rock soderstrom
Tour de France
Thanks! I agree, this amp was poorly designed. I will open a separate thread with some more info.
Fender Pro Junior hiss (question about potential source, seeking advice)
- Thread starter soapfoot
- Start date
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Matador
Well-known member
Just to make sure I'm following: do you mean "Including the Master Volume within the global NFB loop is a beginners mistake"? There are plenty of amps that have master volumes 'before' the power amp, and thus are outside the loop.
Couldn't the phase-inverters plate load resistors be split as well to decrease the 'open-loop' gain of the PI+power amp section? I've had several of these amps howl at ultrasonic frequencies when the NFB resistor is decreased (or even bypassed), which can be tamed by snubbing the transformer primary, however if not done correctly it can impact the distortion characteristics of the output section.
Yes. Global NFB tends to maintain a fixed gain (until the OLG runs out of steam), which somewhat defeats the intention.
Correct. That's where a master volume does its job best. However, for example Marshall 2210 has its master volume as a dual pot between PI and output tubes.
Sure, it's another possibility.
All you say is correct, that's why I suggested caution.
Another possibility, which would present zero-risk, is to decrease the gain of V1a, either by taking a tap on R10, or using a pad between C6 and R12.
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Matador
Well-known member
Yes: it's like pushing the gas pedal and the brakes at the exact same time.
metalockpick
Well-known member
Putting a pot or attenuator circuit in between C6 and R12 (and add another cap B4 R12) will greatly reduce the hiss. That amp was developed to be very low cost, small feature set (two knobs), and to sound great at all volumes with an American Std Strat. No feedback to greatly increase the power amp gain and, as Abbey Rd mentioned, to make the transition into clipping more natural. That high gain, no global feedback power amp is a lot of the tone, but magnifies any hiss from the preamp. As far as hiss/noise goes in a guitar amp, it's a signal to noise issue- turn it up and the signal will (or at least should) drown out the noise.
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Actually it has "some" global NFB, albeit very small, about 4dB, which is nothing to write home about.
I suspect the guy who "designed" the amp didn't really know what everything is doing, so he put that NFB resistor because most other Fender amps had it.
metalockpick
Well-known member
You would be incorrect.
The engineer (John Sams) did that to make the schematic appear to be a traditional Fender design to get it through R&D mgmt. Mktg liked the sound/performance, "Not invented here" was an issue for R&D.
Sams previously worked with the Peavey folks. Great engineer.
The engineer (John Sams) did that to make the schematic appear to be a traditional Fender design to get it through R&D mgmt. Mktg liked the sound/performance, "Not invented here" was an issue for R&D.
Sams previously worked with the Peavey folks. Great engineer.
I knew John last century and I won't openly diss him behind his back. At the time he was working as a bass amp designer for Peavey. His office was downstairs from my mixer design group. Listening to his unmusical wanking on prototype bass amps all day long was unpleasant. I feel sorry for his fellow guitar amp engineers much closer to the sound source. He was replaced by Jim Albert a respectable bass player and things improved all around.
==
Another ex-Peavey guitar amp engineer is working at Fender now. James Brown is Principal design engineer for EVH/Fender. No doubt banking on his Van Halen history (he designed the 5150 series of amps).
JR
metalockpick
Well-known member
... And yet you immediately dis him behind his back.
Hartley once told me "He's a great engineer...when he's not on the phone with his girlfriend!".
I got a kick out of that.
Hartley once told me "He's a great engineer...when he's not on the phone with his girlfriend!".
I got a kick out of that.
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You may be right. Whatever the reason, ignorance or deviance, the end result is the same: an amp with limited attraction.
soapfoot
Well-known member
…except that it sounds pretty cool, and that’s plenty attraction for me, at least (enough to forgive a bit of an elevated noise floor).
Whatever works!
Whatever works!
No problem with that. You're one of the limited group for which this amp is attractive.
Personally I'm a fan of the Blues Jr (I owned three), although I recognize it has its flaws (burning screen resistors, lousy jacks, fragile reverb tank, and this damn tone-sucking capacitor that begs to be removed).
Changing the output section to cathode bias might sound nice,
soapfoot
Well-known member
I’ll posit that it may not be such a minority interest, having been in continuous production for well over 30 years.
It’s anecdotal, but I know quite a few others who really love them (I own about ten very nice vintage guitar amplifiers, and I nevertheless like it)
Well, what I meant is this amp is not for everybody.
In my ecosphere, I know more people who love the Blues Jr than people that love the Pro Jr.
In my ecosphere, I know more people who love the Blues Jr than people that love the Pro Jr.
metalockpick
Well-known member
I’ll posit that it may not be such a minority interest, having been in continuous production for well over 30 years.
It’s anecdotal, but I know quite a few others who really love them (I own about ten very nice vintage guitar amplifiers, and I nevertheless like it)
metalockpick
Well-known member
Matador
Well-known member
I may be in minority, but I haven't seen a single well built Fender, no matter the era.
The old blackface amps have those god-awful wax impregnated cardboard eyelet boards that start to go conductive, leaking voltage all over the place to adjacent eyelets. Not to mention that they buckle after they absorb moisture and crack solder joints. Eyelets are also a major pain to rework (albeit slightly easier than newer Fender PCB's).
The newer amps have cheap circuit boards, and major design flaws. I have repaired so many with blown screen resistors, and cracked plate load resistors. Lifted pads happen all the time, because they don't plate through holes (and many lack soldermask). They also have that section where they derive a bipolar 16V supply from a bipolar 48 volt supply, meaning they need to dump over 8W of power into a 10w resistors, cooking the board, the nearby caps, and even diodes. I've redone that portion of the PSU nearly on nearly 30 amps over the last few years, so often that I actually had to make a replacement PCB with proper TO-220 resistors on a bladed heatsink that can be mounted off-board.
For as much as they charge it's a crime.
The old blackface amps have those god-awful wax impregnated cardboard eyelet boards that start to go conductive, leaking voltage all over the place to adjacent eyelets. Not to mention that they buckle after they absorb moisture and crack solder joints. Eyelets are also a major pain to rework (albeit slightly easier than newer Fender PCB's).
The newer amps have cheap circuit boards, and major design flaws. I have repaired so many with blown screen resistors, and cracked plate load resistors. Lifted pads happen all the time, because they don't plate through holes (and many lack soldermask). They also have that section where they derive a bipolar 16V supply from a bipolar 48 volt supply, meaning they need to dump over 8W of power into a 10w resistors, cooking the board, the nearby caps, and even diodes. I've redone that portion of the PSU nearly on nearly 30 amps over the last few years, so often that I actually had to make a replacement PCB with proper TO-220 resistors on a bladed heatsink that can be mounted off-board.
For as much as they charge it's a crime.
Maybe a cathodyne inverter would be better
And/or
Lower gain tubes like CJ posted
And/ or changing the plate voltage divider like Abbey Road d enfer posted
IIRC years ago at ampage and the blue guitar web site there were ideas posted about using EL84 output tubes and limiting the grid drive and setting the gain in the stages. I think the blue guitar site no longer exists. It also had stuff about trainweck amps.
https://web.archive.org/web/20180324093219if_/http://blueguitar.org/
And/or
Lower gain tubes like CJ posted
And/ or changing the plate voltage divider like Abbey Road d enfer posted
IIRC years ago at ampage and the blue guitar web site there were ideas posted about using EL84 output tubes and limiting the grid drive and setting the gain in the stages. I think the blue guitar site no longer exists. It also had stuff about trainweck amps.
https://web.archive.org/web/20180324093219if_/http://blueguitar.org/
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