original 2tube DI, need help: critique and optimize (on shoulders of REDDI)

[andyfromdenver]

Hi friends.
CJ wound transformers for, and shared a circuit for a 6N1P tube DI (think the above mentioned red box). It’s based upon a circuit from the legendary tome RDH4e the Class A single triode amplifier.
I have expanded upon that design to incorporate an additional tube for a constant current source DC-coupled Cathode Follower (DCCF) and a, familiar looking, treble middle bass arrangement.

I started this a couple years ago, but have been immersed in school.
I finally had a chance to fire up my test circuit and it’s working great except for too much gain (i.e sweet tube distortion). Removing the cathode bypass cap on the DCCF gain stage helped tremendously.

My goal: to have the gain low on the eq section, so a user can optionally use the gain/eq stage or direct into the DI stage and the levels would be close (except if boosting frequencies of course)

Can you please help me evaluate the test voltages and determine my gain after the DCCF? I also would appreciate any suggestions and interest. I need help with a means of reducing gain (perhaps simple divider) without decreasing performance/ hi freq loss.

I have elevated the heater center tap to 50V.

My plan is to fully document a really cool build in the preamp section and help builders make a magic box.

Thank you!

*EDIT: NEW SCHEMATIC FOR TREB BASS WITHOUT THE MARSHALL MID SCOOP IS A FEW POSTS DOWN.  THE ONE BELOW WAS MY VERSION 1.0 AND WAS MASSIVELY ALTERED*

 

[andyfromdenver]

test pic for fun

 

[andyfromdenver]

I guess I am going to either add a volume control  or a fixed divider on the end.  I may just hide it inside like a trim.
In use, it’s a cool option.
I may experiment with splitting the plate load to the gain stage of the DCCF, but i’ll have to do a bunch of reading first.

 

[andyfromdenver]

Incase anyone is following. I am going to build a test circuit for a James tone stack after the DCCF: https://www.ampbooks.com/mobile/amp-technology/james-tonestack-analysis/
and see if i like that more.
The Marshall/Fender tone stack i built is great, but i don’t like the inevitable midscoop when i compare it to the bypassed.

The James is promising because it is treb and bass and with the use of volume, you can have bass/ treb boost/ cut or a mid scoop or mid hump. 
I also think that having just treb and bass will fit the enclosure better.

once i finalize this, i’ll start my build thread in the preamp section.

tbc

 

[alexc]

Very  nice

I am a big fan of the 'tube DI with CJ  se  traffo'  myself and just couldn't 't resist adding a second tube .. up front.

James tone stack is good choice, in my opinion.  My own version is without eq, but I am doing exactly that with my 'Orange 86' build. 

PSU implementation is important, I think - for best results I like to remote it as far the    &BN%4    as I can!

I can see from your pic,  that I am indeed 'preaching to the converted' 

Good luck with it all - it's very definitely worth it!

 

[andyfromdenver]

Thank you alexc!

Prototyping is complete, and here is the final schematic. This is a variation of the james stack from Designing High-Fidelity Tube Preamps by Merlin Blencowe.

This sounds really nice. There are options to run thru the tone stage or bypass by using the 1/4” jacks.
With controls set on the tone stack 5 treb, 5 bass, 5 vol it is fairly flat and coincides with the bypassed level.
Running through the tone stage adds a touch of beef/tube-ness, so bypassing it is a really hard to beat clarity.
But having an option for treb, bass, sparkle boost, and volume is awesome.
With this tonestack you can also dial in a mid scoop around 1k or a mid boost when you max or min the treb/bass and adjust volume up or down.

I can make a youtube video demonstrating and post that.

Please correspond for a hi res scheme.
I put a lot of effort into biasing the DC-coupled cathode follower and calculating output impedance and miller capacitance for the next stage. These values were important in achieving the best tone stack performance, and may be subject to minor changes. 

PS, working title “Gravity Bong”. This seems like a $1k+ studio tool, hehe 

*EDIT SPARKLE BOOST CAP VALUE (on cathode of initial gain stage) 10nf.  The 50volt tap comes at the first node.*

 

[andyfromdenver]

here is a close up of the tone stage add on scheme and also a tidy layout for three pots and one tag strip.

 

[andyfromdenver]

Here’s me testing the prototype using my already built tube di for the power, heaters, and ground.

I don't know how to analyze frequencies in a recording program, so I just go by what i hear.
"They saaaay" lol there is no bad setting with a James tone stack and I would concur; the boosts and cuts are super helpful.

 

[andyfromdenver]

Next step is to execute an intuitive and tidy build in this enclosure. I plan to complete it and document it in the preamp section.
I will go over how to orient your transformers for "ZERO" 60hz hum in a tight enclosure and wire it in a way that would make the 1940s lead dress scientists proud.

You can see that i have the power transformer safely prepared to apply voltage on the primary directly from the wall and listen for hum on the output transformer secondary.

Thanks for watching 

 

[alexc]

Superb ...  did I read right and your calling the 'Gravity Bong'  ? 

What 'a card'  ..  and I mean that in a friendly fashion ..  as in 'light hearted humourous word-play'.

Just having an interested look through your tech detail ...  nice work!

 

[andyfromdenver]

Superb ...  did I read right and your calling the 'Gravity Bong'  ? 

What 'a card'  ..  and I mean that in a friendly fashion ..  as in 'light hearted humourous word-play'.

Just having an interested look through your tech detail ...  nice work!

Thank you!  I like to give my builds names that the cool kids would like  8). It’s subject to change.

 

[alexc]

Very fine.

 

[andyfromdenver]

Here are some finished pics (before labeling)
Output of EQ stage is at bottom of front and you connect in to the DI stage input.

 

[andyfromdenver]

close up

 

[andyfromdenver]

A gut shot. I did the bulk of the wiring outside the enclosure and then tied it all together in there at the end. 
Even in this tight enclosure, the sound is incredibly quiet, with “zero” 60hz hum. Heaters are AC.

 

[Fuzz Face]

A gut shot. I did the bulk of the wiring outside the enclosure and then tied it all together in there at the end. 
Even in this tight enclosure, the sound is incredibly quiet, with “zero” 60hz hum. Heaters are AC.

Nice looking build!!

 

[andyfromdenver]

Nice looking build!!

Thank you so much!  I had a chance to record some bass with it last night. I’m very pleased.
I think I calculated the cathode follower output impedance correctly and the following miller capacitance, but i know the eq section can probably be further optimized, the treble control doesn’t get as bright as i would like, I think this is limited by the miller capacitance of the following stage. I’m very happy for now though, and it’s a great supplement to the already amazing DI.

Here’s the labeling I did by hand.
I may add a frequency graphic in the lower right corner. I want the controls to be super intuitive and all the instructions easily available, incase I sell it.

 

[duantro]

Hey Andy! Looks great. I'm doing a pair of the tube di's with Cj Iron, that I've been sitting on for a while. I'd love a high resolution schematic file of your version, if you are willing.

 

[musipol]

So you have an output jack for the tonestack and input for the DI. Is this for use an unbuffered effects loop.................or something else (perhaps just a direct in bypassing the tonestack? Perhaps you already explained in the posts above.....I only looked at the schemo
Gary

 

[capacitorless]

I usually lurk & absorb info (hence my username, haha ) but have a question. I'm wondering if this design could be modded to use a 12au7, and still have enough gain to be useful?

Edit: physical implementation looks great!