1612 Tubes

[Link555]

So I was so interested in the 1612 Tubes I got to play with...I started a hybrid design. The concept is a vari-mu style compressor using the 1612s as the front end after a Jensen XFMR. I am thinking of then running LA2 style buffer and driver amplifier into a Jensen output XFMR. But the side chain willl be solid state and use either a LOG amp or a THAT audio engine.. Sort of a complex blend of 50's tech with more modern tech...
The panel below is the feature set that I would want...I interested to hear your thoughts or wants for a compressor like this...

 

[Boswell]

This sounds interesting! How are you planning to configure the 1612 in terms of which grids you use for the signal input and the for the modulation?

 

[kmetal]

Looks cool. Dunno if its possible or practical, but an xformer in/out switch could be intersting for tonal options, and a hpf on the detection part, to hit the comp harder before pumping on full range signals could be cool.

 

[Link555]

This sounds interesting! How are you planning to configure the 1612 in terms of which grids you use for the signal input and the for the modulation?

At this point I am looking to do what the Gates limier did as a starting point, which has the control voltage being applied to Pin 5(Grid 3) of the 1612's

But instead of the 6H6 I would use a solid RMS detector and possible a peak detector to drive grid 3 through some form of conditioning circuit. I would run the input to into the TC cap connection.

Looks cool. Dunno if its possible or practical, but an xformer in/out switch could be intersting for tonal options, and a hpf on the detection part, to hit the comp harder before pumping on full range signals could be cool.

Thanks Kmetal at this point I think I need the transformer for the tube voltages, it may be possible to eliminate it but it would take some more thought. The 60hz and 120Hz fitlers are meant to be fixed HPF filters on the detection circuit. I suppose this could be easily made variable though...good thought!

 

[Link555]

Rev2 of the front panel concept

 

[kmetal]

Looking good!

 

[Boswell]

It would not be easy to by-pass the input transformer. It is used as a phase-splitter, and also to sum the input signal and the control voltage, so the signal appears as the difference in voltage between G1 and G3.

 

[kmetal]

My idea of the switchable xformer was pulled directly from the millennia channel strip that allows bypassing of xformer and tube stages. I know next to nothing about gear design / EE. In the millennia demos its surprising how subtle the differences are with the different settings engaged.

My only other suggestion is a ratio control or a 1:1 setting. This stems from my love of using a silverface 1176ln compressor all buttons Out, on vocals for a very nice vocal distortion, ala old blues / rock. The more the singer digs in the more saturation. I much prefer it to the more common approach lately of using decapitator or saturation effect that is a static amount of distortion. The 1176 thing adds drama because it is level dependent.

Anyway just tossing it out there because i felt the gates unit link put a video of would have great vocal saturation characteristics for that type of sound.

 

[Link555]

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My only other suggestion is a ratio control or a 1:1 setting. This stems from my love of using a silverface 1176ln compressor all buttons Out, on vocals for a very nice vocal distortion, ala old blues / rock. The more the singer digs in the more saturation. I much prefer it to the more common approach lately of using decapitator or saturation effect that is a static amount of distortion. The 1176 thing adds drama because it is level dependent.

Anyway just tossing it out there because i felt the gates unit link put a video of would have great vocal saturation characteristics for that type of sound.

Thanks I think I can do a 1:1 ratio...thats exactly the kind of input I am seeking, great idea!

 

[Link555]

here is the first draft of the power supply...I am not planning on regulating the 275Vdc rail, but this concept has worked in the past to stabilize and set the voltage... the plan is to have 4-6 tubes on this so I will need additional 6.3Vdc current..so I think I will need to add another transformer for the tube heaters and split the load across two or three LM317's....

 

[pcrecord]

It's very nice to follow the creation with you like that !

 

[Link555]

First paper Draft of the tube section is done... this far from done...I know are several errors in this schematic .... I will need to figure out the biasing resistors values, but I think I am in the ballpark. I will build up a prototype and add pots to tune and find the optimal bias point. For the output transformer I am not 100% sure I can use the Jensen transformer shown, I am really hopping for an off the shelf solution.... In general I tried to stay true to the Gates 39B circuit....let me know your thoughts. Thanks!

 

[Boswell]

Interesting! I'll be very excited to hear how this comes along.

 

[Link555]

Here is my first thoughts on the side chain control circuit. I am using the THAT 4301 mainly because I have a few, but these could easily be converted to any RMS detector... For the peak detector I just did the a a simple standard opamp based peak detector. I am trying to keep this very simple to start with so the attack and release is very minimal, anyway have a look let me your thoughts.

 

[Link555]

Some refinements and corrections...

1612 Power Supply

1) I added indicator LEDS for troubleshooting.
2) I changed the current limit to trip around 200mA. R5 is now 3.3ohms. I will use an over sized wattage TO-220 type package to minimize heat sink requirements.
3) I fixed an error and connected R5 and R6 as per ESP's excellent article (https://sound-au.com/project151.htm)

The PCB Layout for the power supply is essentially done....

I was careful to separate all common's and star them back the main chassis plane.

I made sure I have decent gap around the 225V and 120V traces.

I have over sized the traces, not so much for current handling but more for rework

I have labeled the pin of the connectors, so I don't need the schematic...as much.. for testing ...

The board is only 2 layer with common planes on both sides with exception of the 225V supply.

Side Chain Control

I have saved one dual opamp and simplified circuit. This simplify the output, less parts= less noise

Tube Section

I added the connector to the side chain section....

At this point I am think 3 separate PCB's. One for the power supply, one for the Tube Section and one for the solid state side chain control voltage.

 

[Boswell]

On a first glance, it looks very neat.

 

[Link555]

Thanks Boswell that means a lot coming from you!

 

[Link555]

Progress update:
1) The side chain circuit has been refined a bit. ... I broke it up over two schematics to try to make it more readable. Having done many repairs over the years on amazing gear with terrible schematics, I really know the value of a decent schematic.
2) I changed the side chain filter into a variable (20Hz to 500Hz) 2nd order Sallen Key high pass filter.
3) I had to use an additional opamp to allow the meter switching network.
4) I added the 1:1 ratio.

5) I laid out the first version of the Side Chain PCB and updated the face plate to match the new layout.

6) I designed the VU Meter Movement driver circuit. I am using a voltage dependent current source to drive the meter movement.

As always love to hear your comments, concerns or abuse

 

[kmetal]

Without any ability to read schematics, i can say the front panel layout looks well done, and i think the hpf frequency range is a useful range.

 

[Boswell]

I haven't had a chance to look in detail at your latest schematics, but I did see in the meter select and drive page that the resistors going to the non-inverting inputs of each of the 2134s should be to ground and not in series with the a.c. coupling caps. The amps need a bias current path!

The current drive to the meter looks a little strange, but it may be OK. You might need a small compensation cap from the meter output to the inv input of the op amp for stability.

A fixed resistor between the cal pot and the rail to avoid problems at the end of the pot range?

What's the function of the 100K resistors from the outputs of the three buffer/gain op amps to ground?