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PostPosted: October 17th, 2016, 4:40 pm 
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Here is my crap for the day -- buy new tubes. :character-oldtimer:

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PostPosted: October 17th, 2016, 5:01 pm 
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I was going to post
"Direct Substitution" but did not wanted to be crapped on
but you did it so get out the TP


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PostPosted: October 17th, 2016, 5:03 pm 
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Pelliott321 wrote:
I was going to post
"Direct Substitution" but did not wanted to be crapped on
but you did it so get out the TP


I hope those were not my initials :crazy:


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PostPosted: October 17th, 2016, 5:05 pm 
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At least your opinions and suggestions are presented in a gentlemanly way


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PostPosted: October 17th, 2016, 10:44 pm 
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All, I went to the horse's mouth and got this response from ARC regarding KT88s in the D76-A:

"There are many brands of KT88s and we have not evaluated them. You may use KT88s in the D76A, but we recommend you avoid the Chinese KT88s as past experience has shown them to be unreliable. We use the 6550WE Sovtek when we service these amps and this tube has proven both reliable and good sounding. We stand behind our tubes because the tubes we select are the ones designed to work the best with the unit."

Interesting!
V/R
James


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PostPosted: October 18th, 2016, 6:42 am 
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I have used Chinese KT88 for years without a failure.
Maybe only because I buy from a relatable dealer that test all the tubes he sells
I would be Leary buying power tubes on eBay. You really do not know what you are getting.
Valve Art, and Pentas have been good performers at a very reasonable price of around $35 apiece.


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PostPosted: October 18th, 2016, 10:40 pm 
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Joined: October 10th, 2016, 10:23 pm
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All,
I'm going to dig into a quad of Gold Lion (new production) KT88's that I had stashed for another project. However the following excerpt from the D76-A manual has the following text:
"This procedure involves measuring (monitoring) the total harmonic distortion (THD) of the amplifier while adjustment is made. To prevent erroneous readings, all test equipment should be isolated from the power line ground. Since the amplifier output is "balanced" with respect to signal ground, all measurements made at the amplifier output should be referenced to the 4 ohm tap only which is internally tied to circuit ground. Never externally ground, either with test cables or test equipment, the "O", "8", or "16" ohm output terminals as this would short out one half of the amplifier's balanced push-pull feedback loop. Use only "floating" loads or test equipment when referencing same to any terminal except the grounded "4"
ohm terminal.

1. Make sure that the output stage bias and balance
controls have been properly adjusted (see DC bias/balance procedure, above) before making the AC balance adjustment.

2. Connect a 16 ohm "dummy" load to the amplifier output of the channel under test.

3. Plug the amplifier into a 120 VAC power source.

4. Allow the amplifier to warm-up for 30 minutes.

5. Center the appropriate AC Balance Control RV3 or RV4.

6. With a low distortion oscillator (~ .01% residual THD) inject a I KHz input signal. Adjust input level for full rated output - 75 Watts RMS.

7. Connect {float) a Harmonic Distortion Analyzer (~ .01 % residual) across the 16 ohm output.

8. Adjust the appropriate AC Balance Control for minimum THO:
< .1%@ 75 Watts RMS per channel@ 1 KHz
< .3%@ 75 Watts RMS per channel @ 30 Hz
< 1 % @ 75 Watts RMS per channel @ 15 KHz"

So, "all test equipment should be isolated from the power line ground" and "Use only "floating" loads or test equipment when referencing same to any terminal except the grounded "4"" and "Connect {float) a Harmonic Distortion Analyzer (~ .01 % residual) across the 16 ohm output."
What does that mean to me? All of my test equipment connects to the wall with a 3-wire IEC connector, with the exception of some older pieces of dubious pedigree and calibration. Also, if my THD analyzer isn't accurate out to 0.01%, can I still find the minimum THD reading with what I have, or am I out of luck?

Many Thanks!


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PostPosted: October 19th, 2016, 8:29 am 
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The "safest" way to do that is with an isolation transformer for the input mains power to all the test equipment and the amplifier. Simply using a cheater will isolate the equipment from ground but if you get a short anywhere in the connected equipment to chassis, touching any chassis and anything grounded in your test area could result in a lethal shock. By using an isolation transformer, nothing in the power delivered to the test equipment is referenced to earth ground and the chance of a shock to ground is minimized. When working with electricity there really is no absolutely safe way to do everything.

Tom


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PostPosted: October 19th, 2016, 9:05 am 
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:text-yeahthat:


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PostPosted: October 19th, 2016, 9:11 am 
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Actually, I think he's going to want to use the isolation transformer for ONLY the test equipment, and not have the test equipment ground connected to the power system ground. Otherwise the D76A ground will be connected to the test equipment ground, which could pose problems since the D76A output is referenced to ground on the 4ohm tap, not the 0ohm tap.

Roscoe

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