Fender Pro Jr IV (the newest one)

[raveonstevie]

Okay. I love this amp. The four series came out in 2018 I think. They improved it a lot. Great little amps.

I want to be clear that the hum I’m getting is only at loud volumes and it’s very usable except maybe for recording it loud. I don’t want to discourage anyone from getting one of these.

With my Les Paul, with minihums, there’s no hum with the amp volume on three (quarter of the way up), which is pretty loud BTW, and guitar volume on ten. At five I hear a little hum. At six (half way up) it really kicks in and gets worse from there. My Strat through it, especially with effects, is a different story and is my reason for wanting to correct it. It’s still very usable for me in a jam setting.

The problem is in the first two gain stages, I’m pretty sure from messing with it. There’s only a 10k resistor on the input and the first triode’s cathode bypass cap is 47uf. I love the tone of this amp and I’m not looking to alter it.

I have a 12v transformer I’m not using. The center tap was cut off (by someone else). I think I’ve decided what to do but want to run this by someone before execution. I’m thinking I’ll use the 12v transformer for at least v1. The transformer is rated for two amps. Can I heat two 12ax7s with the two amp rating?

I’ll also use a hum pot with wiper to ground. I’ll have to cut the circuit board traces to cut off the original heater supply. There’s plenty of room and I really don’t think taking the boards out for a novice like me is a good idea from what I’ve heard of failing soldering joints on ribbon cables and tube sockets. I can do this without removing anything.

Sorry for the long read. Does this sound like a good plan? Any feedback would be appreciated. I might be busy today so responding might be slow but I will be following and I’ll update the thread. Thanks

[thetragichero]

with strat it is likely mains hum picked up by one of the three antennas in the strat
but yes, more than enough heater current to run two 12ax7 at 12v. might as well run it through a bridge rectifier, throw a big ol electro cap on it, and have dc heaters

[raveonstevie]

with strat it is likely mains hum picked up by one of the three antennas in the strat
but yes, more than enough heater current to run two 12ax7 at 12v. might as well run it through a bridge rectifier, throw a big ol electro cap on it, and have dc heaters

Thanks for the reply. Yeah, the thing is it will hum with my mini buckers too. It isn’t that bad but I’ve become obsessed with it, probably from reading internet forums. And the Strat definitely needs work on the grounding. It hums less when I touch the strings.

Can you point to a diagram of the setup your referring to? I was trying to source a rectifier but I’m not sure what to use. I don’t know what size cap and I’m really not clear on how to wire it. Actually, I might have to take that back. I’ve researched this a lot and I think maybe I do have a link. But I’m not sure about that. I’ve saved about 10,000 pages.

[xtian]

Let's get clear about where the hum originates: amp or guitar. With nothing plugged into the Pro Jr, how is the hum, at any position on the Volume control? If there is no hum to complain about with no guitar plugged in, you're not going to improve anything by modding the amp.

[raveonstevie]

Let's get clear about where the hum originates: amp or guitar. With nothing plugged into the Pro Jr, how is the hum, at any position on the Volume control? If there is no hum to complain about with no guitar plugged in, you're not going to improve anything by modding the amp.

Thank you for the reply. That is very good to know. Bear with me here ‘cause I’m running in circles.

With nothing plugged in, there is a faint, low hum. I can live with that. When I plug in there is much worse noise when volume knob is above six (out of twelve). On three (out of twelve) this amp is pretty loud, but the noise is very weak. (These amps are known for the volume maxing out early on the dial. They are also known for being noisy.)

I was just messing with it. I have a feeling this will get confusing for readers because I’m almost pulling my hair out. So, I was thinking the noise was lower without a cable plugged in because the input gets shorted to ground, but I guess that actually makes the case that it isn’t the amp. (There is a faint hum though.) I just discovered a couple of things.

1. I think most of the noise I’m hearing is buzzing, not hum. I just listened very closely. There is a tiny bit of hum with nothing plugged in, but that gets taken over by what I’m pretty sure would be considered a buzz instead of hum when cable is plugged in. I can hear the hum behind or underneath the really obnoxious noise.

2. I think my tubes are biased too cold. I’m reading -13.5 volts. I’m going to look into that now.

3. I just plugged a cable in, no guitar, and I was moving the instrument end of the cable around in air while listening. At one point it got much louder. Further inspection showed that when the cable end got close to my pedals the noise went nuts. And worse when right next to the power supply cable. It’s a daisy chain. Keep in mind I have not been running through my pedals while checking this noise. And the pedal power daisy chain is not plugged into the same outlet, but probably on the same circuit.

Im going to check the bias and unplug pedal power and keep checking stuff but I’m stopping here for now. Sorry for the long post.

[sluckey]

3. I just plugged a cable in, no guitar, and I was moving the instrument end of the cable around in air while listening. At one point it got much louder. Further inspection showed that when the cable end got close to my pedals the noise went nuts. And worse when right next to the power supply cable. It’s a daisy chain. Keep in mind I have not been running through my pedals while checking this noise. And the pedal power daisy chain is not plugged into the same outlet, but probably on the same circuit.

DON'T DO THAT.

[raveonstevie]

3. I just plugged a cable in, no guitar, and I was moving the instrument end of the cable around in air while listening. At one point it got much louder. Further inspection showed that when the cable end got close to my pedals the noise went nuts. And worse when right next to the power supply cable. It’s a daisy chain. Keep in mind I have not been running through my pedals while checking this noise. And the pedal power daisy chain is not plugged into the same outlet, but probably on the same circuit.

DON'T DO THAT.

Okay. I will not do that. I guess that should make noise shouldn’t it? My pedals are noisy though. My heads spinning.

[xtian]

You have confirmed the issue is not the amp (except for the small amount of hum you said you could live with). You can continue the experiment by plugging in a guitar cable only, but shorting the tip to ring with an alligator clip. This would be the same as if you plug in a guitar, but set the guitar's volume to zero. Get it? In all cases, you're hearing the amp only.

As soon as you turn up a guitar, you're hearing EM radiation from the environment--60Hz hum comes from everywhere because of your house AC. We often also hear "buzz" because of the nasty hash (again, EM) radiated by switching power supplies, CFL bulbs, LED lights with switched power, computers, phones, etc.

[raveonstevie]

This is pretty embarrassing but I’m glad I asked. At this point there’s no way I’m putting that thing in. My guitar cable was probably laying right on top of my pedal chain.

I corrected the bias and it made a noticeable difference, for the better. I also removed the NFB loop and I’m pretty sure it’s going to stay that way. I really don’t want to mess with a presence knob.

Now, brace yourselves. Should I start another thread? I seriously want to check out a cathode bias. And I’d rather it not be biased really hot.

So, I’m going to post the schematic for the Pro Jr III series, of which the power amp is the same. I will double check that. I think I understand how to do it. (I actually did it to my Peavey Classic 30 in the early 2000s but I don’t retain data very well.)

If any of you could tell me precisely what I need to do for the cathode bias mod I would sincerely appreciate, and I sincerely appreciate your help thus far, seriously. Thank you

Okay, this isn’t the latest schematic out there but it’s basically the same in power amp. I’ll find the link for a later schematic.
https://el34world.com/charts/Schematics ... pro_jr.pdf

[thetragichero]

r21 r22 go to ground
power tube pins 3 connect to ground via cathode resistor (130-150R is pretty standard, may take some trial and error) and bypass cap (the higher you go, the closer it'll "sound" to fixed bias... i used something like 3300uf for an 18w jcm800 build and it's my go to amp for tracking bass guitar)

[raveonstevie]

r21 r22 go to ground
power tube pins 3 connect to ground via cathode resistor (130-150R is pretty standard, may take some trial and error) and bypass cap (the higher you go, the closer it'll "sound" to fixed bias... i used something like 3300uf for an 18w jcm800 build and it's my go to amp for tracking bass guitar)

Awesome. Thank you. I’ll report back.

[raveonstevie]

Still haven’t done it yet. Maybe today.