23742708   
   From: ruffrecords@yahoo.com   
      
   Patrick Turner wrote:   
   > On Sep 18, 8:39 am, Ian Bell  wrote:   
   >> flipper wrote:   
   >>> On Fri, 17 Sep 2010 10:48:50 +0100, Ian Bell   
   >>> wrote:   
   >>   
   >>>> flipper wrote:   
   >>>>> On Thu, 16 Sep 2010 21:58:37 +0100, Ian Bell   
   >>>>> wrote:   
   >   
   > Snip for brevity.   
   >   
   >> Exactly the same as at the grid - about 2H at -30dB or worse. If I short   
   the input resistor 2H at   
   >> the grid drops below -70dB and at the anode falls to -40dB which is about   
   what I would expect. So I   
   >> was expecting -40dB 2H at the anode and got a lot higher so traced back to   
   the grid and found it was   
   >> the same but that it wnet away with near zero source impedance.   
   >   
   > Your variations of Dn with input series resistance for 12AX7 is   
   > interesting, and not many ppl would be wise to it.   
   >   
   > But what of other tubes?   
   >   
   > In all my tests for line level preamps using 12AU7, 6SN7, and my   
   > favourite, 6CG7, the Dn you mention as a result of high input source   
   > resistance never seemed to occur. What if you change from 12AX7 to   
   > 12AU7? The 'AU7 should work in your circuit with minor load&biasing   
   > changes. I recall measuring a 6CG7 line stage with 100k volume control   
   > before a CC gain tube using 47k anode load then direct coupled CF to   
   > output and at typical levels to drive a power amp at normal listening   
   > levels Dn was always less than 0.02%, or about -74dB, which I thought   
   > would always be inaudible, and completely overwhelmed by the Dn of the   
   > power amp and speakers.   
   >   
      
   I am tempted to use one of my 6CG7 mu followers as a driver but that would   
   only have a gain of about   
   20. To get the 102V pp drive mentioned in my last post would require an input   
   of 5.1V pp or about   
   2.5V peak - my 5mA 6CG7 mu followers bias at close to -5V so it should avoid   
   grid current. I may   
   then need another gain stage to achieve a suitable sensitivity and of course   
   the input signal would   
   become even greater if I use NFB to reduce the white distortion. Of course teh   
   6CG7 mu follower only   
   achieve about 0.4% distortion at 20V rms output itself and I really need about   
   twice this.   
      
   I am tempted to try changing the windings of the transformer to the 6:1 tap to   
   lower the drive   
   requirements.   
      
   Cheers   
      
   Ian   
      
   > BTW, the amp with 6CG7 was tried with 4 different brands of 6CG7 and   
   > with music and with 4 of us listening for an afternoon. There was a   
   > considerable difference to the sound between excellent with Siemans   
   > NOS and not very good at all with Sovtek, but I thought an Aust made   
   > was right up there with the Siemans. The NOS Mullard was just polite   
   > sounding. all tubes would not have had very different Dn profiles.   
   >   
   > But I have never built a preamp with 100k of series input resistance.   
   >   
   > But then there is a another consideration. Many line stages are made   
   > using a gain tube which may or may not have a follower buffer to its   
   > outout. And there may well be a volume control at the input. and after   
   > the volume control the gain tube grid has a 100k series R to its grid   
   > and say a 560k taken to the buffer output. This is a simple shunt FB   
   > network and it gives closed loop gain of about 5x, and if the gain   
   > triode was a 12AX7 with open loop gain = 70x, then gain reduction with   
   > FB = 1/14, and any Dn is also reduced by the same margin UNLESS of   
   > course the series input resistance increases the Dn which would result   
   > in much less Dn reduction than a factor of 1/14.   
   >   
   > Baxandall tone control stages may typically have a CF input stage or   
   > low Rout gain stage driving the input to a shunt FB network around a   
   > tone control gain tube. The potentiometers used for the tone control   
   > are linear pots and easy to get matched, and the Baxandall set up   
   > gives unity gain and low Rout and high Dn reduction and is regarded as   
   > very good practice for tone controls but of course if a 12AX7 is used   
   > the effectiveness of the FB is reduced because the input series R in   
   > the FB network causes so much Dn. I have used 12AX7 in such   
   > arrangements but I have not found the phenomenon of the Dn you speak   
   > of to be a bother, but then I wasn't really looking for it, nor did I   
   > compare the Dn of a simple CC gain stage and that of one with  shunt   
   > FB network to see if I got the expected reduction of open loop Dn.   
   >   
   > If one has a gain stage with open loop gain = 60 with a given RL, and   
   > then one reduces the gain to just under 1.0 with a shunt FB network   
   > which results in the same RL, then one would expect to see closed loop   
   > Dn reduced by a factor of approx 1/60, or a heck of a lot, and   
   > difficult to measure at say 0.1Vrms signal level.   
   >   
   > I have my tone control arranged so that it can be switched in or out   
   > of the signal path and nobody can tell by listening when I have it   
   > either in or out, if I have it set for a flat response.   
   >   
   > Patrick Turner.   
   >   
   >> Cheers   
   >>   
   >> ian   
   >>   
   >>   
   >>   
   >>>> Cheers   
   >>   
   >>>> Ian- Hide quoted text -   
   >>   
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