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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 6th Jan 2013, 3:24 pm   #1
Daventry 5XX
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Join Date: Dec 2011
Location: Daventry
Posts: 135
Default Pye Q.AC38 - rectifier valve

Hi All

Recently purchased a Pye QAC38 (circa 1938) from that well known internet auction site.

After doing all the pre start-up checks and finding nothing amiss I decided to power it up thro' my Variac.

As soon as it reached a sufficient voltage where the valves could function.I was met with an overpowering hum and not much else.

I immediately suspected the smoothing caps (8 & 16 mfd.)housed in a cardboard box to the left of the energised speaker.
As I had no new replacements,I pinched an 8/16 can from a working Murphy A104 which I had lying about,fully expecting this to solve the problem.

On switching on,I still had exactly the same problem so I decided to take a closer look at the rectifier valve.

After removing 75 years of grease and grime I discovered that instead of having a Mullard DW4/350 or Ever Ready S11D as per the Trader sheet (303) and Pye Service notes,I in fact had a Mullard U10.

I checked this against the other two valves in several Data reference books and nowhere does it suggest this as a suitable alternatve.

The two correct valves have max.anode voltages of 350v. whereas the U10 has max.anode voltages of 250v.

I measured the output from the transformer at 345v.

In other words the U10 is running at almost 100v. over max.
Not surprising then that it doesn't work.

I dont have either of the correct valves to put in to see if this resolves the problem,but it seems to me that at sometime in the dim and distant past that somebody has fitted a completely unsuitable valve.

Am I correct ,or am I missing something blindingly obvious ?

Your thoughts and comments please.

Alan
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Old 6th Jan 2013, 4:05 pm   #2
Peter.N.
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Join Date: Jul 2008
Location: Charmouth, Dorset
Posts: 1,865
Default Re: Pye Q.AC38 - rectifier valve

Could be that the rectifier has an internal short but you should be able to see this by looking at it, assuming its a directly heated valve. Do you have any HT on the smoothers?

Peter
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Old 6th Jan 2013, 4:08 pm   #3
PJL
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Join Date: Sep 2005
Location: Seaford
Posts: 2,470
Default Re: Pye Q.AC38 - rectifier valve

Rectifiers are pretty tough so this may not be the fault but you do need to replace before you go any further particularly with the energised speaker and resultant HT surge.

I always put a DMM on the HT when powering up for the first time as you can tell alot from the behaviour. On an energised speaker set it should rise well above the normal operating voltage and gradually fall as the output valve starts to draw current.
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