RCA OP6 Clone

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Well, I guess my question is what choke was being used here.
Maybe it is a custom job.
But MatthisD posted that it was a Hammond.
 
Output transformer is 48K:500, measures 20 dB loss in circuit, with a 1400 ohm DCR primary.    Official OP-6 output rating is +18dBm with less than 1% distortion 50-15K.  Only thing out there with that ratio and level handling currently is the Fairchild signal amp output, and it's meant for PP use and has much lower DCR. 

Parallel feed choke is 200H, 4000 ohms cold.

The attenuator is unobtainium as an off the shelf part, but not impossible to build, you just have to pony up the $$$$ for an OP-6 to have your hands on one for reversing.  It's a custom Daven stepped ladder with feedback tap.  I have all the basic measurements but haven't found the time to take it any further.  It's 2dB/step mostly, with a 3dB and a few 5dB towards the bottom.  There's an extra shorting tap at the 0 (off) position.  There's not really any other tube preamp I know of that uses anything like it. 

90 dB gain quoted in the OP-6.  Generally not a quiet place to go with 3 pentodes.  Most feedback (noise reduction) at lowest gain, least at highest. 

I've been curious why Ben-Hur never brought this one to us, like he did all his other builds. 
 
MatthisD said:
The preservation sound clone uses a Hammond 156C which is explained by the builder.

emrr said:
Parallel feed choke is 200H, 4000 ohms cold.

It's very interesting one can use this relative inexpensive hammond model as a plate choke. I definitely want to experiment with it, see if it actually does full range audio, or has a "vintage" low and high pass roll off. But it does limit the current that can be delivered by the output stage quite a bit.
 
I did some calcs on stepped attenuators the other day and suddenly realized that if you short out one resistor in the pot divider the loss you get is between the two values you got with the switch at either end of the resistor you just shorted out. This means if they used a shorting switch the could get twice the number of steps simply by placing an ident half way between each resistor position. So a 25 step pot becomes a 50 step one.

Cheers

Ian
 
13engrsapper,

Are you saying that you have an actual OP6 attenuator and it is a Daven?  Any other details?

 
I'm about to rebuild one for a guy, or make a new one if the original is hopeless. 
 
Oh, excellent!  So we'll finally get to the bottom of this (assuming you'll share the details, of course).    :eek:

I have been lusting after one of these for quite some time.  Even lost a few bids on ebay.  (I quickly begin losing interest when they pass the $1500 threshold).

I wonder, in the grand scheme of things, how much this attenuator lends to the sound of an OP6.  How much different might the 'preservation sound' hybrid actually sound?
 
The sound is mostly in the iron, and the legend is vastly overblown.  It doesn't particularly sound any better or radically different than the rest of the RCA line.  The attenuator is unique and interesting, but no magic bullet in my eyes.  I'd rather use a BA-2 triode amp than an OP-6, but the OP-6 has always sold the highest because of:

1) self contained package.
2) gain control.
3) big VU meter to fetishize.  (if they bought that option, it was an option.  Hey!  Those without sell for less.)

The BA-2 was always second highest because it meets condition 2 completely, and condition 1 mostly.  The rest generally lack PSU and gain control, but really sound every bit as good if you learn how to drive them. 

90 dB is kind of ridiculous, and out of step with today's needs, but people seem to think super high gain is a mark of extra testosterone or something.  The pentode front end with a gob of negative feedback is not that which most people want out of a tube.  People generally want the open loop triode effect along with gain control, and the BA-2 does it about as well as anything I've ever heard. 

Everything else being equal, the 'Preservation Sound' hybrid probably sounds pretty close to the bren-hur clone.  It'll have the benefit of lower first stage gain, better noise profile, and more forgiving front end clipping. 
 
I really want to try this Hammond 156C as an output stage plate choke. It's only 11 euros! Would be a great marriage to an Edcor WMS 10K/600 or something cheap like that. Just gotta figure out where to use it. I don't need another preamp.
 
emrr said:
...
Tell me about it.  It's hard to stop.
Yeah once you get into the habit to record compressor-less, a pre is the "one-and-only" mojo-box. But blasphemy, I'm in the solid-state camp myself... go figure. I must be cheap.
 
There's certainly no avoiding "mojo" when using two cheap irons in an output stage. Think I'll use some CCCP octal triode. Maybe I'll end up with an active tube EQ.
 
I'd expect the original iron to sound cleaner, the teardowns we have suggest high nickel as the norm in RCA outputs.  Not many (any?) outputs made that way today.  New iron may have broader response but worse distortion profile.
 
EMRR,

My interest in the RCA preamps began when I found a 44BX for sale in cuba for next to nothing.  Operational, but severely lacking in output, I attempted to use all the pres at my disposal with less than stellar results.  I picked up a 77 for way less than market value and obviously had the same issue.  I couldn't understand the hype behind this ribbon thing if the gain required to get them to usable level caused so much noise.   

Picked up a few new pre's - and bought a few kits which are now in various stages of completion.  (I keep buying DIY and then overestimating my abilities and the time it takes to complete them - I've got a pile that I haven't even started just because I have told myself that I need to finish A, B & C first - thankfully, this has resulted in the completion of an Shadow Audio LA-3A pair and an Mnats 1176).  Soon to be finished - a quad set of 312's, although I'm not sure there's enough steam in them to amplify ribbons.

Anyway, I started to wonder:  if my results are this bad using my mic pres, how on earth did they get these things anywhere near line level with 1950's technology??

I thought I'd look in the RCA toolbox to see what was available.  The OP-6 stood out, more for my assumption that the high gain would be required than for the reasons you outlined.  Although, you hit one thing on the head, all testosterone and fetishism aside, the OP6 does have some kinda peculiar sex appeal, for sure.
 
Historically, very few recordings would have been made with an OP-6, since it's meant to be a lightweight portable unit for broadcast.  Possibly the odd radio music remote from various dinner/dance ballrooms.  Most things we hear made with RCA amp are done with either inclusive console amps or the modular plug-in types, all of which are 30-40 dB types.  In a console, you have multiple amps in series, with attenuators and bus losses between each. 

I had a singer on a record project last year who stood 3 feet back from a ribbon that fed a BA-2, which, wide open, consistently delivered peaks approaching +25, which means the ribbon was putting out -25 given her delivery level.  I frequently have a ribbon on guitar amp or as drum room pickup, and have to use a 20 dB pad in front of a 40 dB amp; I get sufficient level to the recorder with 20 dB of net gain.  If I put a ribbon pair 10 feet back from a drum kit, and feed a 75 dB Gates amp, I typically have the volume controls set around -30 to avoid peaking the recorder.

A Shure SM-7 puts out very similar levels.   

So, I'd ask if the 44 and 77 have been serviced since you got them.  They can work, but not be up to snuff, the ribbon material can get brittle with age, oxidation at the ribbon clamps can increase noise, etc etc etc.  They will be lower output than many modern ribbons, but not unusable.

All the ribbons I have sound more correct feeding transformer coupled preamps with unloaded secondaries, they sound lesser feeding anything modern transformerless that I've tried.  I have not tried the new breed of ribbon preamps like the AEA, etc. 

If I were going DIY for ribbons, I doubt I'd ever plan anything with more than 65 dB gain, I'm not wild about the idea of excessive throw-away gain, and in all the high gain vintage tube amps I've tried, the noise floor above 70 dB setting generally renders the amp useless for critical work.  The amps that have that sort of gain are almost always remote amps for voice work sent over noisy telephone lines, like interviews from the new hardware store, etc, situations were the gain is priority, and the noise really isn't. 

Lassoharp did a BA-2 mashup with an additional tube for higher gain, maybe his schematic is still posted here somewhere.  Lots of good options like that for DIY that are much easier to implement than an OP-6.  If one really needs the OP-6 thing, one probably really needs a real OP-6, since the iron contribution is a huge part of the picture. 
 
vineyardgray said:
I thought I'd look in the RCA toolbox to see what was available.  The OP-6 stood out, more for my assumption that the high gain would be required than for the reasons you outlined.  Although, you hit one thing on the head, all testosterone and fetishism aside, the OP6 does have some kinda peculiar sex appeal, for sure.

You need far better power supplies for 90dB of gain than what's generally usually used for these olden days designs. No point in raw gain if the PSU doesn't deliver perfect DC. It's only going to amplify that PSU noise further. So don't worry about any of those lost ebay bids. No point having hissy ribbons with additional PSU hum. Wrong tool, wrong task, failed recording.

If you still absolutely must have 90dB of transformer and tube gain, you could take a look at my Drive-1 design. There's exactly 89,6dB available and a somewhat perfect PSU to go with it. But it's not a classic brand name design so obviously no sex appeal to lure clients.
 
I'd definitely borrow or rent an OP-6 if interested, and I hadn't put one through it's paces first.  The high price tag generally gets you a unit that still needs a full restoration before you can even try it. 

My favorite RCA manual moment is on a scan of the OP-6 manual I saw, where it quotes noise level at different gain settings, and someone in pencil did some math around the max gain noise claim, then wrote the word "impossible" next to the result.  The math looks wrong in one assumption, but it's still pretty funny. 

Oh yeah, here's a goofy looking thing I saw in an ebay auction last summer.  This was a supposed 'Sank Mod' to an RCA industrial line preamp to make it 'like an OP-6'.  I haven't played with it, and I can't vouch for the origin of the idea.  It loads V1 and V2 very differently than the real deal, but maybe it's the almost holy grail......

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