Old Radio Transformers for AX84 project

Tube or solid-state, this section goes to eleven!
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Lukaamp
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Post by Lukaamp »

Hi,can someone explain to me how can i use transformers for the project AX84 from scheme of an old radio on scheme in attach, and how to do it .
I'm interested in a first time of using a power transformer. Rectifier section and grounding is my problem.

Thanks.

AX84 link: http://ax84.com/media/ax84_m217.gif
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deltafred
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Post by deltafred »

Welcome to FSB.

The transformers that you have is designed to use an EL84 output valve (tube) and the amp you propose building uses an EL34.

The EL84 is quoted as 12w max dissipation where the EL34 is 25w so you would have to be careful that you limit the quiescent current so as not to damage either transformer. It would be far better to start with a schematic that uses an EL84.
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Lukaamp
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Post by Lukaamp »

Sorry wrong scheme :)

this is a project : http://ax84.com/static/p1/AX84_P1_101004.pdf

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deltafred
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Post by deltafred »

That makes more sense. :D

If you have never built a tube amp before the best method is to find a layout that is known to work and build yours the same. Incorrect layout and grounding can cause instability problems in tube amps so best start with something that is known to work well.

If the transformers are still in the radio chassis then identify where all the leads go before you remove them or take lots of photos. If they are already out of the chassis then some detective work may be involved to find out what winding does what.

The feedback winding on the output transformer (OT) is not required on your schematic so insulate the leads that come out of the transformer and stow them close to the chassis and away from any small signal carrying components preamp tubes and associated components.
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Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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Lukaamp
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Post by Lukaamp »

OK , Thank you for tips . But I'm a little confused on how to connect power transformer since no CT ? Any suggestion (scheme maybe)?

I know all the windings , which is that transformer still in the chassi.

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Post by deltafred »

Lukaamp wrote:But I'm a little confused on how to connect power transformer since no CT
Ah, I see what you mean.

You could replace the tube rectifier with a single solid state diode, which would give you half wave rectification. A better way would be to use a bridge rectifier, this will give you a smoother supply voltage (less ripple). Feed the transformer into the AC connections on the bridge and connect the "-" connection to ground/chassis and the "+" to your first smoothing capacitor.

See https://en.wikipedia.org/wiki/Diode_bridge
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Post by Lukaamp »

Great, thanks! If I survive I will post a pics . :horsey:

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Post by deltafred »

Lukaamp wrote:If I survive I will post a pics . :horsey:
Be very careful when working on tube equipment, make sure you have disconnected it from the mains before making any changes and don't get complacent, that is when you make mistakes.

R1 in your schematic is there to discharge the smoothing capacitors when you switch mains power off so you should not need to discharge the capacitors.

When I was an apprentice I was always told to keep my left hand in my pocket when working on live equipment, you are theoretically safer getting a shock on your right side than your left because of the position of your heart.
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Post by Dingleberry »

I would definitely leave that radio alone. That's a real HAZARD especially for a beginner. It is really important to understand schematics if doing projects like that. I've been making probably 20 small amps with old tube radio transformers and that radio is not a good candidate by any means. Output transformer and tube sockets could probably be recycled for an amp project and maybe chassis, but: That power transformer has NO plate supply winding. The B+ is taken straight from the primary live phase and half wave rectified. The neutral phase goes straight to the ground. If you insert your mains plug into the wall outlet other way around you have 230Vac in your chassis => in your guitar strings => IN YOUR HANDS. Now when you touch something that has metal outer shelf which is grounded, for example fridge / radiator / kitchen / microwave or kitchen sink you'll get SHOCKED WITH 230Vac and that could be FATAL!

Those kind of power transformers were used back then when copper was very expensive specially in the early fifties, when nearly all the available copper had gone to the war industry for many years (during WW2 1939-1945). Also some cheap american amps used that kind of arrangement in the sixties and some even used oddball tubes with higher heater voltages to allow them to run the heaters in series straight from the 120Vac line or with little power resistor in series. The Silvertone amp-in-a case comes to mind.

I know it really sucks ass when you find a nice radio for cheap or get one for free and get exited about building an amp and then someone you don't even know drags you down and tells you, that is a no good radio for an amp, but you'll get over it and what best stay alive and kicking. Been there and done that, trust me bro. If you live in Europe (what I can conclude because of the 220V primary) you'll be able to find a nice and suitable radio for that project. There are lots of radios that used EL84 output tube (Grundig, Telefunken, Siemens, Philips, Saba, Blaupunkt and Luxor to name a few) out there to get for like 20€ if you're lucky.
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Post by deltafred »

Dingleberry wrote:I would definitely leave that radio alone. That's a real HAZARD especially for a beginner. It is really important to understand schematics if doing projects like that. I've been making probably 20 small amps with old tube radio transformers and that radio is not a good candidate by any means. Output transformer and tube sockets could probably be recycled for an amp project and maybe chassis, but: That power transformer has NO plate supply winding. The B+ is taken straight from the primary live phase and half wave rectified. The neutral phase goes straight to the ground. If you insert your mains plug into the wall outlet other way around you have 230Vac in your chassis => in your guitar strings => IN YOUR HANDS. Now when you touch something that has metal outer shelf which is grounded, for example fridge / radiator / kitchen / microwave or kitchen sink you'll get SHOCKED WITH 230Vac and that could be FATAL!
....
Whoa, I never noticed that the HT is derived straight from the mains! Good catch Dingleberry.

Lukaamp - don't even think about doing this (unless you have a death wish).

Old tube TV sets used to be like that in the 50s and 60s, they derived the HT from the mains and the tube heaters were all in series straight across the mains. The chassis could be live if you plugged them in the wrong way round but fortunately all the knobs were insulated (Bakelite) and the aerial was connected through small capacitors so you only got a tingle if you touched the aerial plug. Ah the good old days, I wouldn't want to go back even if you paid me.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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Post by Lukaamp »

I'm leaving the project and go to standard transformers ( Hammond etc. ). :shock:
I think this has become a very useful and informative post for all the builders who enter the world of tube amplifiers.
Frankly I did not found very important information like this summarized in a few sentences anywhere.

So if you can add on this theme please be welcome !!!

Thank deltafred and Dingleberry .

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