Vibroclone filter caps

tonehoney

Member
Messages
46
I am "Vibroclonig" a 1971 Bandmaster Reverb and would like to know what values of filter caps I should use.

The original Bandmaster Reverb used (5) 20uf filter caps. In other posts, I've read that increasing the capacitance can increase the tightness of the bass, but on the website
http://www.lynx.bc.ca/~jc/fenderAmps.html, here's what he says,



"It is now time to discuss a mystery aspect of the power supply, Because electrolytic caps were costly way back (producing huge savings at the end of the year for a factory that cut costs right), today one can afford to replace the 70yF caps by 100uF bringing the resultant B+ capacity to a typical 50uF - this tightens up the low end a bit and gives the amp a slightly more solid feel without stiff'ing it ... installing overly large values is a bad idea as it can fry the power tranny slowly over time if streesed to high enough levels ... the preamp caps in general are not altered in value since the preamp is very sensitive to them and looses it's "dancing" quality if strapped down too hard ...

...namely the preamp power distribution network ... first the caps, if they're too small in value ghosting and motorboating effects may be induced - if they're too large the amp will sound really dead unless the tnwrok resistance level is high (as in Marshalls) ... in Fenders it is often a mistake to bring the cap values up from their 20-22uF values, some say 30-32uF is good too but generally speaking 50uF and higher filter caps are tone killers in amps that have otherwise the capacity to have the bias dance a little with the signal, a very subtle aspect of tube gear that is overlooked ... "

What values should I use to get that "big bottom" without losing tone?

Thanks,
Jim
 

TheAmpNerd

Member
Messages
1,056
If you are building a clone, then use what the factory did.

http://www.ampwares.com/ffg/

Find your amp there.

Brown used 16uf caps, two paralled for the mains = 32uf
Black used 20uf caps and two seriesed 70 mains = 35uf,
in the AB763

I've not had any problems using up to 68uf for the
mains and 22uf for the rest in most Fender amps
using the GZ34.

With very good new caps you shouldn't have any problems.
Don't go with cheap filters. Also make sure you replace all
the bypass caps (25uf/25V).

*** Your bass response will be fine ***

If need be, change the little .001uf cap from the channel
mix resistors (220k, 220k) to .01uf or .02uf. You can do some
other stuff too. Tweak it to taste. These are fun and simple
amps to work with and mod, thanks to straight foreward layout
and eyelet boards.

The more amps I work on the more I appreciate the 50s - 70s Fenders.

Keep the bias filter at 100uf or below, these were 25uf brown
and 50uf black for their bias filters.

Enjoy and have fun.
 

tonehoney

Member
Messages
46
Thank you for your replies! For AmpNerd, where on the schematic do I find the power supply layout/values?
 

tonehoney

Member
Messages
46
For AmpNerd, where can I buy 70uf filter caps? I've looked at Antique Electronics and at Hoffman with no success. I'm using Sprague Atoms.
 

RussB

low rent hobbyist
Messages
11,170
Originally posted by tonehoney
...where can I buy 70uf filter caps? I've looked at Antique Electronics and at Hoffman with no success. I'm using Sprague Atoms.
Most folks use the Sprague 100uf/350v caps. There is some mention of it at the Hoffman site.

Some even use the 80uf/450v units

I use the 100/350's myself
 




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