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I am trying to make this little amp as cheap as possible (el84 SE), and I am thinking about using a doubler supply straight out of the wall, with no transformer. I am thinking of building in a voltage transient suppressor, and a current inrush limiter(both cheap). I am looking to use two 110uf 450V lytic caps in the doubler supply, with two 3A 600V diodes in series for each leg. Then the supply will follow RCRC, with the next C a 200uf, 400V lytic, then a 47uf 400V SCR Polyp.
Ok, before they shoot me.
Get Two 12v Halogen desklights - cheap (they can be bought for around £5. so you should be able to get them for less).
Dismantle them - they will contain a 12V transformer each...
Now connect one 12V output to one 12V input. You've just made an isolation transformer with about 6dB greater isolation than a true isolation transformer. Then, use the EL84 filaments wired in series (requiring.. you guessed it 12V or so), and using a 12AX7 or suchlike with its series centre tap.. Low and behold, 1 HT supply, and filaments heated too.
...hmmm , doesn't work too well in practice , had some nastiness trying series filaments on 5B/255 and EL38 . Placing power valves in series which are intended for parallel opeartion can sometimes lead to trouble
Yepp. If mixed and matched from batches and years the actual current draw may be quite of (10% is quite some).
But there is a myriad of series heated valves that could be used as power valves as well (gimme a 'P', gimme a 'L'...).
If one has a quad power valve that should have 37.5mA each and 12.6V for bias a 12AX7 filament could be a nice dropping resistor, and the ECC83 will have DC on its filament as well :-)
It's a good idea!
But make sure they are the heavy ones.
The lightweight ones use semiconductor down-convertors rathµéCYhan conventional iron/copper transformers.
No problem with the PSU. The low (extremly low) will be easier to filter (except for "diode spikes" etc.).
But do by all means think more than twice about how to build it.
Firstly you need to isolate all inputs / output. Output is usually solved by itself by the OPT if you don't use feedback. On the input you need an insulating transformer. And of cause the chassis can't be of a elecrical conducting material.
Have a look on Steve's site (linked) for info and warnings.
I think UL84 (EL86 equiv.) or UCL82 would be a more fit tubes if you wanna do away with filament transformer as well and don't use a huge cap for filament a'la TubeCads idea...
The chassis will be hot with line voltage if the plug is reversed. 50/50 chance of electrocution risk - Russian roulette with three bullets. If you use a "full wave" doubler, it'll be hot with EITHER orientation. - six bullets! So your CD player or other input will become hot too. The TVs and radios that were wired like this weren't meant to be hooked to anything - all metal was enclosed, where it couldn't be touched.
Steve Bench has a SAFE line operated amp circuit on his site, using an input transformer for isolation. http://members.aol.com/sbench101/
You can also buy a 50VA isolation transformer (enough for SE 6BQ5s) for $17 from Allied, a 100VA one for $26. Surplus is much less. Or check the circuit on my site, using a $30 toroid.
You may find that isolation transformers are cheaper than other power transformers and are available as 120/240 step-up. Check with Radio Shack and others. Just make sure there are primary and secondary windings and do not use a single-winding step-up which is not isolated. Then for filaments use a separate filament transformer. I rebuilt a guitar amp this way.
I just wondered what the possibilities were. I think this may be the best cause for a transformer, as otherwise I have to float everything else, and hum becomes a problem.
U should do it if your insurance covers acts of insanity. Is your life worth less than a transformer?
This is really very dangerous, unless you know EXACTLY what you're doing.
Even if you did (and if you did, you probably wouldn't have asked),the effort in ensuring safety far outweighs the cost / weight advantage.
Domestic equipment manufactures used this technique for many years on radio, audio and TV, for cost and weight reasons. Most of the makers took great effort to make the product safe - no metal was touchable, isolation components on input and output etc. They also developed special valves/tubes with high voltage, controlled current heaters, wired in series, so a heater transformer was not necessary.
Even so, the performance of the radio / audio using this technique was always inferior: Hum and noise being the main problems.
I spent a few years as a member of a national test comittee, and can confidently say that none of the equipment that used these techniques would pass the current safety tests.
Whilst I am happy (but cautious) working in my current 1400v HT amp, I wouldn't build a "live chassis" 380v one!
Guess what happens if the power plug (or the wall socket) is reversed? The chassis goes live, everthing connected to an interconnect or speaker wire goes to full line voltage, and zap, you're dead.
And don't kid yourself that wall sockets are always correctly wired - get a Radio Shack or Home Depot safety tester and discover for yourself how many sockets have neutral and hot reversed. Just because it violates every electrical code doesn't mean that electricians don't make mistakes. I've even measured *ground* miswired!
Your point about miswired outlets is well taken. It's a good idea to check outlets all the time.
The type of B+ supply mentioned was used in low cost table radios with bakelite cases. A series heater string was used, allowing cheap transformerless construction. My father had such a radio that used a 117Z6 as the voltage doubler.
IF (very big if) the wall outlet has been wired properly, a RELATIVELY safe P/S can be built. Connect the safety ground wire to the metal chassis. Build the "classic" full wave doubler with a 117Z6 or a pair of damper diodes. Wire the "hot" lead to the center of the filter capacitor stack. Wire the "neutral" lead to the 2 discrete diodes, then wire the diodes to the ends of the filter capacitor stack. SS diodes are out, since they have high pulse current capability. Current, not voltage, kills; therefore, proper fusing of the "hot" line as it connects to the filter capacitor stack is ESSENTIAL. A DPDT switch that interupts both the "hot" and "neutral" leads should be used too.
Of course, isolation from the AC mains by transformer is THE WAY TO GO.
> The chassis goes live, everthing connected to an interconnect or speaker wire goes to full line voltage, and zap, you're dead <
Exactly - but maybe it's not the culprit who dies (he could deserve it) but it could a child - his or a neighbours!
With a voltage doubler, your signal ground will be about 150vDC negative relative to safety ground. Anything plugged into this (CD player, TV, etc) will then be at -150vDC relative to anything connected to earth ground (water pipes, damp boards, all other electrical appliances, etc.) DC is generally regarded as more dangerous than AC, because the muscles are frozen and you can't jerk away - you just cook like a hot dog.
No don't do this. Your amp will not be isolated from the mains making it an electocution hazard to you and everyone else.
Two sensible routes would be - either use a 1:1 isolation transformer, one with a separate primary and secondary winding, then use your voltage doubler circuit.
Or use an industrial control transfomer one of those with say a 415V primary to 230V secondary, then run it in 'reverse' with the mains supply to the 'secondary'. This will give you about say 400Vac at the 'primary'. That way you wouldn't need your voltage.
Note the voltages depend on where you are, I've given a UK example. These transformers are designed to be connected across two phases of a three phase distribution system.
Both options are inexpensive using 'off the shelf' components.
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