Mysterious preamps (edit: Program amps)

[Conviction]

Hi everyone,

I stumbled across a pair of preamps today, virtually for free, but I have no idea of their origin. I know they've been used in Sweden (from the word "schema" & "till/från" on the faceplate), just maybe manufactured by Philips-NEFA in the late 50's (?). They could be Danish made as well, though.

All J/S transformers, EF86 (x2) in and EL84 in a PP-config out and a gain factor of 65dB (600Ω in/500Ω out).

Any fellow scandinavian groupdiy:ers recognize these? Who could've used them back then? Perhaps they come from a console, considering the rack-mount?

All the best.

 

[PRR]

EL84 is a heck of a preamp tube. Best-know as the "18 Watt" guitar amp. By analogy to the similar 6V6, I'd expect these to be Program Amps, the last stage in the console, intended to drive long-long lines to transmitter or network. Or depending on output windings, maybe speaker amplifiers.

 

[Conviction]

EL84 is a heck of a preamp tube. Best-know as the "18 Watt" guitar amp. By analogy to the similar 6V6, I'd expect these to be Program Amps, the last stage in the console, intended to drive long-long lines to transmitter or network. Or depending on output windings, maybe speaker amplifiers.

Hah! Indeed. I am familiar with EL84.  Thing is, I thought this was some kind of overkill circuit. I've seen pentode PP-stages in preamps before. Starved.

So. Program Amp.
Transmitter or network... Telephone maybe? That could tell something about it's origin.

Any use? Salvage for parts?

 

[gyraf]

Does look (also mechanically) like the amplifiers for (tube) EMT140 plate reverbs. Maybe second-amp to convert mono plates to stereo?

Jakob E.

 

[Conviction]

Score:

There's taps for 10/50/500 ohms on the output transformer.

The text in the top-right corner says something about "C-amplifier", then "preamplifier" (and a word I can't read).

 

[gyraf]

"televerket" probably means swedish national telephone company?

 

[Conviction]

Yep!

I know they, in the early days, had some broadcasting units (Radio) here and there. I've got a Lyrec AR/TR-2 marked Televerket as well.

Still, I don't quite know what to do with these. Anyone interested?

 

[Landins]

Hejja Sverige!

I may be interested in these for use as is och for parts in preamp-adventures. PSU looks nice, chokeloaded and all, and trafos have nice rating. MU-cans is also great.

Ill PM you so we can talk native.

 

[emrr]

Looks like a very nice playback amp for listening.  I'd keep it that way, you don't find many fully push-pull amp circuits, and every one I've heard has been a fantastic amp. 

We have screen taps on the output primary, so more like a Williamson output stage? 

 

[PRR]

The "quasi 18W" output stage is worked at 232V and lower current so will deliver maybe 6 Watts before clipping.

Gain is set by 75k:100 dividers from 2nd stage plates to 1st stage cathodes. Forward gain is maybe 200*15 or 3,000, NFB reduces this to 750, just 4:1 or 12dB NFB. If the EL84s distort 5% just below clipping, NFB reduces that to 1.25%. Since most studio work needs to be cleaner, we suspect that nominal maximum peak output is much lower, perhaps 1W or +30dBm. (That was the rating on my 2*6V6 program amps.)

Overall gain is 1:10 in the input transformer, 750 in the amp, and 5:1 in the output transformer, 1:1,500 or 63.5dB, similar to a "64dB" number on the sheet. The OT may be 4:1 or 4.5:1 instead of 5:1 as I guessed.

It "could" be a mike preamp. It has a ton of gain and a high step-up on the input transformer. OTOH with uncontrolled input levels it is likely to make HUGE output levels. Because only 2 stages wrapped in NFB, there's no good/simple way to control gain. A pot at the 1st grids throws-away signal/noise ratio. A pad at the output can turn-down "18 Watts" of distortion so you don't smoke the phone-line or tape-recorder, but it will still be distorted.

I don't see the 10 ohm tap (ah, it is not brought out to the socket). If you got that, then it is loudspeaker amp. Damping-factor is not fashionably low (less than 4) but that can be fine. Gain is outrageous, you want 37dB pad in front. That is 70:1. Bridge 470 ohms across the "500 Ohm" input, put two 17K (15K) series resistors to your source.

> probably means swedish national telephone company

Did Swedish Telephone do a lot of long-line audio? The English BBC worked with Royal Post (I think), and the US Bell Telephone System made MOST of their money 1930s-1940s by carrying radio network signals. So high-quality program amps were standard gear in telephone systems (even if the dial-up network got only low-fidelity gear). And that also makes sense of the very high gain. There would be a line equalizer in front, with adjustable loss. You'd go along the long line until 1KC was down perhaps 25dB (trying to use as few amps as possible). At this point 10KC might be down another 25dB, so 50dB loss at 10KC. A resonant EQ would pass 10KC with 0dB loss but put 1KC 25dB down. Result flat to 10KC but 50dB down overall. The 64dB amp would probably have a 6dB pad on the output, 58dB gain. So dial-in another 8dB broad-band loss, the long line is "perfect". Once set-up, the levels are very stable, no wild signals. Working at nominal 8dBm line level, peaks <+20dBm or 0.1 watts, plus 6dB pad is 0.4W from the amplifier, distortion per amplifier is very low and distortion of a long-long cascade of lines and amplifiers is acceptably low.

And it looks like telephone gear. Very plain.

 

[emrr]

Yeah, OK, program amp.  I didn't note the 10 ohm wasn't brought out.  So, call it roughly 6W output, we're around +36 (I'm not gonna look it up - this is similar to the Altec 459A program amp headroom), pad that down 18 dB to typical modern converter input level, we have 46dB gain.  Sounds like a mic amp.  Pad the input -20 if it overloads there, we could be as low as 26 dB total.  I say if you gotta pad both, you are doing something really loud anyway, and that's the sort of gain figure I see in use at my place.  Maybe you don't need the full output pad; use a variable T.  You might only use the input pad for 44 dB total.  It's a preamp as-is, then, if you learn to drive it right.  Put a standard 20dB input pad in an XLR barrel, and a variable T pad with output shunt load in a little box, use either/both when needed.  It's in great shape, don't drill any new holes in it because the collectors will hate you when you sell it, and devalue it accordingly. 

 

[gyraf]

Great analysis - thanks!

 

[radardoug]

It may have been used as a line repeater, in which case it would have a passive line equaliser in front of it to equalise the preceeding phone line. Hence the requirement for huge gain.

 

[ssp3]

Still, I don't quite know what to do with these. Anyone interested?

What a beauties in such lovely condition!

Use them as line amps to give you that specific sound, or, if it's not your cup of tea, put them on evilbay - the Japanese collectors will go crazy over them, trust me.
P.S. Just make sure they remain in original condition. No mods!

 

[Conviction]

So, these are for sale/trade/whatnot. I got too much junk just lying around as it is.

My eBay account, being non-American, requires a second validation if the starting price exceeds so and so much.  :'(
No evilbay, then.

 

[Conviction]

sent you a message.....

PM + e-mail sent!