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
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
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
Matt
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.
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.
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.
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.
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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