From: info@turneraudio.com.au   
      
   Recently I worked on a Fender Deluxe Re-issue which makes 21Watts at clipping   
   with 2 x 6V6 in output driven by 12AT7 LTP.   
      
   The input for clipping at signal input g1 pin2 'AT7= 2.5Vrms, and then you   
   have 0.7Vrms of GNFB from Vo applied to the NFB input g2 pin7 'AT7. The amount   
   of NFB applied < 3dB, and makes almost no change to the sound if the 820r is   
   removed to disconnect    
   the GNFB.   
      
   The owner wanted variable NFB. So, I installed a linear 1k0 pot on amp rear   
   with small knob. pot bottom goes to 0V, wiper goes to top of junction between   
   820r and 47r NFB network. 47r was removed, and replaced with 100r. Top of pot   
   taken to Vo, ie, the    
   whole of the single secondary winding on OPT.    
   The NFB can now be varied between ZERO and about 10dB max.    
   The center of pot gives the original amp's amount of NFB. But a problem occurs   
   where GNFB exceeds 6dB, with the cabinet speaker connected as a load. Above   
   6dB NFB the amp breaks into HF oscillation once past the class A to AB   
   threshold. It was to be    
   expected because the Fender OPT is not much interleaved like a Hi-Fi OPT. The   
   oscillations only occur as HF waves appearing on positive going peaks of Vo,   
   and the amplitude grows larger as clipping is reached and the oscillations   
   have amplitude larger    
   than the LF part of waves even when over driven.    
      
   So I can only guess that in class B during cut off for one 6V6, the circuit   
   changes to that of an HF oscillator.    
      
   The solutions tried were 15r & 0.27uF across Vo, 680pF & 8k2 across anode to   
   anode, placing small C from Vo to pot wiper for phase correction of NFB. None   
   of these worked. Then I remembered the right solution, TWO R&C zobels across   
   each 1/2 primary.   
   The RLa-a load on two 6V6 is about 7k nominally, so I put 4k7 & 2n2 in series   
   network across each 1/2 primary. The pole between 4k7 and 2n2 = 15khz, so at   
   the F which the amp caters for, 50Hz to 8kHz, the networks cause no loading   
   loss of HF in the    
   wanted region of BW.   
      
   So, no matter what sine wave or square wave F I applied at input, there was NO   
   HF oscillations with 10dB FB, so problem solved.    
      
   HF oscillations in guitar amps may be more common in over driven output stages   
   than anyone suspects.    
      
   With the RF present, a high level 200Hz tone becomes harsh, and you can hear   
   the effective cut off and interrpution to sine waves while RF is present on   
   wave peaks. With RF not present, high overdrive sounds "more grunty", as the   
   amp is wrestling to    
   force the speaker to comply with the applied signals.   
   with 20dB over drive, with NFB or none at all, sound is similar because it   
   consists mainly of a series of square waves dominated by the lowest note   
   played.   
   Unfortunately, there is no Master volume control to amplify the preamp signals   
   which have been raised to a desirable level of distortion, and then be able t   
   play the sound in small venues but with a distorty sound.    
      
   Now for a tube warmer.   
      
   Someone wanted a warmer sound for their guitar amp, so I suggested they have 3   
   cascaded stages with a 1/2 12AU7, so 3 gain stages and a CF at output is   
   possible. You want warmth? Well, methinks that means chocolatizing the signal   
   with high levels of    
   harmonics produced **without any stage clipping**.    
   (( nobody has offered any funds for the warmer, but the recipe here is free ))   
      
   How? OK, have a normally biased 1/2 12AU7 with say 22k DC carrying anode load   
   and B+ supply about 200V, so Ia = 3.0mAdc. Ea = 130Vdc. Bypass the cathode   
   resistor. Following grid in next stage has 0.27uF and 390k coupling.    
   Input from a gain control can produce about 42Vrms at onset of grid current   
   and cut off. At onset of g1 I and Co, 2H may be 5% and 3H 2% and other H in   
   declining %.   
   The voltage gain g1 to a = 11 approx. The maximum g1 input could be approx   
   4.2Vrms, and easily produced usually by the guitar amp stage just previous to   
   the power amp section.     
   Now consider you have THREE such stages in series, ( cascaded ). BUT, instead   
   of ending up with gain = 11 x 11 x 11 = 1,331, you use the 390k in a divider   
   network with 11 : 1 signal reduction, using 390k + 39k, and then the gain from   
   g1 to top of 39k is    
   about 1.0. You can have 3 stages in series and still get overall gain of 1.   
   Having 10 stages might be tricky, because if gain is say 1.5, or 0.5, after 10   
   stages the V0 is either too large of too low.   
   One thing is for sure, the sound of un-clipped devices becomes very obvious,   
   and I leave you all to ponder just what that might be like.    
      
   Max Vo after 3 stages is taken from top of last 39k and buffered with CF. So   
   max Vo = Vin and perhaps just an input control is all that is needed.   
      
   Patrick Turner.   
      
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