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Post subject: AC coupling Blue caps in Bandmaster BF 1966, replace or not
Posted: Tue Nov 26, 2019 9:08 am
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I just bought a nice Bandmaster Blackface 1966 head and I'm about to recap all the
possible offending ones. Mostly electrolytics. These will be replaced by Sprague ATOM ones.
But for all the AC coupling Blue tubular caps... Well I tested all of them and after more than 50 years
they have no leakage at all. I know the most critic are the 2 coupling caps just before
both 6L6GC power tubes (0.1uf 400v).
I would'nt want to scrap good capacitors..
Your opinions are VERY welcomed !
JP


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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Tue Nov 26, 2019 10:07 am
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Leave blue caps there. Most of the time they are good for 100 years. These caps are your tone.

Defective signal caps ( like blue caps ) show DC leaking and do some noise.
Bias filter cap ; put a 100 mfd 100 volts, original 25 mfd 50 volts is too small.

Some tubes's plate resistor may do some noise with years, replace them.


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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Tue Nov 26, 2019 10:26 am
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Thanks for your nice advises !
And yes I will replace plate resistors too.
JP


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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Tue Nov 26, 2019 11:52 am
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stratele52 wrote:
Leave blue caps there. Most of the time they are good for 100 years. These caps are your tone.

+1

If it ain't broke, don't 'fix' it.

And I'd replace all the electrolytics (bias supply, filter array, and cathode bypass) with F&T's in lieu of the Sprague's.

Be sure to ditch the "death cap" and install a grounded 3-conductor power cable (bypass the superfluous "ground" switch).

Arjay

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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Tue Nov 26, 2019 12:17 pm
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>If it ain't broke, don't 'fix' it.
I agree.

>And I'd replace all the electrolytics (bias supply, filter array, and cathode bypass) with F&T's in lieu of the Sprague's.
Already have Sprague's..

>Be sure to ditch the "death cap" and install a grounded 3-conductor power cable (bypass the superfluous "ground" switch).
I know that since I made these mods on 2 vintage Fender years ago..
I'll do the same for this baby !

Thanks !!


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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Tue Nov 26, 2019 12:35 pm
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If you already have the Sprague's on hand, go ahead and use them. But check their values before installation. Also, check your power-rail resistors under the doghouse while you're changing out those filter caps. If replacement is necessary, use metal-oxide types for enhanced longevity.

Arjay

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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Tue Nov 26, 2019 2:00 pm
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>If you already have the Sprague's on hand, go ahead and use them. But check their values before installation
Are Sprague ATOM caps susceptible of value changes over time ?
I always put brand new ones in recaping anyway..

>Also, check your power-rail resistors under the doghouse while you're changing out those filter caps.
>If replacement is necessary, use metal-oxide types for enhanced longevity.
I always change them with 2 watts metal-oxide like you say.
In this case 4.7k and 1k.

Thanks for your precision !!


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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Tue Nov 26, 2019 3:51 pm
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About six or seven years ago some new Sprague Atom's started appearing up that did not measure up to their specified values, usually significantly under-spec'd. Such an anomaly could be catastrophic to something like a Deluxe Reverb whose filter array consists exclusively of 16 mfd electrolytics. Other values sold by Sprague at the time were similarly affected. I'm not certain if it was merely a bad production batch or they were outright bootlegs but I stopped installing them and switched to F&T's just to stay on the safe side. I've heard no recent anecdotes about problems with Sprague so likely you're okay. But I'd check them nevertheless before installing them.

Arjay

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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Tue Nov 26, 2019 3:57 pm
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I will do it.


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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Tue Nov 26, 2019 7:46 pm
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Blue MOLDED are very good caps. :)

Image


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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Tue Nov 26, 2019 8:43 pm
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For authentic Fender blackface/silverface tone they're the very best.

Arjay

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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Wed Nov 27, 2019 6:12 pm
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I just checked my Sprague Atom today..
The 100uf/350v tested at around 120uf (+20%) --- (Sprague specs them -10% +75% tolerance)
The 20uf/500v tested at around 29uf (+45%) --- (Sprague specs them -10% +50% tolerance)
The 25uf/25v tested at around 30uf (+20%) --- (Sprague specs them 20% tolerance)
I find the 20uf/500v +45% high for a manufacturer part..
Are they OK for my Bandmaster Blackface recaping ?


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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Wed Nov 27, 2019 6:37 pm
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It's up to you. As I mentioned I no longer use Sprague's.

Arjay

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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Thu Nov 28, 2019 2:05 am
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I use both ; Sprague and but mostly F&T

F&T for power supply filter caps are cheaper than Sprague
F&T 22 mfd 500 V $3.52
Sprague 20 MFD 500V $12.42

For bypass caps, Sprague are cheaper than F&T

https://www.tubesandmore.com/products/c ... gue%20Atom


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Post subject: Re: AC coupling Blue caps in Bandmaster BF 1966, replace or
Posted: Thu Nov 28, 2019 2:14 am
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jpdesroc wrote:
I just checked my Sprague Atom today..
The 100uf/350v tested at around 120uf (+20%) --- (Sprague specs them -10% +75% tolerance)
The 20uf/500v tested at around 29uf (+45%) --- (Sprague specs them -10% +50% tolerance)
The 25uf/25v tested at around 30uf (+20%) --- (Sprague specs them 20% tolerance)
I find the 20uf/500v +45% high for a manufacturer part..
Are they OK for my Bandmaster Blackface recaping ?



For AB763 circuit ?
http://ampwares.com/schematics/bandmaster_ab763.pdf

http://ampwares.com/amplifiers/fender/b ... andmaster/

Where are the two 70 mfd 350 volts caps in your order ? The 100 mfd ? Two of these will work.
I did not see the bias filter cap :(

Yes all of these caps are OK for recaping


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