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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Wed 24, 2019 10:36 pm 
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i have to say, the phono input is excellent in audio output. anything, as expected, into the mics is well distorted.

well, i installed 10mfd@600v C3 new film capacitor. it looks like A TURD IN A PUNCH BOWL.

it sees 520vdc at turn on surge with 117 vac input. the capacitor then stabilizes to 420 working voltage.

as i was investigating, there was another capacitor that saw 520 volts of surge.

it is C4.

i don't know how i missed this, but i am very glad that i always order more than what i need when placing a parts order.

i ordered TWO films 10 uf at 600v to have an extra one on hand for a rainy day.

i have no real choice to use my second new 10@600 in this circuit b/c i can't series two 47s for 22mfd and use balancing resistors. there is just no room.

the 10uf for C4 versus 20uf C4 makes no sonic difference -and- this amp is so well filtered that it will NEVER notice the 10uf less value.

C4 is squared in blue in the schematic. it calls for 20mfd @ 450 and i replaced it with a new film of 10@600.

so, i now have TWO turds in a punch bowl.

...and that darn "turn on surge voltage"... they should use indirectly heated rectifiers in big voltage amplifiers like this... :twisted:

i just wonder how many people are replacing capacitors with 450 volt models when they see well over 500 volts at turn on. i have been argued with a few times on here about that, but that's fine--not my radios/amps.

i will state my claim again--when directly heated rectifiers are used in transformered sets, even a 500 volt capacitor IS NOT ENOUGH in certain instances as seen here. it is IMPERATIVE to use a capacitor that exceeds the turn on surge voltage.

why take the chance ?

steve


Attachments:
Untitled (1).jpg
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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Thu 25, 2019 2:41 am 
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Tid bit of info:
-------------------
( https://www.google.com/search?ei=gQc5Xe ... CAo&uact=5)

"What does it mean to [i]state your claim?
Not "state" but "stake"
STAKE a claim:
​ If you stake a claim to something, you say or show that you have a right to it and that it should belong to you: He marked the spot on his map where he had seen the gold and returned later that month to stake his claim.[/i]"

Don't feel bad... I too have often used phrases that I only heard but never saw written.
I believe the term "stake" comes from the gold rush days when you had to place stakes in the earth at the corners of your claim.

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Thu 25, 2019 2:49 am 
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well then...

i claim to never use 450 volt capacitors on directlly heated rectifiers :!:

steve

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Thu 25, 2019 2:51 am 
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Dutch Rabbit wrote:
i have to say, the phono input is excellent in audio output. anything, as expected, into the mics is well distorted.

well, i installed 10mfd@600v C3 new film capacitor. it looks like A TURD IN A PUNCH BOWL.

it sees 520vdc at turn on surge with 117 vac input. the capacitor then stabilizes to 420 working voltage.

as i was investigating, there was another capacitor that saw 520 volts of surge.

it is C4.

i don't know how i missed this, but i am very glad that i always order more than what i need when placing a parts order.

i ordered TWO films 10 uf at 600v to have an extra one on hand for a rainy day.

i have no real choice to use my second new 10@600 in this circuit b/c i can't series two 47s for 22mfd and use balancing resistors. there is just no room.

the 10uf for C4 versus 20uf C4 makes no sonic difference -and- this amp is so well filtered that it will NEVER notice the 10uf less value.

C4 is squared in blue in the schematic. it calls for 20mfd @ 450 and i replaced it with a new film of 10@600.

so, i now have TWO turds in a punch bowl.

...and that darn "turn on surge voltage"... they should use indirectly heated rectifiers in big voltage amplifiers like this... :twisted:

i just wonder how many people are replacing capacitors with 450 volt models when they see well over 500 volts at turn on. i have been argued with a few times on here about that, but that's fine--not my radios/amps.

i will state my claim again--when directly heated rectifiers are used in transformered sets, even a 500 volt capacitor IS NOT ENOUGH in certain instances as seen here. it is IMPERATIVE to use a capacitor that exceeds the turn on surge voltage.

why take the chance ?

steve

Those two caps are used to reduce ripple on the B+ and avoid HUM.
Any input stage that has to boost the signal a lot is also prone to boosting any ripple as hum.
So the larger the better on those two. I think a couple Ecaps of 22uf or 47uf or even 62 uf or larger @ 450v or 500v will do a good safe job there.
The design engineer speced 450v ... so if he liked it... why not?

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Thu 25, 2019 2:57 am 
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Here's 500volt ecaps

https://www.ebay.com/itm/22uF-500V-550V ... 7508!US!-1

https://www.ebay.com/itm/4pcs-22uf-500v ... Sw6YtZR1d~

https://www.ebay.com/itm/Illinois-Axial ... SwrhhaIKjV

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Fri 26, 2019 4:02 am 
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in the meantime, i have decided to purchase another 10 @ 600 as linked above.

i do have room for it and will parallel it with the 10 that i put in place of the 20. i can swing that nicely and neatly with space, but i have no room to mount terminal strips, series two 47s, and use balancing resistors.

at that point if i were to do that, the underneath will look like HELL and that just can't/won't be.

so i will have three turds neatly installed in a punchbowl, but that is the way it is.

there is absolutely no way i am running new manufactured capacitors more than 70-80% (and that includes turn-on surge) of their rated voltage--especially in this pristine treasure of an amp. also, they have to be of a good named brand. i am using IC Illinois Capacitor.

between the tubes, the parts, and the cost of the amplifier, i already have over $400 bucks into it, which is fine. i have to use quality stock for it and can't stop now.

back in the day, 80s and back, i certainly would trust a good "sprague" twist-lock-lytic for a 50-60 volt surge beyond the working voltage. i've installed many back then and they worked flawlessly. many of those capacitors even stated a 525 or something surge voltage and then they stated a 450 working voltage. those were good capacitors in their rectangular orange, blue, and white boxes. sadly, they are of days gone by.

i will post pics of the amplifier this weekend with the two big 10@600 since i have already taken them. the third capacitor for 10+10 paralleled for 20 will be in on monday.

we are getting there...slow but sure...and it will be nothing less than perfect when done.

one last thing i will have to do after the bottom cover is put on for good is to "roll" some 6AU6 tubes for the mic preamps. one mic 6AU6 channel is normal quiet when it tap on the tube ; the second is just a tad microphonic, but acceptable ; the third 6AU6 is microphonic when i walk across the floor.

i have a pile of about 50-60 6AU6 tubes to roll.

it is just too bad that this amp does not use octal tubes instead of those little frumpy 6AU6 seven pinners. it would look so much more beautiful with three octal 6SN7, 6SK7, 6SL7, 6SC7, or 6SJ7 tubes.

real radios and amplifiers use OCTAL tubes :wink: :P

steve

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Fri 26, 2019 4:31 am 
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Dutch Rabbit wrote:
in the meantime, i have decided to purchase another 10 @ 600 as linked above.

i do have room for it and will parallel it with the 10 that i put in place of the 20. i can swing that nicely and neatly with space, but i have no room to mount terminal strips, series two 47s, and use balancing resistors.

at that point if i were to do that, the underneath will look like HELL and that just can't/won't be.

so i will have three turds neatly installed in a punchbowl, but that is the way it is.

there is absolutely no way i am running new manufactured capacitors more than 70-80% (and that includes turn-on surge) of their rated voltage--especially in this pristine treasure of an amp. also, they have to be of a good named brand. i am using IC Illinois Capacitor.

between the tubes, the parts, and the cost of the amplifier, i already have over $400 bucks into it, which is fine. i have to use quality stock for it and can't stop now.

back in the day, 80s and back, i certainly would trust a good "sprague" twist-lock-lytic for a 50-60 volt surge beyond the working voltage. i've installed many back then and they worked flawlessly. many of those capacitors even stated a 525 or something surge voltage and then they stated a 450 working voltage. those were good capacitors in their rectangular orange, blue, and white boxes. sadly, they are of days gone by.

i will post pics of the amplifier this weekend with the two big 10@600 since i have already taken them. the third capacitor for 10+10 paralleled for 20 will be in on monday.

we are getting there...slow but sure...and it will be nothing less than perfect when done.

one last thing i will have to do after the bottom cover is put on for good is to "roll" some 6AU6 tubes for the mic preamps. one mic 6AU6 channel is normal quiet when it tap on the tube ; the second is just a tad microphonic, but acceptable ; the third 6AU6 is microphonic when i walk across the floor.

i have a pile of about 50-60 6AU6 tubes to roll.

it is just too bad that this amp does not use octal tubes instead of those little frumpy 6AU6 seven pinners. it would look so much more beautiful with three octal 6SN7, 6SK7, 6SL7, 6SC7, or 6SJ7 tubes.

real radios and amplifiers use OCTAL tubes :wink: :P

steve


Two lines of the lyrics from near the end of the song "Creep" by Radio head come to mind:

Whatever makes you happy
Whatever you want


go to 3:12
https://www.youtube.com/watch?v=DDjlaN-X8-0

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Fri 26, 2019 8:01 pm 
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absolutely !


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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Sat 27, 2019 4:05 am 
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under chassis pics

pics of the top side and finished product will come next week.

steve


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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Sat 27, 2019 2:44 pm 
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Very nice work!

-David


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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Sat 27, 2019 4:02 pm 
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thanks much

ocd at its finest.

:P

steve

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Sat 27, 2019 5:16 pm 
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I can see you put a LOT of work in this! It looks amazing! Even with the turds

-Steve

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Sat 27, 2019 6:46 pm 
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it did take quite a while, but it was well worth it.

i have it playing today and it sound great. it sounds just as good as to even better than some of my HiFi amplifiers.

i like the warm and robust sound. perhaps that warmth could be attributed to the 6N7 triode ?

i absolutely love the sound of my 6P5-6AC5 radios.

i would have put 6L6gc tubes in it, but it just needs to have the shoulder GA type in it for looks. it calls for G tubes, but my matched pairs are better matched in the GA envelope.

it kicks the pants off the heathkit A9-C that i had. the A9-C sounded flat and i unloaded that thing in a heartbeat.

i rolled a dozen 6AU6 tubes to find three that were barely microphonic. that turned out well too.

there *is* a slight difference in hum when using a 10 versus the called for 20.

all i need to wait for is that third capacitor to parallel with the 10 to make a 20. i noticed the hum when i played the amp on a large top line imperial/magnificent ?? 1950s magnavox enclosure with the huge 15" woofer with the big bell cover and the very deep horn.

it pounds--pounds hard, sounds deep, robust, clear, warm, and crisp.

the low value paper capacitors were so bad that even the one in the bass control circuit that saw little to no voltage caused the bass to be flubbery and muddy when turned up. it is amazing how the tonaliity cleared up when the capacitors were replaced around the tone controls.

actual amp pics are coming next week.

steve

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Sat 27, 2019 11:28 pm 
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Dutch Rabbit wrote:
thanks much

ocd at its finest.

:P

steve

Funny thing about neatness in wiring.... the electrons don't care. ... neither does my cat.

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Sun 28, 2019 4:30 am 
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funny thing about neatness...

i won't do it any other way.

the rabbit is dead so it does not care either.

:!:

:)

:P

steve

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Sun 28, 2019 5:51 am 
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Well... actually... I have to admit to neatness too... when I feel like it.:
Image

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Jul Sun 28, 2019 2:39 pm 
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right on

steve

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Aug Fri 02, 2019 5:17 pm 
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We had this same Bell at the cafeteria of our church . . . still worked up thru the mid-90s. I sold input impedance-matching transformers for it.

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Aug Fri 02, 2019 8:15 pm 
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were they plugged into the chassis or were they screwed on with the mics ?

my amp has no place for any impedance matching transformers.

steve

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 Post subject: Re: Bell Amplifier Restoration Begins - Page 3
PostPosted: Aug Mon 05, 2019 2:55 am 
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it is done and complete.

steve


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