Cathode Coupled Output???

dbishopbliss · 3858

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Offline dbishopbliss

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on: April 14, 2010, 12:51:50 PM
I have a copy of 2wo's schematic from the Amp contest this weekend and I'm trying to understand the difference between his amp and mine.  The attached image is taken from his schematic.  I think he said the output was cathode coupled.

I haven't been able to find any information on the net about cathode coupled output stages.  Can anybody explain or point me in the direction of how this works.  

Also, what is happening on the input grid?  Is this known as grid biasing?

Thanks,

David
« Last Edit: April 14, 2010, 01:55:09 PM by dbishopbliss »

David B Bliss
Bottlehead: Foreplay I, Foreplay III, Paramour I w/Iron Upgrade, S.E.X. w/Iron Upgrade
Speakers: FE127E Metronomes, Jim Griffin Jordan/Aurum Cantus Monitors, ART Arrays
Other: Lightspeed Attenuator, "My Ref" Rev C Amps, Lampucera DAC


Offline JC

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Reply #1 on: April 14, 2010, 04:02:23 PM
It would appear that the schematic is indicating a negative 18Vdc being applied to the grid (usually the smaller plate on the battery symbol indicates the negative side), so I guess that could be referred to as grid-bias; I never became too familiar with the nick-names for biasing schemes.

As far as the cathode is concerned, I have to wonder if having the secondary of the OPT in series with it doesn't act as a type of degenerative feedback?  It seems to me that you would run the risk of having the speaker cone offset by the amount current flowing through the cathode without signal, but maybe that is counteracted  by the current flowing through the primary?

I'm not sure what it's called, but it is interesting!  I hope someone is familiar with this configuration!

Jim C.


Offline Paul Joppa

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Reply #2 on: April 14, 2010, 08:11:16 PM
You are correct, the winding in the cathode provides a modest amount of local negative feedback. We have experimented with this for a long time; the old ParaSEX used a parafeed version of this concept quite effectively.

Paul Joppa


Offline dbishopbliss

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Reply #3 on: April 15, 2010, 03:53:03 AM
So here's a "I-barely-understand-grounded-cathode" question.

My amp uses a grounded cathode to bias the 6V6 tube.  If I recall, the cathode resistor (250R) keeps the grid biased around 12 Volts.  In the schematic above, the resistor is replaced with the secondary of the output transformer and the speaker.  Does this mean that there will be 12 Volts going across the speaker?  Is that dangerous (to humans or speakers)?

Any have a simple explanation of grid bias?

David B Bliss
Bottlehead: Foreplay I, Foreplay III, Paramour I w/Iron Upgrade, S.E.X. w/Iron Upgrade
Speakers: FE127E Metronomes, Jim Griffin Jordan/Aurum Cantus Monitors, ART Arrays
Other: Lightspeed Attenuator, "My Ref" Rev C Amps, Lampucera DAC


Offline JC

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Reply #4 on: April 15, 2010, 08:13:41 AM
Current in the tube flows from cathode to anode (plate), with the grid in between to act as a control valve for that flow.  By making the grid negative with respect to the cathode, we are said to 'bias' the tube, which means that we are establishing how much current flows from cathode to anode with no signal present at the grid and a specific positive Voltage on the anode.

In your example schematic, what appears to be a negative 18Vdc (negative with reference to ground) is applied to the grid to establish this control or bias point.  this would need to be obtained from a separate power supply or battery.

In your amp with the resistor between ground and cathode, the bias is obtained in a different way.  The current flowing from ground through the resistor to the cathode on its way to the anode creates a Voltage drop across the resistor, making the cathode positive with respect to the grid.  This amounts to the same thing as making the grid negative with respect to the cathode.  Thus, the tube is biased.  The amount of positive Voltage at the cathode will depend on how much current is flowing through that resistor, as Ohm's Law tells us.  The advantage to this type of biasing scheme is that it does not require a separate power supply to establish the bias point.  The trade-off is that that resistor in the cathode circuit is consuming a bit of power which is no longer available for the output of the amp.

You are correct in your assumption that the cathode current in your example schematic will also flow through the secondary of the OPT and the voice coil of the speaker which is in parallel with it.  But, the DC Voltage this produces on that parallel combination is again dictated by Ohm's Law: E= IxR.  So, the Voltage (E) is the product of the current (I) and the resistance (R).  Now, the parallel combination of the secondary of the OPT and the speaker voice coil is bound to add up to a very small resistance, certainly in the single digits.  Given that the current will not even reach single digits (even 1 amp through a 6V6 would be a disaster!), you will see that the DC Voltage across the secondary/voice coil combination will be more in the range of a fraction of a Volt.  For instance, in the example you gave of your amp, the 250 Ohm resistance in the cathode circuit would require .048 Amps, or 48 miliAmps to produce a Voltage drop of 12 Volts across the 250 Ohms.  Well, the parallel combination of the secondary and the voice coil in the schematic maybe results in, let's guess it at 4 Ohms.  So, that same .048 amps would produce a Voltage drop of only 0.192 Volts.

Hope this helps!

Jim C.


Offline 2wo

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Reply #5 on: April 15, 2010, 01:17:20 PM
Hi David,

Everything that Jim and Paul said is correct.  The point was to add a little local feedback to the output stage, I figured with a $17 transformer, it was worth trying.

  The battery does supply -18V to the grid.

When you say that "My amp uses a grounded cathode to bias the 6V6 tube."  That
« Last Edit: April 15, 2010, 05:33:32 PM by 2wo »

John S.