Atwater Kent refurbed but not yet powered up - just checking.

[Scotorvm]

Hi Guy's

Just about finished refurbing an Atwater Kent 20C set.

Has taken me a fair amount of time. Only thing I think is missing is a brass grommet from the rear of the set, everything else appears to be original.

Just checking if I have my head around the voltages especially the -4.5v..

Checked all components and windings, seem ok.

Used a GPO oak box to contain the PSU, all connectors colour matching the new cable attached to the set, soldered onto remains of original cable and wrapped in a cloth sheath before staining to age it, looks ok to me.

Not sure if this is the 7570 or the 7960 version, the tag on the bottom suggests 7570 but not sure, wiring indicates 7960.

Used an HT +90v PSU dropped via pots for other HT voltages ie 22.5v and 67.5v.

For the valve heaters used a separate PSU, the guts from a 5v 2A USB charger.

Used another PSU 12v just to illuminate a LED on the rear to let me know it is powered up.

Voltage wise I have +A Vfil supplied from the 5v usb charger.

-A common black is negative from the HT PSU.

-C (-4.5v) not sure about this, would this be the -5v side from the USB PSU?


+22.5v reduced from 90v on HT via pot.

+67.5v reduced from 90v on HT via pot.

+B (+90v) from HT PSU - connected straight to a tag on the set - speaker + wire also connected to this. -ve speaker connected to tag right next to this which is connected to valve 5.

Thanks
Lee

[Ed_Dinning]

Hi Lee, I doubt if the USB socket will power the filaments, but it may well cause damage to them as 5V could be way too high. I believe there was a thread on this topic a short while ago.

Use of USB for bias supply will probably also not work and again may cause damage to the valves.
Please find out what the filament and GB reqiirements are for you valves and their type numbers.
It would be a pity to destroy an expensive set of valves for the sake of doing some careful checking

Ed

[Scotorvm]

Originally Posted by Ed_Dinning

Hi Lee, I doubt if the USB socket will power the filaments, but it may well cause damage to them as 5V could be way too high. I believe there was a thread on this topic a short while ago.

Use of USB for bias supply will probably also not work and again may cause damage to the valves.
Please find out what the filament and GB reqiirements are for you valves and their type numbers.
It would be a pity to destroy an expensive set of valves for the sake of doing some careful checking

Ed

Hi Ed, thanks for the reply.

The 5 tubes are CX301-A

Filament voltage is 5v. Grid voltage is -4.5v, the USB supply under no load is 5.2v.

Do you think the USB should be OK, are there any other alternatives to supply the voltage?

On the characteristic sheet, it mentions 'the filament in the 01A is intended for operation from a 6volt storage battery. A fixed or variable resistor of suitable value is required to reduce the battery voltage to 5v across the filament terminals at the socket'.

Cheers.
Lee

[PJL]

They are 0.25A each so 1.25A total so should be fine but you could add a forward biased silicon rectifier to lower it as the valves will be costly to replace.

[Ed_Dinning]

Hi Lee, it depends on your USB supply, many will not supply 1.25A and may shut down at switch on when the cold filaments may take up to 5A

Grid bias will need next to no current but do not operate the set without it connected and powered.

Ed

[Scotorvm]

Originally Posted by PJL

They are 0.25A each so 1.25A total so should be fine but you could add a forward biased silicon rectifier to lower it as the valves will be costly to replace.

Hi PJL not really sure about this. Where in the PSU would the diode be fitted.
Don't want to cause any damage, didn't realise how expensive these valves are 2 on ebay just now £200inc post

Thanks
Lee

[Scotorvm]

Originally Posted by Ed_Dinning

Hi Lee, it depends on your USB supply, many will not supply 1.25A and may shut down at switch on when the cold filaments may take up to 5A

Grid bias will need next to no current but do not operate the set without it connected and powered.

Ed

Hi Ed. If I assume the USB supply can handle the switch on load, am I correct in thinking the grid bias for this set is the negative supply of the USB PSU?
The cable labelled -C(-4.5v) on the attached circuit diagram. This then goes to Valve 5, same valve as speaker negative?

I'm still new to all this but trying to learn

Thanks
Lee

[jjl]

A USB PSU won't work as a filament supply for this application. I explained why that is in this thread. As the valves in this set would very expensive to replace if something went wrong, I would advise against trying to use any USB supply.

The -4.5V bias "C" supply can't be derived from the filament "A" supply as it is in opposite polarity. As the set was originally designed to be supplied by batteries, the positive terminal of the "C" battery would be connected to the common connection supply connection with the negative terminal of the "C" battery providing the -4.5V bias supply.

John

[vinrads]

Hi Lee do you have a schematic diagram ? I restored an Atwater Kent some time ago, I think they used a six volt accumulator and used a piece of resistance wire to drop the extra volt, it got me a bit confused at first it just looked like a normal piece of wire maybe a bit thicker , just a thought .I built a five volt, five amp power supply using a lm 317 Five amp version , Mick.

[Station X]

Originally Posted by vinrads

Hi Lee do you have a schematic diagram? Mick.

In post #7?

[red-duck]

This may be helpful.

http://messui.polygonal-moogle.com/valves/VR198902.pdf

I have the 20 4640 and it operates nicely on 5v filament supply .. but as the magazine article says - in the later model there is no longer a variable resistor in the filament supply to the AF valves.

[vinrads]

That sounds like the one I had ,the rf section heaters was via a variable resistors and the output stage ,had an in line resistor . Mick.

[Scotorvm]

Originally Posted by red-duck

This may be helpful.

http://messui.polygonal-moogle.com/valves/VR198902.pdf

I have the 20 4640 and it operates nicely on 5v filament supply .. but as the magazine article says - in the later model there is no longer a variable resistor in the filament supply to the AF valves.

Hi, Nigel
Thanks for the link

[Scotorvm]

Originally Posted by jjl

A USB PSU won't work as a filament supply for this application. I explained why that is in this thread. As the valves in this set would very expensive to replace if something went wrong, I would advise against trying to use any USB supply.

The -4.5V bias "C" supply can't be derived from the filament "A" supply as it is in opposite polarity. As the set was originally designed to be supplied by batteries, the positive terminal of the "C" battery would be connected to the common connection supply connection with the negative terminal of the "C" battery providing the -4.5V bias supply.

John

Thanks John, I will revisit the PSU I have made the attached is the original battery setup.

Not sure if I can alter what I already have. Which is basically a 90v psu (taking the place of battery 'B') dropping for the 67.5v and the 22.5v via pots and caps. If I lose the USB psu, then I need to find a 5v supply capable of at least 1.25A (battery A) and possibly 3x AA batteries for battery 'C'. Someone has constructed a PSU specifically for this set also attached - looks complicated though.

Cheers
Lee

[Station X]

Originally Posted by Scotorvm

and possibly 3x AA batteries for battery 'C'.

Cheers, Lee

These will last forever as they don't supply any current.

For HT I use PP3's in series tapped at the appropriate points.

Perhaps build just the 5V section of the PSU in your post to power the heaters?

[Scotorvm]

Originally Posted by Station X

These will last forever as they don't supply any current.

For HT I use PP3's in series tapped at the appropriate points.

Perhaps build just the 5V section of the PSU in your post to power the heaters?

Hi Graham
I was hoping to electrify the whole PSU, but, yes the 3 x AA batteries for the C battery would be fine.

I was hoping to keep the 90v / 67.5v / 22.5v PSU as I paid for this.

The 5v fillament supply, instead of the USB supply I am going to build as per the attached circuit. Good for 1.5A with a heat sink on the voltage regulator. This will be the A battery.

I have attached a sketch of how I think this will be connected, just not sure re connecting power supplies together!!

Thanks
Lee

[Scotorvm]

OK guy's, this set is finally working 100%.

What I had to do is - replace the grid leak resistor, was measuring 12Mohm, replaced for 2.5Mohm, left original just snipped one leg.

Power supply was a headache, lots of trial and error. Have now achieved a steady PSU.

For HT voltages I used a 90v supply from thecodemachine

Used this to drop down via pots and 100uF 100v caps to the other HT voltages of 67 and 22.

This was my 'B' battery.


For my 'A' battery used a 230vac to 12vac toroidal transformer, 60A rectifier module, smoothing cap, now had 16vdc, this now supplying a buck converter
https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1

Has a handy display to view voltage, this is for the valve filaments. Which are 5vdc

'C' battery is 3x AA battery block..

These were all connected together as per the attached pics.

Hopefully this will save others some time.