Valve Guitar Amp Preamp Modification (VHT Classic 18)

• Valve Guitar Amp Preamp Modification (VHT Classic 18)
  Posted by tomsouthtrader on 21 Sep, 2020 12:26
• Hello EEV Forum!
  Something slightly different for this forum, however its quite clear that the knowledge and experience of members of this forum is second to none
  
  I have been trying to modify my VHT Classic 18 (basically a cheep Chinese amp based loosely on a Marshall 18w circuit).
  Essentially I am trying to completely bypass the awe full input gain stage that uses a TL072. And wire the guitar straight to the input of the first preamp tube..
  I have found the schematic for the amp, and working from that I got the soldering iron out..
  My aim was to completely remove the TL072 from the circuit and wire the input straight into the tube.
  The schematic I have attached shows what I have done to the circuit.
  The amp didn't blow up thankfully.
  And it does make a sound... And The tone itself is not to bad (at least through the LO input).
  However it now has a couple of issues 
  
  The input is now incredibly noisy. There is some sound even when the master is at minimum.
  Also the treble control when turned all the way up makes make an awful noise (almost like some self oscillation). However I had never turned this all the way up before attempting this modification.
  
  So I am asking for you knowlage and advice on how to fix my 'mod'  <3)
  Link to the schematic and my 'MOD'..
  https://drive.google.com/drive/folders/1Q-AdViC60HTd6p0KYzZEajgf1M5A8tYZ?usp=sharing
  
  
  UPDATE..
  Following some advice received for a member of another forum i have placed the input signal after C52 rather than before it...
  This worked and achieved exactly what i set out to do, However..
  Does the input of a 12ax7 need a capacitor inline???
  And what would be the best way to wire in a volume pot for this input? and any suggestions on a value for it?
  
  
  
  Also i have now also attached a schematic and how it is wired now..
  
  

  ---

  VHT Classic 18 Schematic.pdf

  WorkingMOD Classic 18 Schematic.pdf
• #1 Reply
  Posted by gbaddeley on 22 Sep, 2020 12:27
• Replace R76 100k with a 1M audio pot. Disconnect C52 from the preamp IC and connect it to the pot wiper.
• #2 Reply
  Posted by tomsouthtrader on 05 Oct, 2020 18:21
• 
  hi gbaddeley,
  
  Thanks for the reply.
  I have just tried this. And it resulted in extreme noise and the signal cutting out once both end of the pot are reached!
  
  Is C52 necessary for any electrical reason???
  
  Thanks again