711B Biamplifiers

[skeggordon]

Hi everyone,

I'm new on the user's board. Here cause I like Altec electronics, have a bunch of green SS amps, plus a couple of 3124's.

Just picked up some 711B's. Can someone please point me in the direction of a user's manual with a schematic? Also, any popular mod's that I should know about?

Thanx for your help!


JG

[bfish]

Do you mean 771s?

If so, there's no online info I'm aware of.

I don't know of any mods either, other than trading them for 1224s, which seem to be more reliable. If yours are functional, I'd seriously consider replacing all the electrolytic caps. For pro service, IraTech is the guy.

[skeggordon]

Thanks bfish for catching my mental typo. Was wondering mindlessly why Altec would give a receiver and amp the same model number.

The google search was more productive with the right number. So some sources for manuals turned up. I'm an electrical engineer with some amp design experience, so will be looking into a fix for the reliability issue. Luckily the units look quite easy to work on.

Yours, JG

[bfish]

Cool. I forgot the 'hardcopy for sale' sites, but it looks like you found them.

I don't know the particulars of the reliability thing, having never been into one, just know you don't see too many that still function. Sounds like yours are in good hands. Once you've become familiar with them, I'm sure passing on anything you learn here would be appreciated.

[skeggordon]

>I'm sure passing on anything you learn here would be appreciated.

Will do. Worse comes to worse I'll wire in some LM3886's and not talk about it.

Thanx again, JG

[skeggordon]

The spec sheets were interesting, thanx Todd. Ordered a set of original 771B manuals from someone who's unloading his collection. Its always nice to have the originals. Will scan and send them your way.

Many times amplifier protection circuitry affects the sound quality of the amp, yet another trade off. Professional users generally prefer reliability over a slight gain in sound quality, so Altec was going in the right direction for their market.

Yours, Jay Gordon

[Todd W. White]

Still working on getting the GPA site's Vintage Altec Literature Library catalogued, but here's some info that will help:

1219A System Spec Sheets - has info on the 711B

Here's the literature for the 771B and 771X, which were similar to the 711A -

771B/BX Biamplifier Spec Sheet

NOTE: The 711A was a MUCH better sounding amp, as I recall, than the 711B. BUT - it was also prone to self destruct. Altec fixed that in the B, but it never sounded quite as good.

I'm pretty sure I have the 711A schematics, etc., but haven't scanned them in yet. When I get to that part of the list for Bill's website, I'll find it and put it in. If you need it sooner, email me directly.

<font color="#FFFFFF" size="1">[ June 24, 2008 12:07 PM: Message edited by: Todd W. White ]</font>

[skeggordon]

Have had a look at the ~771B~ biamp schematics and they are pretty straightforward. The amp design is a couple of generations back but should work. Question is that I've heard of reliability problems that I'd like to address. Does any one know if there was a typical failure mode that happened regularly? One amp had a shorted axial power supply diode. Is this typical? That would be easy to upgrade.

Thanx, JG

[Old Guy]

The only ones I remember are the "A" models, they tended to fail pretty catastrophically.

The outputs would fry and take out the supply. They were reasonably cheap and easy to fix- but just not heavy duty enough for any pro use. Kinda like Fiats in Italy- they break down a lot, but it's a very popular car so parts are off the shelf and cheap- but you'll be working on it again next week.

[Todd W. White]

Old Guy is right - they direct coupled the output transistors to the speaker line with no protection at all. As soon as you shorted it or, even sometimes overdrove it, the thing blew up.

Sounded GREAT, but they blew up a lot.

The "B" didn't do that, but didn't sound as good.

Compromises, compromises...everything's always a compromise...