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.

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

Re: Building a little practical audio amplifier for experimentation
 

Talking about that, I forgot to say that I intend to fit three diferent types of connectors for external signals. All of them star-connected.

1. DIN-5
2. RCA
3. BNC

This way I will be able to use different types of cables from diferent apparatus

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?

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.

Re: Building a little practical audio amplifier for experimentation
 

Be sure I will :thumbsup:

I remember that I asked the same question when I first tried to build this amplifier. I think it was Paul Sherwin who answered me on that ocassion.

It's to provide a positive voltage reference to the heaters, which otherwise would be "floating".

Re: Building a little practical audio amplifier for experimentation
 

You are going to invert those electrolytics?
:)

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.

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! :thumbsup::thumbsup:

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?

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.

Re: Building a little practical audio amplifier for experimentation
 

And being valve based almost impossible to blow up.

Re: Building a little practical audio amplifier for experimentation
 

Oh, yes. They are there just posing for the picture ;D

Yes I did. In the actual amp, I intend to add a voltage divider to further reduce hum.
And in the worst case it would introduce a 60 Hz noise which is our mains frequency :thumbsup:

That will be one of the purposes. BNC is a very useful and common connector and if I finally go after a homebrew radio I could supply this amp from the radio circuit via a BNC connector. Or I can supply the amp from the signal generator (with a dummy load) to test and adjust the amp itself.

In the picture, you can see the circuit mounted on my HTBB (High Tension Bread Board).

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

Re: Building a little practical audio amplifier for experimentation
 

That is exactly what I will do on this amp. Both for the amplifying valves and for the rectifier.

Here I want to share some pictures of what I did on weekend:

Re: Building a little practical audio amplifier for experimentation
 

You can only do this on the rectifier if you are using an indirectly heated rectifier valve, if it is directly heated the HT voltage will be on the heaters!

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.

Re: Building a little practical audio amplifier for experimentation
 

EZ81

Maybe.............

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.

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.

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