Bush DAC90A Output Valve

[bobhowe]

Hi has anyone used a UL44 valve instead of a UL41 as the UL44 are cheap & i know you have to move the anode connection to the top cap kind regards Bob

[PYE 405]

I have not used a UL44, but I have used a Mazda 10P13 with success.

[Herald1360]

No doubt it could be used but it's a lower power valve (Pa = 5W vs 9W) so biassing would need some careful attention.

[Robert Gribnau]

Also note that for single ended power pentodes the optimum primary load is given by Ra = Va / Ia. For the UL41 Ra the optimum primary load is (close to) 3K. For the UL44 this will be close to 6K.

So if you want to use an UL44 in place of a UL41 in an existing radio, you will get a mismatch of the primary load.

[kalee20]

Originally Posted by Herald1360

No doubt it could be used but it's a lower power valve (Pa = 5W vs 9W) so biassing would need some careful attention.

Originally Posted by Robert Gribnau

Also note that for single ended power pentodes the optimum primary load is given by Ra = Va / Ia.

True - approximately.

Though, in the DAC90a, the UL41 is under-run anyway, so the existing transformer may well be a fair match for the UL44.

Definitely check the UL44 running current, but it probably won't need a huge adjustment to the cathode resistor. to bring it to <5W anode dissipation.

[FERNSEH]

I reckon the the UL44 could well be an excellent substitute for the UL41.
It was employed in late forties small screen TVs such as the Philips 385U, in fact I'm certain no other manufacturer used it.
From the Radiomuseum: https://www.radiomuseum.org/tubes/tube_ul44.html
The UL41: https://www.radiomuseum.org/tubes/tube_ul41.html

DFWB.

[FERNSEH]

From the Mullard Maintenance Manual. Info for the UL44.
Suppressor grid taken out to a separate pin. Cathode, G1 and G2 same as UL41.

DFWB.

[Gabe001]

This thread may be of some interest

https://vintage-radio.net/forum/showthread.php?t=45446

[bobhowe]

Thanks to all who have replied will order a UL44 and see how it goes will start another thread why i want to change the UL41 kind regards Bob

[bobhowe]

Hi got ULL 44 moved the anode connection to the top pin checked voltage it read 178V on my Fluke 112 true RMS multimeter is about right and the radio is playing really well and loud on Talk Sport Radio and the valve looks right at home on tha DAC90A chassis & the bonus the valve is a NOS Mullard and it was only £ 6.00 + Postage kind regards Bob

[Herald1360]

Result!!

[Sideband]

Surprised it was as easy as that since the UL44 is a line output valve. I would have thought that biasing would need to be changed to get it to work on a linear part of its characteristic. Just shows how non-critical some valves are.

'the valve is a NOS Mullard and it was only £ 6.00 + Postage'

Shhh....don't tell anyone or the UL44 will go the same way as the UL41!

[bobhowe]

Hi Herald 1360 glad it worked Hi Sideband was surprised the anode voltage is near enough thought i would have to make adjustments to get the anode voltage correct but the set is working very well even louder if i put my hand next to the wavelength switch dont know if the set is slightly out of alinement kind regards Bob

[Sideband]

Originally Posted by bobhowe

the set is working very well even louder if i put my hand next to the wavelength switch dont know if the set is slightly out of alinement kind regards Bob

No...it's a quirk of the DAC90A. You MIGHT get some improvement if you adjust the aerial trimmer, but its a hand-capacitance effect that affects the aerial tuning.

[bobhowe]

Hi Sideband thanks for reply i will try to adjust the aerial trimmer should i do that on a strong or weak station? and should the radio have been on a while before i adjust it kind regards Bob

[Sideband]

Weak station near the low frequency (500 metre) end of MW and adjust for maximum signal. Weak station as the AGC action will be minimal.

[ms660]

HF end for antenna trimmer on MW (200 meters)

Approx. mid band for antenna trimmer on LW (1,400 metres)

That's assuming the IF and Osc. are aligned correctly.

Lawrence.

[bobhowe]

UPDATE adjusted the aerial trimmer vast improvement very clear reception and been listening to the radio for 4 hours a day since the 22nd of May & it has never faulted running nice and cool with the auto transformer from Ed Dinning & the UL44 valve is drawing less current than the UL41 valve kind regards Bob

[Graham G3ZVT]

Originally Posted by bobhowe

UPDATE adjusted the aerial trimmer vast improvement very clear reception and been listening to the radio for 4 hours a day since the 22nd of May & it has never faulted running nice and cool with the auto transformer from Ed Dinning & the UL44 valve is drawing less current than the UL41 valve kind regards Bob

I bought a UL44, on your recommendation, and fitted it into my 90A last night.

I found the same as you, the volt drop across the 790 Ohm (Trader sheet says 500 Ohm) o/p tx primary was 16.9v with both the old (admittedly rather tired) UL41, and the NOS UL44, corresponding to an Ia of about 21mA.

[bobhowe]

Hi Graham nice to see that you have done the same as me & the UL44 looks at home next to the UY41 hope prices for the UL44 dond go up kind regards BOB