Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Dec Mon 18, 2017 9:49 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 6 posts ] 
Author Message
 Post subject: r390a/urr tube replacement and other amusements
PostPosted: Dec Sat 02, 2017 8:12 pm 
Member

Joined: Sep Sun 08, 2013 11:53 pm
Posts: 25
Hello ... Uncle Fuzzy here :)

Small mishap , BFO went dead .. < dead i am .. signed your friendly neighborhood BFO >
Long story short. Got another tube in , no go , after 10 minutes with the manual + ohmmeter i found an open 33k 1/2 w resistor open.
Changed it , reassembeled the radio and bingo .. back to work he went.

The real question is this .. i got a series of new tubes . several for IF's ( 3x 6ba6w ) the RF amp ( 6dc6 ) mixer ( 2 x 6c4 )
abd a few for the calibration oscillator ( 2x 12au7 ) So having these on hand and since i dont have a full tube complement to replace them all ( forget the 3tf7 there's a 47ohm 10 watt in it's place ) In what order should the tubes be " rotated " and how should we do this ? RF amp is kind of a no brainer .. but for the IF chain wanting the best result , which should be replaced first ? first or last IF ? ( kind of have a feeling it's ist if that gets a fresh one ) .. ? Same for the mixer stages ? .. Has anyone devised a procedure for " refreshing " tubes in the reciever ? ..

Just wonder if there's a " best practice " to replacing the tubes in such a great reciever .

Signed : your friendly neighborhood bear ..

Ric


Top
 Profile  
 
 Post subject: Re: r390a/urr tube replacement and other amusements
PostPosted: Dec Sat 02, 2017 8:22 pm 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4817
Ric,

I wouldn't replace tubes just to be replacing them. Most miniature tubes have a VERY long life but sometimes a tube will suffer from infant mortality (i.e. internal short or leakage or open filament) so it is easy to have replaced a better quality older tube with a soon to fail new tube. For the same reason tube testers are often overused because a tube that tests poor (outside of one that is shorted or severely gassy) may work very well in circuit while a tube that tests excellent in a tester (at DC or line frequency) may fail miserably at RF. When you suspect a tube during troubleshooting that is the time to try a substitute but I would never replace a tube just because it is old in years or even in filament on hours.

The key thing to check with your R-390A is that you have addressed the potential capacitor failure in the IF deck that will take out all of the IF filters because it puts DC on the input transducer when it fails burning out the transducer coil. This is capacitor C553. Acceptable fixes are either to replace that capacitor with a new production capacitor of safe voltage rating OR put a new capacitor with a much higher capacitance rating (i.e. .22uf @ 630V) in series with the existing capacitor to take the DC voltage off of it. With the construction build of some R-390A IF decks the original capacitor can be very difficult to access and the most practical approach is to isolate it from DC by a series capacitor and with a high value series capacitor the impact upon circuit operation is nil. The same repair approach is often applied to the phasing network in Central Electronics SSB rigs which use precision silver mica capacitors but one of these has a high DC differential across it and is prone to silver migration if it isn't DC isolated.

Rodger WQ9E


Top
 Profile  
 
 Post subject: Re: r390a/urr tube replacement and other amusements
PostPosted: Dec Sat 02, 2017 8:52 pm 
Member

Joined: Jul Tue 21, 2009 1:38 pm
Posts: 691
Location: SW WA state
Ric,

If it works, DON'T mess with it! I don't rotate tubes.
For R-390's, you don't necessarily want brand new tubes: The LAST thing you want to do, is put a new tube into an R-390A IF deck, retune it, and have characteristics change and need to align it all over again. It's not fun.
The tubes used should be burned in to age them a bit so the operating characteristics stabilize.
This can be done in a less critical stage (NEVER the first IF tube, better the last...), a different rig, or a jig can be built to operate them at set parameters.
For the uninitiated, IF tubes can be critical to the proper operation of the operation of the receiver: I actually take tubes, swap out the first IF tube, then readjust the gain of the IF deck to spec,
and then check the relative noise level of the tube. This amounts to several minutes spent per tube testing them.
The bottom line is, not all 5749's are the same.. The gain isn't all that important, but the internal noise generated is!
I hand select the best 5749's (lowest noise), and those become my 1rst IF tubes.. Noise makes a big difference in the earlier stages, not so much later.
I've seen some tubes that were markedly better than others... This means you can massage the tube lineup for better performance (or worse, if you put a high noise tube in the wrong spot).
So, there is a lot more to take into consideration that just whether the tubes are NOS, or test the best on a tube tester...
Best of Luck!


Top
 Profile  
 
 Post subject: Re: r390a/urr tube replacement and other amusements
PostPosted: Dec Sat 02, 2017 9:34 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3382
Location: Monterey California USA
There is an old saying - Tubes are not spark plugs. Do not change them needlessly.

_________________
WB6NVH
California Highway Patrol Radio
Bell System Mobile Telephone History
http://www.wb6nvh.com


Top
 Profile  
 
 Post subject: Re: r390a/urr tube replacement and other amusements
PostPosted: Dec Sat 02, 2017 9:59 pm 
Member

Joined: Sep Sun 08, 2013 11:53 pm
Posts: 25
Rodger : The infamous C553 we can call him , been replaced as soon as i have got the reciever in my hands a few years back. Thanks for the reminder
that's one that will never be told enough.


Tom the matter of burn in never even crossed my mind .. and neither did noise measurement.
How is this accomplished ? how do you measure the noise of the tube ?

What i learned reading you both makes me ask serious questions and reconsider the best course of action
which seems to be to do nothing , hands off :D


Top
 Profile  
 
 Post subject: Re: r390a/urr tube replacement and other amusements
PostPosted: Dec Sat 02, 2017 11:17 pm 
Member

Joined: Sep Thu 01, 2016 3:56 am
Posts: 121
Location: San Jose, Ca.
Ric, I agree that the noise level of each tube varies and it will make a big difference in sensitivity. Roger Ruszkowski did a writeup in the 'R-390A pearls' on p 366 of the sensitivity alignment section. Here's a link: http://www.r-390a.net/Pearls/sensitivity_alignment.pdf . The mixers and 1 st IF make a big difference.

Have fun.

Regards, Larry


Top
 Profile  
 
Post New Topic Post Reply  [ 6 posts ]  Moderator: Sandy

All times are UTC [ DST ]


Who is online

Users browsing this forum: Google Adsense [Bot] and 10 guests



Search for:
Jump to:  


















Privacy Policy :: Powered by phpBB