REVISED TUBE CONSOLE SCHEMATIC- Questions???

[HappyHourStudio]

Channel 1 Progress… gonna name this one Spaghetti.

Ended up adding a toggle switch to the center tap on the input transformer. As well as including bus 1,2, & Echo.

Thinking of strapping UTC A-20s across the back of each channel… any thoughts on that?

 

[tablebeast]

What is crosstalk like on the direct outs? What does the rest of the grounding scheme look like?

I have a couple of these RCA 9 pin input transformers just waiting for me to build them a home. Its been a while since I looked at them, but I believe a lot of other brands used the same pinout or at least one that is compatible? Newcomb, Webster, Dukane, McMartin, etc. Could be fun to try different iron!

I don't understand why you're continuing to ignore Gyraf's expertise on NOT grounding the input, especially with the plug-in transformer that you've chosen. That MI-12399 has three taps on the primary, but those taps are for giving you multiple choices of impedance, not for grounding. It has its own shield ground, so any rare need for grounding the center tap is unnecessary with this transformer... leaving no good reason to use it. Here is some good news, though. That switch you've already got in there, NOW you can repurpose it to a useful function and rewire it to select from the two different impedances! Also, I can't be the first person to suggest your wiring is way, WAY too thick. And that tube circuit sub-board needs more support. You've got it dangling like a diving board! Uhhh....

Anyway, here is a similar Newcomb input, the TR-91. It has dual primaries that can potentially give you THREE different input impedances! For your circuit, you would just bridge pins 6 and 7, which won't effect your RCA iron.

 

[tablebeast]

This isn't meant as an insult, but your build needs reinforcement all over the place! Its beautiful, but weak. Anywhere you can flex a part with your hand, you need to make it stronger! I mean what you have there might work fine for now, but you'll potentially develop cold solder joints and oscillation gremlins eventually. Anything half-secured will work itself loose over time. Ask me how I learned that lesson!

 

[Cqwet Dbdfte]

As Gyraf says, you do not need to ground the centre tap of the input transformer.
I assume the secondary of the input transformer is meant to be 50K and not 50 ohms
Your mic/line switch can be simplified to sue just s DPDT switch
As RuudNL says there is something wring with the second triode of the preamp. I think you meant the output to come from the anode not the grid
What is the purpose of the 270K resistor in series with the input to the tone control?
I think there is a problem with the load seen by the anode of the the tone control output stage. The plate resistance of the 12AY7 is about 20K so this is effectively its output impedance. But it is loaded by the 100K plate resistor, a pair of 50K pots, a 50K resistor and a 100K resistor all in parallel. These represent a worst case load of 12K5 and a best case load of 20K. This will lead to higher distortion and also interaction between controls.
The second triode in the tone control gets its grid dc connection via a 1Meg pot. This could lead to scratching noise problems. The preceding cathode follower would easily be able to drive 100K pots so I would suggest scaling the tone control to use 100K pots and make a permanent additional 1Meg connection from the second triode to ground.

Cheers

Ian

The 12AY7 does not have stellar linearity, it is very hard to beat SN7, and its 9 pin equivalents, loaded by a CCS rather than a resistor. Use the source terminal of the CCS for low output Z.
Follow up with a MOFET follower if you need a low source impedance.