65 DELUXE REVERB

[dpm309]

I am working on a friends 65 Deluxe Reverb Reissue that just started making a loud buzzing/hum noise even with volume turned down all of the way on both channels.  I checked all of the tubes by swapping them with good ones and am still getting the noise.  I checked the speaker jacks and found that I am getting continuity with ground on the tip side of both jacks.  Could there be a problem with the jack or should I be looking at the OT?  I do get sound when I have a guitar plugged so I am thinking the OT is working and I have a short to ground somewhere.  Any thoughts?

Thanks,

Dan

[plexi50]

I had a 65 Deluxe reverb reissue 3 months ago and the amp hummed with the guitar unplugged and no volume

I pulled the PI tube and it was still present. It turned out that this PT did not like the power tubes that were in it

I swaped out the power tubes for some old 6V6 Sylvanias and the hum completley dissapeared

There was some sort of power transformer magnetic field or osscillation occuring between those tubes and the PT

I wouldnt have believed it if i had not swapped power tubes in and out a few times just for fun to confirm the difference

A before and after kinda thing

[phsyconoodler]

The input jacks have shorting bars that do indeed short to ground when no cable is plugged in.That's normal and if it DIDN'T short to ground then there would be a problem.
  You need to look elsewhere.There are lots of areas to check,but it sounds like you are not familiar with the workings of amplifiers,so you need more knowledge or a picture of the insides of this amp to share with us.
  You need to have a set of good tubes on hand for swapping and you need to be familiar with grounding and the way the pots and filter caps are wired,etc,etc.It is almost impossible to diagnose an amp with what you are telling us.
 Having access to a good tube tester is a valuable diagnostic tool.
Plexi's description of swapping tubes is the easiest way of eliminating problems.Tubes are the number one suspects in tube amps.Go figure,eh?

[phsyconoodler]

Check the resistors connected to the pilot light socket.If one is bad it will hum like that.Replace both and see if it helps.
  Other than that,there are a lot of helpful hints in Hoffman's Library of information about servicing Fender amps.
 Read it,know it,live it.

[dpm309]

I have some knowledge of tube amps as I have built four from scratch, completly overhauled a old '74 Marshall superlead with a Hoffman board, and repaired a Framus Tube amp.  I understand that these are shorting jacks and I had the speaker plug plugged into the jack when I measured the continuity to ground on the tip.  I have a pair of 6V6s from my home build tweed deluxe that I will try swapping out tonight.  I swapped out all of the tubes last night except the power tubes (it was getting late).  Thanks for the suggestions and I will report what I find after some more tests.

Dan

[sluckey]

I understand that these are shorting jacks and I had the speaker plug plugged into the jack when I measured the continuity to ground on the tip.

The continuity you are reading is the speaker (approx 6Ω dc resistance) in parallel with the OT secondary (probably 1Ω to 2Ω dc resistance). Your meter would probably read less that 1Ω resistance. That's a very normal reading on the secondary of an OT.

[EL34]

Steve has it right.

You would be measuring less than 1 ohm Dc resistance across the secondary and that is normal.

[dpm309]

I just got done going over the amp in more detail.  I swapped power tubes and the rectifier and still have the hum.  I pulled each pre-amp tube, one at at a time without any luck.  See below for the voltages.

Tube                                            Pin Number
                  1           2          3           4          5           6          7        8     9
1 12AX7     154          0         1.0         H         H           157       0      1.1     H
2 12AX7     154          0         1.1         H         H           158       0      1.1     H
3 12AT7     333          0         5.8         H         H           333       0      5.8     H
4 12AX7     160          0         1.1         H         H           156       0       1.1    H
5 12AX7     327        -6.8        0           H         H           286     -6.7      0     H       
6 12AT7     158         35         5.7         H         H           148      37       57    H
7 6V6        NC           H          334       333     -40           NC       H         0
8 6V6        NC           H          336       336     -40           NC       H         0
9 5U4        NC        336          NC         335    NC          335       NC     336

I doubled checked all of the connections, grounds, power supply, etc but the voltages seem a bit low.  I also performed some preliminary checks on the transformers and choke and they all checked out OK.  Any other ideas at what I should be looking at?


Dan

[LooseChange]

You pulled every tube without any luck?  Does that mean the amp hummed with no tubes?
Your voltages all look low. I think the Deluxe B+ should be up around 425v.

[sluckey]

Change the rectifier. Check the filter caps.

[phsyconoodler]

9 5U4        NC        336 

That amp should have a GZ34 rectifier tube.The 5U4 will eat the power transformer.Your voltages are all low.

[dpm309]

The amp still hummed as I pulled out the individual tubes.  I did not pull them all out at the same time.  This is a 2003 '65 Reissue model and it came with a 5AR4 rectifier and was working fine up until the humming started.  As I mentioned earlier, I did swap the rectifier out with a good 5AR4 with no change in the hum level.  According to the schematic, the B+ should be around 396v.  I did notice that the hum increased as I turned up the reverb control and disconnecting the reverb pan cables did not change anything.  I checked the reverb circuit and all of the components tested out.   Is it possible that there could be some sort of short in the power transformer secondaries?  I haven't performed any detailed test on the PT.  Any other ideas or suggestions?

Thanks,

Dan

[LooseChange]

Pull them all out one at a time. Start with the phase inverter. Does the Hum stop then?

[sluckey]

Bad filter caps can cause hum. They can also cause B+ to be low.

[plexi50]

You have to do one thing at a time to eliminate various problems. Like LooseChange said pull the PI tube out and tell us if the hum is still present.

[dpm309]

I pulled the PI tube as well as the rest of the tubes, one at a time, and still had the hum.  I opened up the cap can again and double checked the resistor and cap values.  The resistors checked out but I am not too confident of the cap readings I got off of my EXTECH Multimeter.  They were not even close except for the 3 22uf caps which read 66 uf.  I did a visual inspection and there were no bulges, etc.  Should I be looking at the PT?

Dan

[sluckey]

When chasing hum in an amp and you want to see if the filter caps may be to source of hum, bridging a known good cap across a suspect cap is a very good way to tell if the cap is causing the hum. Your ears will tell you if you found a culprit.

Back in the '60s a 20 or 40µF cap with a gator clip lead was a standard shop tool. The clip lead goes on the negative end of the cap and clips to the suspect cap's negative lead (just use chassis in your case). Slip a piece of shrink tubing or wire insulation over the positive lead, leaving about 1/4" of the lead exposed to be used as a probe tip. Then hold the body of the cap as you would hold a test probe and touch the positive lead tip to the positive lead of the suspect cap. Discharge the cap before moving on to test another cap. (I just touch the positive lead to the chassis. Be aware that you will get a healthy spark.) It will take under a minute to check all the caps under the Fender cap can using this method.

This is a quick and effective way to test a power supply filter cap for hum problems. Since your amp hums with the PI tube pulled, this means the hum is likely coming from the power amp circuit. I would substitute the power tubes first, then move on to the filter caps. There's a very good chance that tubes or caps will fix the problem.

[nateflanigan]

Wow, Cool tip!  I'll have to remember that for the future.

[sluckey]

This cap bridge procedure will not discover all filter cap failures. It is ineffective at finding a leaky cap or shorted cap. But it is very good at finding a cap that has decreased significantly in value. As an electrolytic cap ages, the electrolyte often begins to dry out, and the capacitance decreases, sometimes even to the point that the cap becomes so ineffective that it's almost like an open circuit. As the cap decreases in value, the B+ ripple (hum) will increase. This is a very common problem with old electrolytics and very worthwhile to have a dedicated test cap hanging above the bench.

[HotBluePlates]

I have only once had an amp hum with the phase inverter tube pulled.

The problem was a failed cap in the bias supply (it was a silverface Princeton Reverb). The failed cap put a large a.c. voltage on the grid of the output tubes. Swapped the bias cap and the hum was gone.

[dpm309]

I am assuming the bias supply cap is the one that connects directly to the bias control (C29).  It is a 22uf, 63v radial cap.  I have a couple of 22uf 50v that I can substitute (is 50v OK).  Can I just bridge this cap or should I remove and replace.  I tried getting a reading on the cap but it was erratic but it is not shorted out.  I have also attached the schematic for this amp.  I will fire it up later to see if the voltage at TP36 matches the schematic.

Dan

[phsyconoodler]

There are a lot of other things that can make that amp hum,especially if it seemed to just happen out of the blue.
  I had one that had a cracked power tube socket.I tested everything like you have and a close visual inspection turned it up.
I suggest you investigate further with your eyes and a large magnifier before jumping at anything.
  What about the underside of the Filter cap board?A visual inspection there and some pulling and prodding of components may be in order.I have also located a few bad solder joints by lifting the main board and wiggling components.even getting lazy and re-soldering the boards can work.You would have them out,so it would be a good opportunity to do that.
   And the filament wires run  to the main board,then to the 100 ohm resistors and then to ground.A simple check of those resistors would be advantageous.And the ground of course.
   
 

[dpm309]

I tried the cap bridge procedure on the filter caps using a 22uf, 450v cap and I noticied a significant reduction in hum (~80%) when I bridged C32 (47uf/500v) and C31 (220uf/100v).  When I tried bridging C33, 34, and 35 (22uf/500v), I got a small spark and a small momentary drop in hum.  I discharged the bridge cap each time.  I have already swapped out the power tubes with no luck, lifted the main and cap boards and could not find any bad solder joints, inspected the power tube socket with a magnifying glass and found no cracks or defects, and checked the filiment wires and resistors.  Does this indicate that I have one or more bad filter caps?  Is there anything else I should be looking at?

Dan

[sluckey]

I am assuming the bias supply cap is the one that connects directly to the bias control (C29). 

That's one of two. The other is C36, 100µF/100V (Look near the PT on the schematic). You can bridge these caps too.

Does this indicate that I have one or more bad filter caps?

Yes. Definitely replace C31, C32. I'd probably replace all the caps under the doghouse while in there. Also, measure your B+ when done. I bet it increases to normal levels.

Is there anything else I should be looking at?

Reevaluate after replacing the caps.

[dpm309]

 :grin:I replaced the 2 bad filter caps and the hum and noise is gone.  All of the voltages are now where they are supposed to be and the amp works great!  I want to thank all of you for the great tips and advice.  Now that I have fully retired from a local governmental agency, I will be able to devote more time to what I love doing - playing music, and building and repairing amps.  I have a couple of other projects that I will be working on in the coming weeks so I will definitely be using this forum.

Again, thanks,

Dan

[plexi50]

Glad you stuck with it and found the problem. The accomplishment is a good feeling