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Post subject: Custom '57 Pro tube swap questions
Posted: Wed Aug 01, 2018 10:32 am
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Hello,

in the manual, it describes changing the 12ay7 for a 12ax7 for more gain as well as swapping the rectifier tube for less gain. Are these both tubes that don't require rebiasing when doing so? I'm hoping to not have to do that until the power tubes need to be swapped (and even then, just using the stock ones).


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Post subject: Re: Custom '57 Pro tube swap questions
Posted: Wed Aug 01, 2018 10:55 am
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Joined: Wed Mar 31, 2010 12:48 am
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No re-biasing is necessary for pre-amp tubes or the rectifier......they are "plug and play" in both instances.

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: Custom '57 Pro tube swap questions
Posted: Wed Aug 01, 2018 11:33 am
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A rectifier tube doesn't have to be biased, but in a fixed bias amp, changing to a different type of rectifier tube will change the plate voltage. So the power tube bias should be checked, and adjusted if necessary.

But a quick Google says the Custom '57 Pro is cathode biased. I would assume that if Fender doesn't mention checking the power tube bias, that whatever rectifier substitution they suggest would be within the cathode bias self regulating range.


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Post subject: Re: Custom '57 Pro tube swap questions
Posted: Wed Aug 01, 2018 2:42 pm
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Location: Province de Québec, Canada
From Guitar Magazine

https://www.theguitarmagazine.com/gear/ ... m-pro-amp/

For a while during the late 1950s, Fender had a variety of amp models that were essentially the same amplifier in different sized cabinets with various speaker configurations. If you check out the schematics of the 5E4 Super, 5E7 Bandmaster and 5E5 Pro, you’ll see they are almost identical.

5E5 schematic;

https://www.thetubestore.com/lib/thetub ... ematic.pdf




For a while during the late 1950s, Fender had a variety of amp models that were essentially the same amplifier in different sized cabinets with various speaker configurations. If you check out the schematics of the 5E4 Super, 5E7 Bandmaster and 5E5 Pro, you’ll see they are almost identical.
Where there are differences, they’re confined to the output transformers and one or two components in the bias and negative feedback sections. Of the three, the Pro occupies perhaps the lowest rung on the legend ladder.

The model Fender has chosen here represents an evolution of the simpler Pros of the early 50s and the controls hint at things that were about to pass at the twilight of the tweed era. Already the tone stack had evolved to a Baxandall-type configuration and negative feedback was being used with a presence control in the loop along with fixed bias.

Fender has stayed true to vintage spec with a 12AY7 in the V1 position. Period schematics also specify a 12AY7 for V2, but this reissue has a 12AX7. Since it’s a replica of a ’57 model, the power valves are 6L6s rather than 5881s (used from ’58 to ’60) and they get their juice from a 5AR4 rectifier valve. A period-correct 5U4 rectifier can be substituted for gain and feel that’s truer to vintage Pros and a 12AX7 can be used in V1 to hasten overdrive onset.


more with the link

ImageCapture d’écran 2018-08-01 à 17.42.13


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