Hollow-State Hard-Clipper

Discussion in 'Shock Brother's DIY Amps' started by mrriggs, Jun 28, 2020.

  1. mrriggs

    mrriggs TDPRI Member

    Age:
    40
    Posts:
    18
    Joined:
    May 24, 2020
    Location:
    Vancouver, WA
    I've got boxes of old TV-tubes and find it fun to come up with uses for them. There's like two dozen 6AL5 dual-diodes in there which got me thinking about building a hard-clipping distortion stage using tube diodes. I've also got this old Bogen PA amp chassis that I picked up at a flea market twenty years ago. To make this a really fun and challenging project, I stipulated that I had to use only the transformers, pots and tube sockets that were already in the chassis, and everything else had to be parts that I had on-hand, no buying parts for the project.

    Unfortunately, the 7-pin tube socket had been robbed out of the chassis so no 6AL5 this time. I did, however, have a 9-pin 6FM8 which has two diodes like the 6AL5 AND a high-mu triode for a gain stage. A 6N2P takes up the other 9-pin socket and does the duty of pre-amp and cold-clipper.

    The output transformer has a really low plate-to-plate impedance, perfect for a TV sweep tube like 6DQ6B. This chassis has Novar sockets and I have some 6GT5's which are the guts of a 6DQ6B shoved into a Novar tube. That leaves two more Novar sockets. I dug through the datasheets for the Novar tube in the box and came up with the 6GF7 which has one high-mu triode and one power triode. With two 6GF7's I could use the two high-mu triodes for a long-tail phase inverter. That leaves the two power triodes for another wacky idea I've been wanting to try... screen regulators hooked to a "Volume" knob on the front of the amp to adjust when the power tubes break up.

    After a bit of trial and error, this is the circuit I've settled on.

    [​IMG]

    The hard-clipper is before the cold-clipper and is configured to clip sooner. The tube-diodes can be switched off which will allow the cold-clipper to be overdriven. With the gain turned down and the "Power" knob [screen voltage] turned down you can easily overdrive the power tubes. With the Power knob on zero it's only putting out 400 mW of clean power.

    Just for fun, I recorded a short sound sample of each stage of distortion. Can you identify which one is the hard-clipping, cold-clipping and power-amp?

    http://www.gofastforless.com/junk/HSHCclip1.mp3

    http://www.gofastforless.com/junk/HSHCclip2.mp3

    http://www.gofastforless.com/junk/HSHCclip3.mp3
     
    Last edited: Jun 28, 2020
    FenderLover likes this.
  2. Mexitele Blues

    Mexitele Blues Tele-Holic

    Age:
    42
    Posts:
    995
    Joined:
    Mar 7, 2018
    Location:
    Westminster, CO
    I'll give it a try.

    1 - Hard clipping
    2 - Power amp
    3 - Cold clipper

    There is a lot happening in your schematic. Can you explain in layman's terms what's going on with your screen regulators?
     
  3. mrriggs

    mrriggs TDPRI Member

    Age:
    40
    Posts:
    18
    Joined:
    May 24, 2020
    Location:
    Vancouver, WA
    The pot and resistors make up a simple voltage divider to set the voltage. The regulator tubes are cathode-followers to do all the heavy lifting. You could hook the plates of the regulators directly to B+ to make it a straight forward voltage regulator to get maximum power. I chose to exploit a characteristic of sweep tubes, the low screen voltage, and hooked the plate of the regulator to the plate of the power tube. This way it will act as a voltage regulator only until the power amp is pushed hard. When the power tube plate voltage drops to the regulator voltage then the regulator can no longer regulate and the screen voltage will drop with the plate voltage. Basically, switching the pentodes to triode mode of operation.
     
    Mexitele Blues likes this.
  4. sds1

    sds1 Tele-Afflicted

    Age:
    43
    Posts:
    1,397
    Joined:
    May 4, 2017
    Location:
    Orlando, FL, USA
    I'll go: cold, power, hard
     
  5. Mexitele Blues

    Mexitele Blues Tele-Holic

    Age:
    42
    Posts:
    995
    Joined:
    Mar 7, 2018
    Location:
    Westminster, CO
    Thanks, I will have to read this a few times once i get home.
     
  6. mrriggs

    mrriggs TDPRI Member

    Age:
    40
    Posts:
    18
    Joined:
    May 24, 2020
    Location:
    Vancouver, WA
    You got it.

    I didn't adjust the tone controls between samples but probably should have. For the cold-clipper to sound decent, you really need to pull out some bass.

    That is doable with the tonestack in this amp. Setting both the Bass/Mid and Treble knobs to 12-o-clock gives a pretty flat response. Turning the Bass/Mid knob counterclockwise cuts bass, turning it clockwise cuts mids. The Treble knob will cut or add treble.
     
  7. mrriggs

    mrriggs TDPRI Member

    Age:
    40
    Posts:
    18
    Joined:
    May 24, 2020
    Location:
    Vancouver, WA
    I made a change to the screen regulator circuit this morning. Instead of hooking the lower end of the Power pot to ground, it was moved to a negative voltage supply to get the screen voltage to go down further.

    [​IMG]

    The power tube overdrive is now AMAZING when the screens are turned down. It starts clipping at 0.1 Watts. Very softly at first, and progressively hardens up as it's pushed further. At full square wave output it's still only 0.85 Watts which is a very pleasant volume for playing when the kids are home.
     
    Last edited: Jul 1, 2020 at 2:50 PM
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.