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Old 26th April 2013, 01:46 AM   #1
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Default Will this circuit work ?

I spotted this simple valve amplifier.
The SRPP part I understand ok.
But the push pull part seems a bit odd especially the 47R resistor on the lower el84 grid.

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Old 26th April 2013, 02:05 AM   #2
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So,both output tube cathodes go through a CCS (the LM317). Thus,if current through one cathode goes down,the current in the other cathode must go up,to maintain the same total current through the LM317.
Ground the grid of one tube,and drive the other tube's grid,and it's 'self splitting'.

Edit: Yes,it works.
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Old 26th April 2013, 02:10 AM   #3
TheGimp is offline TheGimp  United States
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Look at it as if it were an LTP, only driving a PP output transformer.

The Constant Current Source limits the operation to class A only.
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Old 26th April 2013, 04:00 AM   #4
Junm is offline Junm  Philippines
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Looks like Single Ended.. the lower parts is there to counter the upper part to null the dc current passing in the opt to avoid saturation...
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Old 26th April 2013, 08:27 AM   #5
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Yes this works .
Sounds clean not so many parts .
But bias both tubes very well.
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Old 26th April 2013, 08:30 AM   #6
AJT is offline AJT  Philippines
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Quote:
Originally Posted by Junm View Post
Looks like Single Ended.. the lower parts is there to counter the upper part to null the dc current passing in the opt to avoid saturation...
that makes it a push-pull....
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Old 26th April 2013, 10:04 AM   #7
DF96 is offline DF96  England
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The 47R is a grid stopper. A straight wire connection could look like a nice VHF resonator to a particularly lively EL84.

The snag with this type of circuit is that you have to set the quiescent bias quite hot so there is room for the change in bias caused by second-order distortion when signals come along. Not a major problem for real music, but could create confusion when sine wave testing is done as it could increase crossover distortion.
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Old 26th April 2013, 10:06 AM   #8
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It will work (I have had similar working) but if you are going to use two triodes in the driver why not just go for a LTP and be done with it ?
Personally I would use something like an ECC99 or 5687 running at 10mA and then you only need one driver valve for the front end. People have also reported great success using an EL84 as the driver !!

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Old 26th April 2013, 09:56 PM   #9
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I have put together a complete circuit tonight including the transformers and have partly laid out a pcb.
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Old 27th April 2013, 02:39 AM   #10
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The impression I get from this is that the driven output tube operates as a common cathode type, while the bottom tune operates grounded grid. The upper tube inverts, and the bottom tube doesn't, so you do get phase inversion. Problem is, the grounded grid bottom tube has a very high plate resistance as compared to that of the upper tube, so I would think that there would be a serious impedance mismatch problem.
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