Fender Pro Junior hiss (question about potential source, seeking advice)

[soapfoot]

The old Fender Pro Junior is a pretty good sounding small guitar amp (especially if implemented with a better speaker)

But it has one persistent issue that you read about everywhere on the internet--it's noisy.

This can be improved with some basic layout things, but nobody seems able to solve it entirely--especially the hiss component.

I recently rebuilt mine on a turret board, and while I was able to improve the hum and buzz, there's still quite a bit of hiss.

Taking a look at the schematic, one thing really jumped out at me:

The voltage divider after the first stage (R5 and R6) places a rather large resistance (470k) in series with the signal between the first two gain stages. Could noise through this resistor be a major contributor to the hiss in this amp?

If so, is there an elegant solution that wouldn't change the sound of the amp too much?

What would happen if I changed R5 for 47k, R6 for 5k6, and R7 for 25kA?

I wonder in particular if @CJ has any ideas.

Thanks in advance!

 

[Tubetec]

If you have a schematic handy post it up for a look .
You might try a high stabillity low noise resistor as anode load in the first stage .

 

[soapfoot]

If you have a schematic handy post it up for a look .
You might try a high stabillity low noise resistor as anode load in the first stage .

Sorry I had forgotten to attach the schematic; it's uploaded now!

Quiet resistor for V1 plate load is always a good idea, but even with that in mind... this amp is much-more-hissy than other tube guitar amps, and this occurs across virtually the entire line (particularly the first several revisions with the unbypassed first stage cathode).

So it seems as though it may be something design-related.

What's interesting is that the hiss is quite loud even with the volume control fully-CCW.

 

[Tubetec]

The tone control stage looks like its very lossy ,
your starting out with 20mV at the input ,after the tone stage only 9mV output .
if you go with changing the values to what you said you'll probably make matters worse .
you could try a different tone control set up that doesnt attenuate as much .

 

[soapfoot]

In that case, it's probably best to leave well-enough alone; the tone control is subjectively great in this amp; one of its biggest assets operationally.

Nothing's perfect, maybe it's okay to let this amp be a little noisy.

 

[Gus]

Measure the heater voltage

I had a cream and oxblood one that C11 went bad in.
I also got rid of the IC brand power supply caps

 

[soapfoot]

Thanks! Every part in the amp is brand new; I've just rebuilt it on a turret board and it behaves pretty much the same as it did before (a little bit less hum, I paid pretty good attention to ground current paths)

 

[Tubetec]

Maybe try taking the first stage anode load resistor upto 220kohms , it will get you a bit more gain and a bit less bandwidth so noise should be less of a problem , a higher output pickup might also help a bit
At the end of the day if your happy with the tone stop worrying yourself over nothing .

 

[soapfoot]

At the end of the day if your happy with the tone stop worrying yourself over nothing .

Probably the crux of it.

It's fine and it's always been fine. But as a recording amp its hiss level is its one weakness.

 

[CJ]

Easiest thing to do is tube substitution. So try that first. Maybe try a 12AT7 or 12AU7 for the phase inverter if you have one. I don't expect a ton of hiss reduction from past experiments but worth a try since it is fast and easy.

Maybe a lower gain preamp tube would help, there has been a trend toward hot gain 12ax7 tubes by ehx and others. I like a lower gain first tube for noise. Like a rca 7025 or maybe a 5751.

Do you have a scope? If so see if you can find where the noise starts.

If no scope, try removing the preamp tube to see what happens to the hiss.

You can also use a .1 if cap with a ground clip lead and short out the various parts of the preamp signal chain to see if you can't find something out.

 

[abbey road d enfer]

This can be improved with some basic layout things, but nobody seems able to solve it entirely--especially the hiss component.

The answer is in the level diagram. In particular the extra 30dB gain provided by V1a.
The level after the volume pot for full power is 16mVrms. It's about the typical gain of a fully cranked up amp.
In order to make the amp gain (and volume) manageable, they have attenuated the signal before the pot, which every half decent designer knows is not good for noise optimization.
The only way to improve hiss is to remove the pre-volume attenuation, by shorting the 470k resistor R5 and removing the 56k R6, then reduce the gain of V1a. It can be done by installing a voltage divider in the signal (mod1) or a tap on the plate load (mod2).
The overall gain will be unchanged, but hiss will be significantly attenuated.0

Of course there is an infinity of in-between solutions, which would make the attenuation shared between pre-volume and post-volume, but I don't think it would change much in terms of overall sound.
I have neglected the effect of the small 22pF cap across the 470k, which provides some treble boost. It can be added to both my suggestions.

 

[soapfoot]

Great insight, thanks so much!

 

[soapfoot]

Looking at this further, it'd be pretty simple for me to implement your "mod 2" on my existing turret board (less so the "mod 1")

It may just be worth a try (I'm out of town for another week or so; maybe when I return)

I'd already deleted the 22pF cap C2 from my turret board version.

As a follow-up question, is there any reason you can see that the designer might have gone with this configuration? Does it hint that they were maybe having problems clipping the input of V1A?

 

[abbey road d enfer]

I believe they took the Blues Jr (IMO one of the best small amps)) circuit and kind of butchered it.

 

[soapfoot]

For some reason I thought the Pro Junior came first?

A brief look online reveals that I'm wrong about that--looks like the Pro Junior made its debut in '93 and the Blues Junior was a couple years earlier.

In any case, I always liked the Pro Junior more (subjectively). There's just something special about it, and I never got quite the same thrill from the (more fully-featured, and less-noisy) Blues Junior.

 

[abbey road d enfer]

For some reason I thought the Pro Junior came first?

Seems you're right. Then I don't know why the electronics are better in the Blues.

I always liked the Pro Junior more (subjectively).

I had 2 Blues Jr's, one that I modded because I thought something was lacking, then I forgot about it. A few years later, I sold the other to a friend, who told me it didn't sound right.
Comparing the two, I just noticed a small difference in HF response, I suddenly realized I had modded mine. It just takes clipping one treble bleed cap, and suddenly it transforms the amp.
I think the Pro Jr doesn't have the oomph to play with a band, unless it is miked. The speaker in the Pro has less bass and less efficiency.

 

[soapfoot]

As a recording amp, the Pro Junior has a lot of potential (except the hiss thing).

As a performing amp, I agree about the speaker, but I think a big part of the issue is actually that the cabinet is undersized for a 10" speaker.

I put mine in a larger pine cabinet (Blues Junior sized, in fact!) and put a Weber 10A125 in there, and it's a completely different story.

The first (blue frame) Eminence AlNiCo that was in the early Pro Juniors was okay, but the ceramic magnet "gold label" that was in the second (blonde tolex) version was terrible. I'm not sure what's in the new ones.

Mine has had a bunch of other stuff done to it, but nothing made as big of a difference as putting it in that larger cabinet.

As the Princeton Reverb will attest, a single 10" speaker at roughly that power level can be plenty (or even "just right") for certain ensemble dynamics.

Obviously some bands are louder and require more power

 

[CJ]

it looks like the blues jr type amps borrowed a bit from James Brown,

he liked to knock down gain at each stage starting back with the 5150.

this makes it easy to tweak the amp at a number of different places,
and allows the usage of caps to further customize the sound.

i like the sound of the blues jr overdrive circuit, for about an hour, then it gets tiring and i am looking for something different, but that is normal for every amp i guess,

who knows, JB might have borrowed from Rivera/Fender as there was some sparse usage of the voltage divider resistor thing.

he continued this with the Kustom stuff>

 

[soapfoot]

I think the Pro Junior was a Bruce Zinky design; Rivera had left Fender by that point

 

[Matador]

It's interesting that many of these Fender amps don't quite have the correct gain staging for modern guitars and their (potentially) high output.

In a distortion channel, it probably doesn't matter, but for players looking for more clean volume, the idea of having 3 or 4 gain stages doesn't make sense when driving a power amp needing only 6-7 Vrms at it's phase inverter grid to drive the output to full power. Almost every modern Fender amp transitions from clean to dirt before 5 on the volume, and that is after dumping half the signal to ground (sometimes repeatedly) between the first tube and the phase inverter.

I'm guessing they did this to accommodate more "sounds", and wider ranges of input signal from weak old single coils to modern hot humbuckers?