Help with hum on Blackface Princeton Reverb

[hatchhh]

I have a loud hum that has developed in my princeton reverb.  It was not so loud at first, but has gotten worse.  The hum is there whether volume is up or not, and pretty damn loud to boot.  There a spot in the middle when volume is increased to say between 4-8 where the hum decreases then increases as you dime it out.  The hum gets louder as you turn up reverb.  The odd part is, with reverb all the way, if you turn the amp to about 5, it plays normal.  I have tried the basic novice things, replaced the older Mallory cathode by pass caps, changed the cables on the reverb, checked speaker connections and swapped tubes.  I didn't mess with the filter cap, it was replaced before I bought it about 3 years ago.  These Princetons have those big single can filter cap.  Any thoughts or direction would be appreciated.

[Mike_J]

Since you have already changed the tubes with no luck I would guess it is the cap can. Other things you can check would be grounds that maybe came loose. Just set your MM to the continuity setting and check all your ground points to chassis ground. If any grounding point is significantly higher than the others make the ground connection sound. This has to be done with the amp off and caps drained. 


Another thing to check is coupling caps. Figure out which pot the coupling caps feed and turn the pot with the amp on. If you hear any static (DC leakage) then I would check the cap by lifting the end that doesn't go to power (commonly connected to a pot). Measure the DC voltage on the lifted side. If it is much higher than 200mV then I would replace the cap. This is dangerous to do so be careful or get a tech to do it for you.


Another possibility is grid wires picking up noise from the heater wires. They need to be kept as far away from each other as possible. Grid wires should cross any higher powered wire at a 90 degree angle or otherwise be kept as far from each other as possible. Using a bamboo stick while the amp is on can help you with this. Be very careful.


Jacks can be a source of hum but since you can dial the hum out my guess is that isn't your problem.


I am sure other members have additional suggestions. Tim Robbins was nice enough to post his article discussing hum issues in reply #10 of my thread titled "reducing hum suggestions". It is well worth reading.


Thanks
Mike

[eleventeen]

The traditional approach (and it's not without its flaws) to troubleshooting a cap can is to bridge the individual sections one at a time with single-unit external e-caps of appropriate ufd value and working volts. You have to be very mindful to solder in one "bridge" cap, observe/listen for the reduction of hum, if any, then when you turn the amp off, do not put it in stby. Let the cooling-down tubes help drain off charge from the cap you threw in for a few minutes. 


You say you have changed the tubes. One at a time or all of them at once? Often something or other can be determined if you start at the small end and pull them one at a time and run the amp minus 1 tube, 2 tubes, etc;


If it was working OK and then stopped and the complaint is hum, you can't really say you've checked things out until the most common culprit, the cap can, has been diagnosed. You can't just think it's OK if it was replaced 3 years ago or was replaced last week.


It is ALSO possible that you have a heater-to-cathode short but these are pretty rare. Yet, with rough handling like you have in a guitar amp that gets dragged around, this too can happen. But you've chged tubes, so....no.


You say there is some interaction with the reverb control and this hum. The reverb driver gets almost the highest volts in the amp and you will see if you try to pull that tube after the amp has been on for a while, that 12AT7 runs substantially hotter than any other little 12A_7 tube. Can easily burn your fingers, not just "hot".


This points to the highest B+, the most likely ecap to fail with the most volts on it. Like it or not, them's the odds.

[hatchhh]

Thank you for the replies.  That gives me some direction.  Now I've just gotta get the time to put your suggestions into action, or breakdown and pay a tech to do it.

[Mike_J]

Thank you for the replies.  That gives me some direction.  Now I've just gotta get the time to put your suggestions into action, or breakdown and pay a tech to do it.

If you do not have a selection of electrolytic caps on hand it may be wise to pay a tech to find the problem unless you want to start ordering parts and make an endless number of amps to stack in your house. Amp building can become very addictive and can start by something as innocuous as repairing a hum problem successfully. Next thing you know your house is filled with your amp builds. Otherwise I am afraid you will need to buy a replacement cap can and hope that is the solution to your problem. My guess is that is your problem.


Thanks
Mike

[hatchhh]

Quick shout guys. Thanks for the replies.  It was the multisection can.  I would have never guessed that.  It took two soldering irons to generate the heat I needed to get old and new/on, but the thing purrs like a kitten now.


Thanks again