A Question for the vintage tube gurus...

[rascalseven]

A buddy of mine got a Gates "Speech Input Console" given to him, and I'm checking it out to see about racking the 5 mic amps, or just using the transformers (BIG transformers) for making tube preamps of a completely different design.

I know nothing about the tubes this thing uses: Each mic amp uses one each of a 6J5 (RCA, black metal tube) and a 6F5 (Tung-Sol, black metal tube) both mounted in octal sockets. The 6F5 looks like a spark plug-- it has a conection on the top that snaps on and is covered with a cap held on by friction.

Each channel also has a large input transformer and an identically sized/shaped output transformer that feeds the routing switches going to the various bussbars. These transformers are not marked with manufacturers and models, but they look like the HA-1xx series UTC's (and, in fact, the output transformer has the H-1 type round pins with identical markings... so I'm guessing they might be UTC's???). The input transformer has 10 pins in 2 rows of 3 and another row of 4.

Sorry to be so detailed, but I'm just wondering if anyone out there is familiar with this series/era of Gates console. I cannot find a model number or serial number, just "Speech Input Console" on the front panel, and he has no documentation whatsoever.

Can anyone shed light on this board or, more importantly, on the basic design of the preamps? Are there any resources that might discuss mic preamp designs using tubes of this type?

Anything will help.

Thanks so much, guys, and peace!

JC

Oh, yeah, there's also a rectangular plastic device that I'm guessing is a capacitor of some type as it was mounted in parallel with a 33.9k resistor. It is flat-ish, about 1.25" long by 5/8" wide, and says "Micamold, Brooklyn, NY, USA, Type 340" on it. It also has an arrow indicating polarity (??) as these are mounted with the arrow facing the same way on all five channels. Any ideas?? Thanks again!

 

[mcs]

[quote author="rascalseven"]Can anyone shed light on this board or, more importantly, on the basic design of the preamps?[/quote]
I can help you with the tubes. The 6J5 is half a 6SN7 (similar to 6C4/12AU7). The 6F5 is a high mu (gain) triode similar to half a 12AX7. The connection on top is the grid.

Best regards,

Mikkel C. Simonsen

 

[PRR]

> I know nothing about the tubes this thing uses: Each mic amp uses one each of a 6J5 (RCA, black metal tube) and a 6F5 (Tung-Sol, black metal tube)

I bet you have them backward.

6F5 is the prototype for each half of the 12AX7, our universal high-gain triode, and makes sense as an input tube.

6J5 is only superficially like 12AU7/6SN7: it tends to be more linear than modern tubes. It is a good choice to drive an output transformer.

There is little mystery about the circuit. Input tranny drives 6F5 grid-cap. There is 1K-2K cathode resistor, maybe bypassed. Plate resistor is 100K-470K; I would expect around 270K. 0.047uFd paper cap couples to a 470K-1Meg grid-leak resistor, and then off to the 6J5 grid-cap. 6J5 cathode bias is 150Ω-470Ω, and really should be bypassed, though if this is old enough they may not have trusted an electrolytic here. Plate is surely fed through output transformer primary.

Heaters are probably wired parallel, so you need 6.3V at 0.6A per channel (6.3V at 3A for five). And I bet they are well-twisted and routed away from grids (that's why the grid-caps!) so it can be fed raw AC just fine. Plate supply is probably 250V (150V-300V will work) and 10mA-15mA per channel (75mA for five). Plate supply probably has to be fairly clean, a 3-stage filter. Output power is 1mW very-clean, 50mW at rising distortion, rated +18dBm program peaks.

To check me: nearly all octals have Heater on pins 2 and 7, each side of the key in the octal center hole. Cathode is always between them, pin 8. Since these are triodes with grid-caps, the only thing left is the Plate on Pin 4. Oh, if metal tubes might be used, they would have grounded pin 1 (shell). And they may have used the NC pins 3, 5, 6 as tie-points for unrelated wiring (though there isn't that much wiring).

The only real circuit question: feedback. Is there a resistor (and maybe a cap) from 6J5 plate to 6F5 Cathode? (Could be that 33K and mica-cap.) Or (very slick) a mystery wire from the output transformer to 6F5 Cathode? If so, you have negative feedback and this is a tame usable amp.

And BTW: the tubes are probably good. They were well-made, never worked hard. Feedback masks any small mis-match or fading. In the very unlikely chance that one has died, replacements are cheap (though you may not get a Premium Brand).

> It is flat-ish, about 1.25" long by 5/8" wide, and says "Micamold, Brooklyn, NY, USA..."

Well, a mica capacitor, obviously. Doesn't it have six dots? The arrow points to the "outside", which should be connected to the less-sensitive side of the circuit. It will work either way, but is self-shielding if you have the arrow pointing the short path to ground.

The remaining question is: what is the output impedance? 600Ω is traditional, but you speak of bussbars which is a little unusual and might hint at something odd.

Do you have test gear? Controllable audio sine source and an AC voltmeter? Rig a couple resistors to knock your source down to about 10 milliVolts. Hang the voltmeter on the output. Fool with input connections until you get output, probably around 1V. Hang 600Ω on the output and measure how much it drops. If less than half or 6dB, then try sweeping the input frequency. Even though it says "speech", I bet it is quite flat over most of the audio range.

Post Pictures!

 

[rascalseven]

Thanks, PRR! Great stuff!

I drew the circuit while I was at his place. I'll compare it with your description (excellent description at that! :thumb: ) tomorrow. I've got some more soldering to do before I turn in.

I figured it was a mica cap ("micamold"), but the arrow threw me. I just bought some mica caps earlier today, and they look like small kidney beans.... nothing like that funky old micamold.

Unfortunately, the thing doesn't work. It doesn't power up at all. It's all there', but it hasn't functioned in a couple of decades, I'm sure. It's just a prop, currently, but I want to get a couple of preamps up and running.

I'll get pics of it next week when I'm back over there and post them.

Without knowing specifics about the transformers, what kind of gain do you think these will comfortably (i.e. sound nice) produce?

Thanks again, so much!

Peace,

JC

 

[PRR]

> It doesn't power up at all.

Small detail. Though you better be very careful!

Basic trouble shooting. Is the wall outlet live? (Hey, done it myself!)

First: power-up for 20 minutes. Do the tubes get warm? (These are metal tubes so you can't tell by the glow.) If they do, you have heater-power, you just have to find the lost DC.

If not: Carefully probe the power cord when it comes into the box. Fuse, switch?

What does the power supply look like? Will be power transformer, rectifier, caps, maybe chokes. If glass rectifier: does it light up?

Take it one step at a time from wall-outlet to preamps, being very careful around 120VAC and 250VDC areas. While you may ultimately have to replace several decaying parts, the "dead!" symptom is probably one failure, revealed when you find power "here" but not "over here" at the next step. (Remember it is AC coming off the transformer but DC after the rectifier.)

> what kind of gain

Fixed gain, and normally 40dB. Since it says "speech" it might be higher, and since it is only a 6J5 output they might have aimed lower, but I'm suspecting 40dB. And you probably can't change that more than a few dB: increase gain and it goes sloppy (higher THD and drooping response), decrease gain and it will go into supersonic oscillation. If you don't need a 5-channel fuzz-box, then dynamic mikes on loud sources and hot-condensers on most modern sources will need a pad in front.

However it sounds like you have a bunch of input windings. It may have connections for 50, 150 and 600 ohms, which gives you a little more flexibility in gain.

 

[NewYorkDave]

Judging by the tube complement and the fact that it has no model name or number, I'd say this is a pretty old Gates (early '50s or earlier).

It upsets me when nice old consoles get cannibalized for racking, but it might be particularly sad in the case of this console. I've seen many Gates consoles (mostly mid '50s and later) and I don't recall ever coming across one exactly like what you describe.

Ever considered selling it intact and using the money to buy or build what you really want?

By the way, "speech input console" was a term used for all broadcast audio mixers at least until the early '60s. I think the term originated with Western Electric. Despite the name, many such consoles were full bandwidth (in broadcast terms, good at least up to 15kHz).

 

[mcs]

[quote author="PRR"]6J5 is only superficially like 12AU7/6SN7: it tends to be more linear than modern tubes. It is a good choice to drive an output transformer. [/quote]
Are you sure about that? The 6J5 has excatly the same data as the 6SN7 in the tube manuals, and the structure looks identical in same-brand 6J5GTs and 6SN7GTs.

Best regards,

Mikkel C. Simonsen

 

[NewYorkDave]

For what it's worth, I emailed your description to a friend of mine who has a large collection of old Gates gear, and here's his response.

This is the olde olde version. Design is circa 1950. I don't have any details on it but I have an original Gates Diamote that has these preamps in it. Circa 1950.

 

[PRR]

> the structure looks identical in same-brand 6J5GTs and 6SN7GTs

Yes. Post-TV-boom, tooling and details were rationalized, and not IMHO for the best audio performance. The 1940-1950 tubes hold Mu at extreme bias point better than the late-1950s stuff.

If you really want linear: strap an early 6J7 as triode. Not a lot of gain or current, so not best for this amp, but killer driver for RC-coupled self-biased push-pull triode outputs where you need huge swing into a high-Z load.

But all quite moot here, or in most audio. The preamp is only swung about 2% of idle current, the output maybe 20%, and at such small swings even a modern AX7 or AU7 is pretty darn linear.

 

[mcs]

[quote author="PRR"]If you really want linear: strap an early 6J7 as triode.[/quote]
And then you have a 6C5 :grin:

At least the old metal 6C5s are triode strapped 6J7s.

Best regards,

Mikkel C. Simonsen

 

[rascalseven]

NY Dave,

It is a groovy looking console, and it is clearly very old, and actually in reasonable cosmetic conditioning if given a little TLC, but it has already been torn up some inside... many dangling wires no longer connected, though I cannot see any parts that are obviously missing. It appears to have all tubes, transformers, attenuators (unmatched), etc... (actually, now that I think about it there's a wattage meter in the center in place of the VU that was most likely there originally. The meter was missing when he got it, so he just dropped a meter he had laying around just for looks... it's not connected).

The thing is not mine, but a friend of mine's. I have no idea if he'd be willing to sell it. Probably so it the price is worth his time and effort to pack/ship/whatever. Do you have an interest or know of anyone who would? I'd be happy to hook you guys up and let you take it from there. Otherwise, he wants to rack some of the preamps.

Peace,

JC