20's neutrodyne valve radio

[unrealdave]

The neutralising condenser is Igranic, and the transformer is a Ferranti type A.F.4.  The tuning caps look like Ormond but the only mark on them is DEVICON

This is home built - possibly from a kit - but the layout is a lot like the American sets from that time.  The case is extremely well made and 4 valves at that time must have cost a lot.  I am going to pair it with a large horn speaker when I get it going as it will look great!

Anyway.......

I replaced the valve holders with more substantial ones as one of the originals was broken.

I took out the 3-4 ohm resistors in the LT+ line as I am going to use 2 volt valves with it.

A few of the soldered hook up wire joints were dodgy so redid them then messed about with a mm.  The LT + was fine until I went to the other side of the switch on the front panel - then read 60 Ohm's.  So I cleaned up the switch in vinegar and copper shone through - all the metal parts of the switch are copper and a black coating accrued over time must have given the 60 Ohms resistance.

I had another (working) type A.F.4 so put that in.

One of the LW coils will not sit in the holder - the pin is solid - ie not split like all the others so won't compress and fit in the hole - I will worry about fixing that later...

That's it for now - when I get a chance I will attempt a circuit diagram but won't be for a while.

cheers guys

[pwdrive]

Looks good so far.

Lawrence.

[Alan Douglas]

the pin is solid - ie not split like all the others

I've found that a large percentage of homemade receivers could never have worked in the state I found them, and I have to assume their owners gave up and bought something else.

I'd take a jeweler's saw and split the pin.

That neutralizing capacitor looks very large.

[Mark Hennessy]

As requested, the posts about drawing schematics have been split to a new thread:

http://golbornevintageradio.co.uk/forum/...p?tid=4863

All the best,

Mark

[unrealdave]

I've found that a large percentage of homemade receivers could never have worked in the state I found them, and I have to assume their owners gave up and bought something else.

I have found the same thing - homebrew with the reaction coil the wrong way round right up to incorrect wiring that could never work - but you don't know how many people have had a go at it

I'd take a jeweler's saw and split the pin.

I didn't think of that - just bought one from Amazon - I'll let you know how it goes

That neutralizing capacitor looks very large.

I will sort out a few different neutrodyne condensers and see what the capacitance is  

I have had a stab at a circuit diagram - loving this ink package.  See if it looks anything like it should

[ppppenguin]

A note on circuit drawing conventions.

In olden days when one wire crossed over another we used to build a little hump-backed bridge to give it a smooth journey. If 2 or more wires met then they were connected. Then the dots got in on the act and the bridges were redundant. Every T junction had a little dot to say it was connected. The problem was with crossroads. Connected ot not? A dot says connected but is easily missed so the modern convention is that a crossroads is never connected with a dot. If you're tempted to doy it, move the wires away from each other. Just like plumbing really where you don't usually have 4 way "X" joints and use a pair of "T".

[Alan Douglas]

Apologies if you've used a jeweler's saw before, but be sure to set the blade to cut on the pull stroke.

[unrealdave]

Apologies if you've used a jeweler's saw before, but be sure to set the blade to cut on the pull stroke.

No - never used one before so good tip - cheers matey

[Alan Douglas]

There are some YouTube videos on using one.

[unrealdave]

I’ve had a look at 4 different neutrodyne capacitors and included a pic – they read (from left to right in the pic)

.75pF – 29pF
.7pF – 15pF
.9pF – 25pF
.3pF – 6pF

As Mike said earlier the neutrodyne was a type of radio using a technique patented by a chap called Hazeltine.  He came up with the idea of neutralizing a triodes grid plate capacity while designing a receiver for the navy in 1919.  The neutrodyne circuit was shown in the Radio news in May 1923 – see page 29 of the pdf.

The interstage coils are wound so the secondary voltage opposes the primary voltage, and is coupled back to the grid by the neutralizing capacitor which is adjusted so the grid plate capacitance is nulled out. 
 A good read is a book called ‘Behind the front panel’ by David Rutland.