What front end for PP - triode, pentode, mu-stage or differential triodes? - Page 2 - diyAudio
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Old 27th May 2007, 01:35 PM   #11
Yvesm is offline Yvesm  France
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Hi ray_moth.

As stated, I run this amp at 8dB global feedback.
Indeed, results are very dependent on how firmly the lousdpeakers need to be damped.

CCS or not CCS ?

In fact it's a matter of available negative rail voltage wich determine the "lenght of the tail"
Since I needed some -115V to feed the cathodes of the CF DC coupled driver I was satisfied with a single 27K resistor.
Also, the symetry of an LTP may be defined as the ratio between the Rp of the tube vs the lenght of the tail. Lo Rp triodes need a longer tail than penthodes !

Since I'm rebuilding this amp in Stereo version, I'll check both way and show the results.

About disturbance caused by screen current, consider it is quite constant as long as the anode volage does not fall too low.
Note that anyway this voltage limit is usually lower that you can obtain with a triode.

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Old 29th May 2007, 03:51 AM   #12
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Before you get too far down the track have another look at SY's Red Light District. The thing that he has used to his advantage there is that with output tubes in pentode mode the Miller Capacitance is very small and he can use high values of grid stop resistors to address blocking distortion without loosing high frequency response. This little gimick is often overlooked when looking at his design because we are too busy looking at his LED bias scheme.
I'm not suggesting you do (since it seems clear that you have some other topology in mind) but his design in toto would easily scale up to EL34 Outputs and a 6SL7 input/splitter.
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Old 29th May 2007, 07:52 AM   #13
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Hi, Ian,

I'm not suggesting you do (since it seems clear that you have some other topology in mind)
LOL! Ya got me, I'm a stubborn SOB.

Actually, there are a couple of other considerations, apart from blocking, that cause me to avoid using a 6SL7 to drive OP tubes directly. I see an impedance mismatch between the high OP impedance of the 6SL7 stage and the low IP impedance of a power tube grid using fixed bias (even a pentode). To me, that's bad practice. (And I really want to use fixed bias.)

Also, having that EL34 grid circuit effectively in parallel with the plate and plate load of a 6SL7 would drag down the impedance at that point, making the plate-plate NFB from EL34 to 6SL7 less effective.

I see a directly coupled cf (or possibly sf) buffer as the ideal solution here, since it solves both of these problems and does the job, also, when it comes to the grid going a bit positive on occasions. A large stoppper is OK for minimizing the effects of blocking, but it still can't give any AB2 headroom. (I realise that the EL34 is not exactly a good candidate for AB2, unlike the 807 or 6146, but I only want it for transients.)
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Old 29th May 2007, 08:02 PM   #14
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Originally posted by Yvesm
Since I'm rebuilding this amp in Stereo version
Aaah, at last! This was long overdue, and the most obvious thing to do. At least I thought so, after having auditioned the original/mono version at Amboulons :-)

If in doubt, just measure.
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