H-Type Attenuator - Altec 1566A

Someone loaned me an Altec 1566A tube preamp that I've been using in my studio for a couple weeks. I've been unable to use this on loud sources, so I'd like to build an attenuator, maybe 20dB. I read the chapter in the Audio Cyclopedia and I think the H-Type will be fine, but as usual, I have a couple questions.

1. What input and output impedence would you recommend?

2. The AC mentioned that a "tap is placed at the exact electrical center of the shunt resistor for connection to ground?"
I plan to put this in a little box I have lying around, so does this mean to connect the physical center of the shunt resistor (not leads) to the chassis, and make a "star" with pin 1 of input and output?

I´m using two 680R resistors shunted with a 220R resistor for my mic inputs. It gives -12dB at 220Z.

Look here for calculating your own:

http://www.uneeda-audio.com/pads/

The Altec 1566A manual gives you ready-made values for U-pads for the input. PDF available here:
425kB PDF

Rick Chinn's site, as linked in Rafa's post, is an excellent resource for designing your own.

Don't use an H-pad, it's unnecessarily complicated for this application, and usually (though not always) found where source and load impedances are equal. For insertion between a low-ish source impedance and a high-ish load impedance, a U-pad is the way to go. Don't worry about creating a "centertap" in a balanced pad; it was never really necessary or desirable in most cases, anyway.

How do you like that preamp, anyway? I've got a broken one that's been sitting on "the pile" for years (as well as enough transformers to build at least two more channels of the same, if I'm so inclined). Fixing the 1566A has been hovering around the lower third of my "projects list" for about ten years.

Dave:

Thanks for the info !!!

I've only had the chance to use this on one source, which was a room mic on a guitar setup. I really did like it. It was very smooth in the mids, but it wasn't really that bright in the top end (which could be a good or bad thing depending on the situation). I tried to use it on a close mic for guitars, but it crapped out... that's why I'm going to make a pad. The guy who loaned it to me said he liked it on distorted guitars, so once I get the pad built I can try it on more sources, and have a better feel for preamp.

I'll post back here...

Yeah, the reason why I haven't been in a hurry to fix it up is because the schematic screams "distortion city" (especially if you try to run it into a low-Z load). But then again, sometimes distortion is what you want. And sometimes circuits sound better than you'd guess from the schematic :wink:

Oh, I almost forgot: Winston O'Boogie posted a nice switchable mic pad here:
41kB PDF

Eddie Ciletti recomends to bypass the first stage. http://www.tangible-technology.com/tubes/1566/1566.html
I have moved the switch behind R1,R2 and it works good for my. Most of the time I record with the first stage bypassed.

Yeah that eddie 1566A pre was my first go at a tube project, its a great little pre amp, & I too usually only use it with 1st stage bypassed.

The 1556 was also my reentry into the electronic diy world and I'm still very happy with that box. :grin:

Hello. I also have a 1566A that I refurbished, and for hot inputs I use an external 20db pad (not as good for s/n performance compared to bypassing the first triode section, but if that's a critical concern the unit wouldn't be a top choice anyhow). I find it a beefy sounding preamp without a lot of headroom, so the input level is quite critical. The high's are a bit rolled off, but in a pleasant way (I believe Scott Hampton tested out the input transformers and found the high frequency performance to be sub par). The main problem in the stock unit I have were: Like other old units using them, the selenium recitifer was pretty shot and only putting out 10v. Rebuilding the filament supply was essential. Also, although the box partitions off the power supply section separately from the signal components, the attenuator is in the front--using shielded cable to and from the pot helps keeps the 60 cycle hum down. You may also try punching a hole on the side panel to get the AC cable away from the rear section, where it sits right next to the cathode follower tube socket. The pre can work nicely sometimes on condensers with rather anemic lower midrange.

All the best,

James

One easy way to reduce input stage overload (at the cost of less gain) would be to remove C2, the 50uF cathode bypass cap on the first stage. Removing this cap will cause the plate resistance of that stage to increase, though, which will make it less capable of driving the fairly heavy load of the 100K pot... But I say give it a try.

If you like the performance with the cap out of circuit, you could make it switchable easily enough. Put a 15K resistor in series between the + side of the cap and the cathode. Add a small SPST toggle, and wire it up to short out the 15K resistor in "high gain" mode.

Hi Dave. I've been meaning to try the pre without the cathode bypass cap--I wanted to use it stock (in circuitry) for a while and just never got around to putting it on the bench again. I do mostly location work and the Altec doesn't get taken out often. I'll let you know how it goes when I get to it.

James