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PostPosted: 27 Jun 2008, 16:58 
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Hi Everyone, The second channel of the amp is in progress. It could be operational by the end of the weekend. Since the last report I did move the CCS to the rear outside of the case (hiding behind the transformer). That works well. My plan to compare the KT88 to one channel of the KT77 was not so good. The set up caused a major ground loop and hum was a problem. In mono the KT88 was fine. I also discovered that the power cables needed to be upgraded. They were getting a bit warm after an hour or so. Not a good thing. My initial thoughts are that this amp sounds like a cross between the 6L6GC and KT77 versions with a bit more output power. Strong clear bass. Good overall balance. It is hard to make any other judgements until the second channel is functional. In spite of the cad program indicating that a 12AX7 would be a better driver, a ECC801 sounded better.

More to follow

Good listening
Bruce

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PostPosted: 07 Jul 2008, 16:05 
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Hi All, This is an update to the project. Both channels are up and running for about a week. I have run into a small problem with one though. It seems that the 4 matched tubes aren't (big surprise right). One is rather slow to stabilize. (like 15 minutes). So accurate measurements have not yet been made. I'll burn it in for a while longer and see if it behaves. I made a few changes to the SRPP driver stage for greater voltage drive to the KT88s. It can now do nearly + and - 70 volts. I'll post the changes later as there could be other small changes. The changes ought to benefit the EL84 and KT77/6L6 versions as well. The response at low levels is quite like the KT77 version (flat from below 20 HZ with about a -.5db at 20KHZ). Distortion was below .33% droping to .2% at both 20HZ and 20KHZ. Hum and noise are not audible with my ear to the speakers (93 db/w Altec Lansings). Very quiet. Good solid sounding amp. The bottom end reminds me of the 6L6GC version only better. The mids and top are like the KT77. I think initially the KT77 has an edge here. I'll wait until a few more hours have passed before committing to that though. There is a small tweak that I want to try that could effect the upper mids an highs, if there is a difference between the two versions, it might vanish.

Good listening
Bruce

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PostPosted: 21 Jul 2008, 21:43 
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I am rediscovering my teenage hobby (from quite a few years ago) and am interested in building a valve amp. I am looking at the KT77 push-pull amp. Is there scope to include bass/treble/balance controls on this amp? Or should I look at a pre-amplifier to do this?


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PostPosted: 22 Jul 2008, 13:58 
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Hi ozzalref, Your question is valid and prompts two different answers. The short answer is that there is not enough gain in the existing amp to add tone controls. It is intended for use with an external preamp delivering about 1 volt of signal. There is no reason why one could not be included though. Source selection switches as well. There are several circuits on the web that include tone controls. If you find some you might want to include, email the circuit or the link and I'll check to see that there would be no problems. The second answer to the question is one that took me a long time to learn and is sort of touchy with many people in the audio community. Simply put, tone controls are used to correct deficiencies in the sound system (source material also). It might be that the speakers don't respond well below 100HZ or the FM tuner is noisy. So I design everything to be properly balanced. I have no tone controls in my system. Does this create any problems, not really as I refuse to listen to anything not up to par. Please don't take this wrong, I'm not a "purist" , only that I want all parts of the system to match. From source (CD, XM, FM, LP, tape) to the speakers, all have flat response in the audio band. I have no need for tone corrections. So if your source material is good and the rest of the system up to par you might not want tone controls. If you include them, I would make a way to bypass them when you actually don't need them. The reason for this is that anything you place in the signal path will introduce some noise, and will have some distortion and may cause unwanted frequency shifts (either phase or amplitude). Welcome to the audio community, listen and enjoy.

Good listening
Bruce

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PostPosted: 01 Aug 2008, 17:03 
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Hi Everyone, An update on the KT88 amp project. There was a minor setback as one of the KT88s failed. (an EH if it matters to you'all). It was not responding to bias changes as it should and would hog the current in pair. So I am waiting for the replacement. In the meantime, since the amp will run KT77s as well I have been trying out other driver tubes. It seems that the 6SL7 is still the best overall, however Sovtek 5751s are a fairly close second. They are a plug in replacement for the 12A_7 series. A little less gain than the 12AX7, and about the same as the 12AT7. The specs are much closer to the 6SL7s than any others I can find. They are fairly cheap as well at $9.50. If you have to use nine pin drivers, they are the ones I would suggest at this time. I'll have more to report when the tube comes in.

Good listening
Bruce

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PostPosted: 15 Aug 2008, 11:56 
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Hi Folks, An update. Lots going on and some really neat things that might work. I still haven't gotten the KT88 back so I've been using the KT77s for test purposes. I recommend replacing the standard LM317 in the CCS with the LM317HV and deleting the series resistor for KT88 applications. It will allow for better linearity and higher drive levels. Most likely it will result in greater output as well. As soon as I get the replacement tube, I'll post again. The HV 317s are available from several sources for just over $1.

Another possibility is to use a power FET as a CCS in the cathode circuit. I have a tentative design for that as well using IRF730s. It appears to work, but will need good heat sinking as there seems to be a thermal drift issue. If the FET works, it promises to have even better results than the 317s. It can go down to nearly zero volts on the cathodes under high levels of drive and get around the 4-5 minimum that exists with a 317.

If someone out there has a good design for a FET CCS (in the .1 to .4 amp range), I would be interested in it.

Good liistening
Bruce

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PostPosted: 16 Aug 2008, 11:02 
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Bruce,

I suggest you take a look at Gary Pimm's site http://home.pacifier.com/~gpimm/
Top left corner are links to some CCS.

Also, see this aX article by Walter Jung http://www.audioxpress.com/magsdirx/ax/ ... ng2778.pdf

Bill


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PostPosted: 29 Aug 2008, 20:51 
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Hi Bill, Thanks for the tip. I'll be tinkering with the FETs over the next few weeks. In the mean time I have made a few small mods to the amps and had a chance to listen to them at some length with the KT88s. The 88s are running at 125 ma per pair, the same as I was using on the KT77s. They can run up to 200 a pair and I will be trying that shortly. The 88s sound louder than the 77s and have a different, but not unpleasant sound. They seem to have a fuller bottom end. The initial brightness in the upper mid range has settled down and is no longer obvious (about 25 hours on the amps now). I expect to do some measurements in about a week after it goes past 50 hours. Right now I would say that it is more "gutsy" (nice tech term there) than the KT77s in the OD225 project and perhaps a little less refined in sound. I have a companion project in the works and will probably write up both at the same time. This is a true pair of mono-blocks. They will be essentially the same circuit but using 12SL7 drivers as I originally intended (in place of the 5751 drivers). They too will be able to use nearly any of the power tubes that share the same pin out as the KT77/88/6L6/EL34. Power output levels will be adjustable with a single resistor in each amp in the CCS circuit.

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PostPosted: 29 Aug 2008, 21:01 
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If you want a very good sounding 5751, check out the late 50s GE blackplates...very good sound. Of course, pricewise you cannot beat those Sovteks!

There are a lot of good at7s out there too...my VTL uses a single in the output stage so I have tried just about every domestic and a few imports.

Definitely interested in this project, I have been noodling with SE-amp cad for a while and just haven't decided to pull the trigger on KT88 SE parts or building something with a little more mojo.


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PostPosted: 29 Aug 2008, 22:00 
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Hi, The Sovtek 5751s are pretty good. I like them better than any of the 12AT7s I have. I originally designed the amps around 6SL7s and only went to a nine pin driver in the one model at the request of another diyer. The 12AT7 isn't really a good sub in this case. The 5751 was a much closer match to the 6SL7 specs and performance. All the new projects revert back to 12SL7s. One because they work and two, right now they are cheap (AES was selling them for $4). I got 6 NOS RCAs. No telling if they have more. The Sovtek 6SL7s are rather good as well. The mono-blocks (I call them Odd Blocks) are really not more difficult to build than most SE amps. The performance is roughly double what an SE can do and the distortion levels (because it is PP and UL) are low. My previous measurements on the KT77 amp were quite excellent, SN and band width as well. Right now for sound quality the OD225 with KT77s has the best sound, but I haven't really finished tweaking the KT88 version.

Good listening
Bruce

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