New Preamp/Poweramp Completed
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.
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.
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. ;)
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.
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.
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.
BTW, what rack cases did you use?
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.
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.
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.
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