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PostPosted: 01 Dec 2017, 15:12 
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Joined: 18 Jun 2016, 06:52
Posts: 19
I would go for 6sl7 tungsol. They look good and have very good sound. But with a pair of Bruce's NOS 12sl7s you're in heaven. The tungsols are more reliable though in terms of absence of noise and microphony. Just my grain of salt.


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PostPosted: 01 Dec 2017, 15:47 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, A couple of answers. The power transformers are big and heavy because they need to be. Smaller ones will overheat and fail.

5751s and 12/6SL7s are virtually identical in performance in the amps. I am not really happy with any of the new production tubes. The best IMO are the Sovtek 6SL7s. Tungsols are likely the same. I particularly found the JJ 5751 offensive. I like JJ tubes, but this one is really off the mark IMO. Some ECC83s and ECC803S ought to be OK, not my first choices, but will function OK in the circuit. 12AU7 and 12AT7 and their variants will work, but the circuit is not optimum for them.

Good listening
Bruce

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PostPosted: 01 Dec 2017, 16:06 
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Hi, Somewhere I saw a question on using EH KT88s in the amps. The reason I don't recommend them is that I could never get a pair to stabilize in the amps. I have not tried any recently so this may have been an issue that has since resolved itself. The tubes would drift nearly 25% over a course of 15 minutes and never would settle in. The amps are designed to tolerate a 10% shift indefinitely. Unfortunately to maximize output in class A amps the tubes run fairly close to max dissipation ( I use 85% in designs). So if a tube is 25% over and stays there it will have a shorter life and if it deviates more it can fail. Depending on the magnitude of the over limit it could fail catastrophically. I don't like any sort of failures so I try to design them out. A catastrophic tube failure (as opposed to just wearing out) could possibly damage the CCS and other components. Yes the resistors in the power supply are sized as a last resort fail safe to protect the transformers, but it ought be a very rare event that they would be needed to do that. So no wandering tubes. This is not to say that the EH tubes are bad, they just don't like the way they are used in the amps.

Good listening
Bruce

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PostPosted: 01 Dec 2017, 16:16 
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Hi, Something else...Psvane brand tubes also do not work in the amps. One diyer tried many of them and they all either would drift, arc internally or just get red anodes. He must have tried a dozen of the them and none would behave. I suspect that they actually can not stand up to the thermal demands the amps place on them. The ratings were the same as other brands.... go figure.

Good listening
Bruce

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PostPosted: 05 Dec 2017, 08:32 
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Joined: 01 Apr 2017, 19:56
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victorzwk wrote:
poty wrote:
Regarding low frequencies distortion: in my case the problem has not occured. Most probably it is the output transformer problem, not LM317.


You are probably right. I am using toroids as outputs. They are very sensitive to current imbalance and could be saturating because if that for frequencies Bellow 25Hz.


Regarding the low frequency distortion, Poty was right. I had some toroids custom made for the build, but they are saturating and distorting the signal below 25 Hz (on full power).

Since I couldn't source other transformers from where I live, I have decided to study a bit, assemble a transformer winding lathe and wind my own thansformer.

It turned out realy great! Brutally large, but great. On the picture the scope is visible showing the output waves from the amplifier. Blue is the undistorted wave from the selfwound output transformer at 20Hz and 25W. Yellow is the output from the toroid transformer, very distorted from saturation.

Now I have to build one more!

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PostPosted: 05 Dec 2017, 10:43 
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Joined: 24 Oct 2010, 07:05
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It reminds me my first iteration of "custom" OddWatt. :-) It was almost 10 years ago and I started with choke filtered B+ and fully regulated heaters, full stereo with Forewatt in the same case. When the weight of the monster approached 50kg (Bruce had predicted this from the beginning of my struggle) I stopped at last and the monster was divided into four different projects. :-)


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PostPosted: 05 Dec 2017, 16:59 
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Hi, The transformers are really key to good performance. If you skimp there then the whole thing is just a so-so amp. The Edcors may not be the best the botique sellers have, but for the cost they are extremely hard to beat.

Only 50 pounds! Yes indeed, that is way all my amps are monos. The KT120s I have are 30 pounds each. I can just see a stereo version with phono and line stage included. Done right would be 100 pounds.

BTW, the LM317s don't cause distortion problems unless you try to push the amps past class A operation. At that point all sorts of fun things go on. After all current limiting means exactly that.

Good listening
Bruce

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PostPosted: 05 Dec 2017, 20:17 
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Joined: 24 Nov 2010, 14:39
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Location: California
[quote="gofar99"]Hi, The transformers are really key to good performance. If you skimp there then the whole thing is just a so-so amp. The Edcors may not be the best the botique sellers have, but for the cost they are extremely hard to beat.

Only 50 pounds! Yes indeed, that is way all my amps are monos. The KT120s I have are 30 pounds each. I can just see a stereo version with phono and line stage included. Done right would be 100 pounds.[quote]

But Poty said 50kg, .....110 pounds....yikes !!! :eek: :)


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