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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 6:08 am
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stratele52 wrote:
Fender screen resistor are always 470 ohms in any amp , replace them if your reading are 920 ohms .

I talk about schematic to be sure the amp is original with original value ; filter caps....


I can tell you that all the caps both under the dog pan and on the bias board were all original paper with following values:

Bias board
Mallory 50mfd-70vdc

Dog Pan
2ea. Mallory 70mfd-350vdc
3ea. Mallory20mfd-500vdc

My replacements are, respectively:
Bias board
Yet to be ordered 100mfd-100vdc f@t

Dog Pan ( already installed)
2ea. F@T 80mfd-450vdc
3ea. JJ 20mfd-500vdc

Now I'm mulling about what best to use to replace off spec 470 oohm screen grid resistors......so I can add to my order with bias cap.


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 6:18 am
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yomammaaho wrote:
stratele52 wrote:
Fender screen resistor are always 470 ohms in any amp , replace them if your reading are 920 ohms .

I talk about schematic to be sure the amp is original with original value ; filter caps....


I can tell you that all the caps both under the dog pan and on the bias board were all original paper with following values:

Bias board
Mallory 50mfd-70vdc

Dog Pan
2ea. Mallory 70mfd-350vdc
3ea. Mallory20mfd-500vdc

My replacements are, respectively:
Bias board
Yet to be ordered 100mfd-100vdc f@t

Dog Pan ( already installed)
2ea. F@T 80mfd-450vdc
3ea. JJ 20mfd-500vdc
Good choice

Now I'm mulling about what best to use to replace off spec 470 oohm screen grid resistors......so I can add to my order with bias cap

Any material will do the job , watts is more important
.


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 6:32 am
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What watt screen resistors would you reccomend?
On a side note, I can gell you that I tend to run the volume kinda high to get a little tube djstortion ( my ears!).


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 6:52 am
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yomammaaho wrote:
What watt screen resistors would you reccomend?
On a side note, I can gell you that I tend to run the volume kinda high to get a little tube djstortion ( my ears!).


Watt for screen resistor have nothing to do with your use of the amp .

I put 2 to 5 watts ,depending on what I find on my electronics store .

Amp need 1 watts minimum if voltage is 400 volts or less , and 2 watts for more than 400 volts

I use short cut to talk about these values


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 7:11 am
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Thanks again!
Got the correct bias cap and 2 watt resistors on order just now.
Will update results when replacement parts installed.

I had a very experienced, and Fender "gold" ranked technician reccomend NOT to change electrolytes if the amp was working fine, but being that the power supply paper caps were 45 years old I thought it better to replace them anyway instead of risking a failure. The amp was'nt humming in the least, but I still felt that the punch and presence was'nt where it should be.

Thanks again for your help!


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 2:25 pm
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Keep or not old filter caps ( electrolitics ) ?

If you gig with this amp I will replace them to trust your amp .

For home use no need but with new filter caps may give you more bass and more punch .

Often ( not always ) you may need to replace all bypass caps ; 25 MFD 25 volts electrolitycs
You need 7 . If they are out of range or dry you loose good some Fender tone .


Be sure to check if your amp have the right fuse .


You are welcome and good luck with your repair


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 3:38 pm
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Matter of fact, I DID order and have the 7ea.25mf25v caps to replace the white Mallory (plastic coated) bypass electrolytes.
Wasn't sure if I should replace them though.....because no way of knowing if they're bad. Being that I want the best performamce from the amp, I might just go ahead and replace them anyway.
Can't hurt.

Cheers


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 3:50 pm
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One question concerning those bypass caps.
There are 6 ea. 25mfd-25v and 1ea. 5mfd-25v electrolytes on the board..............but I ordered 7ea. 25mfd-25v caps.

The 5mfd-25v cap is on the board closest to the power tranny.

Would anyone know if OK to replace the 5mfd-50v cap with the 25mfd-50v cap?


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 4:30 pm
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yomammaaho wrote:

Would anyone know if OK to replace the 5mfd-50v cap with the 25mfd-50v cap?



No , leave a 5 MFD there.


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 7:14 pm
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stratele52 wrote:
yomammaaho wrote:

Would anyone know if OK to replace the 5mfd-50v cap with the 25mfd-50v cap?



No , leave a 5 MFD there.


+1

As for your screen-grid resistors, I think you'll find that the metal-oxide types are much more thermally-tolerant than the original carbon-comp's. However I wouldn't not exceed the power-dissipation rating by more than 100% (I am known to be more conservative than most of my associates here though).

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 20, 2014 7:54 pm
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yomammaaho wrote:
Would anyone know if OK to replace the 5mfd-50v cap with the 25mfd-50v cap?


This bypass cap is part of the vibrato/tremolo circuit. I use a Sprague ATOM 8mfd/150VDC cap. This fits & works fine.

Antique Electronics SKU: C-SA8-150

http://www.tubesandmore.com/products/C-SA


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 27, 2014 10:16 am
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Well.......Recap done.
Much more punch and volume.
Thanks guys!

https://docs.google.com/file/d/0B7J-1Js ... cslist_api

https://docs.google.com/file/d/0B7J-1Js ... cslist_api


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 27, 2014 10:31 am
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yomammaaho wrote:
And yep.
The concensus is to go with the 100mf-100v electrolyte cap for the bias supply.
I have an f@t (100mf-100v) sitting there on an Antique Electronics order form waiting for advise about whether to replace the remaining off spec 470 ohm screen resistors.


I stick with the original values-50uf in this case (47uf is fine). It's a good idea to give yourself a little headroom on the voltage-50uf/100v or 150v is fine.


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Sun Apr 27, 2014 6:54 pm
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Krankster, The 100mfd/100VDC is fine. The bias supply is a constant feed. There is very little current demand. Like the demands on the main PSU. Just be sure you don't over filter this circuit (like 500mfd total capacitance) or put too much regulation onto it. You want the bias supply to be putting out steady voltage, best before, the main PSU kicks in. Esp during cold starts.

Since the bias supply and main PSU tap from the same PT --- changes in the voltage source (outlet) and amp demands during heavy bass runs or transients should follow each other. This is why heavy regulation is not a good thing.


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Post subject: Re: opinions choosing 69 Twin replacement bias cap
Posted: Mon Apr 28, 2014 6:04 am
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O.K.......Fixed the link to the pics.


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