LTP Phase Inverter in a cathode biased amp? Any experiences?

Discussion in 'Shock Brother's DIY Amps' started by tweedy_woodpecker, May 21, 2020.

  1. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

    Age:
    48
    Posts:
    90
    Joined:
    Jun 25, 2019
    Location:
    Austria
    So I am starting my next amp design and am thinking something similar to a 5E3 or 6G3 (Not sure about the tonestack yet.) I really like the sound and the concept of selfbiasing in a cathode biased amp but I would like to try a LTP instead of the regular Tweed style cathodyne PI. (I so far have only experience with the LTP in a fixed bias BF style amp...)

    Is this a crazy, stupid idea? Going through the Fender schematics it looks like the change from cathodyne PI/cathode biased amps to LTP/fixed bias went hand in hand with a few exceptions (Brownface and BF Princetons).

    Is there a reason why these two didn't get used together?
    My main thinking is that the LTP and a 12AT7 would probably give sweeter distortion and shift the distortion more to the poweramp section.
    Does anybody have experience with this or can tell what difference to expect? Something to be careful for?

    Thanks!
     
  2. Mr Ridesglide

    Mr Ridesglide Friend of Leo's Silver Supporter

    Posts:
    2,115
    Joined:
    Dec 11, 2009
    Location:
    Bloomington, MN
    One exception for sure is the 5F6A Bassman - You could experiment around that perhaps. Rob has a micro too perhaps...
     
    tweedy_woodpecker likes this.
  3. Badside

    Badside Tele-Meister

    Posts:
    433
    Joined:
    Jul 5, 2011
    Location:
    Montreal, Qc, Canada
    Vox AC30 and Marshall 18W are examples of cathode bias with LTPI
     
    Last edited: May 22, 2020
  4. tdoty

    tdoty Tele-Meister

    Posts:
    267
    Joined:
    Oct 17, 2011
    Location:
    Winchester, Tennessee
    So.....the Vox AC30 and the 18W Marshall are cathode biased with LTPI?
     
  5. dan40

    dan40 Tele-Afflicted

    Posts:
    1,665
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    Yes they are. Many of the el84 based circuits use that combination. I think Fender was always trying to squeeze more power and clean headroom from their circuits so the use of grid biasing along with the ltpi seemed like the next logical step in the evolution of these amps.
     
  6. Nickfl

    Nickfl Tele-Afflicted

    Posts:
    1,882
    Joined:
    May 24, 2016
    Location:
    Florida
    You can also consider the paraphase pi. More gain than the LTP and a little more old-fashioned, you'll see it in some of the earlier Tweed circuits as well as things like the supro thunderbolt.
     
    King Fan and tweedy_woodpecker like this.
  7. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

    Age:
    48
    Posts:
    90
    Joined:
    Jun 25, 2019
    Location:
    Austria
    Thanks for steering me toward these two amps!

    I feel a bit stupid now with my fender-centric ampworldview :)
     
  8. The Ballzz

    The Ballzz Tele-Afflicted

    Age:
    65
    Posts:
    1,618
    Joined:
    Apr 11, 2016
    Location:
    Las Vegas, NV
    While the 5F6-A Bassman does indeed use a long tail pair PI, it is not cathode bias. The newest Bassman to use cathode bias was the 5B6.

    I know that I've seen some Fender model using LTP-PI, along with cathode bias, I just can't recall which?

    For anyone interested in such a thing (and ready to spend endless hours perusing/comparing), here's a link to most of the early Fender schematics & layouts, from earliest up through the blackface era and a few beyond. Some VERY interesting and inspiring stuff!

    https://vintagefenderamprepair.com/pages/library-schematics-layouts

    There are many amps that seem nearly identical at first glance, but a closer look tells why they often can sound very different, in spite of dramatic similarities. It shows how a few minor changes can change an amps character in either subtle or dramatic ways!

    Just Sayin'
    Gene
     
    schmee and tweedy_woodpecker like this.
  9. The Ballzz

    The Ballzz Tele-Afflicted

    Age:
    65
    Posts:
    1,618
    Joined:
    Apr 11, 2016
    Location:
    Las Vegas, NV
    @tweedy_woodpecker & @Mr Ridesglide
    For example, go to that linked site, look at and compare the schematics for the BANDMASTER 5E7, PRO 5E5-A and the SUPER 5F4. Now, if you can tell me why (besides different speaker complements, of course) these amps sound and respond differently from each other, you are coming close to finding a big clue to character shaping!

    Then look at/compare some of the TWIN and BASSMAN iterations and note the glaring similarities, along with some very small, subtle variations. Always remember though, that the "A" or other letter suffix does not always denote a small revision, but sometimes a whole different, new amp , as with the 5E5 and 5E5-A!

    Just Sayin'
    Gene
     
  10. sds1

    sds1 Tele-Afflicted

    Age:
    43
    Posts:
    1,320
    Joined:
    May 4, 2017
    Location:
    Orlando, FL, USA
    I just found out yesterday some amps use plate load resistors > 100k !!
     
    Last edited: May 22, 2020
    esbrian and tweedy_woodpecker like this.
  11. Commodore 64

    Commodore 64 Friend of Leo's

    Age:
    44
    Posts:
    4,091
    Joined:
    Mar 1, 2010
    Location:
    Kent, OH
    The cathode resistors dissipate a lot of heat on bigger bottles. I usually parallel a pair of 10W resistors when I use cathode bias on a pair 6L6 or EL34.
     
    Paul G. and King Fan like this.
  12. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

    Age:
    48
    Posts:
    90
    Joined:
    Jun 25, 2019
    Location:
    Austria
    :D
     
  13. Wally

    Wally Telefied Ad Free Member

    Posts:
    35,328
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    The Fender Pro Sonic has a LTP and a cathode biasing option.
     
  14. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

    Posts:
    1,006
    Joined:
    Dec 30, 2019
    Location:
    california
    In general, the LTP PI does add gain and the cathodyne PI sits near unity gain. Depending on the preamp design, gain may have to be adjusted elsewhere in the circuit to keep things stable.

    IIRC, Rob Rob has a mod for a 5f6a to switch to cathode bias. Yes, there is a
    *Fixed / Cathode Bias Switch* in his 5f6a mods.
     
    Wally and King Fan like this.
  15. Commodore 64

    Commodore 64 Friend of Leo's

    Age:
    44
    Posts:
    4,091
    Joined:
    Mar 1, 2010
    Location:
    Kent, OH
    As far as gain of the PI though, the Cathodyne allows you to save a triode and use it for another gain stage. I think it's pretty much 6 of one half dozen of the other (regarding gain at least) on number of tubes.

    Also think cathodyne allows for an easy, nice solution for MV to be inserted into amp. To get similar MV behavior on LTP, you need a much more complicated Lar-Mar setup.
     
  16. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

    Posts:
    1,006
    Joined:
    Dec 30, 2019
    Location:
    california
    I rather like the MV inserted after the tone stack in the 5f6a type circuit. That location benefits from the low impedance the cathode follower provides.
     
    Paul G. likes this.
  17. Badside

    Badside Tele-Meister

    Posts:
    433
    Joined:
    Jul 5, 2011
    Location:
    Montreal, Qc, Canada
    To be fair, the combination seems to be more popular with EL84s. I believe a good part of it is that those tubes don't need to drop as much voltage on the cathode resistor so there is less efficiency lost.

    But I've built a few cathode biased 6V6 amps with LTPI myself, even a high gain channel switcher. Works well and loud enough.
     
    tweedy_woodpecker likes this.
  18. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

    Age:
    48
    Posts:
    90
    Joined:
    Jun 25, 2019
    Location:
    Austria
    Thanks for all your input!

    So as of now I am thinking a Tweed style amp with LTP-6l6 Cathode biased, rectifier depending on the B+ I will get from the PT (some sort of 320 to 330 from either side of the CT) but I guess 5U4G or GZ34.

    OT wise I think something in between a 5E5 and 5e3 sized transformer with different output taps so I can use both 6L6 or 6V6s would be nice.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.