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PostPosted: 05 Jun 2018, 13:01 
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Location: Arizona, USA
They need to be isolated. I use ones by CLIF jacks that are plastic and thereby insulated from the chassis. The way you have it may work, but most likely you will end up with a ground loop and hum when you put it in your system.

Good listening
Bruce

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PostPosted: 05 Jun 2018, 14:37 
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Thank you Bruce!! I'm was afraid of this...

The Pot housing also has to be isolated?

Regards!!


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PostPosted: 05 Jun 2018, 15:41 
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Hi, You may be able to use a modest size grommet on the jacks. Sometimes the insulating washers for RCA jacks will fit as well. Often the pot housing can be connected to the chassis. Once in a while it has to go to the signal ground instead. I would leave it alone for now.

Good listening
Bruce

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PostPosted: 05 Jun 2018, 17:19 
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Thank you so much Bruce!!


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PostPosted: 06 Jun 2018, 11:05 
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Hello Again!! I keep bothering with my problems, but I hope the result will be helpful to the next one faces the project...

Finally I put a grommet of isolating material for the jack in each sides of the chassis and I'm done...

My original idea was to put two headphone jacks 6.35 and 3.5mm the former for high impedance Headphones and the last one for low impedance ones (my OPT has 2 taps 95 and 45 Ohm), now I changed my mind and I want to switch the two OPT taps on just one jack (6.35mm) there is a problem to switch the OPT's secondaries taps when the amp is turned on? its unsafe in some way?

Thank you!!!


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PostPosted: 06 Jun 2018, 14:16 
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Location: Prêles, Switzerland
Hi Willy,

In another thread (Pete Millet Engineer Amp) someone warned me to never use a tube amp without loading it. Not sure it applies to this one, I have been doing tests with resistors on my headphone amp and it has been without load from time to time, when switching from resistors to headphone, before I knew about the rule. Nothing bad happened but Bruce is certainly much more qualified to explain what can happen in such a case. I could think about voltage spikes, oscillations, etc, but it's more my wild guess than anything I really know.

Charles


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PostPosted: 06 Jun 2018, 14:35 
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Hi, I have heard that as well. I take it with a grain of salt. Testing an amp with a signal and no load could possibly cause damage. However as often as musicians forget to plug in the speakers and try the instrument....and the amps survive I tend to think that it is not as common a problem as folks think. I test mine under load. It is easy to build a load for an amp.....a few power resistors in a case. For the headphone amp the output power is so low that it seems unlikely that it could be damaged even if left on all the time with a large signal input. It certainly won't hurt the tubes and considering the output transformer is much larger than needed, it should dissipate the energy. I don't really think it can be damaged. I don't recommend it though just as a matter of good practice. In large power amps I can conceive of the possibility of thermal damage to the transformer and possibly over dissipation in the tubes. A short duration oops, I think, based on my experience has never been a problem with any of my gear.

Good listening
Bruce

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PostPosted: 06 Jun 2018, 17:22 
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Thank you guys!!

So, it cames to my mind now, assuming a large power amp, with two outputs secondary, 8 and 4 Ohm to say something, if I put an speaker of 8 Ohm, it's clear that all the secondary is transferring the energy it receives but, what happen if I plug an 4 Ohm speaker, the "upper" half of the secondary is "open air" and it's not transferring the energy it receives... it make sense? or all the energy it's drained thru the 4 Ohm tap? and the "upper" part of the coil gets no energy?

Nice chat!!

Regards!!


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PostPosted: 07 Jun 2018, 21:09 
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Hi, Both sections will get power....the load impedance that the tubes see will be lower though. Exactly what fraction of the power will go to each speaker ought to be a function of the fraction of the secondary windings it sees but the 8 ohm one will be reduced as part of what it would get is now drained off by the 4 ohm load. Beyond my brain power at this hour...but it would seem that the ratio would be around 1/3 to the 4 ohm one and 2/3 to the 8 ohm one. The reduction in effective impedance of the primary side would seem to be about 1/3. BTW, the way transformers are wound, the portion of wire between the 4 ohm tap and 8 ohm tap is not the same as the part between the 0 and 4 ohm tap. The impedance function is a ratio of turns squared. A long time ago I was fooled by that when I attached a 4 ohm speaker between the 8 tap and 4 tap. The volume was way lower than expected. (don't ask why I did that as it was a dumb thing :eek: ). When I moved the wire so that the speaker was between the 0 and 4 taps all was fine. If you look up the math involved you will see why this happened.

Good listening
Bruce

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PostPosted: 08 Jun 2018, 07:42 
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Thank you Bruce!!


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