New Preamp/Poweramp Completed

[roscoefretless]

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.

 

[beans-on-toast]

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.

 

[Passinwind]

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.

 

[roscoefretless]

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.

 

[beans-on-toast]

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.

 

[roscoefretless]

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.

 

[Passinwind]

I have not played with octal small signal tubes.

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.

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?

 

[roscoefretless]

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

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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.

 

[dmrogers]

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!

 

[beans-on-toast]

I've always said that Baxandall had better PR people than James and as a result most people call the James circuit a Baxandall.

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.

 

[Passinwind]

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

Invalid Link Removed

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.

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...