Inverter

I'd MIGHT run a genuine AC/DC set that has not been modded with a capacitive dropper. But double check HT. I'd not run an AC one unless it was a good sinewave. RFI may be an issue.

The wattage is nothing to do with how smooth they are. Marketing speak "modified sinewave" is like "HD Ready", fairly meaningless, it may mean simply not a pure square wave. I think on consumer grade Inverter you need to look at AC O/P with scope when it's driving a transformer with dummy load on HT via rectifier + capacitor compared to real AC and see what DC on load and AC on a 6.3V winding is (with a dummy load),

Transformers and chokes don't like anything other than fairly close to sinewave inverters. A Universal AC/DC set won't care. But do check the HT as some inverters may give higher HT after a half wave rectifier.

Auto-transformer driven heaters with direct rectified transformerless HT count as AC only sets. (KB FB10?)

A modern SMPSU won't care, but early thyristor based SMPSU like in Thorn TX9 chassis might not like it!

RFI can be an issue too. I have a small four stroke generator that's not too bad, though likely unsuitable for an AC motor record player. I've used it for a hybrid 80W transmitter (FT101ZD MkIII).

CPC = Probably decent

Unfortunately there are lots of rubbish inverters now.

I have been looking at inverters in TLC, they have an outlet close to me.
300 watt modified sine wave is about £35. Pure sine wave 300 watt £60.
They are made by Mercury, do you think it is worth going for the £60 one to run a valve radio for an hour or so.
thanks
Mike

do you think it is worth going for the £60 one to run a valve radio for an hour or so.
thanks
Mike

I don't know but Rod Elliot did a good article on them recently, well worth the read before you go out and buy one.
http://sound.westhost.com/articles/inverters.htm

Eddie

I'd only connect an AC/DC set to that, I would call that a poor excuse for a modified Sine Wave. It's a modified square wave. A decent modified sine wave has more steps.

Even then, Check the HT!

Maplin 12V Pure Sinewave 300W Inverter

Code: A26JG
ON OFFER WEB ONLY £79.99

Thanks for the help guys
I will have a think about this.

Mike

A short update on my choice of inverter.
In the end I bought a cheep modified sine(modified square) wave inverter, the radios worked but there was a lot of buzz in the speaker, so I took it back.
I then spent twice as much and bought a pure sine wave inverter from Amazon, and that is only giving 220 volts on a fully charged battery of 12.9 volts and there was still a lot of buzz in the audio. I dont think the extra £40 for the sine wave one is worth it, so that is going back as well.
Oh well back to TLC for the original one.
Mike

Hi Mike, I'm afraid that unless the filtering is REALLY good you will always get noise on the set you are operating, even with a pure (well synthesised)sine wave. harmonics of the switching frequency easily get into the IF stages. If you have deep pockets and go for MIL switching frequencies of 1MHz+, itb then gets into the RF.

One solution, but low efficiency, is to use something like a Wien oscillator at 50Hz and is a pure sine wave . Feed through a transistor amp and output transformer.

Ed

Hi Mike,

Just out of idle curiosity why are you wanting to use an inverter ?
Your not planning a working radio display at Glastonbury festival are you

Marc,

Hi Marc
No, we are planning to go to a boot sale to try and sell some some of the remaining radios from that long list I published a while back. It will be like going home after a visit to the beauty salon for most of them.
I had been considering taking up a stall at a Harpenden swapmeet but then thought, it is easier at a boot sale. You dont have to carry them a 100 yards or so if you cant get close to the hall to park.
We have now decided to just concentrate on the transistor sets, I have about a couple of fold up trestle tables worth and maybe take some of the nicer looking valve sets along to see how it goes.

No rubbish you know.

Small plug, Dunton Boot Sale, Sunday 27/4/14, if the rain holds off.

Mike

I must have a look at my two vintage inverters..

One of them, made by Partridge I believe, has a very elaborate layout inside and is based around a huge transformer.. It hums beautifully like real mains 50Hz hum.. I can't imagine a TSW inverter coming from around 1963, though it was originally intended to provide back up power for a small telephone exchange.. The other one is similarly based around a big transformer, but is nowhere near as sophisticated, and the voltage fluctuates widely according to load.

I also have one of those 800W modern jobbies - all PCB and solid state wotsits with a little fan and LEDs to say it's working.. Definitely not TSW!

Even generators aren't all together ideal.. Definitely not the inverter types, and not the cheap imports of recent years. Some older generators are much better, and I'd recommend a Honda E800 from around 1974(ish) if you can get your hands on one.. I have one of these and can vouch their electrical performance.. They're also quite sought after!

Personally, I find inverters to be quite diagreeable things, and really best confined to lights and small power tools, i.e. resistive loads and asynchronous (compound) motors.

Marion

I think they need to have old slow transistors in them.
They were not too bad for radios.

Hi Mike, I'm afraid that unless the filtering is REALLY good you will always get noise on the set you are operating, even with a pure (well synthesised)sine wave. harmonics of the switching frequency easily get into the IF stages. If you have deep pockets and go for MIL switching frequencies of 1MHz+, itb then gets into the RF.

One solution, but low efficiency, is to use something like a Wien oscillator at 50Hz and is a pure sine wave . Feed through a transistor amp and output transformer.

Ed

Yes, the clean solution is a 50Hz (or up to 80Hz may be ok) osc and a 12V 100W amp driving a mains transformer secondary.
Or a couple of 2N3055 or modern equivalent and mains transformer, drive dual secondary or push pull You can get 65%

The FT101 used an extra winding on its mains transformer for a couple of power transistors for 12V supply on an optional rear mounting pack that was mostly heatsink.

Could anybody come up with a circuit to convert a 100W amp into an inverter. I guess it would need some kind of AVC to keep the output at a constant level.
I have a spare 100w stereo power amp and I probably have the rest of the parts to build this.
Thanks
Mike

Yes, I can.
Regulation is pretty simple actually!

I was thinking about this. Meanwhile, put secondary of mains transformer on amp output, 60W lamp on primary and measure AC volts with DMM. Put scope on amp output. Put amp volume at maximum.
Feed amp from sound card at 50Hz, adjust Sound card volume till you have 220V (not 240V, to allow for a margin). If full volume without clipping isn't enough volts use a lower voltage secondary mains transformer.

Then when AC is 200V to 240V approx without clipping (but near full output voltage), adjust frequency up towards 400Hz. Stop when lamp voltage vs input voltage starts to fall. Any radio should cope with up to 400Hz. It's 50Hz on a marginal 60Hz transformer is the issue. The transformer efficiency & power rating increases with frequency. Certainly 100Hz+ ought to work.

Then try radio instead of lamp.

If the mains transformer you use is about x2 the VA rating or more of the radio and amp is just below clipping voltage on output, then a simple phase shift 2 x RC + transistor oscillator with 5.6V zener regulator via 680 Ohms off the 12V with a 50K gain preset on the output to drive the amp should be OK with no feedback.

To add feedback use peak detector (2 x 1N4148 & 2 x 100nF) driven by 2 x 270K (=540k) & 4k7 (x33 attenuator approx) to give about 5.6V (I may have 540K & 4k7 divider followed by peak detector wrong), compare that with Osc 5.6V HT as a reference to control gain.

I shall try a couple of 15A FETS driving centre tapped secondary with antiphase sine wave drive in class C with tuning on the transformer and class B without.

Hi Michael
I think I am going to need a circuit diagram to understand some of your post.

Then try radio instead of lamp.

If the mains transformer you use is about x2 the VA rating or more of the radio and amp is just below clipping voltage on output, then a simple phase shift 2 x RC + transistor oscillator with 5.6V zener regulator via 680 Ohms off the 12V with a 50K gain preset on the output to drive the amp should be OK with no feedback.

To add feedback use peak detector (2 x 1N4148 & 2 x 100nF) driven by 2 x 270K (=540k) & 4k7 (x33 attenuator approx) to give about 5.6V (I may have 540K & 4k7 divider followed by peak detector wrong), compare that with Osc 5.6V HT as a reference to control gain.

The amp I have is one which I believe works off approximately 35v HT rail. It has 4 x 2N3055's on the OP, per channel, I have a photo somewhere I will go and dig it out.
Will the amp need any component changes to run it off 12v HT? It will have less power output capability.
I wonder if the 2 channels could be bridged.
Is the feedback you mention above for regulation of the voltage OP? What would happen if the amp had a fault on the input, I guess if it was operating near 90% of max it would not cause too much of a high voltage spike.

Here are a couple of photos of the amp I was thinking of re-using, It has 4 x 2N3055's per channel bolted to the chassis under each amp module. It looks like the power amp modules are professionally made, but the way the whole thing has been put together looks home made to me.
The mains transformers dont look like they would do the 4 x 2N3055's much justice from memory I dont think they are much more than 3.5 inches across.
Unless somebody recognises the modules and has a circuit I dont think there is a circuit for the amp.

Alternatively I have an Adastra A68 PA amp with a 100v line OP, so that is about half way there. It also has a 24v DC input, probably giving reduced power because the output from power supply HT is 30v.

Michael
I wont be able to do much with this project just yet as I have 2 radios on the bench at the moment, and need to clear space.

Thanks
Mike

Forget about trying to use a 35V rail amp.

I've not been very well. So not been in lab. I have schematics. Will post simple cct.

Here is part of Yaesu FT101ZD (the earlier FT101 are similar).

DC-DC converter Unit (no.32) is a vented option box that bolts on. It's got 2 x transistors, 2 x resistors and 2 x caps, a giant heat sink plus a plug.

The socket on the back of the radio (which takes about 200W or more) connects to extra windings on the mains transformer in push pull with emitters going to +13.6V vehicle plus (or a lorry battery under the bench) and winding centre tap to -ve as it's PNP transistors. A Jones style socket takes either a cable wired for AC mains or 13.6V battery 5.5A receive (1.1A if transmitter 2 x 6146 valve heaters are off!) and 21 Amps transmit. So about 285W DC in for about 80W RF out! About 60W is purely the heater power for 2 x 6146 and possibly the 12BY7 driver valve. I think the heater switch turns it off too. There is an octal socket on rear that has to have a dummy plug linking heater supply for the 2 x 6146 valves. It's used for optional Transverters for VHF.

The transformer provides feedback for the optional push pull PNP transistors to oscillate. There is no regulation. The output voltage depends on the Battery Voltage.

Mine is very late model, the wires are inside, but no socket for DC converter. Could easily be added.
The cover has 4 flanges with screws to hold it on

Oscillators
http://www.daenotes.com/electronics/dig ... scillators

Actually I have been meaning to build a crystal controlled 50Hz PSU so that mains record player or clock will operate. But most Radio Sets ought to be OK 50Hz to 200Hz and possibly up to 500Hz. Many mobile systems ran at 400Hz and anything AC mains that didn't have a motor was usually fine.