Pilot U650

[Devon60Ben]

Originally Posted by PJL

Measure the resistance of the field coil as Graham suggests. It is probably over 2000 ohms.

Assuming it is high and you are happy with the results then I would leave it as is. If you replace the output valve with one with better emissions it will create another problem by reducing the HT volts.

Hard to say yet PJL all i can pick up is a feint radio 4 and can just get radio 3 on longwave - thats it. Can't get anything through the pick up input yet so don't really know how it sounds.

Want to try to get the pick up working first so i know the back end is ok then tackle the front end / potential alignment etc but with the speaker being 3000 ohms instead of 1400 ohms not sure hoe this impacts things

[Devon60Ben]

Originally Posted by David G4EBT

From my experience, having trodden this path, it's unlikely that the output level of an MP3 player such as an I-pod, i-pad, phone etc would be sufficient to give the required volume level when fed into the gram socket, but much will depend on the signal level required at the gram socket. Back in 2015, I wanted to connect an MP3 player to a Pye Fenman1 and Bush AC41. Even with the set volume and I-pod turned up high, all that could be hear was the faintest sound. I trawled internet looking for solutions, and found all sorts of simple circuits which it was claimed would do the trick, but they didn't. I've attached one such circuit at pic 1.

Rather than dwell on what won't work, I'll say what will. Namely, a small preamp. When I say 'small' I mean 1.5cm x 4cm. They're ludicrously cheap (three for £2.00 from China), a bit more expensive from the UK. EG:

https://www.banggood.com/3Pcs-LM386-...r_warehouse=CN

They have a pre-set volume control on board, so what I did was to adjust the set volume as I would if listening to a station, connected the MP3 player to the preamp and then adjusted the pre-set pot of the preamp to give the required output to the radio.

Velleman do a nice little cheap mono preamp kit No K1803, which I've also used for a set used by my (adult) son.

To quote Velleman:

"This kit was developed as module for a number of audio applications where the input signal is too weak.
Applications such as a microphone pre-amplifier or for signal level correction".

End quote.

£4.32 plus post plus VAT from ESR electronics. You'll find the construction manual at this link too:

https://www.esr.co.uk/velleman/k1803.htm

Pic 1: Useless suggested circuit.
Pic 2: How to connect your device to your radio whether directly or via a preamp.
The two resistors combine the stereo output from your device to mono.
The capacitor isolates your device from the radio, whilst allowing audio signals to pass.
Pic 3: A Velleman K1803 pramp I built into an ABS box.
Pic 4: Box with the lid on.
Pic 5: Showing 100mV 20 kHz p-p sine wave into the preamp, and undistorted 4V out.

It would of course be possible on an AC only set to put the preamp inside the cabinet, powered from the rectified 6.3V heater to provide approx. 9V DC with the preamp input to the gram socket, so that whenever the set is switched on, the preamp is powered up. I'm not suggesting anyone does that as it means modding the set a little, and a separate plug-in preamp can be used with different radios that have a gram input.

Having written this post, I've discovered that I covered this groud in a thread back in 2017! See post #9 here:

https://www.vintage-radio.net/forum/...+to+old+radios

Hope that helps.

Thanks very Much David, will study this and refer to your previous posts -
Perhaps in the meantime i should try connecting ipod to my audio preamp then preamp to radio as the power amp?

[Station X]

Originally Posted by Devon60Ben

The coil measures 3k ohms wired to pins 3&4 of the speaker socket
The output transformer is wired separately to pins 1&2 of the speaker socket

Is the replacement speaker also of the energised type? ie no permanent magnet?

[Devon60Ben]

Originally Posted by Station X

Is the replacement speaker also of the energised type? ie no permanent magnet?

I believe so - pics added

[David G4EBT]

Originally Posted by Devon60Ben

Thanks very Much David, will study this and refer to your previous posts -
Perhaps in the meantime i should try connecting ipod to my audio preamp then preamp to radio as the power amp?

Yes, if you make up a lead as I shown in the sketch in my earlier post to combine the two stereo channels into mono, then a 0.1uF capacitor to isolate the output of the preamp from the input to the radio's 'Gram socket' that should work fine. To make things a bit clearer, I've redrawn the sketch to show the preamp in circuit. The two resistors and the capacitor can of course be inside the preamp box, so all you'd have is a lead from your phone etc into the preamp, and another lead from the output of the preamp into the gram sockets. On the radios I have, the gram input is two wander sockets, so I just use a short length of twin flex from the preamp output with two wander plugs on the end.

Some may argue that the isolation cap is redundant, but I don't like the idea of connecting an expensive smartphone directly into the gram socket of a valve radio even if - as may be the case - there's an isolation cap in the set.

Hope that helps.

[Station X]

Originally Posted by Devon60Ben

I believe so - pics added

It certainly looks like an energised type, but the cone leaves a lot to be desired.

With 3k rather then 1.4k in series with the HT, the HT is going to be low. What effect this will have on the anode and screen voltages of the output valve I wouldn't like to say.

You could try temporarily substituting a 1200R wire wound resistor for the energising coil. The speaker won't work of course, but you could then check the voltage across R25 to see how the anode and screen currents have changed.

Have you tried a simple buzz test? Touching the centre tag of the volume control with the metal blade of a screwdriver should give a loud buzz from the speaker if all is in order.

[Devon60Ben]

Originally Posted by Station X

It certainly looks like an energised type, but the cone leaves a lot to be desired.

With 3k rather then 1.4k in series with the HT, the HT is going to be low. What effect this will have on the anode and screen voltages of the output valve I wouldn't like to say.

You could try temporarily substituting a 1200R wire wound resistor for the energising coil. The speaker won't work of course, but you could then check the voltage across R25 to see how the anode and screen currents have changed.

Have you tried a simple buzz test? Touching the centre tag of the volume control with the metal blade of a screwdriver should give a loud buzz from the speaker if all is in order.

Tried the buzz test but nothing happens other than a small scratch as i tap the centre tag.. no buzz at all!

[Station X]

I'd say you have a problem with the output stage. The fact the energising/smoothing winding has a DC resistance of more than twice the specified value can't be helping much. Try substituting it with a resistor and see what the voltage readings on V5 look like.

Other possibilities are low emission or low gm in V5, mismatching of the speaker to the output valve and problems with the speaker itself.

Extract from circuit attached. L33 is the energising winding.

[Devon60Ben]

Thanks Graham, have ordered a 1.5K wirewound to substitute as a dummy load will then report back

[Devon60Ben]

ok, just noticed something looking at your snip of the circuit Graham, should the heater feed "a" be grounded to the centre tap ground point before proceeding to the valve sockets?
Mine is not, the 2 leads go straight to V5 from the transformer then parallel off from there.

[Station X]

It's usual for one side of the heaters to be grounded.

[Devon60Ben]

Thanks Graham, will run a ground to the pin. Should the incoming -ve power lead be grounded via a safety cap perhaps? Currently it just feeds into the transformer primary.

[Station X]

Positive and Negative have no meaning as far as an AC mains supply is concerned. They change over 50 times a second. The correct terms are live and neutral.

The neutral shouldn't be grounded via a capacitor or otherwise.

As this set has a mains transformer you could earth the chassis via a three core mains lead if you wished.

[turretslug]

Originally Posted by Devon60Ben

ok, just noticed something looking at your snip of the circuit Graham, should the heater feed "a" be grounded to the centre tap ground point before proceeding to the valve sockets?
Mine is not, the 2 leads go straight to V5 from the transformer then parallel off from there.

Bear in mind that theoretical circuit diagrams often tell you nothing about component layout and wiring placement- in particular, they can actually be rather misleading about grounding/commoning arrangements. They are a draughtsman's realisation of neatness, rather than a technician's embodiment of wisdom!

As an example, in the case of the Marconi 559 here, a similar radio from a similar era, the circuit diagram shows much the same thing- yet in practice, the heater wiring is thick twisted pair from the transformer tags linking all valve sockets and kept away from chassis- except for a single link between one side and chassis at the mixer valve socket. This is to keep hum currents out of the chassis, and out of the negative side of the HT rail- which is exactly the problem that would occur if the circuit diagram was interpreted over-literally.

An excellent case of the difference between theory and practice is the Mullard 5-20 amplifier- as well as the circuit diagram, they also published a recommended wiring arrangement scheme. Over the years, innumerable queries have arisen along the lines "I built it exactly to the circuit diagram, yet it hums horribly/hoots/motorboats/makes tweeters smoke...."

[Devon60Ben]

Thanks StationX & Turretslug, thats helpful. Should all grounds be routed to the same point - the centre tap of the transformer perhaps or are several ok? I have 8 currently.

[Herald1360]

If the grounds are as original, even if they're not optimal (and some sets are far from the ideal as built) the set should be fine. As soon as you start modifying you need to understand exactly how any common mode ground return paths might affect things or you may just make things worse.

[Devon60Ben]

Originally Posted by Herald1360

If the grounds are as original, even if they're not optimal (and some sets are far from the ideal as built) the set should be fine. As soon as you start modifying you need to understand exactly how any common mode ground return paths might affect things or you may just make things worse.

Thanks Chris - its difficult to tell how it was originally but will leave as is for now.

[Station X]

I regard a circuit diagram as being as being akin to the "map" of the London Underground. It's designed to be easy to understand, but bears no resemblance to the geographical layout.

It's possible that the heater rail has been grounded at a point remote from the mains transformer. A quick resistance check would show whether it's grounded or not.

[Devon60Ben]

Thanks Graham, resistance confirmed not grounded - but it is now between pin 6 on V2 to chassis about 30mm away which was the simplest way to do it.

[turretslug]

A resistance check is a wise move- you wouldn't want to connect one side of the heater supply to chassis and then find that the other side was also connected after all, say by an inconspicuous wire link hidden under other things. Heater windings can supply a lot of current and might not necessarily blow a fuse very quickly.

Making the heater supply-to-chassis connection adjacent to V2 sounds like a good way to ensure that the local oscillator signal is less likely to leak along the heater chain to other parts of the circuit- the sort of thing that can cause whistles or spurious responses. Even in the mid-'30s, a lot had already been learnt about how to make radios work well, and it's often interesting to take note of what was done back then and take it on board.