Fender Blues Deluxe Hum

[mikeydee77]

I am looking over a Fender Blues Deluxe from the mid 1990s.

It had some problems on the low voltage dropper resistors (R85,86) were 330R and operating at temperature of just under 100 deg c. The 16V zener diodes (CR22,23) in that area had dry joints after coming unsoldered. I repaired and replaced the resistors with 470R as mentioned in many other threads. That supply seems fine now; the temperature is down to 60 deg c and the supply looks good.

It's sort of working fine but there is a low level hum that I am not sure if this is characteristic or a fault. I would love to hear some comments/suggestions of other things to check out from others with experience of these amps.


Some observations/measurements

Switching round the pre-amp valves doesn't seem to alter the noise levels.

With V1 removed the amp is pretty silent.

Transformer leads are in a big not and the heater supplies are not twisted together - I wondered if there are any suggestions on improving this?

B+ is slightly high compared with the schematic at 459VDC as opposed to 427VDC probably within the tolerance. Also there is a 3.5 p-p ripple on the B+ which I think is normal.

With the amp on full volumen and if I watch the B+ on my scope I can see it drifting up and down by a few volts slowly and not in a regular manner. If I connect a guitar and watch the sag I can see that playing a low E causes a B+ sag of 25v until the note decays. Is that normal amount?

The Bias current on the power valves is: 20 and 22mA so that seems fairly well balanced.

[g1]

If removing V1 makes it quiet, then the problem should be before V1, or the V1 stage itself. Have you tried other tubes? Some hum more than others in sensitive positions like the first gain stage. Are all shields in place? (tube shield, chassis installed in cabinet etc.) Also with those amps the position of the ribbon cables can be critical.
Of course, as you say, it may be the normal amount of hum for this model. Pretty tough to verify without an a/b comparison to another of the same model.

[mikeydee77]

Thanks for your reply g-one. Forums like this are a great sounding board when ideas start to run thin. Thanks for reading and replying.

I agree with your theory it's got to be around V1, right? The other thing I forgot to mention was that taking the signal from the pre-amp out to another amp takes the hum with it - so clearly all logic say that this is in the pre-amp.

Yes, I have tried a couple of other new tubes and there is no improvment. This hum is not a gross distortion just something subtle so part of me thinks this could be normal for this amp.

You mention the tube shields - well in this amp we are without tube shields - so should they be there? I would imagine that would improve things because as this amp sits on my bench right now, just bringing my hand near to V1 / the ribbon cable / V1's plate resistor unleashes an enormous hum/buzz so clearly it is sensitive to noise.

I will try a shileld from another amp - must be one around here somewhere . . .

[g1]

Not sure if that model used tube shields or not, I kind of doubt it. But I'm more concerned with the cab. shielding, especially considering what you said about the V1 ribbon picking up noise. There are probably foil shields inside the cab and rear cover. It's possible the amp will be much quieter when it is reassembled and not picking up noise from stuff at your bench. (I'm assuming you only noticed the noise with the chassis removed and on your bench, or did you notice it before you removed the chassis?)

[mikeydee77]

I think from looking carefully the amp doesn't have tube shields. The cab is well shielded with foil as you mention which fits up close to the open side of the chassis.

The hum is there regardless of whether the back was attached although measuring the ac on the speaker connector I measure 22mV with the amp open on the bench and reducing to 9mV with the shielded panel attached properly. I am not that is a particularly meaningful measurement but comparitively it suggests that the screening is important.

Listening to the amp right now all back in the cabinet I can hear some other noises like occasional "whooshing" which I normally associate with pre amp tubes reaching end of life so intend to have one more attempt at swapping them out. If that fails I am going to assume that this fault is just not ready to be found.

Addition:
Just went through all the pre-amp valves again and I think V1 is causing a little occasion "whoosh" like the sound of the sea. The others don't seem to make any difference.

Mike

[mikeboone]

These amp did not have tube shields nor do any of the HotRod series of amps built in that chassis. Since you are picking up the noise in the preamp it's possible that the shorting jack is not making good contact or no contact at all when your guitar is unplugged. I assume you are looking at this and hearing the noise with no input. These amps also have those crappy pcb mounted jacks that a prone to fractured solder joints.

They are also prone to poor solder connections on the tube board as well as the ribbon leads.

Food for thought.

[mikeydee77]

Hi thanks for the confirmation on the tube shields.

I know what you mean about the jacks they are regular source of trouble. On this one I didn't see any evidence - although I have gave them a good ol wiggle and resoldered them just to rule them out - one thing I didn't do which I wish that I had before I put it back in the cab was bypasssed them completely and grounded out the input signal using an alternative path - Hmm.

The hum is there regardless of whether anything is plugged in - so I am not sure if that rules the jacks in or out.

Part of me thinks that replacing the filter caps is worth a shot but actually they look fine - is there any way of measuring their effectiveness or is substitution the only way?

[g1]

If you had any kind of filtering issue, I don't think removing V1 would get rid of the hum. Unless V1 has it's own filter cap.

[mikeydee77]

Good point - that makes logical sense.

Let me check that schematic one more time . . . and ah yes in fact V1 does have it's own filter cap - C46 (22MFD @ 500v). It might be worth a quick swap out if only to rule it out.

[mikeydee77]

Well thanks to the kind people who have read and replied to my first thread on this terriffic forum I have a little update I thought was strange.

First of all I haven't been able to replace the caps just yet and in the mean time I found something else which I find strange so I am pursuing and thought I would share.

I thought I would check what is coming out of the pre amp out socket to see what gives. Connecting the pre amp out to a known quiet practive amp I get a strong sense of hum - fair enought I thought. But what I found strange was that after turning the amp onto standby the hums stays present, turn the amp off and it remains - it is only when you unplug it from the wall do you find that the hum disappears. So perhaps it is an earth continuity problem somewhere. I think I will go back to square one with those troublesome jack plugs and recheck.
Cheers

[booj]

Looking at the back of the amp at the circuit board, do you see a ground wire coming from the board to a chassis ground connection? Just wondering if it's a good connection or if it's even there.

[mikeydee77]

Hi,

Yes the ground wire is there and looks good.

But I want to wrap this thread up as the problem is really resolved.

In the end I tried adding a temporary extra filter cap but that made no difference whatsoever. So I ended that line of enquiry.
The earthing is definitely good so I have ended that one.

In the end I have concluded that this amp's hum whilst present isn't actually that bad - I did a few comparisons with other amps (nothing scientific) just a few old amps knocking about and actually concluded that it wasn't too bad.

I was able to confirm that it is the first triode by grounding the signal after its decoupling capacitor and the amp did become truly silent. I think if I face this issue again I will look to replace the ribbon cable with a screened cable direct from a jack socket and disconnect from the circuit board. I believe that is where the noise is getting picked up from - perhaps the track routing is a little short of ideal.

Thanks to all that read and contributed

[tonequester]

tonequester here. mickeydee77. You've probably already checked, but I thought that I might relay a very similar experience, with my very first amp, 30+ years ago. I was taking
basic electronics at my local ju.co. at the time and didn't know much more troubleshooting than visual inspection for the obvious, and how to check a few things with an old analog meter. The amp was a 1960 Fender Princeton w/o reverb. It looked brand new, but when the seller tried to demo it for me, it had just enough hum to be a little annoying.
The only thing that affected the hum was pulling the plug. It had no standby. I had experience as an electronics assembler on avionics boards and had rigorous solder experience.
In my workplace the number one problem without doubt was faulty grounding at the time, so I had that on my mind. To make a long story shorter, I found the main ground to chassis to be bad. The continuity seemed fine by my meter, but after coming to the end of my experience, I used a sucker and copper wick tape to clean it up real good. Then I used my hottest iron, which was barely adequate, and was lucky enough to get it re-soldered with good flow and a bright shine. The seller let me buy it as is for $50.00. The hum
was gone for good and I had an amp worth considerably more, for many years. I wish I had neverlet it go. If by chance you haven't checked out, it would be an easy try for you,
especially if you have an iron hotter than 40 Watts. I actually used a propane torch to heat the tip up more, then quickly cleaned and re-tinned it before soldering ! Good luck
with the hum, and all your endeavors. I hope that I didn't insult your intelligence. Truly trying to help, with admitted lack of brain power. tonequester.