Bush DAC90 with U40 valves. Was such a set ever made?

[Robert Gribnau]

Originally Posted by FERNSEH

Hi Bob,
to be honest after looking at the Mullard information for the UCH42 and UCH81 there is very little difference in the performance of the two valves.
At 170 volts the conversion conductance of the UCH42 is 670uA/V and the UCH81 is 750uA/V.

I think that the difference mostly counts when using the UCH81 in other ways than as a frequency changer/oscillator. Beside the seperate connections for the grid of the triode section and grid no. 3 of the hexode section, the UCH81 has a suppressor grid, which is lacking in the UCH42. According to Philips the suppressor grid is necessary for preventing secondary emission when using the heptode section as a HF amplifier.

[turretslug]

That was my impression too, the xCH42 being very similar in equivalent noise resistance in mixer mode to the xCH81 as well as conversion conductance, both valves benefitting from the intensive wartime developments in precision grid alignment and electron lensing to minimise noise compared to their late '30s predecessors. The xCH81 development seemed to be towards optimising performance in the (circa) 10.7MHz IF amplifier application- the Germans were early and rapid adopters of VHF/FM broadcasting with a number of sets using the xCH42 AM mixer/oscillator as 1st FM IF amp and this application might have highlighted the shortcoming of what was effectively a suppressor-less tetrode with the xCH81 representing a "tweak" for this particular task rather than a revolutionary all-round improvement.

The xCH81 heptode also generally needs one less resistor in a typical circuit as the screening grids can be fed with a single resistor to HT rather than the xCH42 hexode's advised potential divider!

[FERNSEH]

The Mullard amplifier circuit diagram corrected and now has 0.2amp CL33 output valves in the line-up list.

The Mazda 10C1 is pin for pin compatible with the UCH42. 28volt heater.
http://www.r-type.org/exhib/abc0009.htm

DFWB.

[ms660]

The bottom line with a UCH42/ECH42 in sliding screen grid mode under AGC control is reduced selectivity due to the secondary emission circuit damping the primary of the IF transformer when the screen grid voltage shoots up due to AGC control, supplying the screen grid via a potential divider to reduce the effect of a sliding screen grid was recommended, the conversion trans-conduction figures given in the valve data are with the screen grid being supplied via a potential divider, the arrival of the suppressor grid in RF/Mixer/IF Vari-Mu valves largely got rid of the problem.

Lawrence.

[bobhowe]

Hi David Hi Lawrence Ed Dinning has kindly found me a a 10C1 Valve will report how it performs when i collect it next week kind regards Bob

[FERNSEH]

Hi Bob,
the Mazda 10C1 is a triode-heptode so there will none of the secondary emission effects found in a hexode mixer valve.
The 10C1 was marketed as the X118 by GEC and Marconi.
Extract from from a Murphy radio circuit diagram clearly shows the 10C1 with it's suppressor grid.

DFWB.

[bobhowe]

Hi David have to give it a try in a Bush DAC90A another modification to add to the list kind regards Bob

[FERNSEH]

So convinced that a close to end of production variant of the DAC90 was made I've gathered all the parts to needed to construct a DAC90 which employs the U40 series valves. UCH42, UF41, UBC41, UL41 and UY41.
Attachment shows the partially assembled chassis. Note the adaptor plates to fit B8A valveholders in place of the octal sockets.
DFWB.

[bobhowe]

Originally Posted by FERNSEH

So convinced that a close to end of production variant of the DAC90 was made I've gathered all the parts to needed to construct a DAC90 which employs the U40 series valves. UCH42, UF41, UBC41, UL41 and UY41.
Attachment shows the partially assembled chassis. Note the adaptor plates to fit B8A valveholders in place of the octal sockets.
DFWB.

Hi David that looks a lot different to what it was like yesterday kind regards Bob

[FERNSEH]

The special DAC90 is ready to be tested and then aligned.

DFWB.

[bobhowe]

Not be long for its First Squeak kind regards Bob

[Heatercathodeshort]

Why not just stuff a DAB chassis inside it and save all the hassle? John.

[bobhowe]

Originally Posted by Heatercathodeshort

Why not just stuff a DAB chassis inside it and save all the hassle? John.

The vintage repair shop did a DAB conversion not impressed

[FERNSEH]

Testing the radio set. Final checks confirmed there is no wiring errors. Connect up the frame aerial.
Connect to main supply, greeted with a working audio amplifier and IF amplifier.
No other activity though, the frequency changer is not working. Tried two other UCH42 triode-hexodes but still no results. No negative voltage on the triode oscillator grid. No oscillation. Connections on the valve base confirmed to be correct.
The continuity of oscillator coils is OK. Try reversing the anode winding, voila! stations coming through at last. Medium waveband performance is excellent but absolutely hopeless on the long waveband, in fact medium waveband stations are breaking through.
The IF amplifier must be aligned to the correct intermediate frequency, 465Kc/s.

DFWB.

[bobhowe]

Hi David have to post some pictures of the radio

[bobhowe]

Been to Davids shop this afternoon to give him a DAC90A chassis for another project and I listened to the DAC90 U valve conversion this afternoon and its LOUD even with the repaired speaker cone Evan the reception of radio 4 is very good as well

[FERNSEH]

Hi Bob,
the medium waveband oscillator coil is to be repositioned to allow easier adjustment of the tuning core.
This is first DAC90 I've ever come across which has adjustable cores for the LW and MW oscillator coils.

DFWB.

[bobhowe]

Going to sound really good when the chassis is in the cabinet

[FERNSEH]

The completed Bush DAC90 with U40 series valves.
The receiver was constructed using salvaged parts. SUG3 chassis, cabinet, valveholders, valves and bits and pieces from scrap DAC90 and DAC90A receivers.
Third attachment shows the relocated oscillator coils.

DFWB.

[bobhowe]

Hi David looking nice what did you use to polish the cabinet with