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Replacing tubes in a Fisher 800C receiver

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Posted on September 23, 2014 at 06:28:10
j.white
Audiophile

Posts: 1
Location: SE U.S.
Joined: September 23, 2014
 Can the 6kg5 tubes in the FM front end and in the beacon circuit
be relaced without affecting the FM alignment? Also, can the 6au6 tubes in the FM circuitry be replaced with new tubes withiut reuiring a
FM alignment? Thanks for your help
j.white

 

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No. (nt), posted on September 23, 2014 at 07:06:43
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000


Eli D.

 

RE: No. (nt), posted on September 23, 2014 at 07:37:30
truep
Audiophile

Posts: 375
Location: Texas
Joined: March 28, 2004
 I somewhat disagree Eli, Yes if you replace the tubes the alignment won't be perfect but in most cases it will work fine. I worked on tube TV's for years and never had to do an alignment after replacing tuner or IF tubes.
Give it a try and see if it works!

 

RE: Replacing tubes in a Fisher 800C receiver, posted on September 23, 2014 at 07:45:38
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 I rebuilt dozens of these Fishers plus aligned the tuner sections. Swapping in new tubes without realignment does not cause any issues.

However, when performing a sweep type alignment, ELI is correct that the EXACT center of band-pass will be off slightly with different tubes. You will not hear the difference.

I would prefer healthy tubes vs weak tubes in the tuner without an alignment.

Some Fishers stay reasonably aligned and others are far enough out that weak stations are compromised with regard to signal to noise ratios.

 

So.., posted on September 23, 2014 at 08:47:34
airtime
Audiophile

Posts: 11287
Location: Arizona
Joined: February 4, 2003
 If the tuner was originally aligned with strong tubes, than wouldn't putting new tubes in bring it a little bit closer to original alignment.

Just a thought.
charles

 

RE: So.., posted on September 23, 2014 at 09:11:17
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 Not to your reply. Other components drift over time.

BTW- I believe the second fore-mentioned tube is only the relay driver, thus does not effect alignment.

 

RE: Replacing tubes in a Fisher 800C receiver, posted on September 23, 2014 at 09:20:20
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 You got a lot of answers - none of which is MAYBE - which I believe is the proper answer. It depends on a lot of different things. You can try it and see what the results are, but as Eli would advise - get it properly aligned if you want the BEST performance. It may still work - even work well - with a tube change, but it's unlikely to work as well as if it were aligned. Whether that difference is important to you is for you to decide.

 

RE: So.., posted on September 23, 2014 at 09:42:55
airtime
Audiophile

Posts: 11287
Location: Arizona
Joined: February 4, 2003
 Ohhhhhh yeah. I forgot about those 50+ year old caps and resistors????

 

RE: Replacing tubes in a Fisher 800C receiver, posted on September 23, 2014 at 22:24:17
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Exactly
So many think tube health means simply putting a tube in a decent tube tester and if it registers good,that's all there is to it..Most don't take into consideration the inner electrode capacitance which can greatly affect the behavior of these small signal tubes in the way they relate to these tuned circuits along with other specs and this can affect center tune as you say but also I have found that it affects separation.When I change any tubes,I always set the top and bottom core of the 38kc coil and then the 19kc phase coil so I can at least maintain 30db separation.I always peak the IFs as well but changing the 6GK5 he probably won't notice any issues..
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

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