I have done some calculations on the feedback network. In the setup below, the gain Rm is ca 130k ohm and Rout (each pentode) is ~300 ohms. If the theory is close to the practical world, this means that I probably don't need loop feedback to lower the Zo.
To sort things out, I have made a spread sheet to do the feedback calculations.
It can be found here: http://www.veiset.net/miniPP/Shuntfeedback-Excel97.xls
One thing: What is the rp for EL84 in UL? In triode mode rp =~2k and in pentode configuration rp is 35-40k, but I have not found data or curves for EL84 in UL. My guess was ~4k.
An externally hosted image should be here but it was not working when we last tested it.
To sort things out, I have made a spread sheet to do the feedback calculations.
It can be found here: http://www.veiset.net/miniPP/Shuntfeedback-Excel97.xls
One thing: What is the rp for EL84 in UL? In triode mode rp =~2k and in pentode configuration rp is 35-40k, but I have not found data or curves for EL84 in UL. My guess was ~4k.
Hi jane,
Can't load your spreedsheet !
Will try later.
You probably don't need UL taps too !
Yves.
<edit>
Once I did that (for another purpose indeed)
http://www.dissident-audio.com/SE_845/DCCoupled.gif
Was fine.
</edit>
Can't load your spreedsheet !
Will try later.
You probably don't need UL taps too !
Yves.
<edit>
Once I did that (for another purpose indeed)
http://www.dissident-audio.com/SE_845/DCCoupled.gif
Was fine.
</edit>
Hi jane,
Depends on exact operation point and moreover, on exact UL ratio (a few percents off can make a considerable difference). And changes quite considerably under signal conditions (which means, over plate swing), too.
If you are very interested to know, I could try to make a limited plot around your iddle OP point to compute µ, rp and gm.
From your circuit, I see:
Vg1 = -10V
Va = Vg2 = 250V
Ik = 34mA
What I need to know additionally:
1) Ia at op point above (I have only a pair of mildly used JJ/Tesla EL84 at hand, so I must do a sanity check of Ia/Ig2 ratio against your sample first).
2) UL ratio.
But if it is not this important to know, please spare me the hours it would need do rig up/down & draw/compute
Tom
P.S.: Please note your tubes run at a quite unusually low plate Pd at the moment, probably less than 8 Watts or 70% ....
Depends on exact operation point and moreover, on exact UL ratio (a few percents off can make a considerable difference). And changes quite considerably under signal conditions (which means, over plate swing), too.
If you are very interested to know, I could try to make a limited plot around your iddle OP point to compute µ, rp and gm.
From your circuit, I see:
Vg1 = -10V
Va = Vg2 = 250V
Ik = 34mA
What I need to know additionally:
1) Ia at op point above (I have only a pair of mildly used JJ/Tesla EL84 at hand, so I must do a sanity check of Ia/Ig2 ratio against your sample first).
2) UL ratio.
But if it is not this important to know, please spare me the hours it would need do rig up/down & draw/compute
Tom
P.S.: Please note your tubes run at a quite unusually low plate Pd at the moment, probably less than 8 Watts or 70% ....
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