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revintage 6th May 2009 11:38 AM

DC-coupling of tube stages
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Below are four of many possible solutions of DC-coupling a driver to an output tube in a SE-poweramp.

The anode is DC-connected to the grid of the presceding stage.

To keep bias potential OK (no need to bother about tube-aging) a CVS(gyrator) is used instead of a CCS.

But are they really DC-connected;) ? One must take a close look at the PSUīs to determine.

What do you think?

revintage 7th May 2009 06:23 AM

Hey MJK,
Want your comment on this as it concerns the same topology we are both building:cool:.

Shoog 7th May 2009 07:14 AM

I hate to see such awful wastefullness on the cathode of the output stage. I tend to use a negative rail for theinput stage and DC couple without the huge cathode in the output.


revintage 7th May 2009 09:46 AM

Hey Shoog,

Check again, there is only waste in ONE of the circuits ;) .

The others are a smart way to get rid of the input transformer you use, by just moving the ground connection.

But the question isnīt about waste, it is about if DC-connections really are DC, as the other pole through the PSU must be taken in account.

Shoog 7th May 2009 10:02 AM

I see what you mean - clever work. Still needs two power supplies which is a bit of a pain.
I am not really qualified to answer your question, but I think they are DC coupled because the two +B in your circuit are effectively at AC ground.

One observation I would make is that those CCS can take a brutal burst of voltage on switch on and many are not up to the job. I had to use the IXY 900V chips in my Tabor clone.


revintage 7th May 2009 10:17 AM


One observation I would make is that those CCS can take a brutal burst of voltage on switch on and many are not up to the job.
Sorry my mistake, there should be a RC link added from B+ to CVS. In this particular case the CVS only needs to be fed by 250-300V. DN2540 is what I stock and they can take 400V.

jueic 7th May 2009 10:42 AM

OldEurope has an interesting circuit for DC coupling.

OldEurope's circuits surprise me from time to time.. this is one of

revintage 7th May 2009 10:46 AM


OldEurope has an interesting circuit for DC coupling.
Hi Jueic,

Sorry, that one had nothing to do with the subject. It is still about the PSU pole, not the DC-coupling between tubes.

jueic 7th May 2009 12:02 PM

Hi, Revintage

Are we reading the same thing ?? or my english reading is out of wack ?

By looking at T3, the input feeds thru PLATE, and the output taken from CATHODE.. the coupling tube works as a diode which
has an adjustable plate voltage set by cathode resistor (sort of auto-bias).

Maybe you are right, the OldEurope's circuit adds one more tube stage is not pure DC-coupled. but it does not need
the coupling cap we normally have.

To me, it is really an interesting circuit.

revintage 7th May 2009 12:14 PM

OK, get your point. Still it is not about what I want to discuss here;).

My question and idea is that there seldom are such things as DC-coupling as the pole through the PSU in fact mostly is cap-coupled. In other words we are fooling ourselves:D!

One have to use a cap-free, lowZ series- or shunt-regulated PSU to achieve this.

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