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Old 17th Apr 2019, 1:08 pm   #21
Miguel Lopez
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Default Re: Building a little practical audio amplifier for experimentation

Quote:
Originally Posted by Andy
what are those trimmer pots for?
To adjust the center "tap" of the heaters, both on the amplifier and rectifier valves, to reduce hum

Quote:
Originally Posted by Andy
are you still using aspirin as flux?
I am. But those soldering that you see on the PCB were done using the solder that you once sent me. That's why it looks shinning. That solder is reserved for important projects only.

Quote:
Originally Posted by David
use insulating 'top hat' bushes on the screws.
I will use insulating sleeves and bakelite or fiberglass washers.
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Old 13th May 2019, 5:56 pm   #22
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Default Re: Building a little practical audio amplifier for experimentation

As Mr Time, the "Implacable", has no mercy of this humble mortal, I'm advancing very slow with this project, as usual. Anyway I have done some advances that I want to share with you. It is already working althought I have not tested it with audio nor a speaker, just a 1kHz signal from my signal generator. The sine wave was clean for a power of about 3W, which is more than enough for what I want. I powered a 10 ohms dummy load.
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Old 13th May 2019, 9:02 pm   #23
Sideband
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Default Re: Building a little practical audio amplifier for experimentation

That is a very impressive construction method....looks like a piece of restored military kit! Well done!
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Old 18th May 2019, 7:56 pm   #24
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Question Re: Building a little practical audio amplifier for experimentation

I'm probably mistaken about this, but looking at R9, the phasing connections of the O/P transformer and the grid-anode phase relationships of the valves, isn't that feedback via R9 in-phase with any signal appearing on the grid of the first triode - thus increasing the chance of instability?

Al.
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Old 18th May 2019, 11:17 pm   #25
Paul JD
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Default Re: Building a little practical audio amplifier for experimentation

The feedback is going to the cathode of the input stage so it needs to be in phase with the signal on the grid in order to produce negative feedback.
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Old 5th Jul 2019, 3:26 pm   #26
Miguel Lopez
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Default Re: Building a little practical audio amplifier for experimentation

I'm advancing very slow on this project due to lack of available time. Anyway I have begun to build the input block with the three different plugs, which will be connected in "star".
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Old 30th Jul 2019, 9:31 pm   #27
Miguel Lopez
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Default Re: Building a little practical audio amplifier for experimentation

On last weekend I did some advances on this project. I mounted the shield between the transformers and the signal lines (input conectors and volume potentiometer), and also put the input connectors on their place. I played music with the amp for the whole weekend althought it is not finished yet. I have to build the front panel and the built-in speaker cabinet. See some pictures
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