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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Mon May 30, 2016 4:29 pm
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Aspiring Musician
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Great !

While inquiring about the replacement F&T's I was given the following advice:
--------------------------
There are a few things you can do, but #1, don't just put 500v caps in the power supply (the first two caps) and expect them to hold together long term. The amp will exceed their limits while on standby and they will fail. Install the first two caps in series with 220k 1w+ equalization resistors across each cap. Most guys use 2x 100uf 350v caps in series, which will render 50uf 700v total capacity.
--------------------------

:D


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Mon May 30, 2016 5:31 pm
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That might be a good way to go. Ppl also, have used the Sprague 20mfd/600VDC in place of 500VDC rated ones. With pretty good success. Item number: C-SA20-600, in Antique Electronic Supply catalog.

The amps that I have which used 22mfd/500VDC F&T have been fine. SF DR & VLR. Blonde Tremolux. Make sure you replace all main filter caps. I also replaced the resistor in main rail with either wire-wound 5-watt or metal oxide 3 watt resistors.

If you decide to go to the two caps in series, for first section --- use 100mfd/450VDC caps. Major overkill. But, you won't need to worry about popping any caps --- when flipping power switch. Use 5-watt ww or metal oxide, 2 or 3-watt 220k-ohm leveling resistors across those caps. You'll be good to go, forever.

One caveat... Going to greater voltage rated caps, esp in first stage, will change the tone. You'll prolly get more B+ on the plates. And the transient response will be faster and greater with the overkill caps. The tone may become tighter and bassier with the higher voltage caps. Lose some warmth in the mids. Esp when compared to the nice, soft, brown tone of the original brown and blonde setups.

I'd go to the 600VDC rated Sprague ATOM caps. In parallel, like the original setup. Replace the rail resistors with metal oxide ones. This will prolly tighten the bass somewhat (compared to the dried out OEM Mallory or Astron caps). But, at least you may avoid changing the tone from a brownface to more of a 1970s era SF tone.

One other thing (always one other thing) :)

Use good 6L6GC. I mean, this era amp sounds the worst with new Russian mil spec 6L6GC, than any other era Fender amp. If you have RCA BP, you are good as gold. GE mil spec 7851A, Sylvania fat bottle 6L6GC or STR tubes. Even, Tungsol (USA) 5881 or RCA, Sylvania, Tungsol 6L6WGB are going to sound better.

I think it's the way the front end is made. Not as hot setup as in BF and SF era amps. Esp with that 7025 phase inverter.

Good luck! Keep us informed.


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Mon May 30, 2016 6:06 pm
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Thank you.

I was told that the 600VDC rated Sprague ATOM caps are too big to fit under the original cover.
Is this correct?


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Mon May 30, 2016 7:44 pm
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Here are the dimensions:

http://www.mouser.com/ProductDetail/Vis ... 252bdR0%3d


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Fri Jun 10, 2016 6:04 am
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The tech just called me today to ask if I want a bias pot added to the amp, same as in BF circuits!??

What do you think?

:)


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Fri Jun 10, 2016 12:12 pm
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Yes, add a bias pot. This will make changing 6L6GC much easier and the amp safer.

Have him drill a hole in the chassis and mount it same place as in blackface amps. That way you can adjust the bias with amp in cab or out of cab.

:)


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Fri Jun 10, 2016 1:31 pm
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BMW2002Ti wrote:
Yes, add a bias pot. This will make changing 6L6GC much easier and the amp safer.

Have him drill a hole in the chassis and mount it same place as in blackface amps. That way you can adjust the bias with amp in cab or out of cab.


PFFFFT!

If you've installed a grounded, 3-conductor power cable, you have no further need of the ground switch. Remove it and install your bias pot there. That way no hole need be drilled into an otherwise pristine chassis and, if desired, the mod will be totally reversible.

Arjay

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"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: Fender Pro Amp 1961 - thoughts?
Posted: Fri Jun 17, 2016 9:51 am
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I just bought a late 1961 brown face pro and it is one fantastic amp. I ran into it by chance and am really glad I did.
Thanks John


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Fri Jun 17, 2016 1:05 pm
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Retroverbial wrote:
BMW2002Ti wrote:
Yes, add a bias pot. This will make changing 6L6GC much easier and the amp safer.

Have him drill a hole in the chassis and mount it same place as in blackface amps. That way you can adjust the bias with amp in cab or out of cab.


PFFFFT!

If you've installed a grounded, 3-conductor power cable, you have no further need of the ground switch. Remove it and install your bias pot there. That way no hole need be drilled into an otherwise pristine chassis and, if desired, the mod will be totally reversible.

Arjay

That's what I did to my 69SFSR took out the ground toggle and put the bias pot right in the hole and can bias it from there with a screw driver
Image
mud


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Fri Jun 17, 2016 3:42 pm
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mud wrote:
That's what I did to my 69SFSR took out the ground toggle and put the bias pot right in the hole and can bias it from there with a screw driver
Image
mud


Perfect!

8)

Rawk on!

Arjay

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"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: Fender Pro Amp 1961 - thoughts?
Posted: Fri Jun 17, 2016 6:25 pm
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Cheers guys, as per Arjay's advice I've instructed my tech to remove the ground switch and put the pot right there.

:D


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Fri Jun 17, 2016 6:26 pm
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powerwagonjohn wrote:
I just bought a late 1961 brown face pro and it is one fantastic amp. I ran into it by chance and am really glad I did.
Thanks John



We'd like to see your amp, you know that ... 8)


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Fri Jun 17, 2016 6:52 pm
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NT, yes a much better spot for bias pot.

Good luck with that!

:D


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Fri Jun 17, 2016 6:58 pm
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An interesting opinion from my tech .. the 240V Classic Tone PT is close to that spot; he thinks this may cause some hum.

:mrgreen:


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Post subject: Re: Fender Pro Amp 1961 - thoughts?
Posted: Sat Jun 18, 2016 12:38 am
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Don't think it'll be a problem, if he routes the wires to and from the pot away from PT lines.

However, the ground points of the resistor and bias filter cap may cause a ground loop --- since it's a distance from each other, with the pot being placed on the back of the chassis.

Guess you'll find out. :)


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