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PostPosted: 12 Mar 2020, 09:42 
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Thank you very much Bruce.


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PostPosted: 21 Mar 2020, 09:44 
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Slowly all parts are finding their way to my home. I was contemplating a feature that I would like to incorporate. I would appreciate your opinion !
I was thinking that instead of the 4 test points for each channel I could use two small analogue voltmeters build into the top panel with a 4 pole toggle switch to switch between channels.
Is there a reason not to do it this way ?


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PostPosted: 21 Mar 2020, 14:32 
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Hi, You can use meters, but they need to be rather sensitive as you are measuring about 50 millivolts. I use (as I do this sort of thing often) a pair of inexpensive digital panel meters. 200 millivolt full scale. They are available from many sources. I built them in a plastic case with 1 foot long test probes coming out. The biggest issue is that thet need 9 volt power and are not isolated from what is being tested. A pair of 9 volt batteries and a DPDT switch solves this nicely. I have been using it for about 6 years and have not yet had to replace the batteries. With two meters you can watch as one goes up and the other down to ballance the amp.

Good listening
Bruce
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meter.jpg


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PostPosted: 21 Mar 2020, 14:58 
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I was thinking of using two of these, their range is 1 Volt and they don't require power, except maybe for back lighting. I think they look very nice. I don't understand the not being isolated part, I'm afraid.


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PostPosted: 21 Mar 2020, 21:06 
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Hi, they will not be nearly sensitive enough to measure what is needed. They miss by about a factor of 200:1. They would need amplification to work. The isolated part means that the meters measure across the sense resistors and can not be ground referenced. Both terminals of the meter will be at some voltage above ground. In this amp about 10-15 volts. Measuring to the ground would give a meaningless value. Thus to power the meters I used, each required a separate supply as you cannot connect between the opposite sense resistors either. Any values you would get from that would not reflect the performance of the individual tube being measured. Since nearly all builders have at least one test meter and the amps don't need frequent checking (after the initial day or so) the set up is not really all that inconvenient. I check mine about every six months and usually they are still quite close (10% or better). You will find that many tube amps require similar checking since any that use fixed bias (as opposed to cathode bias) have the same issue at roughly the same intervals. I figure it is a small inconvenience in light of the high quality reproduction they deliver.

Good listening
Bruce

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PostPosted: 22 Mar 2020, 01:07 
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Yes, I see now they are not sensitive enough. I also understand you measure the voltage without ground reference. But I don't understand why the meters have to be powered. If you use a normal DVM it's not powered either when measuring voltages is it ?
Sorry to be so dense.


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PostPosted: 22 Mar 2020, 08:42 
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Hi, A DVM has its own internal power. The digital meters I used in the test device also need to be powered, thus the individual 9 volt batteries for each one. Sorry for the confusion.

At one time I considered having the meters built in the amps and switched on and off by a switch or delay relay so I could check them at start up. A delay of about a minute or so would be fine. I gave it up as I would also have to come up with power for them. Yes it could be done when they were off via a set of rechargeable batteries that also were switched by delay relays. To get it all functional would IMO complicate a simple design into a monster with lots of things that could go wrong. BTW I would not want the meters active when it was playing music as it is one more source of contamination of the signal since the second output tube gets its signal from the cathode circuit. Picky for sure...but I wanted (and still want) the very best sound possible.

Good listening
Bruce

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PostPosted: 22 Mar 2020, 08:55 
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Ah, I thought the battery in a DVM was only for measuring resistance.
What would be the measuring method ? Do I measure one tube in one channel and try to get 41mV, then take the next tube in the same channel or in the other channel ?
I better start looking for a second DVM. I have a very old Fluke that works very well, I'll see if I can find another second hand one.


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PostPosted: 22 Mar 2020, 16:00 
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Hi, Two meters are helpful. But not really needed. The first time you check see what each value is. Add them together then divide by 2. That will be the setting desired. The constant current source is really pretty constant and the total will not change significantly regardless of what you do. So if the first time the total is 82 then you can be pretty much assured that for the next few years it will be the same. It makes setting simple. In one of the earlier kit amps we only included one pair of test points because they were so consistent and the 2-3% difference that could occur due to parts tolerances made no difference in the sound.

Good listening
Bruce

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PostPosted: 22 Mar 2020, 17:12 
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Thanks Bruce. No doubt I'll be back.
Stay safe and healthy.

Martin


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