| burnedfingers |
http://www.audioasylum.com/forums/v...ges/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? |
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| Wavebourn |
| Silly article (sorry Joe) describes some good ideas found in the article he criticizes. |
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| Mark A. Gulbrandsen |
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 |
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| burnedfingers |
| quote: | | Silly article (sorry Joe) describes some good ideas found in the article he criticizes. |
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.
| quote: | | 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 designed 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. |
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. |
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| burnedfingers |
| Seems to be a lack of interest in this animal. Do I give up the idea? |
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| kevinkr |
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.. |
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| burnedfingers |
Kevin,
Thanks for the reply. Do you have any ideas for the bias section? |
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| kevinkr |
My service information is buried beyond quick rediscovery, any chance you could post a schematic or link to one?
IIRC The main issue with these amps was the input transformer and how to get around it.. :D |
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| Wavebourn |
| quote: | Originally posted by burnedfingers
[B]
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.
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I said already, he criticized good ideas, and thanks to his comments they are well documented. :cool:
The amp was designed great with that parts, however later more parts became available so I believe original designers would use them with a pleasure. |
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| burnedfingers |
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. |
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| dshortt9 |
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. |
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| Wavebourn |
| Go for FETs. Ask SY, he knows how. |
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| burnedfingers |
| quote: | | 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. |
Heck, I thought the problem was the small output transformer.
Now I'm really confused:bigeyes:
Any way to drive this baby without the driver transformer? |
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| burnedfingers |
SY
Do you have any ideas to re-invent the bias of this animal? |
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| burnedfingers |
SY
Do you have any ideas to re-invent the bias of this animal? |
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| mwiebe |
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 |
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| burnedfingers |
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. |
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| mwiebe |
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...1QQcmdZViewItem
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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| Dave Cigna |
First of all, I agree with mwiebe. If it were mine I would get it working and listen to it as designed before I started making wholesale modifications. From there I might make some mods, in stages where ever possible. Study what others have done, especially those with experience with this amp. Take what you want from each, but make sure you understand their reasons for any change before you try it.
Anyway, I'm not sure why you want to get rid of the center tapped choke loading the driver. In my opinion it's one of the better and more elegant parts of the design. Some here have reported that the Peerless 17173 is a weak spot for the low bass. It's not air gapped or tiny, so it should have more than enough inductance. I wonder if those people that have found it to be lacking had the driver stage well balanced. A little bit of DC current imbalance will cause the choke to saturate early, which would be noticed mostly at low frequencies.
I do know that Mike Lefevre of Magnequest has in the past sold 'reissues' of the 17173 (he owns the Peerless name and engineering documents.) I also know that the Magnequest EXO-173 is based on the Peerless 17173. I'm not sure what changes Mike made and I shouldn't speculate (it makes him mad) but I suspect that one of the differences is increased interleaving between the two sides. That alone will not get you much more inductance - a little in this PP situation, but not much. In fact, informal reports are that less interleaving in a PP choke sounds better, possibly because of the lower interwinding capacitances. In any case, he does offer the choke with different cores. I wouldn't suggest an all nickel core, but the pinstriped version would probably have more inductance than the straight m6 version. Go over to the Magnequest forum and ask Mike yourself for better advice on that. You might want to do a search on '17173' and/or EXO-173 before asking any questions that have already been answered.
As for the bias arrangement, I suppose you might want something adjustable, but to be completely honest I have more faith in the Altec engineers knowing what they were up to than I do in Joe Rosen being able to figure it out. They weren't complete idiots; they built some very good equipment and there might be something more subtle going on with that 600 ohm resistor being used like that than a simple cost cutting trick. Again, I'd get it working as designed first, then see if your own ideas work better or not.
As for swapping out the 6W6's for EL34's. Okay, I suppose. I'd try 6L6's first if I wanted more beef (as already said, it should be a plug-in replacement and matched pairs are cheap enough to try) but others have gotten good results with EL34's so who am I to argue. All of this follows my less (change) is more (likely to work better) philosophy on this.
-- Dave |
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| burnedfingers |
Thank you for your input and suggestions.
Do you have any ideas concerning the input capacitors that can be shorted out or left in? |
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| mwiebe |
As always with input caps, it depends on if what is upstream is outputting a DC offset. If you plan to drive this amp with a preamp that is cap coupled or has an output transformer to its output then, no, there is no reason for input caps. If you are unsure you can always measure for DC offset. Input caps are just there to make sure the first grid is sitting at the voltage dictated by the amp's design. It will not hurt to leave them in until you figure things out a bit.
Matt |
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| dshortt9 |
| I had a pair of these and modified them quite a bit. I changed the rectifiers to solid state, used 812's 572-30's, 572-10's, changed the input tube to a 5687, changed the output tranny to $440 custom made ones, changed the 6sn7 values, replaced the 6w6 with 5881's and finally realized that the weak point wth these amps is the mystical swinging choke used with the 6w6's to drive the outputs. SO after much ado here is the best thing to do. Make sure that you bypass the 70 Hz filter in the input circuit and connect the input directly to the 12AX7 grid. Use 812's with an appropriate change in the cathode bias resistor, I'll come up with a value if you are interested. The 812's are more linear than the 811's, have a lower plate resistance for better dampening. That's all you can do to improve the sound to a value point. The output transformers are actually pretty good in the bass if you do not expect the full 165 watts from them. Great mids and highs but slightly mushy deep bass due to the swinging choke. I used a differential 5687 with 1% components throughout and matched sections in the tubes so balance was good in the choke. Then experiment with a way to eliminate the swinging choke drive, probably by substituting a Mosfet in the circuit as suggested earlier. I would use it from the 6SN7 and eliminate the 6W6 circuit. More if interested. Check Tubelab for more info. http://tubelab.com/index.html Vacuum Tube Valley did a article regarding the Altec 1570B. Check the site for the specific issue. |
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| burnedfingers |
dshortt9
It sounds like you did a lot of modifications. Was it worth it all? |
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| burnedfingers |
I believe that I will try the amplifiers as they are to start with. It would be nice if someone had a copy of the Sound Practices that contained the information on the mods would be kind enough to send a copy. I would gladly cover any cost.
As mentioned in a prior post the amplifier is subject to subsonic disturbances such as record warp. I believe I may be able to use a 24db per octave filter prior to the signal reaching the Altec amplifier and thus eliminate any problem.
So far I am totally unable to make any decision on this amplifier. Some feel the driver choke is the problem and others the output transformer. Others yet feel the driver tubes need to be replaced with something else.
Granted this is all very good information. Unfortunately no one has probably graphed any frequency response changes for before and after. I am very gratefull for any and all information on this animal. Please if you have more ideas I am more than willing to listen and learn. |
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| dshortt9 |
What do you plan on driving with these? The 24db filter is not needed as low frequency rumble will not make it through the amps as they are. When I have a chance I'll look up the article.
Was it worth it - yes for the experience. Not for the final product. I would have made a more conventional Williamson Ultralinear, or a 300B PP. It is hard to beat 300B's for sound even in Push Pull. I have been a diehard Tube fan but my last project was a Krell Clone and it is really good. |
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