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mendipviews
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I have been given three Audio Amplifiers from a clear out they are unusual in the fact they contain a Rotary Generator and look to be designed for battery use.
They are made by "Express Radio" of Portsmouth, they are missing the valves :| but contain a lineup as follows
2 x 6L6 and 2 x 6N7
Controls are Fil, gen Tone and Volume

I was wondering if anybody knows anything about them as Google doesn't seem to turn up much

Many Thanks
Luke
2E0VHV

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Topic starter Posted : 08/05/2013 12:43 am
Katie Bush
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Rotary Inverters were quite common in aircraft usage, I believe, to provide the necessary HT supplies, and unless I'm mistaken, and I often am, even NASA were still using rotary inverters in some of their space vehicles into the late 1960's.

Typically, you will have a motor end, driven at say 12V or 24V and then the "inverter end" outputting whatever voltage you may require.. Usually, I think, DC to DC inversion, but there were some DC to AC versions as well.

As for the amps themselves, they're outside of my scope of knowledge, but can only assume they'll ultimately be battery powered via the rotary inverters.. I this case, I imagine the valve heaters will be directly powered from the batteries, and the HT, logically, from the inverters.

Marion

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Posted : 08/05/2013 12:59 am
mendipviews
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Hi Marion,
thanks for the reply.
Indeed I have a power pack for my 19 Set thats rotary powered so to speak.
Interestingly enough I just checked the valve specs and the heaters are 6.3 volts and can be AC or DC
I will have to get the covers off and see if its a 6volt or 12volt power supply required, and then have a rummage for some valves and see if can get some life from the unit.

Many Thanks
Luke

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Topic starter Posted : 08/05/2013 1:15 am
Katie Bush
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Hi Marion,
I will have to get the covers off and see if its a 6volt or 12volt power supply required, and then have a rummage for some valves and see if can get some life from the unit.

Many Thanks
Luke

Hi Luke,

I would expect it to be 6V supply.. The valve numbers give the clue here, 6L6, and 6SN7.. If it were have been 12V, the valves would have been 12L6 and 12SN7, I believe.

I wouldn't imagine there being any value in using 12V batteries, then dropping the voltage to supply the heaters.

Marion

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Posted : 08/05/2013 1:22 am
mendipviews
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The plot thickens....
I have just taken the cover off one and the rotary generator is 12volts DC Input with 300Volt 115ma Output
The valves look to have the heaters in series straight off the 12volt line
It has the usual swelling capacitors but looks to be in reasonable nick, some of the capacitors are marked April 1945 so early than I thought.
I also answered a question I was going to ask about speaker impedance but the unit has a Output Transformer with selectable outputs of 7.5Ohm/15Ohm/62Ohm/250ohm & 500ohm by moving a tap all neatly marked on a panel, the transformer is made by Ardente Acoustic Laboratories Ltd

Pictures to follow tomorrow

Many Thanks
Luke

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Topic starter Posted : 08/05/2013 1:28 am
Katie Bush
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The plot thickens....
I have just taken the cover off one and the rotary generator is 12volts DC Input with 300Volt 115ma Output
The valves look to have the heaters in series straight off the 12volt line

Pictures to follow tomorrow

Many Thanks
Luke

That makes sense and obviously allows the use of 6.3V valves from a 12V DC source.. Two series chains, albiet short series.. If it's four valves, that would give a two chains of valves at 6V per valve heater.

Marion

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Posted : 08/05/2013 1:41 am
mendipviews
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Indeed there are four valves, two are in series and then the two sets are in parallel.

Is there anything I need to know about the generators prior to giving them some power as I have heard from various sources these can be quite fragile things??

Many Thanks
Luke

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Topic starter Posted : 08/05/2013 1:46 am
Anonymous
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Pairs of identical valves in Series / Parallel. Common on 12V car Radios and ALL Military valve gear.

Pity the 6L6s missing :(

By about 1946 Rotary Converters or Dynamotors only used on Transmit, then not at all, the Vibrator pack (dating from 1930s actually) replacing them. Then in early 1950s the vibrator pack replaced by 2 x Ge Transistors (on Military gear) with an extra pair of windings added to same transformer the vibrator drove, to drive bases. I had a pair of late 1950s amps with 4 x EL34 each that ran off 12V (via vibrator) or mains, using an extra winding on mains transformer. 19" rack mounting. From P.A. for a Mill (in the cloth factory sense).

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Posted : 08/05/2013 1:53 am
Anonymous
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I never heard of the generators being delicate. Think of 2 x Black & Decker drill motors on one shaft. One is 12V and the other 240V. That's all it is. Normal issues of brushes and commutator and bearings as with any motor of that size.

Noisy inefficient beasts. You need a car battery. Startup current is fierce, as with any loaded DC/ universal motor of that size.

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Posted : 08/05/2013 1:57 am
Katie Bush
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Is there anything I need to know about the generators prior to giving them some power as I have heard from various sources these can be quite fragile things??

Many Thanks
Luke

I've never had one in bits, but I would imagine it useful to check and if necessary, clean the commutators, especially the 'inverter' end.

Insualtion would also be worth looking at, on the inverter's rotor windings in particular.. I would imagine it will be all OK, but it's too late after things have gone wrong.. Are the ends of the inverter enclosed? If so, this will have protected the innards from dust and grit, but could also have trapped moisture over the years.. If there are removable covers, it would make it easier to check all of the above, and would be useful to leave them off for a while to help get rid of any dampness..

I am just wondering if it would be wise to run up the inverters at a lower input voltage to begin with, rather like waking up a mains set via a variac.

Would it have any info regarding rotor speed (RPM)? I've heard of these things running at up to 3000 RPM, which could make it interesting if anything were to let go.

Marion

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Posted : 08/05/2013 2:00 am
Anonymous
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All the ones I've seen are a casting at each end for the brushes/ bearing/commutator and metal tube over the rest.

I'd run at full 12 to 13V

     /-----------------
   /                     
 /                         
[                           ]
                          /
                        /
     -----------------/
12V motor               240V dynamo	 
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Posted : 08/05/2013 2:06 am
Cobaltblue
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Although Dynamotors are not particularly delicate they can be killed.

They were common surplus in the 70's and I used one to run a 40 W light bulb and a Regentone DW2 during the blackouts of the 70's. I ran another 40W light bulb from the Vibrator pack of an old Car Radio.

Check for suppressor caps they sometimes lurk in the frame next to the brushes. They are frequently leaky and or ineffective.

Try to avoid arcing on the brushes on the secondary side or shorts as the high voltages that can be generated by this can result in shorting turns on the secondary side, I think that's how I managed to kill two of mine.

Personally I like Dynamotors but they are not very efficient.

Cheers

Mike T

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Posted : 08/05/2013 10:47 am
jimmc101
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Up to the early 60's there were several Government surplus / electronics shops clustered around the junction of Lake road and Fratton road in Portsmouth.

At that time 'Express Radio' was a shop in Lake road, as I remember, more radio / hi-fi than surplus.
(It later moved to Fawcett road and became more hi-fi oriented although they still sold components.)

I wonder if your amplifier was designed to take advantage of vast quantity of Government surplus parts available?

Re Dynamotors', like Mike, my experience of them is of failure caused by insulation breakdown on 'high voltage side'. I think this is where the reputation for fragility comes from.

Jim

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Posted : 08/05/2013 6:01 pm
boyblue
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Hi. Were Ardente makers of hearing aids?. Could this amp have been for driving an inductive loop?. Most loops were lower impedance than the OP of this amp though. The dynamotor doesnt quite fit either. Peter

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Posted : 08/05/2013 8:43 pm
Refugee
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Would it be a PA amp from a passenger ship?

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Posted : 08/05/2013 9:00 pm
mendipviews
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Thanks for the replies everyone
Here are some pictures so everybody has an idea what I am on about.

Hope it helps

Many Thanks
Luke

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Topic starter Posted : 08/05/2013 11:23 pm
Katie Bush
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Well.... That's certainly the device I was always brought up to know as a DC-DC rotary inverter (or perhaps, "converter").

Not fully enclosed, and no covers, by the look of it.. I would expect it to clean up quite well, and will mostly likely function as it should.. I would say particular attention should be paid to cleaning and servicing the commutators and brushes - especially the high voltage end.

I'm wondering about the speaker outputs, and in particular about them being high voltage jobbies?

Marion

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Posted : 08/05/2013 11:39 pm
Anonymous
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UK: Rotary Converter
US: Dynamotor (which is exactly correct, for once!)

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Posted : 09/05/2013 12:05 am
Refugee
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What on earth is that huge 47K resistor connected to?

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Posted : 09/05/2013 12:52 am
mendipviews
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What on earth is that huge 47K resistor connected to?

It is connected from the Main HT Smoothing Capacitor to a tag board which splits between the large capacitor next to the resistor (This connects to the chassis and is only rated at 12V DC 75MFD) and a connection on the Microphone Transformer at the other end of the chassis

Strange
I think the next step is to trace out the circuit diagram

Many Thanks
Luke

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Topic starter Posted : 09/05/2013 12:58 am
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