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8th Mar 2019, 5:19 pm | #1 |
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
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Building a little practical audio amplifier for experimentation
Hello everybody
I want to share with you the steps on building a little and practical audio amplifier, based on Soviet valves. I decided to build this little device in order to use it as a tool in future experimentation on valve electronics. I have always planned to build a radio using valves, but in my modest opinion, first it would be very useful to have a little and compact amplifier to amplify the signals if I'm able to tune and detect them. The circuit in question is the one which attracted my attention to vacuum electronics about 7 years ago. In that moment I tried to build it but things went improving, knowledge too, and I finally ended building a bigger stereo amplifier which is currently in use at home. See: https://www.vintage-radio.net/forum/...t=77762&page=6 I've been a bit distanced from vacuum electronics in the last couple of years due to time availability, but I think that now I can resume my experimentation (and fun) playing with glass and electrons, so I decided to go for it. Anyway, I will make some variations of this circuit as I want to use a vaccum rectifier. All of the valve-based circuits that I have built, have silicon rectifiers, but I'm interested to use a vacuum full-wave rectifier to experiment with the behaviour of this kind of power supply. Problem is that I also want to use a transformer that I winded for another project (to save time and labour) which has a single HT winding. Then I decided to use the circuit on Picture #2, which appears on The Valve Wizard page about rectifiers. This way I can have the behaviour of a full wave vacuum rectifier while I have a single winding transformer. I know, I know.....it will not be an "all-vacuum" set but after all, this is for experimentation.......and fun. The other thing to have into account is: SPACE. Not outer space, but space in my workshop, my bench and my shelves. My shelves are at full capacity of "probably-I-will-never-use" stuff, as it should be on any DIYER workshop, so space is something very important to have in mind. So, I decided to build the chassis in tower style, in order to save space. This will be a very compact (if I can say so) audio amplifier with a built-in speaker.............. Oh! I forgot to tell you that it will have a built-in speaker. Well, for the same considerations of space and usefulness, I will not use a separate speaker but I will insert it into the chassis too. As the chassis will be build on steel plate, which is not good for a speaker cabinet, I will try to build a small wooden cabinet for the speaker inside the steel chassis. What I have done in the chassis can be seen on Picture #3.
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8th Mar 2019, 6:34 pm | #2 |
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Re: Building a little practical audio amplifier for experimentation
A very practical and workman like design ideal for the workshop, as they say "does what it says on the tin"* and you have the option of connecting whatever you like to it. Fun is what hobbies are all about after all.
*A reference for doing a proper job https://en.wikipedia.org/wiki/Does_e...ays_on_the_tin |
8th Mar 2019, 7:36 pm | #3 | |
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
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Re: Building a little practical audio amplifier for experimentation
Quote:
1. DIN-5 2. RCA 3. BNC This way I will be able to use different types of cables from diferent apparatus
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8th Mar 2019, 9:08 pm | #4 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,169
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Re: Building a little practical audio amplifier for experimentation
Miguel, it may be a good idea to add a 600V series capacitor to the input.
It will prevent DC voltages from the signal source altering the bias on the input valve. It will also provide some protection and isolation. What is the function of R11? |
8th Mar 2019, 9:43 pm | #5 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Building a little practical audio amplifier for experimentation
I was wondering that as well. I'm guessing it is to provide some sort of reference to the heaters which would otherwise be floating. This would normally be done with either a centre tap on the heater winding or an artificial centre tap created with a pair of resistors which would then be connected either to ground or an elevated DC voltage (such as the power valve cathode). I don't recall seeing it done like this before.
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8th Mar 2019, 9:45 pm | #6 | ||
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
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Re: Building a little practical audio amplifier for experimentation
Quote:
Quote:
It's to provide a positive voltage reference to the heaters, which otherwise would be "floating".
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8th Mar 2019, 10:28 pm | #7 |
Heptode
Join Date: Jan 2009
Location: West Lothian, UK.
Posts: 762
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Re: Building a little practical audio amplifier for experimentation
You are going to invert those electrolytics?
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George |
9th Mar 2019, 8:33 am | #8 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,670
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Re: Building a little practical audio amplifier for experimentation
Nice work mate. Re R11 one would think it would introduce 50hz onto the cathode but you said in an email that the amp is very quiet. Did you fit it on the amp when you built it on the BB Miguel?
Andy.
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9th Mar 2019, 11:00 am | #9 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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Re: Building a little practical audio amplifier for experimentation
A small amplifier with its own speaker, plenty of gain and a high input impedance is a very useful thing to have in the workshop. A valve amplifier can be a rewarding project in its own right anyway, and can fulfil the above criteria, so this is a double win!
I'm guessing the purpose of the BNC connector is so you can wire the amplifier in parallel with an oscilloscope, so you can see and hear exactly what is going on in a circuit?
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9th Mar 2019, 11:23 am | #10 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Building a little practical audio amplifier for experimentation
As it is just a reference there will be no current flowing through it to introduce noise. It could just as easily be connected to ground but using a positive voltage such as the cathode can help to reduce heater hum.
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9th Mar 2019, 7:05 pm | #11 |
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Re: Building a little practical audio amplifier for experimentation
And being valve based almost impossible to blow up.
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11th Mar 2019, 1:34 pm | #12 | |||
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
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Re: Building a little practical audio amplifier for experimentation
Quote:
Quote:
And in the worst case it would introduce a 60 Hz noise which is our mains frequency Quote:
In the picture, you can see the circuit mounted on my HTBB (High Tension Bread Board).
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11th Mar 2019, 2:48 pm | #13 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Building a little practical audio amplifier for experimentation
A much better way of referencing the heaters would be to do away with R11 and create a virtual centre tap using 2 low value resistors (100 - 220 ohm). One end of each resistor is connected to each leg of the heater supply and the other ends are connected together to form the centre tap. This can then be connected either to ground or to an elevated DC voltage which could be the power valve cathode or from a voltage divider off the HT supply (30 - 60v would be typical).
Lots of good info on the subject here http://www.valvewizard.co.uk/heater.html |
11th Mar 2019, 3:02 pm | #14 | |
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
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Re: Building a little practical audio amplifier for experimentation
Quote:
Here I want to share some pictures of what I did on weekend:
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11th Mar 2019, 3:22 pm | #15 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Building a little practical audio amplifier for experimentation
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11th Mar 2019, 3:32 pm | #16 |
Guest
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Re: Building a little practical audio amplifier for experimentation
I love the transformer mounting method. Another thing to add is a switched input so you can listen to the radio (or any other source) without unplugging to go into workshop mode.
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11th Mar 2019, 3:42 pm | #17 | ||
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
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Re: Building a little practical audio amplifier for experimentation
Quote:
Quote:
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When electrons move, things happen. There's nothing you can do that can't be done. |
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16th Apr 2019, 4:49 pm | #18 |
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
Posts: 617
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Re: Building a little practical audio amplifier for experimentation
I have prepared the PCB for the amplifier but I want to apply a layer of paint to the chassis before begin to assemble. I want to use gray, as this color hides the dirt.
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When electrons move, things happen. There's nothing you can do that can't be done. |
17th Apr 2019, 7:32 am | #19 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,670
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Re: Building a little practical audio amplifier for experimentation
Looking good Miguel, what are those trimmer pots for? Also noticed how shiny your solder is, are you still using aspirin as flux?
Andy.
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Curiosity hasn't killed this cat...so far. |
17th Apr 2019, 7:44 am | #20 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,902
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Re: Building a little practical audio amplifier for experimentation
With that transformer mounting method, use insulating 'top hat' bushes on the screws. Otherwise the screws and the outer laminations (and the metal chassis) create a conductive loop through which part of the flux of the transformer passes. It's not obvious, but it gives a shorted turn effect. It increases losses and acoustic noise, but there is often bad enough contact and enough series resistance to stop it being dramatic.
David
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