Unusual alterations

[quantum]

When you acquire a set do you sometimes find that alterations or adaptions have been made to extend its use? By this I don't mean changes as a result of repairs (such as replacing a side-contact valve with an octal type, or changing an energised speaker for a PM model) but added extras. For example, one or twice I have come across an extra switch fitted on the side or back of a set to switch off the internal speaker so only an extention speaker would operate. On one set I found a switch to silence a set if a gram was connected, on a set when the P.U. terminals were permanently 'on' and there was no gram setting on the wavechange switch.

The most unusual adaption I've found was a set where underneath the chassis there was a 'flying huddle' of components with a single transistor, which appeared to be in grounded base mode and connected to the P.U. input. In such mode the input has a very low impedence, and a high output impedence, and I wondered why it had been added. One thought was perhaps the set had at one time an extension speaker in a room which had now become a baby's nursery, and the extension speaker now became a microphone using its ordinary cable to the low input impedence of the transistor pre-amp (thus minimising hum pick-up and matching the speaker to the P.U. input). By switching the set to gram the parents could use it as a baby alarm. If I remember rightly the tranny received its power from the top of the cathode resistor of the output valve.

Has anybody else come across unusual alterations?

[Sideband]

Hi Quantum.

I've not come across any as unusual as your transistor mod. Yuk!! If I'd come across that it would GO! The most common thing I find is as you say...a switch added for an external speaker. My Pilot 75 had a hole cut in the back to take such a switch. When I fully restored it, the switch went. Still got the (small) hole in the back but at least the set is now original.

I also came across this on a Ferguson 461 which I was repairing for someone else. Trouble is the 461 has a live exstension speaker socket...it is coupled to the anode of the output valve and chassis for a high impedance speaker. For safety I disconnected the socket completely and removed the switch mod. My customer had young children so I did'nt want any nasty accidents from prying little fingers!

Rich.

[Paul Stenning]

My Philips Reverbeo which I'm working on at the moment had the infamous speaker switch on the back too.

The transistor preamp circuit for the delay line (original Philips circuit) is powered off the top of common cathode resistor for the o/p valves. It seems to be a common way of getting the right sort of voltage at a couple of mA for free.

The transistor mod in Quantum's set sounds like it was fairly well thought out and designed (common base cobfiguration is not exactly something we do every day, and the voltage feed is sensible), even if the implementation was a bit of a mess.

[Norm_Leal]

Hi

Noticed unusual modifications in radios received from Europe. Most of the time replacement parts can be found in the US.

Had a radio where the ballast tube (valve) was replaced with a wooden form wound with high resistance wire.

A #75 (triode) was used in place of #78 (pentode). It worked since filament, plate & grid #1 were on the same pins & cap.

Found were volume control shafts were extended by soldering on another piece of metal.

Norm

[quantum]

It is not uncommon to find sets which used a ballast (or barretter as they are known here) replaced by a large resistor - sometimes these were fitted as 'vintage' replacements by servicemen after WWII when parts were in short supply for older '30s sets.

One unusual valve replacement I've seen was in a Pilot which should have had a 6A8 heptode frequency changer but was fitted with a 6K8 triode-hexode, but no circuit changes had been made. The set worked perfectly well, and I didn't realise the error until I looked at the circuit diagram!

[Norm_Leal]

Hi Quantum

Even though the circuit diagram looks very different between 6K8 & 6A8 they are similar valves. The 6K8 has the oscillator shown as a separate section. These valves are interchangeable in many radios.

We also use wire wound resistors to replace barretters. In this radio a person wound their own on a wooden dowel. Lucky it didn't burn up.

During WW2 adapters were used when a specific valve wasn't available. I have some commercial adapters made during that time.

Norm

[quantum]

During WW2 adapters were used when a specific valve wasn't available. I have some commercial adapters made during that time.

Norm

I still have a set of side-contact (CT8) to octal adapters put aside for the day when I get a set with CT8s in it - however, I have yet to come across such a set! It was Mullard/Philips who made a lot of these sets, the valves weren't so popular with other manufacturers.

I think I'd probably be inclined to drill out the base and fit an octal socket anyway, as I'd always be suspicious that an adapter would introduce losses or crackling!