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PostPosted: October 19th, 2016, 11:06 am 
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Given further thought, if the signal source & measurement device are the same box, he might be screwed anyway. Assuming separate boxes, only the measurement device should be on the isolation transformer....

Roscoe

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PostPosted: October 19th, 2016, 3:58 pm 
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After looking at the schematic, the solution is actually quite simple. The input jack shell is connected to the common which is chassis ground. The 4 ohm tap of the output transformer is also connected to common or ground. The 4 ohm tap is actually the center tap of the output transformer. If you take all your signal and measurement equipment commons and reference them to the 4 ohm tap, all the grounds will be common. Then take your measurement probe positive terminals and measure the 0 ohm tap. If you need to get the absolute value of output voltage at terminals other than the 4 ohm tap to determine output power, use an isolated AC voltmeter. However all the percent values will be correct at the 4 ohm tap as both the signal voltage and distortion products will be reduced proportionally relative to the 8 and 16 ohm tap. It does not matter if the speaker or dummy load are on another tap because the current through the secondary will essentially be the same through the whole winding because the zero tap is at one end of the secondary and all loads pass through the zero tap and past the 4 ohm tap. The amount of additional load that the test equipment places on the secondary through the 4 ohm tap is totally negligible. The only current that should flow to ground through the output transformer might be a very slight amount of feedback current as feedback is taken from the two ends of the output transformer back to the input circuit which is referenced to ground.

Tom


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PostPosted: October 23rd, 2016, 8:59 pm 
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All,
Many thanks for all the insightful hep with regards to the D76-A. I bought a Chinese 300VA isolation transformer (the torroid wasn't even properly bolted to the chassis). The output grounds are connected to the input ground, so I have to take care of that, perhaps with a switch? But I assume that I do need to separate the output ground if it's going to work with my 3-prong test equipment on the D76?
V/R
James


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PostPosted: October 23rd, 2016, 10:11 pm 
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Using the isolation transformer on all the test equipment but not the amp will prevent ground problems. If the test equipment has grounds they should not be tied together and also left floating from earth ground which is connected to the amp chassis ground and also the 4 ohm output tap. On some of my test equipment like the signal generator you can either use it balanced with no ground connection or else tie the negative output to ground with a shorting strap at the output jack set for an unbalanced configuration. However that is not the case with all signal generators where the negative output is sometimes always connected to the chassis. With other pieces like the scope you cannot lift the negative signal input from the chassis ground which is more problematic.

Remember with your amp, the 4 ohm output is always connected to chassis ground. The shell of the RCA input jacks are also connected to chassis ground at some point and therefore are connected to the 4 ohm output. If you connect a signal generator to an input jack the negative side of the generator will be essentially connected to the 4 ohm output. If it cannot be configured in a floating mode and always has the negative side at chassis ground that chassis is connected to the 4 ohm tap. Likewise if the scope cannot be lifted from ground unless it is connected with with a true differential probe, it's chassis will be at chassis ground. If you connect the ground side of the probe to the 0 output of the speaker, it is not at the same potential as the ground of the signal generator. In that case above, the ground side of the input signal generator will be at the same potential as the 4 ohm output and the chassis of the scope at the 0 ohm output.. If the chassis of the signal generator and the scope should touch, you will short out the 0 to 4 ohm set of windings on the output transformer.

It is probably safer with that amp to always connect the negative sides of all test equipment to the 4 ohm tap. That way even with equipment where the test connections cannot be floated, you will not short out the output transformer. In general, it is also not a good measurement practice to leave the individual chassis floating as leakage and induced voltages can affect the measurements. If you connect all the test equipment chassis together and MAKE SURE all the negative connections are either to the chassis on the input side and the 4 ohm tap on the output side you will minimize induced noise problems. What the isolation transformer will do for you is provide some protection if a piece develops a short to the chassis as there is no way for it to get back to earth because of the isolation. The reason I said some protection is that if by some extreme fluke one piece of equipment gets a short from one side of the isolation transformer to chassis and another piece gets a short to the opposite side of the isolation transformer the chassis on the two pieces will differ from each other by the full output voltage of the isolation transformer. If for example the signal generator negative (in an unbalanced configuration) connects to the shell of the RCA jack or ground and the scope negative connection is on the 0 tap of the output transformer, the full 120 volt output of the isolation transformer would travel through the negative test leads from each test piece and the output transformer from the 0 ohm to 4 ohm tap. That is not a good situation, although the chances of that happening are extremely remote.

I still think the best way to do the setup is to not use an isolation transformer and connect all the chassis grounds together including the test equipment. You will then only make negative connections on the input side to chassis ground and the output side to the 4 ohm tap. In that case measurement noise is minimized and there will always be a path for a short to chassis in any piece of equipment to travel safely to ground. However, you MUST, MUST, MUST make sure that the connection connection configuration is ALWAYS maintained as stated above. I really hate working on designs like that amp.

Tom


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PostPosted: October 24th, 2016, 6:47 pm 
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So I have a 2-prong signal generator, so I assume that will not pose problems, or can we assume that the signal negative jack is at the same potential as the neutral wire, which is at ground potential. Is it as simple as measuring resistance between the signal "ground" jack and the neutral wire and seeing if it is open or short?


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PostPosted: October 24th, 2016, 7:59 pm 
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On any properly designed equipment the neutral wire of the power connector should NEVER be connected to ground. The neutral is a current carrying wire and ground should never be used for that purpose. With home wiring, the ground and neutral should only be tied together at one spot inside the main distribution box. If sub panels are used, the ground and neutrals should be separate as they return to the main box. Any appliance that has the neutral and ground connected should immediately go into the trash.

If your generator has only two outputs the negative one is either floating or connected to chassis ground. When disconnected a simple resistance measurement will tell you if the negative is connected to the chassis as you guessed.

Tom


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PostPosted: October 24th, 2016, 11:05 pm 
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Yes, my reference to neutral being at ground potential was in reference to the neutral bus bar and the ground bus bar being connected at the main panel.
JJJ


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PostPosted: October 25th, 2016, 8:43 am 
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On a two wire appliance that is not double insulated, ignoring the old isolated chassis TV sets, neither side of the line should be connected to the chassis. However there may be capacitors that connect the chassis to the line. In any case that still does not answer the question of whether the negative signal lead is floating or connected to the chassis. If there is any chassis metal exposed the resistance check will answer that.

Tom


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PostPosted: November 15th, 2016, 12:46 am 
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I've made my custom cables for the amp and test equipment (bias current is measured via a 1/4" phone plug.....) and will start fiddling soon. As a side note, I just installed a McIntosh MC240 and it produces as much bass (albeit a tad tubbier sounding) as my Marantz solid-state amp. Why is this relevant? I've only owned a few amps in my life before the D76 and the McIntosh: Dyanco ST-70, Dynaco Mk III, Marantz 16. Neither of the Dynaco's produced much bass, even with stiffened power supplies. This slanted my opinion of the amount of bass a tube amp could produce. I really had no idea how bad the bass was on the D76-A until hearing the McIntosh. So at least I have a yard-stick now.
V/R
James


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PostPosted: November 15th, 2016, 6:05 pm 
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My Mac-30s had the most bass of any of the tube amps I had used, even the ARC products. The size of the output trannies is pretty substantial compared to the ARC amps. I know that size isn't always or even most-ways the defining variable, but the tube Macs just seem to have more low-end than the other amps. Getting ready to try some vintage W5M and W4-AM amps in the near future so that will be interesting. It should be noted that Dave Gillespie has spoken on this topic a number of times, master that he is.


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