Bush TV24C

[`FRANK.C]

Hi Trevor
Great result there. It would have been a real shame if it wasn't rescued.
They are good sets I have one myself. The tube in it is pretty good too.
I keep it in the workshop and use it or testing sometimes.

Frank

[Murphyv310]

Thanks Frank.
Yours looks much better than mine as the cabinet although complete really needs to be refurbished, not one of my strong points TBH.
A little of the veneer is split below the speaker. No doubt linseed oil would help but it would be good to repolish it and do the darker strip as well.

[`FRANK.C]

Hi Trevor
The most important thing is it preforms well and it certainly does that.

I recaped my one but haven't done anything to the cabinet apart from a wipe of a cloth. I was fortunate that it came that way.
Mine takes an age to get going. I haven't checked but I assume that it is the thermistor in the heater chain.
As it works I didn't do anything about it. I am in no hurry wit it heating up ant it is probably kinder to the valves.

Frank

[Geordie McBoyne]

These sets take a long time to warm up especially if the original PY81 boost diode is still present on the chassis.
Early PY81 valves have a ceramic tube inserted between the heater and cathode.

Geordie McBoyne.

[ppppenguin]

I know that David and Trevor are very familiar with this, but others may not be so aware of the special problems faced by a boost/efficiency diode such as the PY81.

The cathode has very high voltage pulses, several kV, during flyback. Hence the cathode is the top cap. Thes diodes also need exceptional insulation between heater and cathode. A side effect of this is that it takes longer for the heat from heater to warm up the cathode. The boost diode is usually highest in the heater chain since it can easily withstand the voltage difference between heater and cathode.

Before specialist boost diodes such as the PY81 (also, later, PY800, PY801, PY88, PY500) were available, clever designers repurposed other diodes in different circuit configurations that reduced the pulse voltage on the cathode. For example PZ30 in the Bush TV22.

[Murphyv310]

Hi.
Jeffrey makes a very good point. In fact when an old scrap TV22 came my way a few years ago I went down the road of fitting a jelly pot line transformer. The PZ30 buckled under the strain of the much larger pulses from the new set up. The valve line up then changed from PL38-30P4, PZ30 - PY33/PY81. This and the line stage then ran on 105v HT plus 9kv EHT stabilisation with 9x1kv avalanche diodes.

[LENINGRAD T2]

Hi,
I saw a picture of an amateur TV from the 1940's that does not include an EHT tube rectifier. However, I noticed that an extensive selenium rectifier was installed. Were there serially produced TV sets incorporating such solutions during this period? I assume that the selenium rectifier could have been fitted at a later date.

[ppppenguin]

Going back just a little further, energy recovery wasn't done at all in TVs until the Pye B18T in 1948. So all the energy put into the line scan coils in the forward stroke had to be lost as heat somewhere. Damper diodes were sometimes used to improve the scan waveform. With small CRTs and small deflection angles this didn't matter too much. Ultimately, in first generation colour TVs in 1967, the circulating power in the line timebase was around 3kVA! Energy recovery, with a PY500 boost diode, reduced the power consumption to a few hundred watts.

The energy recovery line timebase was actually invented by Blumlein in the 1930s.

[Geordie McBoyne]

Hi,
I saw a picture of an amateur TV from the 1940's that does not include an EHT tube rectifier. However, I noticed that an extensive selenium rectifier was installed. Were there serially produced TV sets incorporating such solutions during this period? I assume that the selenium rectifier could have been fitted at a later date.

That was the ViewMaster TV.  This was a TV receiver designed for home construction. The EHT rectifier was a selenium device although an EY51 could be fitted instead if desired.
The boost diode was also selenium with no provision to fit a valve.
The pictures are of a ViewMaster I acquired in 2016. it has since been fully restored.

Geordie McBoyne

[Murphyv310]

Hi,
I saw a picture of an amateur TV from the 1940's that does not include an EHT tube rectifier. However, I noticed that an extensive selenium rectifier was installed. Were there serially produced TV sets incorporating such solutions during this period? I assume that the selenium rectifier could have been fitted at a later date.

In fact my Dad made a console version of the Viewmaster Adam. He gave it away in 1961 when he bought a 17 Inch Defiant. A few years later in 1966 we saw his old set for sale at a Labour party summer fete for £5 I really wish I had it now. Dad also made three TVs with the green screen VCR97, My older sister remembers it well. In fact he made one for the land lady they at hat the time in Walton-on-Thames for three months free rent!