Simple cheap 2A3 stereo amp

[jim]

I have seen comments a couple times on the JoeList where a common cathode R was preferred in a simple amplifier.

Here are some projects using a common cathode R :

Projects

Jim

[Tube_Dude]

Here are some projects using a common cathode R :

I was talking about shared cathode resistors...but in two diferent chanels...

Two diferent chanels runs diferent signal programs...

If it is used elsewhere....my only coment is:

-like in politics a lie doesn't become true by being repeated to many times.

The only reason for using it is if you can't afford one more resistor and a capacitor....

Jorge

[planet10]

Originally posted by jim
Here are some projects using a common cathode R :

Projects

Yes, that is one of the guys. Another interesting point he makes is that in a 2-stage amp, he likes the B+ for both stages to come from the same point (ie top of the anode resistors tied together)

dave

[valveluver]

A single ended 2A3 is only good for a few watts. I would like to know what type of speakers are used because most modern speakers are verry inefficient. My hi-fi system is probably a joke when compared to some of the high end stuff out there. My JBL 4412s take some power to make them talk.

[Gabevee]

Tube_dude,

-like in politics a lie doesn't become true by being repeated to many times.

There are many circuits, including stereo push pull (and I have a few) that use one cathode resistor for all output tubes, designed by much more qualified engineers than I.

Decware's famous Zen amp uses one cathode for both output channels, AAMOF.

The reason for the use of the common cathode resistor is for fixed bias. For one channel, the tubes of the other channel and the cathode resistor act as a fixed bias. This tends to tighten up bass. Yes, it is cheaper to use the tubes that are already in the circuit that already dissipate the 5-10 watts rather than getting a 20 watt 2K resistor to do the same job. Pure simple techology, not lies.

BTW, I have already run tests and crosstalk is lower than the power supply noise....

As for the other questions from others:

I have listened to my 300B amp with AC, and then changed to DC. No noticable difference, except no hum whatever. Beautiful silence when no music is played. I suspect that the 60 Hz and harmonics are somewhat enhancing some of the music to make it seem to sound better.

I use Hammonds 273JX for power, and One Electron's output transformers.

Valveluver,

I did not see such a transient with my 300B amp. In fact, since cold starting causes the filaments to seem like zero ohms, there should also be almost zero volts. I again suspect the same to be true for the 2A3's.

As for cathode bypass... why non-polar? The cathode bias is in relation to B+.

Gabe

[Tube_Dude]

The reason for the use of the common cathode resistor is for fixed bias. For one channel, the tubes of the other channel and the cathode resistor act as a fixed bias.

Using a resistor in the cathod is for cathod bias...be it one chanel...two or three........

This tends to tighten up bass

If you say it!...

Yes, it is cheaper to use the tubes that are already in the circuit that already dissipate the 5-10 watts rather than getting a 20 watt 2K resistor to do the same job. Pure simple techology, not lies.

In one of yours first post you say that the decision of the shared cathod resistor is for reasons of economy not sound quality!

BTW, I have already run tests and crosstalk is lower than the power supply noise....

At what frequency??...50 Hz ???

So you like the sound and the project...you are in your right...

But anybody need to agree???

Jorge

[fdegrove]

Hi,

But anybody need to agree???

I don't agree, stereo separation is severely compromised.

Cheers,

[Tube_Dude]

I don't agree, stereo separation is severely compromised

And the intermodulation that this unwanted ( or may i say Wanted ? ) crosstalk will produce!

Jorge

[Gabevee]

In one of yours first post you say that the decision of the shared cathod resistor is for reasons of economy not sound quality!

The use of a shared supply for both output tubes is for economy. This necessitates the use of one cathode resistor for both tubes. Cathode resistor is for cathode bias, but one can also make it a fixed bias by putting a resistor from B+ to the cathode such that it and the cathode resistor can form a divider. Hence fixed bias.

As for sound quality, you brought that up, so I replied with my experience.

As for crosstalk being in relation to the power supply noise, I was referring to amplitude not frequency.

As for agreeing... I am merely pointing out facts of electronic science. I didn't think that that could be argued or disagreed with. My mistake. I should know better.

Now if I said it sounded spectacular compared with... whatever... then that would be an opinion that no one needs to agree with.

Fdegrove,

I don't agree, stereo separation is severely compromised.

No it isn't. I have already done it. I have made the measurements. Again, how can one argue with measured results???

Again, my mistake. So I will let it go here until someone else actually does it and can put in an opinion based on experience.

Gabe

[fdegrove]

Hi,

The fact remains that the two channels are only separated by the heater/cathode resisitance.

The same effect can be achieved by connecting two separate channels through some resistance, it may give a pleasing effect but has nothing to do with Hi-Fi.

Sorry to downtalk but facts are facts,