TLA turret layout with Vus (unfinished)

[Jatchley]

i tried posting this earlier but somehow broke the post when trying to upload a higher resolution layout. oops sorry about that.

So i'm trying to build Ian Thompson Bell (ruffrecords) Twin Line Amp but turret/eyelet style

i'm having trouble with the lead dress/layout. i understand how hard of work it must be to design turret layouts. but any help is appreciated.
i want to use this device as a make up gain stage after my 16 channel balanced passive summing mixer. i decided to do it eyelet/turret style because ians pcbs are for tiny euro cards and i have a big 2u chassis and like turret style and want to practice making small custom eyelet/turret boards.

i'm also unsure about how to add some cheap (not real) Vu meters.
the meters i have are 500uA 630ohm pulled from an junked akai reel to reel.
i put some new 12v bulbs in them and they look nice when lit with a 9V battery.
im uncertain where to tap signal from. is there an off the shelf driver that takes balanced in? or do i tap part of the unbalanced audio pre OT? would that mess with the load?
i wanted to have the audio path into them on a switch to bypass them entirely but to keep the lights of VUs ON (i want to use the VU lights as my "powered on" lights)

also with the output control do i ground 1 side of the primary and the secondary of the OT and just run the signal wire thats left through a pot to the xlr?
also about the XLR... what do i connect the Cold Pin to if theres only 1 lead coming from the transformer?

another thing im unsure about is the heater voltage doubler. i got that from some guy who uses it on his 5v winding for dc heaters and said since im doing 6.3 to 12v i should probably use a dropping resistor or 7824 regulator or both to get a solid 12vdc? any thoughts here?

i haven't bought audio transformers yet (availability is sparse out there, might go edcor but wait times are just as long for carnhill)

my PT is a Hammond P-T261G6 45VA 250V C.T. @ 130 ma, 6.3V @ 2A (i think i'll be ok on current)
ian states the tubes use .5A @12vdc on heaters and 15mA on HT

sorry for all the questions. i usually just work on guitar amps and i wanted to try and spice up my mix bus a bit. 

i also have 2 tip/ring switching trs jacks i found in a box. i was wondering if i could use them as output inserts post OT? (they are not mentioned in layout.)

[Jatchley]

here is ians documentation.

https://drive.google.com/drive/folders/0B_n67A1hN3qtfmdGbTBZZWlkeFVmTzJGVjM3T0g5dEJhcE9pS1JWZFYtVS1vUGpCZS1YNW8?resourcekey=0-MAjgxX-rZ43QUwYdRtef_g

so yeah any suggestions on lead dress or any of my questions above would be most helpful. thank you.

[glass54]

Hi Jatchley,
Interesting Project 
I will attempt to answer some questions based on my experience.
Just for Clarity, Ian's connector is a DIN41612 BC, 64 Pins used and true, too high for a 2RU chassis. Some manufacturers lay these down for a stereo device BUT this may limit your space for power supplies, etc. Your alternate would be a DIN 41623 C/2 (half length) with 32 pins (2x16).
Anyway continuing on, I have used a DC heater (schematic attached) very successfully and it it important the input voltage always stays above 15.5V. Typically for this project, I would use a transformer with secodaries at 240V@125mA and 12V@2A. As long as your voltage doubler is capable of 16V plus, no problems and you may need a heatsink on you LM7812T. Your meters would be best run off a small 47R ? resistor and a 100uF 25V Electro capacitor. (I'd go for LEDs in the meters, plenty of colour choices, no heat). With your HT (+250V) supply. I don't see any real problems as I have used a similar circuit. You could split and decouple L and R +HT if you wanted to.
If you use Output balancing Tx, feed the 2 wires to pins 2 and 3 of XLR, same colours for same pins ie maintaining correct Phase. Pin 1 to chassis is common. If you end up with Earth loops that will require a rethink of input/output ground connections.
It would be good if you could draw up your final schematic, as I'm unclear about you "Pot" question.
You DO NOT run a lead from Pot to XLR as you already have a gain control. Thus a schematic would give us clarity 
Trust this helps a little.
Kind regards
Mirek

[Jatchley]

Hey there. The second post of this thread has Ian’s documents with schematic and configs. But I will attach what I have been referencing most here for ease.

[glass54]

....my apologies, got carried away 
My opening comments re DIN41612 Connector are incorrect!! Ian actually used the DIN41612 HALF LENGTH CONNECTOR, B+C.
Just for Clarity, Ian's connector is a DIN41612 BC, 64 Pins used and true, too high for a 2RU chassis. Some manufacturers lay these down for a stereo device BUT this may limit your space for power supplies, etc. Your alternate would be a DIN 41612 C/2 (half length) with 32 pins (2x16).
Too much Christmas Cheer, can't count past 9 
I looked up Ian's info/sch and as I understand it, you are wanting to build your own customized version, therefore a "final" schematic would be useful (for all of us).
Kind regards
Mirek

[Jatchley]

Here’s my best shot so far.

The 12V filament dropping resistors will be calculated from real world voltages. When I actually start building it.

I’m unsure where to connect VU audio inputs

I’m unsure how much current the VU buffers will actually draw.

I think the buffer circuit is the JLM one. Unsure though.

Any tips or corrections would be helpful. Thank you.

[glass54]

This will be a big project!
You realise the Jensen JT115k-e is a microphone input Tx? https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-115K-e1.pdf
What OP Amp are you using to drive the meters?
I don't like the pot on the line output to adjust the output level. (Share some ideas later)
Are you going to use the +75V for a (regulated) Phantom power supply?
I wouldn't run the Op Amps off the same node as the heaters.
I do have more questions BUT we need to weed out things you don't need and create something useful rather than an R&D monster.
Alternatively, you build Ian's project 
On the question of LEAD DRESSING, a very good example can be seen in the following link (by Sluckey) https://sluckeyamps.com/VAC15/ac15.htm Just scroll down a few pages.
We'll battle on 
Regards
Mirek

[PRR]

...You realise the Jensen JT115k-e is a microphone input Tx?.....

That's in the documentation, reply #3.

While described as a "line amp", this is really a lowish-gain "mike preamp", adapted for the special chore of recovering signal from a large passive mixing network. Mix loss will be 12dB to 30dB, overall gain might wish to be 6dB, so up to 36dB needed. The boys who were doing this around the turn of the century just used mike preamps, turned down.

IMHO, the Jensen plus the high-NFB amplifier will be cleaner than Ivory Soap and you may as well use a 5532 chip.

The question about meter buffer current draw really depends on the chip used there, and I don't see that specified anywhere. Is it the JLM? Should be easy to check.

[Jatchley]

I believe the opamp used is a tl072. So That’s 3Ma of draw? 1.4 each side. So far my total draw (I think) is 503Ma on the 2A filament winding. I guess I’ll also need to check the lamps but I don’t expect them to be much. Am I correct to think I need to know total current draw of everything on that 12v path to calculate the 12V dropping resistors?

Also I have a hard time understanding whether or not the tubes draw are .5A or 1A. Ian says the circuit uses .5A at 12vdc. Ok. but I read when using a voltage doubler circuit like that my current draw is doubled? But then I read that when using dc for filaments current draw is halved. So does it just come back to .5A?

Why shouldn’t I power the buffer from the 12VDC filament path?

The 75 volt tap is just part of Ian’s PSU design. I won’t be using it. But I figured if I had a had too hard a time with the dc filaments I could try to use it as a CT standoff for elevated ac heaters. Not really sure which would work best in this scenario. (Ian says 12vdc in series is the way) but I know there’s debate for ac filaments and 6922 as well.

My passive mixer has 40-45db loss.

I was under the impression this circuit does about 50db including the +20db from the IT and the -6 from the OT

My understanding is that I’ll be pushing this thing nearish its limits.

Would a Hammond 1140-MN-A for the input trafo give me more color?

My plan was carnhill VTB 2291 for the OT. (Never used one but was told it’s pretty colorful)

I do want it pretty clean but a little bit of color when driven wouldn’t hurt. (I have a ssl style bus comp with a switchable OT and a pair of eqp1a down stream)

I decided on a 1k 4 gang 24 stepped rotary switch for the outputs. And a 2 gang 24 step 10k for the gain. Rather than ganged pots. But I’d rather not draw all those resistors in a schematic.

[glass54]

Hi Jatchley
As I am not familiar with the 6922, I looked up the RCA (design centre values) for Heater requirements and best I can tell its a shared heater for both triodes and rated at 6.3V @ 300ma. Somebody may be able to confirm this and pin 9 is an internal shield?
You are looking at using a 12V DC (nominal) heater supply, so this needs attention. Obviously no problems with 12AX7 ad you can do AC or DC 6.3V or 12.6V. OP Amp current will be negligible compared to Heater current 
Tube Heater Current will become obvious once you sort out voltage requirements for 6922.
My personal preference would be the NE5532 (as its capable of a higher current output if required) +1 with PRR. It's readily available, cheap enough and well proven for the application. I would use the TL072 if I was using a Peak indicator LED ie no great demand on IC.

Why shouldn’t I power the buffer from the 12VDC filament path?

I don't like running ICs from Heater supplies without some form of isolation/PS regulation esp in Calibration equipment.
No Problems with your 4 Gang, 24 step Rotary Attenuator (and Gain control), just a lot of work BUT potentially very accurate.
Regards
Mirek

[Jatchley]

Ian provides this for the heater wiring for 6922. It’d be the one on the left.

So for regulation I could replace the dropping resistor after the doubler for a 7812 regulator.