GATES RADIO tube Dual Limiter Compressor

Has anyone got a schematic for this limiter compressor. Looks to have 2 6SK7 tubes

http://cgi.ebay.com/GATES-RADIO-tube-Dual-Limiter-Compressor-vintage-audio_W0QQitemZ310156486952QQcmdZViewItemQQptZVintage_Electronics_R2?hash=item4836c46928&_trksid=p3286.c0.m14#ht_500wt_1182

yes, and it's a giant multi-fold blue print that definitely won't fit a scanner.  It's a cheaply made stereo BA-6A knock-off, for the most part.  Many basic value differences, but close to the same basic drawing in many ways. 

Hi!Thanks for your answer!
Do you know if there's attack and release time adjust?
Regards

fixed attack and release. 

Ok!Thanks!

This limiter uses 6K7 tubes (not 6SK7 like RCA) as the variable gain stage (in the early version). The voltage amp is a 12AX7 and the output is a 12BH7A. It can be used in linked stereo or separate dual channel configuration.

There were two versions that were produced. The early version was a true vari-mu limiter, the later version came out with an added PCB mounted under each amplifier circuit board that contained a transistor attenuator - otherwise they are identical.

GATES marketed this as a limiter for stereo FM stations or as a limiter for mono co-located AM/FM combo operations.

The one pictured in the auction link is an early version and that was actually my auction, I hated to sell it but I am thinning out my collection. I have the manual for both versions and the smaller sized schematics that will fit on a legal size page. The auction generated a lot of interest and by popular demand I am scanning the vari mu version manual to .pdf. Mail me at palmer at c l g w dot net and I will forward a copy.

Thank you Palmer and welcome to the forum!

Good to see you here Palmer!

Doug, I have the manual scanned and condensed the schematic to letter size. Do You need a copy? btw I have lost your email address in a recent computer crash.

I've been getting into tube projects recently and would love to take a crack at this eventually, email sent.  Maybe you should email a copy to the group diy gmail account.

For those who have the schematic

Why does this have such a fast attack time compared to a Sta Level which has a more powerful output/sidechain amp??
Is it because 6K7 requires less negative grid volts to achieve significant limiting??

I understand that the Sta Level's time constant R and C limit its attack time (56k resistor in CV path in particular) but can the Sta Level attack time be made faster or is it designed for the fastest attack that it can achieve?

The manual states that the attack time was chosen (1-1/2 to 2 Milliseconds) so that almost any pair of 6K7 tubes could be used to give thumpless operation. Manual further states that changing attack time makes the limiter less tolerant to differences in tube matching.

Sure the Stalevel could be faster.  For that tube, we're talking about the Collins 26U limiter.  Compare those two, and throw in the 26J for good measure. 

the Sta Level and the Dual Limiter were designed for different purposes when they were produced. The dual limiter is just that, a limiter, hard slope limiter for the prevention of over modulation of a broadcast transmitter. The Sta Level is a soft slope compressor, it was developed and marketed as an automatic gain control. The time constants were optimized for the intended use at the time. Each one is very easily modified.

Just finished servicing the second version of the Dual Limiter.

What changed in 1966 was the addition of a SS daughter board underneath, which comprises a variable U pad after an additional input transformer.  It is inserted between the input attenuator, and the original input transformer.  The original CV no longer goes to the 1st tube stage for gain reduction, but is rerouted to a SS buffer on the new board.  The original vari-mu stage is now just a fixed gain stage.  Early version has 3 transformers in signal path, later has 4.  Later version is really a SS variable U pad attenuator followed by a fixed gain tube amp, and not a variable mu tube compressor.  Based on the dates, I suspect most of the Dual Limiters out there are the later version. 

I neglected to mention previously I'd added variable attack and release to it which required a good bit of switch rewiring.  In stereo mode the right side attack and release control everything.  Some pics from 2011.







Serviced this one again last week.  The output tube sockets had gotten in such bad shape they would hardly hold the tubes, leading to some red plating in one from lack of grid connection. Replaced the sockets, also burnt cathode resistance on one side, went ahead and replaced all cathode resistances on both output sides as all were significantly out of spec. 

Discovered it would not align for gain reduction that matched in both dual mono and stereo.  It was down to the 2N1307 shunt transistors not matching for HFE, along with associated resistances having drifted significantly.  Germanium not matching, imagine that!  Surprisingly there was a relatively inexpensive source of NOS pieces, so I bought 10 to select 4.  3-4 were borderline DOA, the remainder just barely got me acceptable matches. 

R501 (39K) on the transistor daughter boards is the threshold setting, this is not documented anywhere.  This unit had been modified to differing thresholds in each channel, both lower than stock.  Going back to 39K pushes the threshold much closer to the headroom of the amp, so transients from slower attack times get distortion/limiting from hitting max output.  From listening 28K or so seems like a reasonable threshold for use with an attack modification such as I did, keeping transient overshoot further away from max level. 

While the iron looks small compared to a BA-6, it's all pretty good.  The 1:1 interstage measures 256H on each side at 120Hz with a handheld meter, which is about double the inductance found in the Sowter BA-6 repro transformer with the same meter.  With the SS daughter boards in there we have 4 transformers in the signal path, and response is just about +/-1dB 20-20K. 

I once noticed a unit being sold with sales copy stating 'improved with deluxe UTC A-18 interstage transformer'.  I think that was just all someone could find to do a replacement with after one died.  For one, the ratio is wrong, and assuming it was connected as a step-down like many limiters, would lead to a different threshold point. 

Comparing manuals and units is a bit confusing, as the BA-6 style Bal 1 / Bal 2 controls are no longer critical components since this one is a fixed gain tube amp with a SS shunt limiter front end.  The SS front end seems most particularly to have increased attack time to a point that called for critically matched 6K7's in the vari-mu version.  I imagine this was the #1 reason Gates made the change.  I added a few new docs in the Gates thread of the Technical Documents area.

The client (Strange Weather) liked the sound and did not feel a need to explore changing it to vari-mu operation.  I would agree, not having heard the vari-mu version either.  I have been told it sounds good too. 

I’m close to wrapping up another of these, this one had a rough life and in the end both output transformers were found to be in varying states of going open on one side of the primary. It worked but bandwidth was off with ugly breakthrough distortion.
The owner opted for Sowter Ampex 351 replacements, which fit ok and work well.

Both meters were bad too!

Someone had already done the cap job, then it got over their head and sold as ‘working’.

This time it was requested to convert the SS compression back to vari-mu, and that turned out to be much more complicated than envisioned. The selector switch wiring and layout changed at some point, and (yay!) this unit turned out to be a different schematic (or modified) from either I have, there were no meter shunt resistors at all and I had to add standoffs and wiring to put them back.

Having seen several of these now, my opinion is these come from one of the ugliest transitions of technology, PCB’s with a very large point to point wiring loom, taking advantage of the ability to shrink the footprint yet also making it very difficult to get at certain points and assess. Crusty old wiring looms are always stiff and challenging, add some corrosion and you can frequently wonder if you remove a wire connection, will it break, will it resolder or not, and if it breaks how long will it take to find where it goes. I’d shop very carefully and want to see one in person if I was really gonna buy one. Never in anyone’s wildest imagination would we be keeping these things going in 2022.

They do sound good, either gain reduction method, and without one of each side by side, I can’t say what if anything sounds different from memory, both have the same basic action once variable attack and release are added. Ironically the SS version with the additional transformer probably sounds more vintagy!

I lowered the threshold on this one to make headroom for the envelope mods, ratio will be slightly reduced but it’s a good tradeoff. Done by ear, was stock 33v bias, now 28v.

I found the manuals on the internet. I don’t know if these have been posted here before.

Heikki said:

I found the manuals on the internet. I don’t know if these have been posted here before.

Click to expand...

Should go in tech docs if not there already

Thanks for sharing Doug. Another interesting piece from Gates! These few years of hybrid tube/SS designs are always fascinating to me!