BA-6A time constant board design

[Martan]

Hi all,

I am wanting to build a small pcb for the Time constants for a BA-6A build. I have a collective cases case which has 8 setting marks for the time knob which I assume are for the 2 on the original BA-6A (the bypass is on a separate switch) and 6 spots derived from the Fairchild 670 time constants. I plan on designing the board in Eagle and using OSHpark to fab. That way I can upload the files for public use and contribute a little here. I am thinking I will build around a Grayhill rotary such as this one: Grayhill rotary and treat the BA-6A as in the attached block diagram where it is a pretty simple no-contact on position 1 for the BA-6A "single" setting and an RC to ground for the "double" setting.

I have two questions i am hoping someone can help with:

1. The Fairchild schematic (attached) doesn't have the time constant options going to ground. How would I treat these for this project? Should they just go to ground like the "double" setting?

2. Would there be better options to use for the time constants in this project other than the Fairchild ones? I've read that some of the release times on those are very long.

 

[Heikki]

Take a look at fairchild 660 schematic. It is simpler and the time constant components go to ground.

I wouldn't mess with the BA-6A time constans too much without thinking about it first. Just sticking in the fairchild RC values might lead to some unwanted effects since it's a different amplifier driving the rectifier. Those capacitor coupled rectifiers are hard for me to think about.

 

[Martan]

Ah, thanks for the tip! I get what you are saying about a possible incompatibility. I honestly am assuming someone in the past tried this since there is a provision for it on the case. Maybe it was not a prevalent build/modification?

 

[JMan]

That case was designed specifically for the Drip BA6A project, which had an optional extra board for a Grayhill switch like the one you mentioned above in order to build in the Fairchild time constants. However, Gregory stopped making that time constant board a long time ago, and now doesn't even sell the BA6A board anymore, so the project is pretty dead. I have a near-complete one without the time constant board sitting on a shelf right now and I hope to finish it up one day soon. But to your point, you are spot on about why those extra time constant positions are there on that front panel.

If you are interested, I could send you the [somewhat incomplete] part of the Drip BOM that lists the Fairchild time constant components, so you could compare.

 

[Martan]

Thanks for the offer. I am actually working off the Drip pcb but don’t have the time constant board.

 

[Martan]

I’m in your boat, too, in that I started this one a couple years ago and am just now finishing (hopefully) up. I’m testing it in single mode without any time constant circuit. It passes sound but the input is wonky. Good luck on yours when you start again.

 

[JMan]

Much appreciated!

I seem to remember that one of the sticking points for me before I shelved it was that something was weird with the balance A and B — I sort of seem to remember that the pots were called for but the switches weren’t built into Greg’s design (maybe these were just meter display switches though and don’t actually affect the calibration? I can’t quite remember).

 

[Martan]

None of the meter options except GR are on the board. The calibration is fairly straight-forward just trying to match the voltages around the 6Sk7's with the balance trimmers. My issues at the moment are that the zero-adjust does nothing, my input pad does nothing (really weird) and the audio disappears when I turn up the Input knob. Have to go through and do a thorough re-check of connections and maybe do some tube swapping.

 

[SubhumanRecords]

I have a drip 660 board with the time constraint pcb, I could take some pics for you or copy it to sprint layout and make the Gerbers for it, if needed.
Myke

 

[Martan]

Thanks for the offer! I actually have a copy of that pcb as well to get ideas for the layout. I’m using this as an excuse to try laying out and fabbing a pcb. I will definitely remember your offer if I get stuck.

In terms of troubleshooting I found a couple mistakes that I made but still no change of symptoms yet.

 

[SubhumanRecords]

No problem at all bro, good luck.
Myke

 

[Martan]

Update: the boards are off for fab at OSH Park. Will post the link to them if they work out in my build.

 

[JMan]

Question for you, Martan, since you are building the Drip board with the Collective Cases panel. Did you implement a bypass switch, and if so, how did you do it? (Actually, I wonder the same thing about the in/out pads as well).

 

[JMan]

I mocked this up based on Greg's drawing of the original time constants in the build guide. Does this look like it would work correctly?

It basically just splits the bypass function and the time constants out on to two separate switches instead of one -- since the front panel controls look slightly different on the Drip build than on the original, this seems to be necessary to maintain the aesthetic balance (four knobs).

 

[Martan]

Yep. That's basically how I am going to implement it when I get the time constant pcb. I had to order 3 of them, btw, if you need one.

For the pads I'm using dpdt switches with t-pads built on the switch as in the attached screenshot (though the image is for a V72 input with 200 ohm impedance). The input and output impedances of the BA-6A are both 600 ohms (unless using the 150 ohm input). There are online calculators to find the resistor values you need for this impedance (T-Pad Attenuator Calculator - Electrical Engineering & Electronics Tools). I based my attenuation amounts on the discussion here: RCA BA6a attenuators - Gearspace.com

Using what resistors I had I ended up somewhere around 18 Dbs for both of the pads.

 

[JMan]

Thanks for this info! I have never implemented a pad like this, but it turns out it's pretty simple!

Where do these pads actually sit in the circuit? Input pad after input transformer before dual 25k pot? Output pad after output transformer, before or after attenuator? Or do these both literally just sit on the XLRs?

 

[Martan]

I have my t-pads between the input xlr and pcb, and between the output pad between the output transformer and the output attenuator. I’m using 3 watt resistors on the output t-pad.

 

[Martan]

I don’t have a pad on the 150 ohm input, thinking that these would be mic level sources that likely won’t need attenuation

Edit: Mic level sources

 

[JMan]

I'm very slowly beginning to think about this build again, piece by piece. Martan, this thread has actually inspired me to try to complete it after a long time of letting it sit idle.

One thing that crossed my mind today as I was looking at the thing was that I don't know what type of indicator lamp is intended here (bulb, LED...?) or what the best practice is for adding it in to the circuit. I don't see anything in the schematic or in the build docs (am I missing it?). It seems like a very basic question but for whatever reason I just don't know the answer.

 

[Martan]

It seems like the front panel from collective cases was designed for a smaller LED type of bulb. I went ahead and drilled that hole out bigger for a standard fender amp style jewel light. My Simpson meter runs on 6 V AC for the lamp anyway, so I just daisy-chained them to the heaters for most of the tubes. I’m running with the preamp tubes on a separate transformer for 6.3 V DC.