Gates Radio SA-39 limiter restoration and modification

[emrr]

Some pics of a recent restoration and mod.  I must have done about 6 of these to date.  They are all slightly different, it seems, with at least 4 different circuit revisions I've seen, along with 3 different input transformers, 3 different output transformers, 2 different power transformers, 2 case styles, and 4 different basic appearances.  They did build them for 23+ years, after all.  And then there's the commonly seen field upgrade to the power transformer, once the undersized original blows up.  And the early SA-38 variation, which adds more advanced metering capabilities. 

This one is a 1960ish model, as evidenced by the 'B' version simplifications, the steel case, the pre 'Harris' logo, the UTC input transformer, the smaller version output transformer, the simplified input and output attenuators, and lack of DC filament revision. 

The mods lose the headphone jack on the front, move the power switch to the back where the release control was, the release control to the front where the headphone jack was, and add an attack control where the power switch was.  The release control expands from the original 6 positions, maintaining the original slowest 3 timings and expanding the range below that into faster territory than stock.  Stock attack is fixed for fastest possible, and the added control simply inserts a linear resistance chain to let varying amounts of transients through.  I seldom use the stock attack. 

I followed joe-electro's suggestion of using zeners in place of the neon regulator bulb, which tends to turn into an audible oscillator on these units.  Seems to make meter warm-up more stable too. 

The filter caps really need to be 500V types, which is getting to be a bitch.  B+ tends to ride at or above 450 at their positions.

Some of these really require the input wiring to carry a shield to the ground tap, and some do not.  I haven't sorted out exactly why there's variation there. 

party....

 

[MikoKensington]

Nice stuff, Doug.  I've always wanted one of these.  They're just getting harder and harder to find.  How many do you have personally?

 

[emrr]

I have three, and I should really off one of them, since I'm in the hole on some RCA tube limiters at the moment.  I have another whacky super-rare Gates limiter that I should do show-and-tell on too. 

 

[MikoKensington]

I have another whacky super-rare Gates limiter that I should do show-and-tell on too. 

Yes!  Maybe one of these days I can upload some files for you to process?  You know, when I actually have time to work on something.

 

[Biasrocks]

Really nice Doug, I bet these thing will kill on bass gtr and vocals, not too mention agtr.

Makes me pine for my BA6A that I sold.  :'(

Mark

 

[joe-electro]

Very nice, Doug!  So, when do we get to see porn of this "super-rare" Gates limiter you're teasing us with?

Joe

 

[abby normal]

Great job as usual Doug! I like the "date code" drawn on your replacement caps. Good tip, think I'll be doing that from now on since me memory she ain't as good as she used to be. 

 

[emrr]

I am loving these things on piano for pop and rock songs.  Around here, probably sees acoustic guitar the most, then bass after that. 

I've been doing the cap date thing for years.  Has always bugged me when you stumble across a recap, and can't tell when it was done. 

The other Gates limiter got some pics updated in another thread, but no reason to point that out, better to wait until it gets it's own post. 

 

[echorec]

Looks beautiful, I have one SA39 (working) and one SA38 (project...) The 38 is missing the little A4 lamp, can I just buy the newly made NE45s found on ebay or should I look for an old one? Or should I get a Zener, which one would you suggest?
Regards from Stockholm,

Gunnar

 

[emrr]

Zener for sure.    I stuck in a 1 watt whatever.    I don't think the new bulbs have quite the same characteristics, since so many turn into oscillators so quickly. 

 

[echorec]

Thanks!

 

[stitch-o]

Holy cow!
What are the attack release times? Where you able to modify them?

NICE!

 

[emrr]

All stock release times are still present, just double resolution in the bottom two stock steps, plus 3 faster than stock.  The fastest borders on no compression, gets a little bit into fuzz from side chains inability to fully charge. 

Attack.  Dunno; 10 slower settings, picked by ear.    Slowest was slowest that seemed to be useful on bass heavy bass and drum tracks. 

 

[tubologic]

Zener for sure.    I stuck in a 1 watt whatever.    I don't think the new bulbs have quite the same characteristics, since so many turn into oscillators so quickly.   

Sorry,but I can't fully agree with you on that point. I also restored and modded (7) GATES SA-39's and tried the Zener conversion with very poor results. The A4 (G.E NE-45 neon bulb) is used in this circuit as a voltage reference for the stabilized power supply regulator and has a direct influence on the output voltage. It is a well know fact that  zeners diodes have a  poor temperature coefficient,and at higher VZ voltages this problem rises dramatically. Conversely,neon bulbs are relatively unaffected by temperature changes and have a very stable breakdown voltage when used within specs. Also,they are much more reliable than zeners in high voltage/high t° environment like tube circuits. I never encountered oscillation problems with genuine G.E (Or Westinghouse) NE-45's but I admit I've never tried the modern replicas as the originals are still available. The temperature inside a working SA-39 with closed door is quite high and regulated voltage drift from cold to thermal equilibrium with a (1.5W) Zener diode is unacceptable: I measured nearly 20 volts versus 4 volts with the good old NE-45. An added bonus is that the neon bulb glow will tell you visually that the power supply is operating. For all the above reasons I can't see any valid reason to use a Zener to replace A4,as this will lead to degraded performance and reliability. IMPORTANT !!! Actually most of the power supply regulator troubles in the SA-39 are not due to the NE-45 bulb (wich can reach a 10,000 hour lifespan in this circuit) but to a defective R44 (33K/2W) resistor in the output divider/potentiometer chain. On all the (7) SA-39's I've restored this resistor has drifted dramatically  and I measured anything from less than 500 ohms to many megohms ! YOU MUST ALLWAYS CHECK R44 AND REPLACE IT WITH A NEW 33K/3WATTS MOX RESISTOR. If the old resistor has dropped in value (less than 10K) the NE-45 may have been damaged due to the increased current and must also be replaced.  After the repair don't forget to check and adjust the regulated voltage with the R43 pot. to precisely 275VDC measured at the cathode of the pass tube (V10,pin8). If you can't adjust this voltage to 275V the regulator doesn't work properly and must be repaired.

 

[tubologic]

By the way,another very useful mod. to the SA-39 is to add a variable TRESHOLD control. This can easily be done by varying the bias voltage at the R29/R30 node which is fixed and set at the factory to 42V by the R27/R28 divider. Just remove these two resistors and install a pot with two end resistors to limit the voltage range from about +10 to + 70 volts wich gives a mid-point reference close to the factory setting,and plenty of treshold level adjustment range above and below. This,and the variable Attack and Recovery controls already described will dramatically extend the possibilities of the SA-39.  Only 3 parts needed for this mod: great deal ! 

 

[emrr]

Well, agree to disagree.  I have yet to see an SA-39 operate properly for more than a year with an NE-45 installed.  New, old, whatever.  All eventually turned into audible oscillators; can't use the limiter consistently.  On one unit, I went through a half dozen or so old pieces, and a similar number of new NE-45's, with none fixing the audible problem; every other circuit constant measuring correctly.  With so many bulbs, I was initially convinced it was some other cause, but none could be identified.  All circuit values (6SJ7, 6Y6/6L6, 6X5 included) around the bulb checked out properly, whether original or through replacement.    I have more stable 275 VDC readings with a zener, consequently, I have faster and more stable meter warm-up.  With bulb, it always tended to drift up and down +/- 8 volts or so.  The bulb, or zener, sees 55 volts, and I don't think I've seen a (properly ventilated) SA-39 reach more than 110 degrees internally.  Certainly not 120.  There is a bit of vent space around the bulb socket too, whether it's blowing or sucking air, I'm not sure. 

This may (only slightly) echo the situation people report with LA-2A builds, where the specified neon bulb fails to deliver the required voltage, and zeners prove to be the fix.  I believe it was even reported than UA had gone to zeners for the repro units. 

 

[tubologic]

Well, agree to disagree.  I have yet to see an SA-39 operate properly for more than a year with an NE-45 installed.   New, old, whatever.   All eventually turned into audible oscillators; can't use the limiter consistently.   One one unit, I went through a half dozen or so old pieces, and a similar number of new NE-45's, with none fixing the audible problem; every other circuit constant measuring correctly.   With so many bulbs, I was initially convinced it was some other cause, but none could be identified.   All circuit values (6SJ7, 6Y6/6L6, 6X5 included) around the bulb checked out properly, whether original or through replacement.    I have more stable 275 VDC readings with a zener, consequently, I have faster and more stable meter warm-up.  With bulb, it always tended to drift up and down +/- 8 volts or so.  The bulb, or zener, sees 55 volts, and I don't think I've seen a (properly ventilated) SA-39 reach more than 110 degrees internally.   Certainly not 120.   There is a bit of vent space around the bulb socket too, whether it's blowing or sucking air, I'm not sure.  

This may (only slightly) echo the situation people report with LA-2A builds, where the specified neon bulb fails to deliver the required voltage, and zeners prove to be the fix.  I believe it was even reported than UA had gone to zeners for the repro units.  

Strangely enough, I had many various problems with SA-39's but never an oscillating NE-45 in the power supply,even with  45+ years old blackened original bulbs... I tried various Zeners and allways ended up with more long term DC drift than the old NE-45,thus I gave up and went back to the neon bulb. Neon bulbs have a fairly long life (5.000 to 10.000 hours) if used properly and if they fail within a year something else in the circuit must be wrong and overloading the bulb. They could (and sometimes do) oscillate but only if decoupled with a capacitor which doesn't exist in the SA-39 circuit. A zener could be substituted because it is cheap and easily available,but not necesarily better in this application. (except of course for an infinite life) Meter (zero compression) drift is usually due to bad (gassy) 1612 tubes. This can be checked by changing the recovery time from the shortest to the longest position: the needle should not move with good tubes.
BTW,could you specify the exact type (and voltage) of the Zener you are using in this circuit.  IIRC, I used BZX61's and some others... I think I would try again. 

 

[emrr]

When I turn on an SA-38/39 with a neon bulb, I have to have the door open and grab the zero adjust so the meter doesn't slam past full scale.  Then I have to continually re-check the zero point for several hours.  With the units I've put a zener in, the meter has exhibited less deviation, and the zero point stabilized much faster.  Meter drift was not a significant factor after warm-up with a neon, but there might be a bit of slow up and down.  The zener's have locked in quicker, and stayed put. 

The neon 'failure' in all my experiences has been entirely based on audible oscillation.  Nothing like an audible tone mixed into the audio!    I've seen frequencies between 14.5K and 26K.  In all cases, 275VDC could still be dialed in, and the voltage divider string was in spec. 

I'll check the part # I used.    Joe-electro may also have something to add. 

 

[mpc3000limited]

Excellent piece of gear! What is the approx. value of these unit's? It is nice to know there are people that spend countless hours figuring out how to keep these beasts living for a few more sessions! 

 

[lassoharp]

Don't really have an opinion on the zeners as I haven't used them but I've had the same neon bulb in my unit since I bought it in '97 and luckily haven't experienced any audible oscillation problems. If it is happening supersonically it hasn't been enough to show on LED meters.  The voltage was measured at 275V just after recap in '97 and when last checked after a new pair of 1612s was dead on 275V.  Always had problems with meter zeroing properly with unknown 1612s. New 1612s allowed for proper zero but still overshoots slightly at warm up then drops to about 1/2db down. My unit is a later B version.

As for old carbon comps drifting one way or the other - I've measured too many NOS unused ones that were already out of spec too far to trust.