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New Preamp/Poweramp Completed

Discussion in 'Amps [BG]' started by roscoefretless, Dec 6, 2013.

  1. roscoefretless


    May 5, 2012
    I finished up my latest preamp last week. I used Fdeck's HPF concept, but used a 12AT7 instead of the FETs or opamps. (He encourage me to post.) Took about 3"x3" of turret board space. Used the TCJ Filter Calculator with 24db Butterworth filter. Works like a champ. Made it switchable to hear the difference. Next amp won't have the switch :). Did splurge and use NOS Mullard 12AT7s.

    Preamp itself is a 12AX7 in the V1 position with different options for cathode resistor/LED and 3 different choices for bypass - none, full, and partial. Tone stack - James with mid-shift control. Output - paraphase using 12AT7 driving XLR to Hypex 1200W(peak) power amp (2 400W modules bridged w/SMPS). Also, driving a Jensen DBE with a cathode follower using the other 1/2 of the output 12AT7.

    The power supply is John Broskie's PS-1. Have used these several times and for the money, sound pretty good in a reasonable amount of space. Absolutely no hum.

    Total weight - 14 lbs. I am getting rather old and my all-tube heads (TraceElliot and DIY) are staying home much more frequently.

  2. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Congratulations on the build. A build like this takes a lot of planning, thought, and work. Even better when it works.

    So how does it sound? The PS-1 is a good design and affords a quiet amp.

    What B+ are you using?

    How effective is the tone stack mid shift? When implemented within the baxandall circuit, I find that this feature works but the range can be limited.
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  4. Passinwind

    Passinwind Charlie Escher Supporting Member

    Way cool. I've posted a number of DIY builds here using either Broskie's Aikido line stage or power supply boards -- PS-14 in the last one. Nearly went the Hypex route for my current in-progress bass amp build, but ended up using an ICE module I picked up used instead.

    Hope to hear some clips of your stuff eventually. ;)
  5. roscoefretless


    May 5, 2012
    It works well :) The sound - subjective as always, but I am a fretless player that wants to sound fretted much of the time (can't play frets - started as an upright player) and want quick attacks with warmth. This preamp has the most kick of any that I have built so far - this is incarnation 6 or 7. I play mostly R&B, gospel and funk. And some BeBop on the side :)

    B-strings work well with this design - no mud and the HPF filter helps get more power to the cab without wasting power on subsonic string movement. The cone no longer moves when I push on the strings. Thanks Fdeck for the concept!

    The mid-control has a wide range. I replaced the "normal" 120K or 180K resistor in the James stack with a 1M rev-log pot. Seem to get about 2-1/2 octaves this way. The extremes are not terribly useful. It moves the center frequency of the stack from around 200Hz to about 1kHz. I like it around 400Hz or so. The tone stack was an experiment - have a customer that wants a ukelele amplifier and we will be testing the tone stack with several of his ukes this weekend.

    I am running 300V from the PS-1 and then have 3 separate Solen Cap filters, one feeding each tube. Each tube is seeing 280V. Running ~1ma per 12AX7 section and 4ma per 12AT7 section.
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The filter on the front end is nice to have.

    Thanks for the feedback on the mid tone feature. I find that I tend to set the mid and leave it. Using different instruments is a good reason to have it in there.

    I like to keep the B+ voltage as high as possible.

    I've bought some 6N1P tubes to try but I haven't got to them yet. They are known for their clarity.

    I'm wondering if you considered a 6SL7 octal tube for the preamp. Being a fan of the B-15, I like the incredible tone of these tubes. Also have some RCA 1620's waiting in line to be tried.
  7. roscoefretless


    May 5, 2012
    I have not played with octal small signal tubes.

    I have used 6922 (not quite the same as the 6N1P, but similar) in hi-fi stuff over 25 years ago. It would be interesting to hear them. Cascode is what I used back then. Don't know how they would do with bass, but might be interesting with guitar - pentode-like without microphonics :)

    I did pick up some 6cg7s to play with the next round - supposed to be similar to the octal 6SN7, just not as high a voltage rating.

    Also, hard to beat the gain of a 12AX7 in simple circuits :) They certainly are easy to design with for cleaner applications.

    My last preamp had an 3-way-active-Baxandall using 12AX7s and 5965s - too much gain. I am hoping to revisit this in my next attempt - hence the 6cg7s.

    Thanks for the interaction.
  8. Passinwind

    Passinwind Charlie Escher Supporting Member

    FWIW, my last couple of builds have used active bass and mids with a passive treble stack following one stage later. I used a 6SL7 and two 6SN7s.

    BTW, what rack cases did you use?
  9. roscoefretless


    May 5, 2012
    I used the Sescom stuff. Wolgram is now the owner of the name and manufactures them:


    They are a pain to ground. Each panel needs its own wire to the earth bond point.

    Also, by default you get solid top and bottom pieces. For a little extra $, you can get the punched top and bottoms. You just to have to ask - not listed on the website. Good folks to work with.
  10. dmrogers

    dmrogers Supporting Member

    Jan 26, 2005
    Eastman, GA
    Wow, those are works of art.

    I have so much respect for the folks here on the forum that can built such as what you have done.

    I love just reading the posts.

    I wish I had stayed involved in electronics when I transitioned to the computer/network field.

    Great job!
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I've always said that Baxandall had better PR people than James and as a result most people call the James circuit a Baxandall. :p

    The 6N1P is used in the REDDI, a DI that many here consider top shelf. A key part of the design is a big output transformer which helps it deliver its incredible tone. They use a highly filtered power supply, the single tube, and the transformer. A simple signal path.
  12. Passinwind

    Passinwind Charlie Escher Supporting Member

    Ahh, thanks. I used some of their stuff in the past, and wondered what happened to that part of the business. I mostly have been using Par-Metal for cases these days -- their alodine ones make grounding pretty easy. As always, convenience costs though...;)

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