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PostPosted: 08 Jul 2009, 21:34 
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Right, so maybe I won’t bother exchanging the 220uF then, but may try anyway if I can get at it easily. Although sound is hard to judge with the single channel, there is a definite lacking in bottom end bass and probably not much (if anything) below 150Hz. The same speaker on an SS amp has a much lower reach, although I realise that is one of the main drawbacks with tube stuff anyway. I was thinking that replacing this cap may be the answer to better bass response, not just because of the characteristics, but also because of inefficiencies caused by the higher than necessary voltage rating (100v). I saw some NOS 25v 300uF Spragues locally which I was considering as another option. Am i right in thinking that a slight increase in capacitance here could boost the bass a little? Sounds like it might not be worth tampering with though.

Are there any other factors that affect the bass? I notice that there is a fuller range after half an hour or so warming up. Is there also a more general break-in time? The tubes are Gold Lions/NOS 12SL7 btw.

Regarding the coupling cap, I’m using is an Obliggato Gold Premium (film) cap from HK. It looks nice, but any improvements over the previous PP are impossible to pick out with all the other changes that have gone in too. Cost about $7 so hopefully it is an improvement!


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PostPosted: 10 Jul 2009, 10:42 
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Hi Bruce, thanks for the reply.

@cannst77 - the only cap that should affect the low end response would be the 0.33uF coupling cap. As long as you use that value or larger you should be fine. What did you use here?

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PostPosted: 10 Jul 2009, 11:54 
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Hi cannst77, I may have missed it, but what type of output trannies are you using. My experience with the GL KT88s and virtually all 6L6GCs is that there is almost too much bass. The JJ KT77s are better in the mids and upper ranges. What type of speakers do you have? I got rid of my subs because the bass was so good that they were not needed. Actual measured response of the originals, the preproduction and final kit amps is very close to identical and all go below 20HZ easily. Most go flat to about 10HZ. You may wish to check the orientation of the NFB connections. If they are backwards, then it can cause a lack of response. The best way to check is to actually reverse one set. If you wish, PM me a schematic with any parts changes you made. The coupling cap has a major effect on the sound. I am not familiar with the ones you used and can't comment on the effects they might have. My second choice (over the K40s) is Auricaps. Others will certainly work, but I have not tried many. If someone has used others - either good or bad experience, chime in and let us know.

Good listening
Bruce

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PostPosted: 10 Jul 2009, 22:32 
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Hi Guys,

Thanks for the suggestions. I used a 0.33uF as per spec, and all other components actually. Although the test at http://www.humblehomemadehifi.com/Cap.html is for speaker filters, they seem to like the obbligattos, so they shouldn't be a problem.

Regarding the orientation of the NFB, I just had a look at the diagram that came with the OT, and the Blue primary wires are shown in the upper windings, and the Brown in the lower windings. I wired it like this, with the blue wires going to the upper KT88. It is hard to tell from the pic but it looks like Bruce's may have them the other way round.... Bruce, can you confirm?

For testing, the CXPP25-MS-8K is driving a 6ohm 2 way bookshelf speaker from its 4 ohm tap, but it looks like it could be a false alarm. Shifted the amp into my living room and just tried it with my 3 way floorstanders (also 6 ohms), and the difference was amazing :up: The bass is definitely there, just not as exagerated as I'm used to. Definitely absent on the bookshelf though. Maybe something to do with the characteristics, and its matching amp (mini system) boosting the bass to compensate for a known lack of response :confused:

Anyway, although I think it sounds good, I'm still curious about the orientation of the OT, does the blue side go to the top or bottom KT88??? It is a bit awkward to experiment on it at the moment....

Cheers,


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PostPosted: 11 Jul 2009, 21:27 
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Hi, I believe you have just identified the problem. The drawing supplied with the transformer just show the color codes for the connections. It does not specify which one goes to which tube. In this circuit for the feed back to be negative. The tube connected to the .33 uf cap is connected to the the Brown set of wires. With the low amounts of feedback employed, it will still apparently work ok the other way and fool you. I checked with the original amps and one of the production kits to verify the connection. Now depending on how you wired the outputs. You can swap the connections if it is easier. Use the yellow wire as the speaker ground, the orange for 4 ohms and the white for 8 ohms. Be sure to put the NFB connection on the white wire and the ground going back into the amp on the yellow if you do this. Keep us posted on your progress.

Good listening
Bruce

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PostPosted: 12 Jul 2009, 21:35 
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Spot on, thanks for that Bruce :up: Swapped the OT outputs and now the bass is much more pronounced (as well as the rest of the sound)! Sounding great now, even in mono :D Can't wait to get my next set of trannies now.

Out of interest, how could I have known which way to orient the OT? Is it just trial and error?

Cheers,


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PostPosted: 12 Jul 2009, 23:07 
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Hi, The orientation of the windings depends on how many stages and inversions occur before the music gets to the speaker. The actual orientation in this case was determined after the build. It is easy to tell if you get it right. If correct the volume will decrease a little, if backwards it will not and might goe up a little. With out test gear this is as simple a way as I know. If there were more stages in the amp or the tap was to a different location, the results could be different.

Glad it worked

Good listening
Bruce

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PostPosted: 20 Jul 2009, 11:46 
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Hi All. First of all, thanks for all the help during this project, it has been very educational! My trannies arrived today after some time in customs, and I now have two functional monoblocks :D

However :( I made a small wiring error in my eagerness to get things running (I have waited months) and wired the c/t of the HT secondary, to the ground instead of n/c. This lead to a blown fuse, but I then also noticed a hole in my 50x50 cap :eek:

From the pic can someone please let me know if this could have been cause by my wiring error, or is more likely caused by a stray soldering iron/splash of solder?

Given that the cap still works fine, and the amp has no hum, is everything ok? or is imminent catastrophic failure awaiting me? The hole is only in the insulation from what I can tell, and I would just replace it, but I will have a lot of trouble getting a replacement and will only do so if absolutely necessary!!! I will not use it any more until I hear back!

Thanks,


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PostPosted: 20 Jul 2009, 14:06 
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Hi, As long as the hole is only in the plastic cover there should be no problem. If it goes through the metal then yes, by all means replace it. In answer to the other part of the question. it is unlikely that the hole was caused by the wiring error. The most likely consequence of this error is failure of the rectifiers, since there is no hum, i would guess that all is well. The fuze did the job.

Good listening
Bruce

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PostPosted: 21 Jul 2009, 09:26 
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Thanks Bruce! I'm now listening to the amps on my propper speakers and they sound excellent!

My final question has to regard the temperature of the PTs though... Both monoblocks are the same but the temp is up very high. They're the dual 120v trannies now, but the temp is simmilar to what I had at 120v. Is this ok? I have now isolated the PTs from the amp to reduce vibration etc. but they still get very hot. Is this ok? Cheers,


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