1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Early preamp breakup … 5879 preamp tube … PI … ?

Discussion in 'Shock Brother's DIY Amps' started by NSB_Chris, Oct 23, 2020.

  1. NSB_Chris

    NSB_Chris Tele-Meister

    Age:
    55
    Posts:
    303
    Joined:
    Sep 2, 2017
    Location:
    New Smyrna Beach Florida
    Hello all! I am hoping to get some discussion going on some aspects of some classic amp preamp configurations. There are two aspects that I am interested in below.

    1. Use of preamp pentode for early breakup but more specifically the 5879 used in early classics like the GA-20 and AC30. Also used in channel 1 of the /13 RSA23 and FTR37. The GA-1RVT uses a different pentode in the preamp but has a wonderful growl. I can understand the early amps utilizing what we now consider more exotic tubes because in those days everything but toasters had vacuum tubes, but it is intriguing that /13 and other modern builders went out of their way to use NOS 5879 tubes. If pentodes are used in the preamp it seems they are always used as the first preamp stage. Can they really be the source of the breakup we hear, or is that the next stage which is either a 12AX7 type stage or a phase inverter like the one used 1960 AC-30, RSA23 and FTR37?
    2. Is the AC30 phase inverter configuration a source of early breakup? In researching use of preamp pentodes, it occurs to me that the main breakup character is probably coming from the next gain stage, not the pentode. I have seen Tim Pierce demo channel 2 of his RSA23, which starts breaking up at 1 or 2 on the volume. I suspect channel 2 of RSA23 is similar to the FTR37 that seems to have an unremarkable fender like preamp driving the AC30 phase inverter with cool biased 12AX7 and a big 47k tail.

    I am going to build something to play around with these circuits. Interested in any words of wisdom before I dive in!

    Hand drawn FTR37 schematic in this thread: https://el34world.com/Forum/index.php?topic=15506.0

    1960 AC30 reference: https://www.thetubestore.com/lib/thetubestore/schematics/Vox/Vox-AC30-1960-Schematic.pdf

    GA-20 reference: http://www.0rigami.com/gg/ga20t.pdf
     
  2. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    I think your intuition is right. Unless the bias has been hijacked, it is unlikely that a first stage is going to be overdriven by a puny guitar signal.
     
    Tom Kamphuys and NSB_Chris like this.
  3. NSB_Chris

    NSB_Chris Tele-Meister

    Age:
    55
    Posts:
    303
    Joined:
    Sep 2, 2017
    Location:
    New Smyrna Beach Florida
    I continue to read up on options ...
    • debate on two 12AX7 stages rather than one pentode at the input
    • claims that the pentode adds a "wide" or "thick" frequency response
    • more tendency for microphonics with pentodes
    • differences between EF86 and the 5879
    Guess I am just going to have to build something and see what I end up with. My plan is going to be to provide a pretty standard 6V6 power amp section and separate the boards so that I can make significant changes. I plan to have two channels so I can AB the two. Probably will drill lots of holes for sockets, pots and switches so that I can make changes without having to make chips. If I end up with something I want to keep, I will make face plates. I will make this a head to be able to try different speakers and to isolate for microphonics if needed.
     
    andrewRneumann likes this.
  4. andrewRneumann

    andrewRneumann Tele-Holic

    Posts:
    593
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    I found it hard to come up with good reasons to not go with 2 12ax7’s (or other twin triode) instead of one pentode at the input. They both take up one socket. With the 2 triodes you get so much more flexibility... more total gain, more tone shaping options on the interstage coupling and how you handle anode and cathode of second stage especially. You can overdrive the 2nd stage if you don’t attenuate the first stage too much. Yeah it’s the way thousands of amps have done it—probably because it makes the most practical sense? I have to agree with Merlin... if given the choice, save the pentode (or cascode) for a stage that can be overdriven.
     
    NSB_Chris likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.