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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Sat 07, 2020 11:36 pm 
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Joined: Nov Fri 10, 2006 12:24 am
Posts: 2702
Location: Thornhill, Ontario, Canada
I'd replace all paper caps... my experience they are either bad or will go bad. All my audio reparations are for "over 50 future years" and I suspect your amplifier has been around for that time already. It is vital to replace the coupling caps to the O/P tube grids right away... as was said above.
I used to reform electrolytics, checking leakage, value and low ESR... if good I kept them. Now I don't! New electros are reliable and cheap enough, so I replace them. Of course, never parallel an old cap of any type with a new one!
Cheers,
Roger
EDIT: ... and replace any ceramic caps in the signal path with film caps. They are only good for RF and AC supply filters.

_________________
Roger Jones,
Thornhill, Ontario
Ontario Vintage Radio Assoc. http://www.ovra.ca


Last edited by engineer on Mar Sun 08, 2020 9:34 pm, edited 2 times in total.

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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Sat 07, 2020 11:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20495
Location: Warner Robins, GA
Plus the new caps often have better specs such as lower ESR and some may even have higher ripple current ratings.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Fri 13, 2020 7:53 pm 
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Joined: Nov Wed 23, 2011 4:40 pm
Posts: 328
Location: Somewhere on Planet Earth
Thanks to all for your valuable comments and tips. They gave me perspective and things to think about before deciding what to do.

The amp was working amazingly well as it was. All original.
So far I replaced the coupling caps on the grid of the output tubes and the resistors I mentioned before that were cooked (plate 10ohm and grid 2 100ohm).

There is a question among the latest posts related to the use of the jukbox. No, it will not be used to collect money. Just to enjoy one song from time to time in the dining room. Fortunately I tried a lot of coins which are currently running in my country and the slot takes one of them (the smallest in value hehe). I need to get a bunch of those to distribute among people who come visit to feel the experience.


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 Post subject: Re: Jukebox tube amp general questions
PostPosted: Mar Fri 20, 2020 6:14 pm 
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Joined: Aug Tue 04, 2009 2:15 pm
Posts: 459
Location: Elmira, NY
As far as replacing output tubes:
I would Test the tubes in a good tube tester(for shorts, leakage. And mu. If the tubes are operational but weak, I would modify the bias circuitry such that each tube can be biased separately.( individual bias resistors), then I would adjust each bias for best linearity to achieve symmetrical non clipping output. Then attach a note of the modification and adjustment values. This way you can get the best operation and make tubes last longer.
As for resistors and capacitors, study the circuit and consider how critical they are. Bias resistors can be critical. Grid leak, grid stopper and plate load resistors may be less critical. Capacitors such as power supply, and decoupling can be less critical than coupling capacitors.It depends on the circuit function. Need for reliability varies as jukebox vs pacemaker.
As stated new capacitors are inexpensive. “Inexpensive”is relative to your budget.
This is my opinion, others arguments may be valid.


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