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Old 29th Jul 2021, 12:37 pm   #1
pentoad
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Default UL46 as a substitute for UL41 in Bush DAC90A.

Has anybody done this, Is it possible without modifying the the connections?
Pin 4 is used as tag for R12, R13 and C18 this is shown as i.c on some data sheets but is empty on others.
Is pin 4 internally connected to anything in the UL46?

It is described below:

The UL46 is a fairly uncommon valve because it is essentially a UL41 audio output pentode constructed with improved internal insulation to enable it to function as a video output pentode, or a line or frame output amplifier in TV service.

http://www.r-type.org/exhib/aaa0376.htm
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Old 29th Jul 2021, 1:00 pm   #2
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Default Re: UL46 as a substitute for UL41 in dac90A

A quick forum search just now showed that this topic has been discussed a few times in the past. I seem to think that it was actually discussed quite recently, but couldn't immediately spot the thread in the search results, so could have been part of a ongoing thread on the DAC90A. A study of the search results should confirm what you need to know.
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Old 29th Jul 2021, 2:03 pm   #3
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Default Re: UL46 as a substitute for UL41 in dac90A

There was a recent thread on using a UL44 as the audio output. That seemed to work OK despite the anode being a top-cap. The UL44 is a line output valve with a top cap but seems to work OK in this application. You would need to provide a top-cap connection for it to work.

Theoretically the UL46 should be a direct replacement for the UL41 but you might have to watch the connection to pin 4.

Here's the link https://www.vintage-radio.net/forum/...1+in+dac90afor using the UL44 in a DAC90A
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Old 29th Jul 2021, 2:11 pm   #4
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Default Re: UL46 as a substitute for UL41 in dac90A

Quote:
Originally Posted by Sideband View Post
Here's the link
That's the recent one I was thinking of, but couldn't immediately spot it - 'can't see for looking', as they say! There's a few much older discussions on the subject that come up in the forum search.
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Old 29th Jul 2021, 2:16 pm   #5
pentoad
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Default Re: UL46 as a substitute for UL41 in dac90A

I couldn't find a thread which was conclusive about whether on not it was a direct substitute just speculation, in particular pin 4 which I mentioned before.
however it is mentioned on the valve museum site that "ic Internal connection. Nothing must be connected to this pin of the valveholder."
so I guess the answer is no?

Key to abbreviations:

http://www.r-type.org/static/connect.htm

Last edited by pentoad; 29th Jul 2021 at 2:24 pm.
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Old 29th Jul 2021, 2:56 pm   #6
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Default Re: UL46 as a substitute for UL41 in dac90A

A 10P13 valve is another option that's often discussed in this setting. A forum Search will yield useful information
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Old 29th Jul 2021, 3:48 pm   #7
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Default Re: UL46 as a substitute for UL41 in dac90A

Quote:
Originally Posted by Sideband View Post
The UL44 is a line output valve with a top cap but seems to work OK in this application. You would need to provide a top-cap connection for it to work.
Five minutes with a steel can out of the recycling bin and a pair of scissors and a drill bit to act as a former got me a UL44 top-cap connector that looked the part.

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Old 29th Jul 2021, 3:56 pm   #8
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Default Re: UL46 as a substitute for UL41 in dac90A

You can use a UL46...been there and done it (before I joined this forum) BUT you have to check pin 4. I suggest you isolate pin 4 which is what I did...(carefully remove any components connected to pin 4 and reconnect them in 'mid-air'). There's no danger of any shorts as the components will support themselves. I know that the DAC90A in question is still working some 20 years later and still with the UL46 fitted.

The problem with pin 4 is that it was often used as an anchor point for other components. However some UL41's had an internal connection so I tend to isolate it anyway. My own DAC90A has been running with a UL41 and a mid-air connection for around 10 years.
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Old 29th Jul 2021, 7:05 pm   #9
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Default Re: UL46 as a substitute for UL41 in dac90A

Hi Pentoad use a UL44 Valve you can get NOS Mullard one for £6.00 plus postage & lift the Anode connection from pin 2 on the valve base & connect it to the top connection on the UL44 job done . Done this conversion on my 1951 DAC90A on May 20th used the radio every night with out any issues kind regards Bob
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Old 29th Jul 2021, 10:52 pm   #10
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Default Re: UL46 as a substitute for UL41 in dac90A

Would it be possible to remove the offending pin on the valve as an alternative to modding the radio? Carefully grinding it off with a carborundum cutting disc using a Dremel-type drill, or using microshear-type cutters should prevent damage to the glass seal.

Last edited by emeritus; 29th Jul 2021 at 10:53 pm. Reason: typos
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Old 30th Jul 2021, 7:13 am   #11
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Default Re: UL46 as a substitute for UL41 in dac90A

Neater would be to use a socket saver, and adapt that instead.
9-pin-socket-saver

Then you don't have to think when swapping tubes.

Last edited by Richardgr; 30th Jul 2021 at 7:18 am.
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Old 30th Jul 2021, 3:09 pm   #12
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Default Re: UL46 as a substitute for UL41 in dac90A

There is a solder tag on the main tag board connected directly to pin 4 only 2 cm away (C18) so why didn't Bush use that instead of pin 4?
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Old 3rd Aug 2021, 1:23 pm   #13
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Default Re: UL46 as a substitute for UL41 in dac90A

I don't know about a UL46, but i have just modded my DAC90A from a UL41 to a UL84, and i must say iam very pleased with the performance, there is almost no detectable audio hum from the speaker when the volume is at minimum, a far cry from when it had a UL41, i have 3 other DAC90A's around my house, and iam going to mod them all for UL84's, should have done it years ago!
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Old 3rd Aug 2021, 1:36 pm   #14
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Default Re: UL46 as a substitute for UL41 in dac90A

A UL84 conversion was the Mazda recommended solution when the UL41 became obsolete. Nowadays people tend to see it as a last resort because originality is lost, but it's possible to make the mod easily reversible by mounting the B9A holder above the B8A original rather than simply changing it.

You normally need to increase the cathode resistor to stop the UL84 overrunning the output transformer.
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Old 3rd Aug 2021, 3:04 pm   #15
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Default Re: UL46 as a substitute for UL41 in Bush DAC90A.

Hi the reason why i used a UL44 as they are cheap no cathode resistor value to change or valve socket & all you do is solder a wire from the anode terminal on the output transformer & connect it to the top connection on the UL44 & its kinder to the UY41 Valve & output transformer as it draws less HT currant & easy to revert back to the UL41 if you want . Here is a picture of the UL44 Valve does not look out of place kind regards Bob
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Old 3rd Aug 2021, 3:21 pm   #16
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Default Re: UL46 as a substitute for UL41 in Bush DAC90A.

I've used the pentode part of a UCL82 as a replacement for both the UL41 and the UL84; the heater voltage of 50V isn't really an issue - indeed it can possibly be seen as a help because mains-voltage round here regularly goes up to 255V!

[The 'free' triode section can be put to use as a BFO if you're doing the job on a receiver with shortwave coverage but which lacks the ability to resolve CW/SSB as standard]
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Old 4th Aug 2021, 9:18 am   #17
crusher19860138
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Default Re: UL46 as a substitute for UL41 in dac90A

Quote:
Originally Posted by paulsherwin View Post
A UL84 conversion was the Mazda recommended solution when the UL41 became obsolete. Nowadays people tend to see it as a last resort because originality is lost, but it's possible to make the mod easily reversible by mounting the B9A holder above the B8A original rather than simply changing it.

You normally need to increase the cathode resistor to stop the UL84 overrunning the output transformer.
Yes, very true, i increased the cathode resistor from 150 Ohms to 220 Ohms.
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