How much hum is normal? and valve change DAC90A
 

I am restoring a Bush DAC90A and after replacing a few obviously faulty parts, I have it running outside the case. The sound is very good but there is a low hum heard with the volume down. It is almost unnoticeable when it is playing a strong station.
I know I still have a few more parts to replace, mostly waxy capacitors and a couple of resistors, but I have no idea if I should be getting any hum or not when its all back together.

If I have to resort to swapping valves, is there a technique to use to get them out, they look very well held in ???
Allan

Re: How much hum is normal? and valve change DAC90A
 

I'm working on a similar Bush (an AC91) and replacing the main reservoir and smoothing electrolytics has completely removed audible hum.

Small modern electrolytics are usually easy to hide beneath the chassis, and I now tend to replace the originals as a matter of course if there is any hum at all.

Re: How much hum is normal? and valve change DAC90A
 

If you have sensitive hearing you might hear some hum on a DAC90A. It shouldn't be objectionable but in no way will compare to a modern hi-fi which uses totally different techniques. Your best bet is to put test caps' of about 4-8uF across the smoothing can and listen subjectively as to whether it make a difference. If so you may well find the smoothing caps' faulty.

As to removing valves be very carefull, I learnt the hard way that the spring fixing rings seized up and can take the glass seals off when handled. I now carefully remove the spring clips before attempting to remove a valve.

Re: How much hum is normal? and valve change DAC90A
 

I had the same problem with a DAC90A I was restoring. I snipped pin 4 from the UL41 and this made a big difference but eventually had to replace the valve as it was suffering internal leakage.

Re: How much hum is normal? and valve change DAC90A
 

Yes, one of the pins on the UL41 valveholder is used as a convenient mounting tag. Even worse, it's grid-related circuitry! So, any internal leakage within the UL41 has maximum effect.

If you isolate this pin, you'll be doing yourself a big favour.

As for HT smoothing, you can add as much extra capacitance as you've got room for, to the smoothing capacitor (which unusually in the DAC90a is 16µF and less than the reservoir capacitor at 32µF). Don't increase the reservoir or the UY41 won't thank you long term. (You could however tack on 100µF 300V, and run it for a few minutes, just to prove or disprove whether it's HT ripple or not).

Re: How much hum is normal? and valve change DAC90A
 

I`d heard about breaking the glass pimple off the valve when removing it, taking the spring off seems the most logical but I havent really looked at them yet to see if they are easy to remove. I have a spare UL41 valve, I dont want to break the old one if I need to swap it. I will also be replacing the smoothing electrolytics.
I was just a bit hesitant about the valves, I havent removed one from a set since around 1964!
Allan

Re: How much hum is normal? and valve change DAC90A
 

Very easy to remove the springs Alan. Just slip a screwdriver blade under it and ease it away from the valve. After that gently rock the valve gently to get it free from the holder.

You only make the expensive mistake of breaking the pips off once.

Re: How much hum is normal? and valve change DAC90A
 

Thanks, sounds easy enough.
Allan

Re: How much hum is normal? and valve change DAC90A
 

I think there is a thread on this somewhere. Some DAC90's don't have the springs on the valveholders. If yours does and you want to retain them, it's best to clean and grease them before refitting. Most people just remove them. It's OK if the set is not moved about or bumped too much and as most are in static collections, it will be fine.



Rich.

Re: How much hum is normal? and valve change DAC90A
 

...or just temporarily remove them whenever you want to remove/replace a valve, then pop them back on afterwards :)

My DAC90A barely hums at all.

Good luck.

Re: How much hum is normal? and valve change DAC90A
 

I'd also be interested to hear from anyone who has the back off their set in the future as to an actual measurement of hum from their set, i.e. taken in volts RMS (or peak to peak if easier) across the field coil of the set or anode of the output valve. When I get back from my hols I intend to have a quick service of my set.

My set also hums a little (messing with filtering components exhausted but not tried the UL41 tag dodge). The trouble is I've no idea what is a 'little hum' having only heard any one elses DAC90A playing in a crowded auction hall where the hum is completely masked by background noises.

Cheers

Dom

PS: As to the expensive valve pip breaking excercise - some idiots take more than one go to work out why all their valves are going gassy - three ECH42s in a Pye P76F before I clocked!:-[

Re: How much hum is normal? and valve change DAC90A
 

There is softly audible hum on my DAC 90A when it's off-station. It does not vary with the volume control and is almost a comforting reassurance that the set is working.

When tuned to any station it is completely inaudible, however, so I'll leave all that's well alone.

Re: How much hum is normal? and valve change DAC90A
 

It would be an extremely rare ac mains set that had no hum at all. I have never come across one. All 11 of my DAC90A's have a slight background hum as do my DAC90's. The TV22 from the same stable also has a slight audio hum which I asked someone for a second opinion on and they said it was perfectly normal.

I agree with johnlees. I find it quite pleasing.

Re: How much hum is normal? and valve change DAC90A
 

I assume then that a 32µF + 32µF See here would be a good replacement for the original?
Allan

Re: How much hum is normal? and valve change DAC90A
 

I had a look at a DAC 90a that I picked up recently, Changed grid Capacitor etc. The Hum was very bad and I had a reading of 7 volts on the grid of the UL41 (Valve faulty) Snipped off pin 4 the sound was hum free and the grid voltage went down to 5 mV. It the first time I had ever tried this mod and was amazed at the results.

Re: How much hum is normal? and valve change DAC90A
 

Yes, a very good replacement. Hum will be lower, at no detriment to the rectifier valve. It'll even fill the chassis hole nicely!

Re: How much hum is normal? and valve change DAC90A
 

After replacing all the wax capacitors and testing the resisters, which, spookily, were almost all within tolerance, I was left with the humming, which got a little louder when the chassis was back in the cabinet. I ordered a new dual section electrolytic cap and while waiting for it to arrive I fitted a new UL41 and the hum has gone, well, if you put your ear on the case you can just hear a very faint background hum, which I assume is normal.
It now has a beautiful rich clear sound that puts many modern sets to shame.

Thanks for everybodies input

Now for a beer :beer:

Allan

Re: How much hum is normal? and valve change DAC90A
 

Definitely leakage within the UL41 then. But, do the pin 4 mod and prolong the useful life of your new UL41.

Is there any chance of having the old UL41, if you've got no further use for it? I'll pay p & p of course!

Re: How much hum is normal? and valve change DAC90A
 

Your very welcome to the old one. PM me your address and I'll send it to you.
I'll treat you to the P+P.
Allan

Re: How much hum is normal? and valve change DAC90A
 

Posted Friday, should be with you early next week.
Allan