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Old 26th Jul 2004, 10:33 am   #1
newlite4
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Default DAC90A - working at last!

I bought the DAC90A locally for £45 over a year ago. No discount could be negotiated despite the hairline crack underneath, the missing mains lead and the fact that it didnt work!

When the back was removed, I found that the chassis contained two UF41s! A bonus you may think, trouble was that one of them was where the UL41 should have been. Anyway, I bought a new UL41 at the March 2003 Harpenden for £10. On fitting the valve, the heater showed no signs of life. It turned out that although it was a brand new valve, the heater was short circuit.

Not to be outdone, I changed the base and wiring so that I could fit a UL84. At least I had an audio output but still no signal. I recapped the set and a faint signal was just audible until I connected C17 the tone control cap from C18 (audio coupling cap) to ground. I tried for many hours to get some improvement all to no avail.

After a years gap, I got back to it this weekend. I checked the aerial circuit, that was all OK. The only other culprits were the two IFTs. It turned out that the secondary of the second IFT was o/c. Upon removal of the IFT and its can, the litzt wire from the secondary winding had corroded and become detatched. The wire was re-tinned and reconnected, the IFT was reassembled and refitted. On test, the audio output was more than ample. C17 was reconnected and a brief re-alignment followed. On final test, the benefits of the earlier recap and rewire could be heard to the full.

I think that this set has cost me so much in terms of parts and time that I will not be selling it on, it will serve as a reminder that those less obvious faults are out to get you.

Neil Deacon

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Old 26th Jul 2004, 11:14 am   #2
quantum
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Default Re: DAC90A - working at last!

Bad luck with the UL41! When you replaced it with a UL84 did you make any circuit alterations for it? I was wondering that since the UL84 does not have the same characteristics as a UL41 - it takes a higher HT current at a lower voltage than a UL41 - you made any alterations to ensure the Ht consumption was kept down? I was concerned that output trannies on DAC90A sets have somewhat of a reputation of getting o/c primaries, even on the HT current of a UL41, and the UL84 could be much higher...

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Old 26th Jul 2004, 11:31 am   #3
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Default Re: DAC90A - working at last!

OK, I know it's not quite the same circuit, but I recently did this in a BUSH AC11. All that was required was to add an extra 200 ohms to the cathode bias resistor. Ia=34 mA with new valve and worked very well.

I piggybacked the new socket on top of the old one in such a way the mod could be removed in the future if someone discovers a warehouse of UL41's! Of course there is plenty of room in an AC11, rather less in a DAC90A methinks. Messing about with the screen grid voltage didn't seem to make much difference. I have sent the report (with photos etc.) for said set to Paul, so watch the main site.

Cheers
Tim

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Old 26th Jul 2004, 11:50 am   #4
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Default Re: DAC90a - working at last!

The conversion was relatively straightforward. The Ediswan valve data book (1965) was very helpful. It shows in detail how to convert from UL41 to UL84 (which it views as an equivalent). The book, dated 1965, regards the UL41 as obsolete and that this conversion is the best option, no component changes were mentioned. Although it would suggest that a straight changeover would be sufficient, I recognise that changes to some values may be beneficial. The radio was running for two hours with no problem, however any hints on how to improve the output stage further are most welcome. It would be good to see a write up on how to do this conversion on 90a's and other sets so that we are no longer tied to the - by now - very expensive UL41 .
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Old 26th Jul 2004, 12:02 pm   #5
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Default Re: DAC90a - working at last!

Hi All

A short while ago I ran a UL84 through the AVO VCM at 200V anode and 100V screen (nearest to a DAC90A) at various grid voltages. Measuring Anode current and gm.
I have a small excel table which has also the calculated anode diss and cathode resistor.

Anyone is welcome to a copy of it just let me have an email address by pm. Or if Paul S has some way of posting it via his web site.

It does seem to show that a straight swap would be ok in regard to the anode current.

It was a new old stock UL84
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Old 26th Jul 2004, 12:11 pm   #6
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Default Re: DAC90a - working at last!

It's a bit naughty for a valve book to suggest that a UL84 is a modern equivalent of a UL41!

Of course, in most sets where valves are run well within their capacities swapping one output valve for another will be OK nine times out of ten, especially as valve circuits are far more tolerant of circuit variations than s*l*d st*te stuff. The UL84 does have a lower HT max than the UL41 with a higher current, and the anode load is lower too which means theoretically that a output tranny with different ratios is required, but in reality sets seem to work perfectly well without these worries.

I'd certainly use the UL84 in place of the UL41 if the latter was not available, but as Tim says, I'd be inclined in the case of a DAC90a to raise the bias resistor value just ot make sure that the HT current is kept to less than 40ma, as the output trannies in these sets are small and have this reputation for going o/c.

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Old 26th Jul 2004, 12:49 pm   #7
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Default Re: DAC90a - working at last!

ISTR the anode current in the AC11 with the UL84 in place and the circuit un-modified was about 45mA. I felt this was too high so added the 200 ohms to "fix" it at about 35mA. Anode impedances won't matter too much as these sort of power levels and no MW radio is HI FI!!

Might be worth checking your anode current Niel, although I gather most DAC90As aren't well known for having high HT, and probably not as high as the AC11, which has a mains transformer.

While we're sort of on the subject, I used a 10P13 to replace a UL41 in a BUSH DAC10. It worked perfectly, with an anode current of 19mA, and the set sounded fine. Plugged straight in, but the DAC10 has a low HT and screen grid voltage, and the 10P13 is only rated at 29mA anode current so I wouldn't recommend it for other sets.

Tim

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Old 26th Jul 2004, 1:25 pm   #8
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Default Re: DAC90a - working at last!

My 1970 Mazda Data Booklet lists the UL41 as obsolete and recommends a UL84 as a substitute (actually it recommends a 10P18, the Mazda equivalent). There is a warning to check operating conditions though.

Oddly, my 1969 Mullard Data Book lists the UL41 as current, the only B8A radio valve included in that edition AFAIK.

Best regards, Paul
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Old 26th Jul 2004, 6:39 pm   #9
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Default Re: DAC90a - working at last!

If I have to substitute the UL41 in the DAC90A for a cheaper valve, I use a UCL82 (on an adaptor of course, to preserve originality). There is plenty of headroom, even on the DAC90A, and 100 Ohms of extra cathode bias is needed in the adaptor. The triode anode and grid are connected to its cathode, in turn connected to the pentode cathode.

I too would be concerned about the excessive anode current of a UL84 frying the miniscule output transformer on the DAC90A. The UCL82 sounds fine, and is cheap.

Regards, Leon.

 
Old 26th Jul 2004, 7:47 pm   #10
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Default Re: DAC90a - working at last!

Thanks Leon I'll remember that one. I think you have mentioned this before but I didn't have a new old stock one in at the time, the three UCL82s I had at the time were all "well used", whereas I had 5 or 6 UL84. I thought I had better "experiment" with a known new valve, in case I based my experiments on a used valve and someone replaced it with a new one later.

Cheers
Tim

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Old 26th Jul 2004, 8:17 pm   #11
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Default Re: DAC90a - working at last!

To us, our radios are valuable (even the common ones) and as such attempts should be made to keep them as original as possible. However, this was not always the case. In their normal lifetime they were just another consumer item. When a valve was deemed obsolete it was normal to substitute another. No Engineer would search for an obsolete valve, he'd root round the workshop for something "that would do".

Manufacturers would recommend one of their own valves as a replacement, not one of the competitors. That is why the Mullard equivalent lists in particular are pretty useless. The DAC90A was towards the end of valve set production, so there are not many easy substitutions. There are some, though, probably originated by resourceful (desparate?) Service Engineers rather than Manufacturers. Now we have BVWS and eBay, UL41s are once again available in small quantities.

Providing a set is clearly labelled, I would suggest that effective substitution is keeping alive the tradition of generations of Service Engineers rather than the shady bodge it is sometimes seen as. I wouldn't recommend it for your V2 though!

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