Resurecting Gates STA LEVEL and SA-39A ...it's alive...it's alive...

[Kilohertz]

HI boys and girls,

I have had this Sta- Level for about 15-18 years, and am just getting around to putting into use. It's pretty rough, but seems to be all there. The input xfmr had come off the back, hanging by the wires, and much of the insulation had fallen off. I managed to get heat shrink installed and that's all fine, hopefully got all the wires to where they should go, but I noticed there are 3 extra wires coming out of the xfmr, should these just be insulated and not used? I can't find any good inside pictures on the web, the schematic only shows 5 wires being used, 2 in, 2 out and a shield.  What are these extra 3 wires for?

2. For studio use, +4dBm, do you remove the input/output pads or leave them in place?
3. It's been re-wired for 6BZ8 for V1, I'll try it as is before searching for a 6386.

Otherwise it's all original and should probably be re-capped, at least the PS caps.

Any help will be much appreciated.

Cheers

 

[Rob Flinn]

I have some pictures that may or may not help you with the transformer.  PM me your email address and I will send them to you.


I have also made 5 or 6 point to point sta levels over the years.    In my experiences I found the input pad unecessary.  The output pad needs to remain & some sort of attenuator is useful on the output.  I didn't use the original i/o transformers so this info might not apply to you. 

 

[Kilohertz]

I have to thank Rob for replaying so quickly and sending me the pictures, confirmed the 3 wires are not connected to anything, just insulate and leave alone.

Then I slowly powered it up, as it hasn't been on in who knows how many years, after a minute or so I started to hear the headphones singing and away it went. Simply amazing! Works (seemingly) perfect! I ran it for a while with the 6BZ7 which turned out to be quite microphonic so I tried a 6ES8 and 6BC8 and all 3 seemed to work about the same, 6ES8 may have sounded a little better to my ears, I was able to get 40dB compression no problem with the input attenuator in place, driving it with around 0dBm.  As this is my first time hearing a Sta-Level I have nothing to compare it to as far as the sound of the 6386, but for now, I am happy with the sound of these 3 tubes.

Based on Robs pics, it also looks like this may have been recapped, although the caps look original, solder tarnished etc. Did these come with Sprauge Orange Drops?

Anyway, she is up and running, now onto my RCA 86B and SA-39B, I know the RCA works, the Gates needs a power xfmr.

Cheers and thanks!

 

[Rob Flinn]

The Sa39 needs well matched valves in the audio part of the circuit or you get some very interesting problems.....  Be careful with it becaue you have quite high d.cvolts on the meter terminals.  I normally use a load of insulating tape on them while I do work on an Sa39.

 

[Kilohertz]

Thanks Rob, will do.

I found a xfmr in one part of the shop, 300-0-300 with 5 and 6.3, still have 2 or 3 other places to search. Most of my xfmrs are Hammond old school wrinkle black, heavy duty. I could use the 300-0-300 if I used solid state replacement rectifiers, but I want to keep this original.

Back to the hunt.

Cheers

 

[Rob Flinn]

Thanks Rob, will do.

I found a xfmr in one part of the shop, 300-0-300 with 5 and 6.3, still have 2 or 3 other places to search. Most of my xfmrs are Hammond old school wrinkle black, heavy duty. I could use the 300-0-300 if I used solid state replacement rectifiers, but I want to keep this original.

Back to the hunt.

Cheers

yes, but you would need a dropper resisitor to get down to the correct H.t voltage

 

[Kilohertz]

yes, but you would need a dropper resisitor to get down to the correct H.t voltage

The original transformer is 350-0-350 according to the schematic, and if I put in 2 diodes (those epoxied looking things that simulate a tube and fit into the original socket) wouldn't that be close to the original 438VDC, using the 300-0-300?

600X.707=424VDC using silicon
700X.707=494 - 25v/plate drop=444VDC using 5V4

My tube rectifier theory is quite rusty, correct me if I'm wrong.

And, I did find another transformer, seems almost perfect, it's a 375-0-375 with 5 and 6.3 windings, looks like it will bolt right in place on the back, correct form factor and all. That one may need a dropping R, or a larger drop rectifier tube. More to play with.

Your thoughts?

Cheers

 

[Kilohertz]

Well I decided to proceed with the 375-0-375 xfmr today, extended all the short wires out of it to reach all the terminals inside, replaced a few tubes that were missing and powered it up through my light bulb test fixture, it slowly came up, and in about a minute was hearing audio out of the headphones. YEAH!    Let it sit for about a minute on this low power setting on my fixture then bypassed the bulb and gave it full willy and it works great. The measured DC voltage at L1 right out of the rectifier is 424VDC, pretty damn close to the 438 as shown on the schematic so I was very happy with that. I will now have to go thru it and test all the tubes and clean switches and such as there is a bit of distortion, not much but enough to be noticeable.  I also need to set or check all the tube currents on the 6 or 8 jacks on the back.

That's it for now, more later.

Cheers

 

[emrr]

now onto my RCA 86B and SA-39B, I know the RCA works, the Gates needs a power xfmr.

SA-39's use an under-spec power transformer, most have already been replaced in the field with the next size up in the Triad line.  The filter caps at turn-on go slightly over-voltage until the 6V6's warm up, so they must definitely be as spec'd or higher.  You may want to use a series pair with bleeders as many instrument amps do.

See my SA-39 thread for a lot more commentary.  Neon bulb being one of several. 

 

[Kilohertz]

Thanks Doug,

It's been operating in the bench for a few days now, seems to sound okay with 3-5dB of compression, any more and the sound becomes distorted, it seems like there is a LOT of gain in this. Listening on AKG 240 headphones. I haven't checked any of the tube currents yet but did check/ change the tubes until I found nicely matched 6L7s and 6SJ7s, all tubes measure like new or better on my B&K 550. I haven't brought the AVO tester in yet as it is large and cumbersome and slow to use.

I'm thinking possibly leaky caps and I should just shotgun all the paper caps in it.

I'll reread your thread, I read it once last week and garnered some good info.

It sure doesn't sound like the STA LEVEL though, the STA LEVEL is much smoother and clean sounding so something is still amiss.

Cheers

 

[Kilohertz]

And another picture

 

[Kilohertz]

Well thanks to all on board here who have helped me.

I referred to this thread that Doug mentioned, I think this is the one you meant, https://groupdiy.com/index.php?topic=36432.0
and started poking around the regulator stage and found the voltages going up and down, some of the voltages were way off, Neon was 70-74 with about .75VAC component on it, causing all sorts of weird stuff on the 275VDC line so out to the shop to the zener drawer and I put in 1N4756A and a 1N4742A to get me to 55VDC. I also changed R37 from 5.1M down to 220K so that there is enough current to keep the zeners running properly. With the 5.1M there was barely enough current to get them to turn on. The regulator operates 100% now and is steady at 275VDC, the meter seems to stay nicely at 0.

Now it sounds AMAZING and I haven't even changed any caps yet. Oh ya, R44 was 15K instead of 33K, fixed that too. There is still one voltage that is not right and that is pin 5 on the 6L6, which is also the plate on the 6SJ7 is only 195 and it should be 250. It seems to be operating fine so unless I hear from someone that says it should be 250, I'll leave it for now.

Now I suppose I should change all the paper caps next, but it sounds so good as is.

Cheers and thanks!

PS is it R25 and R26 that controls the attack time in conjunction with C6 and C7?

PPS I noticed that my unit has NO output attenuator boards, the xfmr just goes to the output pot. I should probably at least terminate it in 600 ohms.

 

[emrr]

You have an SA-39, not an SA-39B, the circuit is different in many small places.  Find the SA-38A manual that's out there, it's closer to what you have.  Should be a ladder on the output, no load needed. 

 

[Kilohertz]

You have an SA-39, not an SA-39B, the circuit is different in many small places.  Find the SA-38A manual that's out there, it's closer to what you have.  Should be a ladder on the output, no load needed.

Well I'll be go to heck...you're right. It's an SA-39A, the model number plate inside is M-3529A, and has an input attenuator not a pot, and an output attenuator, not the AT1 AT2 and AT3 boards and a single pot. Someone has put in an SA-39B meter over the years then as that is what's in there now, that's what threw me.

So for the attack and release mods, did you leave R34,35,36 in place, and split R32,33 in half and then changed R31 as well to make it a faster release? And for attack I would think I could replace R25 and 26 with a nice quality good tracking dual section pot, somewhere 500K or so? I know you went with stepped attenuator, which I could do as well, would be easier to return to a fixed setting. Do you recall what R values you used?

Thanks Doug!

Cheers