powering up M8 tube amps

[Electric-Hair]

I need help in powering up the tube amps off a Akai M8 reel-to-reel.

Each of the tube amps has its own transformer, but what consfused me was the voltage selector transformer which sets the appropriate voltage for particular country. Being an Australian we use 240 volts and I don’t have this selector and transformer. And I’m not sure if this transformer is a step up or down one either?

Can I connect 240v power to the amps main transformer? On one side I assume is the primary, there are 2 groups of 3 terminals – 250v, CT, 250v @ 60mA D.C. and 117v, 100v, 0v. The other side of the transformer has 2 terminals for each – 5.7v(0.3A), 6.3v(0.6A) and 6.3v(1.8A).

I’ve got a fully working Akia M8 reel-to-reel to compare, as well as the service manual. It’s a little hard to trace things because the wiring is spaghetti and clamped too well. The mono blocks look identical and complete in comparison to the reel-to-reel, except for the power selector transformer which sit’s up higher next to the motor.

This link suggest if my power is 120 volt then I could wire it up as 117volt tap, neutral to 0 tap and ground to chassis?
http://messageboard.tapeop.com/viewt...=asc&&start=15

and,

http://beyondsanityproductions.com/i...ask=view&id=13


But I’m using 240 volts power, in this link he is asking whether it’s safe to have both 117v in series.
http://www.groupdiy.com/index.php?topic=40846.0

Here's link to a restore
http://www.briarsfield-hifi.com.co.uk/akai_M8.htm

Maybe all I need is simply a commercial step-down transformer that both amps could plug into?

thanks

[ukcol]

The 0v-100v-117v winding on your amplifier transformers will be the mains primary winding, all the others are secondaries. So you can't connect 240v mains to your amplifiers. Although connecting the two amplifier transformers in series may work on a nominally 240v supply it is a very bad idea. The two amplifiers will have to present exactly the same load and impedance to the mains which is not likely and certainly won't happen under any fault conditions.

Here in the UK power tools on building sites have to be 110 volts and transformers for that purpose are available here new and second hand. I don't know if these transformers are available in Australia but they would be suitable for your amplifiers. This 110 volt supply would be safe with your amplifiers set to 117 volts input. You would need to select a transformer of suitable VA rating of course.


Are the amplifiers seperate or part of the M8? Your first sentence isn't clear to me on that point.

Colin M

[Chiltern]

Only the other day I consigned one of these Akai transformers to the skip, having sold on the amps.

Here in the UK we also use 240v and this transformer (from memory now) was multi tapped on the primary, 240,220.200, (120?) 117 and 100, the secondary 117 and 100, I seem to think that the motor on these Akai models is a 100v motor. It is a fairly small transformer (size/weight) and I would think that if the consensus is, that running the amps in series is not a good idea, a simple step down transformer to 120 would suffice.

I have a Teac 2300S, originally supplied in the US and 120v only, this has now a toroidal transformer fitted inside, to step down our 240v. The Teac has a stepped pulley to enable the change of supply frequency and a frequency change over switch.

Alan

[Electric-Hair]

Originally Posted by ukcol

Are the amplifiers seperate or part of the M8? Your first sentence isn't clear to me on that point.

Colin M

The amps are seperate, I picked them up a later thinking they might make a good versatile amp. The have all the sockets on the front to make all soughts of connections.

[Electric-Hair]

117v, 100v and 0 is the primary winding. 250 -0-250 is the HT secondary in another thread. In the schematic it looks like the 250 one is connected to a tube.

I think I might be able to pick up a commercial step down unit here that I can just plug into like any other American appliance.


thanks

[Electric-Hair]

What about these step-down units?

50VA 240VAC to 115VAC Stepdown Transformer
http://www.jaycar.com.au/productView...r&form=KEYWORD


120W 240 - 115V Isolated Stepdown Transformer
http://www.jaycar.com.au/productView...r&form=KEYWORD

[Radio Wrangler]

A couple of comments about power tool transformers just in case they cause anyone some surprises:

1) Their output may be 55-0-55 with the centre tap grounded

2) With one in use, faults on the secondary circuit won't trip RCDs (ELCBs) in the 240V supply.

David

[Refugee]

I have two cinema amps also 110v only. These used to have one transformer each when new as the motor and lamp were also 110v. The transformer is a non isolated one with a selector rated at 1200W. I run two amps from one transformer. I also thought about wiring them in series and quickly discounted it for all the above reasons. My transformer also has a little 130v power supply with its own transformer that was a retrofit for magnetic sound track films in the same case.
I have two of these and 4 amps to enjoy here.
The yellow site transformers may cause hum due to the odd location of the earth. The best would be an isolated one so that you can try it with reversed secondary if there is a hum issue. I am pretty sure that an ordinary 110v transformer would be cheaper if you have to buy one.

[Electric-Hair]

From the M8 manual the complete unit takes 100VA. So without the motor the amps would take less and individually I suppose half.

I could buy a transformer but I thought it might be simpler and quicker to just get the commercial unit, especially if work needs to be done on the amp. Some of the wiring needs to be reduced or redirected and the capacitors need to be replaced too I suppose. I assume the more expensive isolated transformer unit is the preference for such as potential hum but non-isolated may be O/K. On second thought, maybe a 240v to 100v worth considering?


It’s a bit of a brain tease for me this voltage selector combination. The power plug active goes straight to the selector and its transformer has 6 taps. 2 of those taps, bottom(ground) and top and power plug neutral go to a plug socket. On the amp transformer the 250v section looks like it goes to a tube and there are a few wires connected to the 100v tap and 0 tap. The tubes the amps take are EF83 *2, 12AX7 *2, 6BQ5 *2, 6X4*2, 6ARS *1. That last tube is only on one amp(the top one I think). There are 2 additional smaller transformers in each amp, lots of capacitors and resistors. They are 6 watt/8ohm amps.


Far as controls and sockets in the amps, there is your volume knob, one tone knob with a bias knob in the centre of that. It sounds best on the highest bias setting. Each one has its own VU meter which reads the speaker outs I think. Far as sockets are concerned, there is a line-in socket and microphone socket. There is also a turntable-in preamp socket and preamp out(line out) socket. Each have their own speaker-out socket. Each has a power switch. Finally, the top amp has the record light and “push to motor” push-button switch, and the bottom amp has “push to sound on sound” push-button and headphone socket.

[Electric-Hair]

Success tubes are alive.

I managed to pick up 240v to 100v step down transformer. The only catch was the step down transformer had Japanese sockets but I managed to get a plug adapter too.

All tubes look alive as well as the VU meter and there does not appear to be any burning smells. It’s the bottom amp with one less tube and less wires. This one had a power lead that is meant to plug into the top one.

I suppose the next step is to connect some audio and speaker to it. There might be a slight catch there cause the motor is meant be in record for some audio to pass through. Well there is a switch in the back for the push button switch lever.


Mmmmmmm ……………

[Electric-Hair]

I get audio now that nice groovy tube sound.

Next thing to do is reduce the wiring in the top one to be the same as the bottom one.

Then it is to make a cabinet for both. I suppose the stepdown trans should sit outside.

[Refugee]

The extra valve on one of the units is likely to be the bias oscillator and will not be required if you are just using it as an amplifier. Also without an erase head connected it may not behave itself.

[Electric-Hair]

Originally Posted by Refugee

The extra valve on one of the units is likely to be the bias oscillator and will not be required if you are just using it as an amplifier. Also without an erase head connected it may not behave itself.

Im starting to reduce the wiring on that one. Iv taken that tube out as well as an oscilator coil. The 12ax could be the preamp.

[mrmagnetophon]

Do you still have the oscillator coil?
-Chris

[Electric-Hair]

Originally Posted by mrmagnetophon

Do you still have the oscillator coil?
-Chris

Ive taken it out along whith the other stuff like a few extra resistors and capacitors. Because the tubes were in a different order I had to move one so it would be exactly the same as the other.

There is a switch at the back on both that was operated from the front pushbutton which I need to mull over. I think it is about configuration. The turntable preamp and the mic/line in preamp can be operated seperately if one chooses and that is the way beyondsanity.com have done it. I think they are concentrating more on microphone preamp tasks.

I might want some speakers of the full range 10 watt/8 ohm variety. I was sought of thinking 5 inches but there is not many cheapy choices about. Iv noticed some tube amps have a 4,8 or 16 ohm switch at the back which really would be handy.