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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Wed Jul 03, 2019 8:14 pm
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Okay, sorry to beat this topic to death but in reviewing the fender webpage, it states that the Custom Pro is a 5E5A circuit and going to ampware webpage, the info states that one of the differences is between the 5E5 and 5E5A in the pro is that 5E5A is a fixed bias. As posted, there is only one way to tell but I guess there is only one way to be sure. I do however want to understand the lingo a little better.... Does one inherently mean that it has a fixed bias and the other a cathode bias? Let me, emphasize that this is helping me comprehend a lot. Thanks again.


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Thu Jul 04, 2019 7:15 am
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If the 26 watt output power is accurate, that strongly suggests it's cathode biased. In fixed bias, a pair of 6L6's normally push 40~60 watts.

There are several ways to reduce power, but most of them would be even more "historically incorrect".

Since it's a "custom" amp rather than a "reissue" there's no guarantee that any of the original schematics are 100% correct.

Magazine writers aren't always 100% correct. Fender tech reps are minimum wage people who read from scripts to answer commonly asked questions -- uncommon questions often get wrong answers. So no guarantee you'd get a correct answer from Fender.

I think only one person on this thread has access to a Custom Pro. It would take us just a couple of minutes to do the measurements Tim described, but we can't because we don't have a Custom Pro to measure.

(If you're uncomfortable about working on amps, it's a simple quick check so an amp tech might do it on the spot for a low fee.)

If you could post pics of the amp's wiring, it's possible Tim or Stratele52 could tell you if it's fixed or cathode.


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Thu Jul 04, 2019 7:58 am
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Hmmmm. The comment on the ampware page suggests that both the 5e5 and 5e5a were 26 watt amps.

If ampware's power spec is wrong, their assertion that the 5e5a switched to fixed bias may not be trustworthy.

And what's the ampware comment about a "selenium rectifier is used in the 5e5a bias circuit"? What sort of bias circuit has a rectifier?

I haven't looked at any of the schematics (pain to read on my phone).

Has anyone actually looked at the 5e5a schematic?

And has anyone actually had their hands on a 5e5a? (There have been cases where the schematics have been wrong.)


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Post subject: Re: '57 Pro Amp fixed or internal bias and what does means?
Posted: Thu Jul 04, 2019 8:43 am
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You are correct in your assessment that it is an "A" version.
The original pro only had a tone control, not treble, bass, and presence.

In looking at the gutshot of a 57, there are a few fixed bias components to the left of the three gray main filter caps.
The hex nut there may be the bias adjust control with screwdriver access.
The selenium rectifier would be obsolete and is considered hazardous waste nowadays. The reason its called a rectifier is because selenium has a limited reverse voltage rating. So several diodes plates are stacked in series to build a rectifier package with a useful voltage.
It would be replaced by a silicon diode.

There are several differences between the original and the A models. Besides the different tone controls, there is a 5 meg resistor NFB loop on V2. On the modern version, it would be 4.7Meg.
Opening up this loop would increase the overall gain.
The preamp coupling caps are different. The A model uses .02, while the original had .1 for more Bass.
The two stages of V2 are direct coupled in the A model, improving tone by eliminating the coupling cap.
There is a bright cap on the A model volume control on channel one.
There are also mixing resistors to isolate the two channels at the volume control. This would reduce the effect of the Bassman Trick.
Any of the 4 input , 2 volume control amps of this era have crosstalk between channels. This is used for a creative effect I call the Bassman Trick because the 59 Bassman has the most.
If you plug in to one channel, turn up the other channels volume until you hear your guitar a bit. Then turn up the channel you're plugged into. The two are now out of phase and the tone changes dramatically with minor changes in comparative volumes.
Overall there is no clear winner between models, even though they would sound different when compared side-by-side. Both sport clean tone controls, designed before the tone stack was adopted. This era of amp will pass square waves without distortion. So they are excellent for harmonica and other reed instruments. Their clean tones are well suited to Jazz and Country. Other pickers just dime the volume for power tube distortion.
Have Fun.


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