Graves Vulcan

[radiosoul]

Hi folks,

I have recently acquired a Graves Vulcan by J G Graves Ltd (photo attached) which appears to have been manufactured in 1926. At some point I'd like to test it out but I'm not yet familiar with the valves used and what working battery voltages were applied. Clearly, it would be great to obtain a circuit diagram if anyone knows where I might get one. I would also like to know what impedance a speaker would be to work with this set.

The valves are:

2 x PM1LF (Max filament Volts 2.0 V, Max anode voltage 150 V, Max filament current 0.1 Amp)

1 x PM2 (Max filament volts 2.0 V, Max anode voltage 100V, Max filament current 0.15 Amp)

If anyone has information on this set I would be grateful to hear from them.

Kind regards


Radiosoul

[Nuvistor]

Could be this one, obviously cannot be certain.
https://www.service-data.com/product...73/3407/s12573

[Ed_Dinning]

Hi when you initially power this unit up, test with LT only first and see if the valves light.
Make sure you have a working grid bias battery fitted (I believe the output valve is supplied from this battery) and use the recommended or a higher voltage. NEVER be tempted to operate the set with HT on without GB. Do not even change taps on the battery with HT connected as this can easily result in excessive HT current that will damage valves and transformers.

That said they are a nice little set that performs well on local stations with a good aerial.

Ed

[60 oldjohn]

Hi Radiosoul, I think the info in the link is for the wrong set, it is for a two valve set and yours is three valve. The same info is available here https://www.americanradiohistory.com...ld-1928-12.pdf, scroll down to #26. Please let us know if you find anything relevant to your set elsewhere .


John.

[Herald1360]

Originally Posted by Ed_Dinning

Hi when you initially power this unit up, test with LT only first and see if the valves light.

Given the comment on the labels on these valves "there is hardly any glow....." it might be easier just to check the filament resistance with a multimeter.

The original LT supply would have been a single lead acid accumulator cell, hence the 2V. DON'T feed them with more! HT wise the output stage would likely be 120V though 90V would be fine for initial playing about. The earlier stages probably 60V, though again 90V won't hurt. Ten PP3 batteries clipped in series is handy- you can pick off 60V part way up if you want. They will bite if you're careless! You'll also need a grid bias supply- see Ed's notes.

A loudspeaker needs to be high impedance so to use a modern one an output transformer will be needed. Pretty much any LS and TX from an old mains valve set will do for starters.

[radiosoul]

Hi Folks

Sincere apologies for the late reply - been out on call-outs for the last two weeks!

Many thanks indeed for your comments. Ed I shall take note of the LT application!

I have a Circuit Specialists bench power supply with current limiter that I can use to begin with. I need also to get a suitable bench power supply for HT. I want to be able to tie these together to produce grid bias and HT

Frank, I've just ordered the service data CD so we'll see if that gives me a suitable circuit diagram. As John has pointed out, the set is a 3-valve set so I'll need to do some more searching. John, I did check your link and it leads to the 2-valve version. That link is packed with interesting information though!

I have just purchased a BTH horn loudspeaker with impedance of about 2k Ohms. I'm assuming, at this stage that it should work with the set.

Next on the list is to get the valve pin allocations although I guess I could work that out by inspection - will try to get the diagram first though.

High voltage kicks don't bother me - I'm a sparky, among other things..

Radiosoul

[HamishBoxer]

Speaker should indeed be fine.

[radiosoul]

Some photos attached.

There are three battery terminals - from what I make out. So these need to be identified.

What looks to be the output valve (a PM2) has a twin cable attached to one of its terminals so I have guessed that this is one of the speaker leads.

As you will see there's a shot of some external terminals, one is "E" presumably for Earth and the upper two are A 1 and 2. My assumption is that these are speaker terminals but that conflicts with my observation about one of the speaker leads being connected to the PM2. Inside the set I have another shot of the external terminal connections where there is a 0.0002uF capacitor across terminals A1 and A2. A2 is connected by a straight wire to what look like two resistors on the baseboard.

[radiosoul]

one more photo of the interior showing the external terminal connections on A1 and A2

[radiosoul]

If the radio museum in Switzerland (https://www.radiomuseum.org/r/graves_vulcan_iii.html) is correct then I have a Graves Vulcan III. Now casting around for a diagram.

[Herald1360]

All B4 based directly heated triodes have the same pin arrangement, AFAIK.

https://www.radiomuseum.org/tubes/tube_pm1lf.html

shows the PM1LF.

Most of the manufacturers' datasheets don't seem to bother, presumably because everyone knew it at the time anyway.

[TrevorG3VLF]

My guess is that E is earth and A1 and A2 are aerial inputs, they are connected together with a capacitor, chose the most sensitive.

You can find the correct power input connections by measuring the connection to the valve terminals. Do the filaments first and check with 2V maximum.
The second HT input will be connected to the primary of the intervalve transformer.

[radiosoul]

Thanks to you both.

I found the PM2 pinout on the Valve Museum site and it's the same as the PM1. I guess it's time to lift the set out of its case and do a few tests. That will give me an outline of a circuit. I still need to understand how the LT and HT are connected in practice.

[AC/HL]

A1 and A2 are for selectivity adjustment to compensate for different aerial lengths. The maximum allowed was 120 feet, but not many had gardens that long!

[radiosoul]

Actually pulled out a book I picked up from the local second-hand book store. Title is "The Book of Practical Radio" written by John Scott Taggart. He goes into a great deal of detail about operating a reaction receiver including the antenna coupling coupling capacitor adjustment for the compromise between signal strength and selectivity. Yes, if I strung a 120 foot antenna out the back of our flat I'd attract attention, no problem!

[radiosoul]

Just drawing out the circuit diagram. A while since I've done this but I may as well start with a simple circuit. Cold filament resistances are: 1st PM1LF = 7.5 Ohm (Max anode voltage 125V, 2nd PM1LF - 4.8 Ohm (its max anode voltage is higher than the first PM1LF at 150V. and the PM2 = 3.6 Ohm. On first inspection it looks like there are two grid bias voltages, one for the 2nd PM1LF and the other for the 1st PM1LF. I'll go over my drawing and attach it a bit later. I have a Peak LCR45 tester with which I'm testing the capacitors. The Telsen inter-valve transformer measures: Primary 396 Ohm (It's marked at "P") and secondary is 3.05kOhm (marked as "S"). The main HT capacitor is a Dubilier 2uF and it tests out at 2.85uF @ 1kHz.

[snowman_al]

Hi,
You do indeed have 2 Grid Biases.
See the picture with the 'curly' leads and Green, Black and Red plus plugs.
One will go the PM2 and the other to the 2nd PM1, they are the audio frequency amplifiers.
The First PM1, the one that connects to the tuning coils, will not need a battery bias. That is the radio frequency / detector valve .

You mention building a power supply.
As the set you have has a bracket installed to hold a grid bias battery, the frame on the left of the same picture, I would reproduce a battery (6 AA cells) to fit the space and keep it more original. They will last for ever as the grids draw tiny amounts of current when in use. Search Grid Bias Battery and you will see how easy it is.
The PM2 will probably use the 7.5 or 9 volt 'tap' and the PM1 4.5 or 6 volt tap. Always start high though and adjust after if needed, if the reproduction is noticeably distorted.
Oh I'm sure you realise already; the +ve of the bias is connected to the filament (LT) -ve connection.

There are also plenty of circuits using LM317s (ouch) to give you a stable 2 volt supply for the filaments.

Filaments again, strange one PM1 has a different resistance to the other?
Did you check the 2uF cap for leakage as well as capacitance?

Good luck, Alan

[radiosoul]

Hi Alan,

Was just about to post! Thank you indeed for your comments. I'll continue below since I was idle for too long and site timed out.

Here's the first foray into the set - now out on the workbench (Photo 1). Initial circuit tracing is showing that the circuit is very much like the Vulcan 2 (Photo 2) but the PM2 circuit adds more up-front. No surprises there. I'll double-check what I have drawn up for the circuit diagram and then upload it. I can't tell if the two components in Photo 3 are resistances or a resistor/capacitor combination. Testing them with the Peak LCR45 tester in "auto" LCR mode reveals shows that they are both resistors, the one marked "Anode" being 20.41k Ohm (PM2 anode) and the unmarked one being 2.21 M Ohm (1st PM1LF grid). If anyone has seen these components before I will be happy to be enlightened. Photo 4 shows the grid tuned circuit capacitor which tests out as 28.2pF (or 0.0000282uF). Don't know if that value is good. Photo 5 shows the three battery leads.

[snowman_al]

Yes they are resistors. Values sound reasonable too.
The 2.2 meg ohm one is the grid leak.
The capacitor across it is normally 200pF or 300pF. So if that is the one you are referring to, it will need replacement. There may well be a value on it somewhere? You can often 'hide' a modern one in the old one or fit it under the base board.

It is a nice looking set you have.

[ms660]

Out of interest there's a catalogue entry for an Ediswan RC coupling unit here (16th picture):

http://www.valve-radio.co.uk/literat...esome-3-valve/

Lawrence.