Blue Junior Hum even with all controls at zero - help with PD, please?

[LondonSi72]

Hi folks,

So I'm new to this forum, but have been looking around any forum I can find that has information on Fender Blues Juniors and tube amps in general, and this one seems to be one of the best. I'm hoping someone here can help guide me in some PD on a second hand BJ (cream board) amp I got a few months back. I love the sounds I can get from this little amp, but from the day I got it, I noticed a quietish but still annoying, persistent slightly buzzy hum. I've read enough forum posts to know there are plenty of people with BJs that hum, and describing the hum and knowing if it's 'normal' or excessive, is half the problem. I checked against a new BJ III in a local shop, and it was dead quiet compared to mine, so I don't think I'm just being picky and think this one does have a problem.

Anyway, I'm hoping someone here can help me in some problem determination, and can at least tell me if I am being too picky and what I think is hum is normal for one of these. I dabbled in electronics and have built bread board circuits from designs (even built my own effects pedal a loooooong time ago), but I'm no expert. I like a bit of a challenge, but I don't want to spend more than it would cost to buy a new BJ III trying to fix this one.

First the obvious and my PD so far: The amp is pretty silent, except for a very quiet hum from the PT, until it warms up and then the hum with a bit of buzz is heard from the speaker. This is with nothing plugged into the input jack and all controls at zero. I've tried the amp in different rooms to discount any local EMF from other equipment and the hum is the same wherever it's plugged in.

The hum doesn't change when any of the controls are adjusted - but it does start to hiss when vol and master are wound up (as I'd expect) and that does have a bit of an effect on the hum, but it's marginal.

I have replaced all the tubes with new JJs including matched power tubes (bought from a tube specialist on-line) and tried swapping the pre amp (V1, V2) and PI (V3) tubes around - with no change to the hum. I've pulled V1 then V2 and there's still no change - pulling V3 and it does go quiet, but again - as expected.

I even sent the amp to a local amp tech and got him to replace the Power cap (and upgrade from 47uF to 100uF to tighten the sound a little) and the three filter caps with new good quality caps - and to try and fix the hum/buzz but he couldn't find the cause. He also reflowed the solder joints on the tube connections to the PCB. All lovely, buit no change to the hum. Though the bass does sound better, at least

In an attempt to work out if it's 50Hz or 100Hz hum I'm hearing (I live in the UK), and because reading a few other sites it seems an oscilloscope is pretty much mandatory for really understanding your amp, I got a handy, but basic little digital oscilloscope (eBay is my friend) called a DSO Nano for £30 so I could check the signal in the amp itself.

Connecting the scope to the speaker blue terminal and ground (the amp chassis) I get a very clear picture of 50Hz hum, with a bit of distortion which pretty much matches the sound I'm hearing - a buzzy hum. According to the scope, this hum 'signal' is 8.37mV vrms - Vmax is 11.8mV and Vmin is -14.7mV. Is this reasonable? Am I being picky?

See picture below:



So - I've confirmed it's 50Hz, which is great, but what do I do about fixing it? What else can I check? Any hints on where in the amp this hum could be coming from based on the info I've given, or suggestions on where to look would be much appreciated.

I have tried Bill Machrone's suggestion of putting a sheet metal shield between the PT and OT to try and divert any leaked magnetic field from the PT from entering the OT, but that didn't make any difference.



(PS The scope is useful but it is limited - it only has DC coupling, so reading AC signals with high DC is tricky as I have to set the probe attenuation to 10x and the scope to match and then I can't easily read small AC on top of the 300+Vdc that is inside this amp in some sections. That said, I did manage to identify and fix oscillation in the circuit that matched what Bill Machrone describes as PI oscillation. I found a 41kHz signal at the input to the OT transformer and fixed it by putting a 100pF cap across the R30 resistor. Worked a treat. But still didn't fix the damned hum!)

Many thanks in advance
Si

[nickb]

8.37mV equates to about 0.01mW of power. Yes, you are being too picky I would call that exceptionally low.

[LondonSi72]

Thanks for the quick reply nickb. Seeing the 8.37mV stated back to me made me think that can't be right - i get more from that just out of the guitar pickups (strum medium hard and get 30-40mV vrms!), and that's not enough to drive a speaker directly. I must be reading the scope wrong, or placing the scope probe on the output to the speaker is maybe the wrong way to measure it.

I've attached an audio file so you can hear what it sounds like on power up. There's no way to objectively assess the volume from this (I just used an iPhone microphone placed by the speaker grille) but you can hear the sound I'm trying to describe.

Is there a better way to measure the signal at the speaker? I have an 8ohm resistor I could use as a dummy load instead, but not sure that would help.

BJ Power up 2.m4a

[Tom Phillips]

Thanks for the quick reply nickb. Seeing the 8.37mV stated back to me made me think that can't be right - i get more from that just out of the guitar pickups (strum medium hard and get 30-40mV vrms!), and that's not enough to drive a speaker directly...

Actually that is plenty of voltage to drive a speaker to a sound level that is easy to hear. However, a pickup cannot generate that voltage if it is hooked up directly to a speaker. This is because the pickup cannot supply the current required into the speaker load. The speaker is just about equivalent to a short circuit across the pickup.

You can do an easy experiment by observing the output of a pickup with your scope. Then leave the pickup connected to the scope and clip on a speaker across the terminals. You will find that the pickup output signal pretty much disappears. However, be aware that, if the room is noisy the speaker will be acting like a microphone and the scope will display the ambient noise that the speaker picks up.

Have fun with the physics,
Tom

[52 Bill]

Well, you have pretty much isolated the hum pickup to somewhere after the tone controls and before the phase inverter. Have you tried moving the ribbon cables in/out/up/down, etc? Is the rear panel foil intact and in contact with the chassis ground?

[LondonSi72]

Tom - thanks. My electronics knoweldge is old and patchy, so what you said makes sense. I guess I keep comniog back to that quiet BJ III I compared mine to. I know it is louder than that, so I guess it's become my yard stick.

52 Bill - yes I tried dressing / adjusting the cables as one of the early attempts to reduce the hum/buzz, and the sound doesn't change when the back is on or off. The foil protection on the back is visibly intact - hard to tell if it's electrically intact. The amp did have the classic problem with these BJs, in that the blue wire that is input to the OT was right over the ribbon cable to the power tubes, and moving that / adjusting the ribbon cable significantly reduced the hum. I'm wondering if maybe what I'm hearing is the distortion / harmonics that are on the 50Hz (look at the photo of the wave form in my original post), and although the signal is apparently small, they seem louder as they are higher frequencies. (One of the things I DO remember from physics - a pure 50Hz sine wave is very quiet, but a 50Hz square wave is a horrible, seemingly much louder buzz.)

One of the things I was looking for guidance on is, after all the parts I've eliminated from the PD I've done so far, where else should I look? (Thanks for your suggestions Bill) Is this likely to be hum induced by AC in filament heaters, or is that unlikely as it's unaffected by the amp controls? Does the fact it's unaffected by the controls suggest it's PT / OT field coupling?

If I were to trace the signal through the amp with an oscilloscope, where would be the best places to look for where hum could be getting in?

Thanks for your comments.

[52 Bill]

Does the fact it's unaffected by the controls suggest it's PT / OT field coupling?

If the hum stops when you pull V3, then it would seem unlikely that would be the case.

Have you routed all of the ac transformer wires away from all of the audio wires? Where is the negative feedback wire routed? Is the power transformer mounted securely?

[LondonSi72]

If the hum stops when you pull V3, then it would seem unlikely that would be the case.

That's useful to know

Have you routed all of the ac transformer wires away from all of the audio wires? Where is the negative feedback wire routed? Is the power transformer mounted securely?

There aren't a huge number of options for dressing the wires in a Blues Junior due to the PCB layout but I've tried to separate the signal and transformer wires as best I can. The OT wires are bunched together, but I don't know if that makes any difference, and it's not really possible to separate them. As I'm in the UK my amp is an export model and the PT wires are more complicated/messy to allow for various voltages. I have adjusted mine for 240v not the standard 230v that these amps come with for Europe (rest of Europe is 230v but UK is 240v)

See the photo below for my layout. (Ignore the temporary / botch job 100pF capacitor on R30 in the photo - I only recently put that on when I found the PI oscillation problem I mentioned earlier - I have yet to solder it in place permanently - it's not the cause of the buzz)



The PT is mounted firmly, so it's not vibrating - though it does definitely hum!

[52 Bill]

I don't see anything out of the ordinary there. The two primary output transformer wires are normally twisted together. That might also help your high frequency oscillation problem.

For the price of a cable tie, I'd try moving the 4-wire bundle of the fat switch wires and negative feedback wire around to see if that makes any difference.

[Steve A.]

I listened to the MP3 file. If it was my amp and I wanted to mike the speaker I would add a Vox-style Cut control which is a crossline treble-cut tone control in the power amp section. It consists of a 250kA pot wired as a variable resistor*** in series with a .01uF/600vdc cap, and would normally be added between the PI-side terminals of the 1.5k grid stoppers on the EL84 output tubes.

With those grid stoppers apparently on the PCB it would be easier to solder the leads to the Cut control directly to the pins of the tube socket. (I do not know if locating it there would create any problems.)

The value of the cap along with the setting of the pot will determine the frequency of the filter. (Perhaps someone here could recommend a different cap value to specifically address the frequency of the hiss in the MP3 file.) BTW if there is a specific setting of the pot which works well you could replace the pot with a fixed resistor.

*** The Cut control is wired so that it cuts treble as you turn it clockwise.

Steve Ahola

[Enzo]

Your DC only scope? Put a cap in series with the probe tip. I have a lot of 0.047uf 630v film caps, so I use one of them, but value is not critical. The cap blocks DC and lets AC through to the scope.

With a scope or a signal tracer, just follow the signal path and see what hum signal is present.


How even are the plate voltages on the phase inverter? measure current through each power tube, are they the same?

[LondonSi72]

I listened to the MP3 file. If it was my amp and I wanted to mike the speaker I would add a Vox-style Cut control which is a crossline treble-cut tone control in the power amp section. It consists of a 250kA pot wired as a variable resistor*** in series with a .01uF/600vdc cap, and would normally be added between the PI-side terminals of the 1.5k grid stoppers on the EL84 output tubes.

With those grid stoppers apparently on the PCB it would be easier to solder the leads to the Cut control directly to the pins of the tube socket. (I do not know if locating it there would create any problems.)

The value of the cap along with the setting of the pot will determine the frequency of the filter. (Perhaps someone here could recommend a different cap value to specifically address the frequency of the hiss in the MP3 file.) BTW if there is a specific setting of the pot which works well you could replace the pot with a fixed resistor.

*** The Cut control is wired so that it cuts treble as you turn it clockwise.

Steve Ahola

Thanks Steve - I might try Enzo's suggestion below before I put more circuitry in the amp so I can hopefully narrow down where the distorted signal is coming from.

As you listed to the mp3 - did that sound like hum or buzz, in your opinion? Is that typical of the sound you'd expect to hear from a tube amp with no input?

[LondonSi72]

Your DC only scope? Put a cap in series with the probe tip. I have a lot of 0.047uf 630v film caps, so I use one of them, but value is not critical. The cap blocks DC and lets AC through to the scope.

With a scope or a signal tracer, just follow the signal path and see what hum signal is present.


How even are the plate voltages on the phase inverter? measure current through each power tube, are they the same?

Thanks Enzo. I have seen that suggestion of putting a capacitor in series with the probe in other posts on this subject. There was a suggestion that getting the uF value was critical to it not influencing the signal you're trying to observe, so I wasn't sure what value cap to put in. I have a spare 1pF and 100pF 1kV caps I could try, but would either of those work or are they too small a capacitance? More than happy to buy a capacitor if I know what value to use if it helps me see the signal more clearly...

Thanks...

[nickb]

Thanks for the quick reply nickb. Seeing the 8.37mV stated back to me made me think that can't be right - i get more from that just out of the guitar pickups (strum medium hard and get 30-40mV vrms!), and that's not enough to drive a speaker directly. I must be reading the scope wrong, or placing the scope probe on the output to the speaker is maybe the wrong way to measure it.

I've attached an audio file so you can hear what it sounds like on power up. There's no way to objectively assess the volume from this (I just used an iPhone microphone placed by the speaker grille) but you can hear the sound I'm trying to describe.

Is there a better way to measure the signal at the speaker? I have an 8ohm resistor I could use as a dummy load instead, but not sure that would help.

[ATTACH]39618[/ATTACH]

Why do you think your measurement is wrong? Do a little experiment to test it. You can connect your scope to a known level and see if it reads correctly. Between one of the heater pins and ground should give you around 3.3VAC (from schematic). For further confirmation you can attenuate that same voltage to the original range by making a potential divider using a 100K and a 220 ohm that will give you 7.3mV rms.

Edit:
I'm not sure which version of the nano scope you have, and what probe you are using it with, but the spec I just looked at specifies a max of 80Vpp on the input. If you probe HV lines and use a capacitor to block the DC then there will be a short period of time when you first connect the capacitor where it will apply excess voltage across the input to the scope. You might consider adding a couple of back to back 24V zeners or TVS diode across the scope input.

[LondonSi72]

Why do you think your measurement is wrong? Do a little experiment to test it. You can connect your scope to a known level and see if it reads correctly. Between one of the heater pins and ground should give you around 3.3VAC (from schematic). For further confirmation you can attenuate that same voltage to the original range by making a potential divider using a 100K and a 220 ohm that will give you 7.3mV rms.

Partly becuase I find it hard to believe the buzz/hum I'm hearing is normal

I don't have the equipment to properly calibrate it accurately, and I get slightly different readings each time I measure, so it's not completely reliable. I wanted the scope mainly to view the signal as it passed through the scope to see where the buzz was coming from in the circuit. I did a very rough and ready measurement against a 'known source' - a 9v battery that I could also test with a separate voltmeter and it did vary from the voltmeter reading, but I am trying to figure out a 'good' known source I can use as a calibration for it...

Edit:
I'm not sure which version of the nano scope you have, and what probe you are using it with, but the spec I just looked at specifies a max of 80Vpp on the input. If you probe HV lines and use a capacitor to block the DC then there will be a short period of time when you first connect the capacitor where it will apply excess voltage across the input to the scope. You might consider adding a couple of back to back 24V zeners or TVS diode across the scope input.

The DSO Nano I have is version 2 with the BenF firmware. But I bought a separate probe that has 10x attenuation so in theory, the scope should be able to handle up to 800v. (The probes that come with the Nano are definitely not for high voltage and I wasn't going to chance hooking those up to a 300V+ amplifier circuit!)