Gates Sta-Level restoration and modification

[plumsolly]

I got an original Sta-level today. I sold some things for a radio station in exchange for it. Its in pretty good shape - there are some caps that will need to be replaced and someone has mucked about with the time constant area a bit. I have a few questions and I wanted to open up the discussion on a couple of modifications I was thinking of. I plan on giving it a good listen before I change anything, I just wanted to go ahead and get some feedback on these ideas. Also, I wont be doing anything that can't be put back if the need arises and I won't be drilling any holes in chassis or anything of that sort.

First, those black Aerovox caps with a band on one side are elecytrolytics, right? Assuming they are, is ok to replace them with film caps? They are in signal coupling potions (C1, 2, 5 & 6) for the most part. How about C13 on the PT secondary? Ok to replace with film?

Here are the modifications I was thinking of, roughly in order from most to least sensible:

Add a balance pot for the 6386 in place of R10 and 11 - This is taken right from the Collins 26-U schematic

Replace the input pad with a bridging arrangement - 5-10k/600 U-pad

Replace the output pad/attenuator with a constant 600 ohm attenuator - I want to be able to go all the way to zero attenuation

Add variable attack and release controls - the arrangement would be the same as the 26-U or the UA 176 - these would probably live on a separate panel. I'm not sure whether I will incorporate the single/double switch or not - I still don't quite understand this, but it seems similar to a modification done to the RCA 86-A by NBC which is explained in that manual.

Make the input switchable from line to mic - This thing has 60db gain, it would be a shame to let it go to waste - The mic position would switch out the pad and possibly switch the input transformer to a 150-ohm input - What do you think? Should I switch to 150 ohm primaries for the mic? The extra gain would be nice. I did this on my Collins 26-C (just switched the line pad out) and it works great.

Add ratio/threshold control - this would be a variable R32 - the effect of this is explained in the manual - not sure if this would be useful or not

Ok this is the wacky one, I think, so bear with me: variable feedback in the output stage - this would be variable R19/R20 - distortion might sound nice? This may not even work, let alone be useful.  

Here's the schematic: http://www.waltzingbear.com/Schematics/Gates/Sta_Level.htm


And the manual: http://www.google.com/url?sa=t&source=web&cd=7&ved=0CFIQFjAG&url=http%3A%2F%2Fsujan.hallikainen.org%2F~harold%2FBroadcastHistory%2Fuploads%2FGatesStaLevel.pdf&rct=j&q=sta-level%20manual&ei=G3GvTdT7CpGusAOT3ISSAw&usg=AFQjCNEPwFNlVK44DnkuunSmag3Jyii6cA&sig2=TQcrkA9qVD_mquPy4GQfqA&cad=rja

Thanks for your thoughts.

Ben

 

[alexc]

Must be nice to have a real one to play with!

I'm doing a build from scratch with Edcor traffos - based on the Sta-Level with some mods, similar to what you describe.

Sort of a Sta-Level with a dash of 175B and a twist of CCA-1D, stirred with my own swizzle stick.

The adjustable threshold and switched recovery time from the CCA-1D
Input atten, output atten and variable attack from the 175B
6BC8 tube, cathode and plate  balancing from 175B

and my touches :

Regulated heater for varimu and driver stages
Interstage traffo Edcor 15K:15K
6SN7 driver stage
2 channels in a 3RU, tubes and traffos on top
Independent psu for each channel (same power traffo)
Relay bypass boards
VU and GR meters

I'm doing a 175B stock with drip pcb and sowter traffos  for comparison at the same time.


I like your suggestion for the variable feedback. I think it would make a difference.

I would like to try some kind of 'power soak' thing on the output, to allow driving
those 6V6s and output traffo a little harder. To soak up maybe 7W or so of power.

Good luck with it

Cheers

 

[lassoharp]

Make the input switchable from line to mic - This thing has 60db gain, it would be a shame to let it go to waste - The mic position would switch out the pad and possibly switch the input transformer to a 150-ohm input - What do you think? Should I switch to 150 ohm primaries for the mic? The extra gain would be nice. I did this on my Collins 26-C (just switched the line pad out) and it works great.

I like the idea of doing this even though it isn't popular.  As you said - 60db of gain, 150r mic tap, fat PP circuit, and I believe you could add a 'limiter out' switch to remove the SC ala the Collins 356/26J.  Who knows, it may turn out to be a better mic pre than limiter on some things. 


The aerovoxes sound like films - the band is the foil indicator.  Film caps would be the choice in those positions.  Anything less than 1.0uF is usually always film type.

Add variable attack and release controls - the arrangement would be the same as the 26-U or the UA 176 - these would probably live on a separate panel. I'm not sure whether I will incorporate the single/double switch or not - I still don't quite understand this, but it seems similar to a modification done to the RCA 86-A by NBC which is explained in that manual.

I just used a modified BA6A double TC network in a varimu knock up I'm working on and it's working out very well.  I'd say wire it out to a breadboard and try as many combos and variations as possible and keep what you like.  They (complex dual networks) either seem to work great or sound really bad when the combination isn't right.

good luck

 

[plumsolly]

That sounds like fun, Alex. I am curious about the recovery circuit in the CCA. What's the advantage of switching the caps as well as the R's? All the revisions of the UA 175-177 and the 26-U switch only the R's. You got me thinking about that threshold control - check out the Collins 26-W and 26-J for similar examples. It would actually be great to have a threshold control for when I plug a low-level signal (mic) into it. As far as a power soak, I plan on just putting a variable t-pad after the transformer. Jeffs are 1/4W, but the original Mallory's (which can definitely be found) are 4W.

Hey Lasso - For attack and release, I am tempted just to do rheostats as per the 176 - do you think it's worth doing something more complicated? I think a 'limiter out' switch would be pretty easy - check out the 176.

Thanks and Best,

Ben

 

[lassoharp]

I'm guessing emrr will have the best advice on that.  I had a sta-level years ago but never attempted any mods.  I remember it being fairly smooth and slower release sounding on both settings.

 

[alexc]

I have some Mallory 600 ohm attenuators (4W) for this project.

6V6s in push pull should do around 10W or so (like a tweed deluxe!) so I'm hoping I can figure out how
to drive the Edcor 8K:600 10W traffos reasonably strongly without destroying the next unit in the chain.

Otherwise, it's a switched attenuator with 5W resistors or so  

Don't really understand how it's done in the real Sta-Level. I mean, does it drive the output traffo lightly or has it got a hi watt attenuator? Or is all that power used up by the side chain?

I guess I'll find out when I get there. I've collected some interesting info from the forum contributors (thanks emrr and others) on the subject, so I'm sure I can figure something out.

Not sure about the diff between using the 175b potentiometer release vs the CCA 1D switched R-C recovery.
I used the switched R-C recovery in the poorman 670 with good result - it releases nice and smooth.

On the other hand, I never much cared for the potentiometer release style in the 1176 as it seems to be very 'lumpy'.
(could me my diy tho' - as I recall, the real deal revB didn't)

In the end, I like to try different ways for the learning. Switched R-C seems sexier, somehow.
As I said, I will compare with the 175B as I go.

I think I will add a HiZ input for the hell of it. The poorman with an external DI sets the standard for awesome
cleanish-crunchish guitar for me - I'd like to get something similar going with this unit.

And the 'SC out' switch is a great idea. Definately going to do that (now) !

All good - since my first varimu 'experience', I mostly don't use anything else  ;D

 

[plumsolly]

This is interesting from the Retro 176 manual: "The Attack knob has a pull switch that disables the Retro 176 program-controlled time constant. The original UA 175B and 176 have a single time constant. By pulling the Attack knob OUT you can have that mode. The program-controlled time-constant (with the knob IN) makes the Retro 176 gain reduction more natural, better controlling dynamics with less distortion." Hmmm...

There is a section in the PDF version of the manual that is titled 'Schematics and Drawings' but it is blank. I assume its included in the hard copy, so if anyone has one and would be willing to scan it and email it to me (I understand Retro's concern for having it floating around the internet and I applaud them for including it at all) that would be awesome.

Thanks and Best,

Ben

 

[plumsolly]

Here's the relevant material from the RCA 86-A manual additions.

 

[plumsolly]

Sorry, I forgot the drawing from the 86-A manual. Here it is:

 

[emrr]

I wouldn't mod an original if it was in okay condition.  You kill a lot of the value. I'd sell it and then start from scratch with the proceeds. 

 

[bitman]

Can I come down the mountain and just gape at it?

 

[plumsolly]

I wouldn't mod an original if it was in okay condition.  You kill a lot of the value. I'd sell it and then start from scratch with the proceeds.  

Hey Doug. It will need a little work to get it back to original as it is. I don't plan on doing anything to it that can't be easily undone. I don't think I can let it go - I am already attached. I will put it back to original and play with it some before I do anything drastic to it.

Can I come down the mountain and just gape at it?

Absolutely! It might be more exciting once it's up and running. You can come check out my Collins 26-C as well, which is quite an impressive beast. Send me a pm.

I emailed Phil Moore at Retro Instruments, and he was kind enough to get back to me with some pointers. He said the Aerovox caps have got to go. He also said the Sta-Level usually does not require a balance pot for the 6386. Interestingly, he said the power supply caps are usually still good - though I plan on replacing them just to be on the safe side.

Best,

Ben

 

[lassoharp]

Interestingly, he said the power supply caps are usually still good - though I plan on replacing them just to be on the safe side.

I'm not surprised.  Some can be indefatigable.  Send 'em my way if you're going to throw them out. 

My SA-39 still had the original octal plug in PS cap that was still going strong when I added some new ones under the hood.

 

[emrr]

The thing with the old lytics that measure fine, is that when they do give it up, they can take out the power transformer. 

 

[plumsolly]

I found this useful bit in another thread: "The TRIPLE position is the same as DOUBLE, but with a faster attack. The value of R35 is switched to achieve this. The RECOVERY is switching between 7 different values of R36." Thanks Flundran! I would bet that what is done in the Retro 176 is that pushing the attack knob activates a "double" time constant and the attack and release controls are just as per the 176 schematic. This seems like a really versatile way to do it. I will experiment with this once I get it up and running. I ordered my caps so hopefully I can power it up next week.

Ben