Atwater Kent Model 30 Question

[DavGoodlin]

I built a power supply for my AK from a vast supply of salvage parts.

It makes an A-voltage of 4.8 volts DC at 1.5 amps for the filaments
and B voltages of +95, 65, 22 volts for the plates of the 71A, 01A's and 12A respectively.

Question: Would I be able to apply the filament voltage as positively grounded and derive the C voltage from the A supply at -4.5 volts?

If not, I will add a second rectifier bridge to the A supply transformer.

Thanks - This is my first time doing this. Fortunately there are two AM stations that still play music here!

[Reece]

I have no conclusion, but cast you adrift upon these waters:

http://antiqueradios.com/forums//vie...?f=12&t=143001

What you could do is put three AAA cells in a holder and connect them for C. They would last forever as there is no current drain on them.

[polyphase]

Applying the fil supply at positive ground would throw off the grid bias of all the tubes.
A 'C' supply is required. The AAA cell solution suggested above is the way to go IMO,
as really clean Vdc is mandatory here.

[DavGoodlin]

Thanks Guys, I will use the 3AAA battery solution for C.

I ran a 120 ft antenna outside and applied power.

We got filaments lit and measured correct B and C voltages but have a 120 hz hum from the full wave bridges I built. Do I need a B voltage choke like on the AK 43?

No matter how much filtering capacitance is applied to the 90 v B supply
(500 mfd) or the 5v A supply (36,000 mfd) it is overpowering the audio. We are getting reception, but a check of the antenna choke confirmed it is open.

[DavGoodlin]

The battery solution has been implemented with help from R.S. battery holders @ $2 each.

The hum is still there and can only be from the B voltage source. I will try an choke.
Antenna choke is definitely needed. Does anyone make these as reproductions?

[polyphase]

A choke is not required. 500uf is way more than enough for the B+ supply of this radio. Please check the filters on the 67v and 22v supplies. Each should have a cap to ground, maybe 50uf, probably less. The detector supply (22v) is particularly sensitive to supply ripple.
A good earth ground, with the caps mentioned above and the C+ supply connected to it, hum will disappear.

footnote: One of my '20s radios had hum just because the line powered B supply was too close to the radio.

[DavGoodlin]

Thanks Polyphase, I will try this over the weekend. The choke is open, so the antenna goes right into the grid of the first 01A, maybe a hum issue there too. I need to ceck the resistors and caps when I pull out the chassis.

Might just gang ten 9-volt batteries together and call it a day...

[vintagecollect]

Just to chime in here. Try use use those 500uf caps when possible in power supply. I have got a 1924 AK 20 Big Box set running off an AC supply. I used big caps to quash any AC hum, really needed since these are DC sets to avoid hum showing up in audio.


That supply needs smooth as possible dc. Smaller caps used--- as big cap technology not there yet or too costly long ago.

[teevee]

Take care of that front end RF choke first. You can sub in a resistor temporarily, I think you'll find the hum is gone. Try a 1k or so.

[polyphase]

I looked at the schematic for this set, and noted the following....

There is a cap from the 67V supply to ground. If this is the original,
it is probably leaky & should be removed. No need to replace.

The -A input is not directly grounded, but rather passes thru a fil rheostat to
ground. This means your A supply must float with respect to the other supplies.

It also ocurred to me that your A supply, if it does not include a regulator, could
be a source of hum. Even with 36000uf, at 1.5A draw, ripple would be on the order of .2Vpp a quick calculation shows. This is enough to cause trouble because the filaments are also the cathodes, and any noise here would be amplified the same as if applied to the grids.

FWIW, my preferred fil supply is a small (10Ah) 6V lead acid battery. Can be charged with a wall wart or even a solar panel.

[DavGoodlin]

The Antenna coil is toast, some varmint feasted on it before my time. The original was a 35 milliHenry and, based on 01A grid capacitance, resonates at 2 MHZ, so I could go down to 20 mH... Maybe a 500 K or 1 Meg resistor for temps?

For now, the A supply is 4 D cells but maybe use a 7805 regulator on the supply later. The B supply will have about 500 mfd in each source, 22v, 67v, 90v. Will check the caps and grid leak resistors, eliminating the extra cap on the 67 B leg. Of course, I left the schematic I was drawing at the office friday...

Pics to follow

[DavGoodlin]

Here are the pictures. The antenna coil is the little item sticking out the upper left end of the chassis.

[polyphase]

Nice looking set. Here is the schematic, also available at nostalgiaair.org

[teevee]

Just use a 1k to 5k for now, the important thing is to not have the 1st RF grid hanging open. To test the theory, remove the first 01'a and see if the hum is gone...

[DavGoodlin]

I am making some progress. The grid leak is open. All other resistors are OK. Its a wonder I got anything before.
Just ordered grid leak and the coil from oldradioparts.com (PTOP)

I will try it tonight with resistors subbed in. This is way more fun than AA5 tinkering! (AKA shooting fish in a barrel)