Altec 1570B project

[Deleted User]

Hello,
I've finally finished the second amp, I checked out the frequency response, distortion and gain yesterday. Everything seemed fine, connected it to the system this afternoon. So far the sound is excellent, first time I've heard directly heated triode output stages.
Enjoy, Ken

george daniel

I love the looks,, how do they sound:)

[Deleted User]

Thanks, George,
I still have some work to do on the overall appearance. I have one set of brass hexagonal feet that attach to the bottom. I have to save up, a bit, to buy the second set. Then I have to come up with a front plate, something transparent to let the tube glow come through. Those 811A do glow!
The sound is everything I was hoping for and more. I know it's hard to be objective when you've built something from the ground up. But, I also know what sound I like and won't blow smoke up my own skirt. At first I wasn't 100% sure, it sounded a little too "solid state" for my taste. I was listening to a blues CD from Mapleshade and it was very "clean". Then one of the cuts had a soft piano accompaniment and I was hooked. It was exactly like a piano sounds, the tonality was on the money. It sounded like a real piano.
Then things just got better, I played the Tom Petty MoJo and was captured. Previously the big tube amplifiers I've had experience with were the ARC D150 and a great sounding tube amp from Audionics. They had a way of controlling a speaker, making the speaker respond effortlessly. When a musical passage got complex and dynamic they could hold everything together and allow the speaker to simply follow the changing dynamic.
The Altec is doing the same thing but with less tube coloration. I can always tell when things are sounding good, I start playing "air drums" and have a way of making a "did you just hear that!" kind of face. That's what I'm doing now, really digging the music.
And this is just with CDs (no offense intended digital guys), I've got to get some air compressed for the turntable. Big clean, musical tube amps with fast transient response and no coloration and wide dynamic range!
Audio bliss in B-more!

george daniel

Thanks for the response,,I know a couple of folks who owned these,and they will probabally own them for a very long time,,I would appreciate your impression with vinyl or tape.Amazing how gear that was made for stadiums,greyhound terminals are finding there way into home audio setups,and it's what,,190 watts of all tube power:)

madmax

Wow, that inside shot of the chassis on post #15 shows pure art and craftsmanship! Wow!

[Deleted User]

Thanks for the compliment! My wife says I'm a "sound artist" and that approach lets her overlook all the audio gear that's strewn across her living room and dining room. We had talked about moving the sound system upstairs into what is the "TV room". Putting the TV in the bedroom would free up that room. But it's 12' x 12' a problem with resonant nodes. Plus she said, "Then we wouldn't be able to hear the speakers downstairs!" Boy, am I a lucky guy! So, we're going to try and move the records and CDs out of the dining room and recoup some space for her.
I really feel bad about taking up so much space with audio stuff.

[Deleted User]

Solved a really weird problem. Everything was going along smoothly, loving the amps. But, one of them (the most recently finished one) started acting funny. The DC bias is supposed to be around 4.0/4.2VDC across a 22 Ohm resistor. You adjust the balance pot until both sides are the same then adjust the bias pot to read 4.0 VDC. I get everything set and after a period of time I check the one side and it's 19.0VDC and the other side is -2.2VDC. Okay, no panic unhook the amp bring it down to the test bench. I look carefully at one of the GE811A and there's a crack down the seam of the plate, about .25" long. Okay, order a matched quartet of Taylor select 811A from California. Get them on Wednesday and put them in, set the bias and everything looks good. This is with the amp sitting directly on my test bench and an old towel as protection against scratches. Then this morning I lift up the amp and prop it up with a wood block so I can measure the voltages. After making a few measurements I check the bias and "ugh" they are way off again. One side high and the other side low.
Put the amp back down, turn it on and now things are okay. Prop it up and back to wrong. I think, "must be a cold sold joint that I missed and some wires must be moving when the thing's down flat". I get out the magnifying headset and go over everything. I re-solder some joints but nothing's looking suspicious. I turn on the amp and the bias is off, I start pushing on wires, nothing changes. I push on the .22 V-Cap that feeds the EL34 and ta da, push on the body of the cap and the bias goes to 3.7VDC let up and 10.0VDC. It's a leaky cap that swamps the bias voltage.
Time to contact V-Cap....
Ken

[Deleted User]

My apologies to V-Cap, it turned out that wasn't the problem at all. I removed the V-Cap and put in an old Wonder Cap and the problem remained. Push down on the cap and everything's fine, release the cap and a problem. The board where the two interstage caps are have a mounting screw in the middle of where the two caps are. By pushing down on one cap the other cap is lifted off very slightly. This led me to compare resistances between the two "sides" of the circuit. I kept on measuring until I found where something changed when the board was pulled away from the chassis. What was happening was that the spacers I had used to mount the board were not "tall" enough to prevent the underside of the mounting turret to touch a chassis screw holding the phase splitter inductor. I put a piece of Teflon sheet that I keep around under the bottom of the turret and the screw and all's well.
Now I can re-solder the V-Cap back in place.
Ken

george daniel

Ken,, the purpose of the EL 34 in your amps is/are,, ? I read it but cannot remember,,something in the power supply?

[Deleted User]

Hey George,
They are used in triode configuration as a driver stage before the 811A output tubes. The amp is a Williamson type design with 811A tacked on for power.
Ken

[Deleted User]

Recovery room, operation successful.

george daniel

What is the value of that big resistor--I'm assuming it is the :load" while you are working with the amps.

[Deleted User]

Eight ohms, 250 Watt.

Face

George beat me to it, that thing is a beast!

megasat16

Digging it back from the yesteryears! I am now in possession of a pair of 1570b that need the love and magic that Mr. Swauger performed. Awesome job and inspirational thread, Ken!

I will start with complete stripping of the chassis and powder coated everything (I prefer factory green color though). More to come as things progress from here and out.

[Deleted User]

Congratulations on your new project! Good luck with everything! The Altec's are an amazing sounding amplifier.