http://www.audioasylum.com/forums/vintage/messages/4477.html
This is an excellent article on the 1570's and modifications that have been reviewed by Joe Rosen in Oct of 2001
I am trying to get some ideas for re doing the bias circuit. I have some feelers out as I am trying to locate two of these amps with the 16492 output transformers in them. I had several of them about 15yrs ago and like a fool let them go.
Anyone have any ideas?
This is an excellent article on the 1570's and modifications that have been reviewed by Joe Rosen in Oct of 2001
I am trying to get some ideas for re doing the bias circuit. I have some feelers out as I am trying to locate two of these amps with the 16492 output transformers in them. I had several of them about 15yrs ago and like a fool let them go.
Anyone have any ideas?
I used to repair alot of these amps as they were used in many Movie Drive Ins. Almost all of them eventually experienced either power transformer failure or output tranny failure. Since the power supply is quite high voltage on them BE SURE YOU GROUND THE CHASSIS VERY WELL AND EXCERCISE EXTREME CAUTION and be sure you have the proper size fuse in place!!Almost all of these amps that were used in the Drive Ins ended up in the garbage dump. This Altec was a cheaper way to powering all the ramps at a Drive in... there was always a main amp and a backup amp and for good reasons. Better drsigned Drive Ins used multiple small tube amps and there was a single backup amp that could be switched into any ramp in case one went down. All in all they are a fire hazard in my opinion.
MArk
MArk
Silly as it may seem it is still some type of information and it seems it is all I can find on the 1570's. I would be more interested in your positive input and comments as a result of your vast experience. Anything you would care to share would be most appreciated.
Well I guess different people in different areas of the country do things quite differently. We never used 1570 for drive in theatre amps around here.
Its amazing to note that there are still a number of 1570's in service in large facilities like football stadiums and convention centers. Within the last several years the company I work for did a large new system and replaced 20 some 1570's that had given excellent service and were still functional and had lived their life without incident. I guess there is a lot to be said about the quality of any installation and maybe that was the problem with the drive in theatres in your area.
Heck, if you have some ideas on the 1570 please do share them as I'm sure others would be as interested as I am to hear them.
Hi burnedfingers,
I wouldn't give up on the idea. I haven't seen too many of these used in drive ins either. It's been a number of years since I last saw or worked on one.
I always understood them to be quite reliable, and the ones I rebuilt needed very little work to get them going again. Not really sure where Wavebourn is going with this one..
I have worked on at least a dozen of them over the last 15yrs or so, and I have not seen one with a bad power transformer or output transformer.
Unfortunately in this neck of woods they now command pretty princely sums of money. I have seen functional pairs of 1570B selling for well over $2K in the last year or so.
They sound pretty nice when not overtaxed on the low end..
I wouldn't give up on the idea. I haven't seen too many of these used in drive ins either. It's been a number of years since I last saw or worked on one.
I always understood them to be quite reliable, and the ones I rebuilt needed very little work to get them going again. Not really sure where Wavebourn is going with this one..
I have worked on at least a dozen of them over the last 15yrs or so, and I have not seen one with a bad power transformer or output transformer.
Unfortunately in this neck of woods they now command pretty princely sums of money. I have seen functional pairs of 1570B selling for well over $2K in the last year or so.
They sound pretty nice when not overtaxed on the low end..
strangely, 12th down, under power amplifier
http://www.altecpro.com/products/vintage/index.htm
1570B Power Amplifier Manual ZIP
scema-
http://images.google.com/images?svnum=100&hl=en&safe=off&q=altec+1570
154
http://www.altecpro.com/products/vintage/index.htm
1570B Power Amplifier Manual ZIP
scema-
http://images.google.com/images?svnum=100&hl=en&safe=off&q=altec+1570
154
Attachments
I wouldn't think the 15095 transformer was a degrading feature on the amp. Its very simple to bypass.
Pull the tranny and Jump 6 to 7 and jump 1-8 and it should be in business. Or if my memory is correct use the terminal strip and send hot on 2 and ground on 1
You could even change the tranny and use a 15335 and do a minor rewiring of the socket.
Pull the tranny and Jump 6 to 7 and jump 1-8 and it should be in business. Or if my memory is correct use the terminal strip and send hot on 2 and ground on 1
You could even change the tranny and use a 15335 and do a minor rewiring of the socket.
I had a pair of these and modified the h out of them. In the end the problem was in the driver choke, rolled off in the lowest frequencies. Below 40 Hz or so it gets a bit wooly.
To avoid the input transformer just connect directly to the 12AX7 grid and pull the transformer. I think there is a grid leak resistor to ground, if not just add a 100K or so.
I used 812's, 572-30's and 572-10's.
The best bang for the buck is to use 812's with about a 100 ohm resistor from the center tap of the filament winding to ground for some bias, and replace the 6W6 with 6L6's. You can just drop in the 6L6's with no other changes and get a better sound.
After that you really need to re-do the driver circuit. The easiest and probably best way would be to get Mike Lefevre or someone knowledgeable to wind better driver chokes for you.
To avoid the input transformer just connect directly to the 12AX7 grid and pull the transformer. I think there is a grid leak resistor to ground, if not just add a 100K or so.
I used 812's, 572-30's and 572-10's.
The best bang for the buck is to use 812's with about a 100 ohm resistor from the center tap of the filament winding to ground for some bias, and replace the 6W6 with 6L6's. You can just drop in the 6L6's with no other changes and get a better sound.
After that you really need to re-do the driver circuit. The easiest and probably best way would be to get Mike Lefevre or someone knowledgeable to wind better driver chokes for you.
I’m not really sure what you mean reinvent the bias? The 811’s are running with positive bias and there are only a handful of options for that. You can replace the center tapped choke with a constant current supply and you will have to bias the CCS to match the choke. You could also stand the drive cathode on a resister but you would need a negative supply then to get the bias to where it was. You could also use a pair of interstage transformers and bias the secondaries to where you want.
If you want to switch the output tube then there are other options. Magnaquest’s EXO-173 is a close replacement for Altec’s 17173 if you need a replacement. That would be the easiest I expect.
Matt
If you want to switch the output tube then there are other options. Magnaquest’s EXO-173 is a close replacement for Altec’s 17173 if you need a replacement. That would be the easiest I expect.
Matt
Matt,
I am ashamed to admit that I do not understand the engineering behind the Altec's bias setup. That is why I am seeking help with it.
I would like to replace the 6W6's with EL34's and individual bias controls. I would like to get rid of the 600 ohm resistor and L1 driver choke transformer. I have never seen anything like this and therefore I don't know how to change it. I don't have any experience with the 811.
Also, I am worried about the subsonic oscillation problems I have read about.
I am ashamed to admit that I do not understand the engineering behind the Altec's bias setup. That is why I am seeking help with it.
I would like to replace the 6W6's with EL34's and individual bias controls. I would like to get rid of the 600 ohm resistor and L1 driver choke transformer. I have never seen anything like this and therefore I don't know how to change it. I don't have any experience with the 811.
Also, I am worried about the subsonic oscillation problems I have read about.
If your 1570B is not broke I would sure enjoy it a bit before doing the changes you suggest. I don’t have any experience with the amps but I’ve heard they are nice. I think there was some mod info in an issue of Sound Practices which you can still order on ebay,
http://cgi.ebay.com/SOUND-PRACTICES...oryZ3284QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Digging around in that amp, like most amps, can be dangerous, especially with close to the 1570B’s 1KV on tap, so be careful. I’m not sure why you want to drop in an EL34 in place of the 6W6 as replacing 6W6s are inexpensive. Be aware that an EL34 will take more filament current, bias differently and might require the socket to be rewired.
Read around on A2 operation a bit, Steve Bench has a primer,
http://members.aol.com/sbench/a2part1.html
and you should be able to get a feel for why Altec used the cathode drive, cathode choke approach, and what you will need to pay attention to if you change tubes, and or bias schemes.
Matt
http://cgi.ebay.com/SOUND-PRACTICES...oryZ3284QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Digging around in that amp, like most amps, can be dangerous, especially with close to the 1570B’s 1KV on tap, so be careful. I’m not sure why you want to drop in an EL34 in place of the 6W6 as replacing 6W6s are inexpensive. Be aware that an EL34 will take more filament current, bias differently and might require the socket to be rewired.
Read around on A2 operation a bit, Steve Bench has a primer,
http://members.aol.com/sbench/a2part1.html
and you should be able to get a feel for why Altec used the cathode drive, cathode choke approach, and what you will need to pay attention to if you change tubes, and or bias schemes.
Matt
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