailed definition of class-A on our FAQ. There is also a way to judge an amp to figure out if the claim of class-A is true.

Have fun
Kevin O'Connor

[clyde]

Just my $.2, correct me if I'm wrong

Output conduction of one half of p-p output vs input on that half

360 degrees = Class A
>180 degrees but < 360 degrees = Class AB
180 degrees = Class B

[Enzo]

Yes, and so when a class AB amp is running at low signal levels it is running in class A - the waveform is not large enough to get past the limits of either side. Point being that class of operation is more a matter of what is happening than it is of being a certain circuit.