REVISED TUBE CONSOLE SCHEMATIC- Questions???

[HappyHourStudio]

I revised and rounded out the schematic for this tube console I’m building to be the workhorse mixer in my studio. I’ve got all the parts and I’m slowly approaching the time where I’m gonna start cracking at it. As that time creeps up, I wanted to try and wrap my head around it as much as possible. Curious what you guys think and if any of you have the knowledge and time, would love to hear your thoughts on the following questions…



•Do the resistor values on the 11 summing resistors of Bus 1 & 2 need to be 50K instead of 100K so that the total value = 100K at that juncture instead of 150K? To match the break away point between the Tone Control Stage and Direct Out Cathode Follower?



•Would the echo send need to be unbalanced? If not, where would the (-) and ground run to for the return into bus 1 & 2? (I’ve left that part of the schematic blank until I figure out where that’s running)



•Are there any easily identifiable improvements to the circuit that might improve the overall quality of the audio? Be it frequency response, etc.



•At the input stage of the preamp, I have a line level switch that will have 10K resistors running in parallel and a 240K resistor bridging the two right after. Are these values standard for that kind of level switch? Are there other methods? Are there values that might be better suited for a line level switch?



•The power supply I’m using is actually just an old Heathkit power supply where I can choose the precise voltage I want. This worked really well for the first rendition of this console I built; however, it was only 5 voltage amplifier/tone control channels running into a two bus much like the one in the schematic I posted. I’m wondering if the Heathkit will be able to power an additional 5 channels. It’s essentially 20 12AY7s & 13 12AU7s.



•I was told by a buddy that often times output transformers don’t really help in any way when dealing with a circuit utilizing cathode followers in this manner. I’m sure that might be generalizing a bit, but is there any truth to that? Out of curiosity, why?



Anyway, thanks ahead of time!!! Love reading all of your thoughts and input. I’ve attached a photo of the schematic, the loose block diagram, and one of the channel strips.

From top to bottom: Direct Out Volume/Bus 1 & 2 Volume/Treble/Bass/Phase Switch/Mic&Line Level Switch/Gain/Echo Send


Mucho Love,
-Kyle

 

[gyraf]

..you probably dont want to ground your input transformer primary centertap..

For line level, I would use two 4k7 then a 470R between. This gives ca 10k input impedance, and presents some 400 Ohms to the input transformer. See e.g. the G9 project.

For power supply, you dont give us figures, and I'm too lazy to do all the calculations and adding up. But the G9 psu scales really well, and can probably be adapted..

The output transformer "not helping" does not compute? If you need balanced or isolated outputs, there's really no way around..

500K for mixamp input pots is unnecessary high and thus noisy imo. I'd probably use 50k and live with the slightly reduced gain of this stage. And yes, I'd make the mix resistors smaller also. How small? Depends on the output drive capabilities of the preceding stage(s) - remember it should be able to drive all possible sumamps in parallel..

/Jakob E.

 

[RuudNL]

What is the function of the 'right' part of the 12AY7 in the preamp stage?
(Both anode and cathode aren't going anywhere.)

 

[thomasdf]

Just stopping by to say I am intrigued by your project and would very much like to see pictures of the console !

 

[ruffrecords]

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

 

[HappyHourStudio]

..you probably dont want to ground your input transformer primary centertap..

Hey!!! Thank you for your help! As far as grounding the center tap, this is what the original schematic that I’m basing this circuit off of called for for some reason. I’ve built a more simple version prior to this and it worked really well.

For line level, I would use two 4k7 then a 470R between. This gives ca 10k input impedance, and presents some 400 Ohms to the input transformer. See e.g. the G9 project.

Thank you!!! I will try this out!

For power supply, you dont give us figures, and I'm too lazy to do all the calculations and adding up. But the G9 psu scales really well, and can probably be adapted..

Groovy!

The output transformer "not helping" does not compute? If you need balanced or isolated outputs, there's really no way around..

I have quite a few 600:600 and 500:500 transformers laying around that I think I could throw on the back end of each channel, maybe I’ll try that out. The previous version seemed to work fine without them.

500K for mixamp input pots is unnecessary high and thus noisy imo. I'd probably use 50k and live with the slightly reduced gain of this stage. And yes, I'd make the mix resistors smaller also. How small? Depends on the output drive capabilities of the preceding stage(s) - remember it should be able to drive all possible sumamps in parallel..

I hear ya! Thank you! You are talking only about the 500K pots on the buses, correct? Not the 500K pot at the beginning of the direct out stage?

Thank you for all your help!!!

 

[HappyHourStudio]

What is the function of the 'right' part of the 12AY7 in the preamp stage?
(Both anode and cathode aren't going anywhere.)

You are right! I drew that up wrong! These things are hard to get right while chasing my 4 year old daughter around at the same time. ha! I’ll fix it shortly!

 

[HappyHourStudio]

Just stopping by to say I am intrigued by your project and would very much like to see pictures of the console !

I can show you a picture of my early version! But the new version will take some time! I only have some concepts drawn up.


(THE OG)


(THE NEW VIBE)

 

[thomasdf]

beautiful ! thanks for sharing ! It looks ... sturdy

 

[HappyHourStudio]

As Gyraf says, you do not need to ground the centre tap of the input transformer.

For some reason, the old schematic I’m basing this off of did this. For what it’s worth, it does seem to work well on my previous build.

I assume the secondary of the input transformer is meant to be 50K and not 50 ohms

You are totally right. That was a typo! Thank you!

Your mic/line switch can be simplified to sue just s DPDT switch

I’ve honestly already purchased all of the 4pdt switches a while back, so I’ll very likely roll with that method. Thank you for the advice though! I’m curious how one would wire a DPDT switch that way?

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

100%. I’m going to correct that today and will repost an updated version.

What is the purpose of the 270K resistor in series with the input to the tone control?

I am not entirely sure what its purpose is, but this part in particular is, again, something that was in the original schematic. It could be a sort of buffer that the manufacturer put in not knowing exactly what the consumer would do with the product because it is based on an old modular system. I’ve just combined the schematics together from each module.

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.

Referencing the old schematic again, there is a section in the manual that talks about summing multiple preamp/tone controls into a single cathode follower and they use 100K resistors. That’s the only reason I was trying to make it work around that number. So you would recommend a lower value resistor coming off this post tone control point on each parallel? Each being the direct out/bus1/bus2/echo send.

The second triode in the tone control gets its grid dc connection via a 1Meg pot. This could lead to scratching noise problems.

This wasn’t really an issue on my previous build, that I can recall.

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.

So you’re saying replace the two 1MEG tone control pots with 100K pots? Where would that 1 MEG ground connection break away from?

Thank you for all the advice!!! All of this is really helpful! Truly!

 

[HappyHourStudio]

In reference to the centertap ground….

..you probably dont want to ground your input transformer primary centertap..

Here is my transformer layout and the original.

 

[HappyHourStudio]

Here is an updated schematic with some of the things folks mentioned.

 

[ruffrecords]

I also notice that you bus 1, bus 2 and echo send feeds are all pre fader. Is this what you really want? They would normally be post fader.

Cheers

ian

 

[HappyHourStudio]

I had a friend recommend doing it the way it is currently laid out, but I’m not opposed to trying it post. Would I just tap it directly after the direct out cathode follower? Would adding output transformers to each direct out change the location of the tap should I decide to add those?

I’m entertaining the idea of not bothering with the busses and echo send, but they were fun to mess with on my old version of the mixer. I would very likely not be mixing through this board and only tracking… the option may be nice though.

 

[HappyHourStudio]

What is the purpose of the 270K resistor in series with the input to the tone control?

Here is the technical data for just the cathode follower section.

 

[thomasdf]

side note - where did you find the RCA Daven big knobs ? Are these vintage ones or repro ? if repro , they look legit !
I have 4 vintages ones and I plan to build a pair of RCA BA6A one of these days ... so I am looking for 4 more, without selling my kidneys (I still need them)

 

[HappyHourStudio]

side note - where did you find the RCA Daven big knobs ? Are these vintage ones or repro ? if repro , they look legit !
I have 4 vintages ones and I plan to build a pair of RCA BA6A one of these days ... so I am looking for 4 more, without selling my kidneys (I still need them)

I just watched EBay for a while. The ones in the pic are true vintage, but I have a few repro that look pretty legit! Got most of them pretty cheap, but paid more than I would have liked to for a couple.

 

[ruffrecords]

I had a friend recommend doing it the way it is currently laid out, but I’m not opposed to trying it post. Would I just tap it directly after the direct out cathode follower? Would adding output transformers to each direct out change the location of the tap should I decide to add those?

Yes that would be the place to tap from. It would also solve the loading problem I mentioned.

Echo is normally post fade simply because you want the echo to reduce as you bring the fader down - you don't usually want to hear echo from sources whose fader is right down

I’m entertaining the idea of not bothering with the busses and echo send, but they were fun to mess with on my old version of the mixer. I would very likely not be mixing through this board and only tracking… the option may be nice though.

If you are no planning to mix with it then perhaps better to optimise it as 10 mic pres.

Cheers

Ian

 

[ruffrecords]

Here is the technical data for just the cathode follower section.

Interesting. For some reason the grid bias resistor is only 22K so they add a 220K series resistor to increase the input impedance. Odd way of doing it.

Cheers

Ian

 

[gyraf]

Note that if you insist on grounding input center tap, that you'll compromise

- Phantom power will be shorted
- unbalanced (e.g. minus to ground) will not work
-your cmrr can be significantly impaired depending on the precision of transformer winding and symmetry.