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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 31 Aug 2009, 18:32 
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Joined: 26 May 2009, 18:51
Posts: 130
Location: Lilburn, Georgia USA
Hi everyone, I finally finished my amps and found my voltages were high on the heater bias and also on the SRRP section. I replaced my 48k resistors in the filter section with 68k ones and that brought down the SRRP to 225v on one amp and 236 on the other one. Then I replaced the 220k heater bias resistor with a 270k one and that brought the heater bias down to 80v on one and 83 on the other. After that I hooked up some old speakers I made up back in the 70s with 4 ohm 12 inch guitar amp speakers and no midrange or tweeters. I played CDs for 2 hours and all was well. These amps really sound fine and pump out lots of bass and volume! Then on the 3rd CD I noticed one channel died. I looked down at the amps on my coffee table in front of me and saw what looked like a small fire burning inside the 12SL7 tube. What could cause this? All the voltages are dead on spec. Thanks, Robert Parsh

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PostPosted: 31 Aug 2009, 19:29 
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Joined: 14 Oct 2008, 17:35
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Location: Dartmouth, Nova Scotia
a REAL PYRO FIRE!!!!! :hot: :hot: :hot: :hot:

I sure hope you are not running that amp any more if that is true! :eek: If you ask me it sounds like a bad tube if it is your plates are burning, which also means you most likely have some kind of short in that tube if your voltages are still dead on.

Better yet take some pictures of your creations, and the tube problem. ;)

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PostPosted: 01 Sep 2009, 11:23 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, This is the first instance I know of like this. I suspect a bad tube. Since the bias on th heaters is in the right range, it should not be a heater to cathode failure. That will happen if there is no H-C bias. I would check again to make sure the bias is correct and that there are no shorts to the B+ gound in the heater circuit. Please let us know what you find. Also PM me as well so that I can update the data we have on the amps. There are quite a few out there and if something can be improved, great. BTW, depending on the various filter caps, rectifiers etc, we have found that 68K and 75K for the resistors is often a better value.

Good listening
Bruce

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PostPosted: 01 Sep 2009, 11:53 
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Joined: 26 May 2009, 18:51
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Location: Lilburn, Georgia USA
Hi Bruce, I replaced the bad 12SL7 and turned the amp over and rechecked the voltages while it was playing music. The B+ is 455v, the SRRP 220v, and the heater bias is 82.9v. After I turned it off and let the filter caps go down to zero I checked the resistance between the copper ground loop and both sides of the heater circuit and came up with 1.86 megohms on both sides of it. So maybe it is just a bad tube. A 65 year old tube gone bad in just 3 hours. I ordered 5 more of them just the same. AES now wants 8 dollars a tube,TTS gets 5.75. By the way I ordered the JJ KT88s two at a time from AES on sale. The first two came 2 boxes taped together like they do pairs and they were loose in the bottom of a 17X10 inch Hammond chassis with no padding around the tubes. One was broken and the other was hit so hard the inner structure of the tube was way off to one side of the glass envelope. They paid return UPS shipping and return shipping of two replacements. Any way I was glad to hear that you liked the sound of the JJ KT88s on your system. Thanks, Robert P.


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PostPosted: 02 Sep 2009, 20:31 
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Joined: 14 Oct 2008, 17:35
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Location: Dartmouth, Nova Scotia
Ooooo, I don't rememmber those pictures being on your first post, that or I was not looged in so I did not notice there were any, but those are really nice looking amps!

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PostPosted: 02 Sep 2009, 21:39 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Niiiiice :up:

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PostPosted: 19 Sep 2009, 16:52 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Nice looking amps. Have the new tubes been OK? The 12SL7s I have are all NOS RCA and have been singing away for nearly a year now. The actual kits use 5751s which sound virtually identical and will run on the same voltages and circuit values. For anyone building these amps the choice is yours. The 12SL7s are still a good buy. I have found that the new production Sovtek 5751s are sometimes better than the NOS ones out there. I use the NOS Phillips ones in the kits.

Good listening
Bruce

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