Help troubleshooting hum in paramour II.

[jrebman]

Ok, so I replaced the one rca input on the one of my new paramours that has been giving me some hum problems, and was also able to strip the jacket back a bit further and solder a new end into the jack, and I still get the hum in this amp.  Realize that I don't have a manual handy, what else should I check at this point?

The symptoms are as follows:  I make all the connections to the amp, insert the tubes, plug in the power and then turn the amp on.  It has a very mild hum -- hardly audible -- for about 3-5 seconds, then there is a small pop, followed by a much louder 60 hz hum.  When the source is playing, there is no residual music behind the hum, and the hum balance pot has no effect whatsoever.  Now, these amps were supposedly working perfectly (and I have no reason to doubt the seller on this), so any ideas what may have happened?  This same set of symptoms is consistent with every power up cycle.  Also note that this is the amp that did not have a loose output transformer, so I don't believe any physical damage occured on this input side of the amp, and everything, as far as I can tell, looks ok on the other side by where the plate choke was loose.

Thanks,

Jim

[JC]

Does anything change if you swap out the tubes from one amp to the other?

[jrebman]

Jim,

No, sorry, should have mentioned that tube swaps have no effect.  Also, the hum isn't so loud that sounds like it's anywhere near full volume, but certainly louder than residual hum from AC filaments.

-- Jim

[Doc B.]

I would guess that the mild hum/pop/louder hum sequence would tend to indicate that the hum is becoming more audible as the 12AT7 heater warms up and the tube conducts. So that seems to indicate that the problem is ahead of the 12AT7. In other words it kinda sounds like there is an open circuit somewhere between your source and the 12AT7 grid. Are you checking this with the input of the amp shorted? If the input is not shorted, it could be a bad interconnect cable. If the input is shorted, maybe the grid load resistor has broken or lifted at one end, or the grid stopper resistor has opened up.

[jrebman]

Dan,

That was my thinking exactly, but I'll have another look around on the input circuitry.  I'm just swapping these amps in in place of my Fis, so I know everything up to the new amp is working.  It did the same thing on the other side of the system, which is also now playing fine with the other amp.

I'll take a closer look at and around the tube socket, the input wire was broken off at the rca hot terminal, so maybe something else broke with a hairline crack.

I'll report back after some more testing.

-- Jim

[JC]

OK, it sounds as if you have been doing a good job of narrowing down the field! 

The only other question that comes to mind, and you may have   already answered this as well, is: 

Are you pretty well satisfied that the hum you are hearing is 60 Hz?   You know, as opposed to 120?

Either way, I'll look forward to hearing about the outcome.

[Paul Joppa]

If it's only 3 to 5 seconds, it may be the 2A3 grid that is floating, so I'm suspecting the grid stoppers - check both of them. Perhaps measure resistance while poking or wiggling them.

[jrebman]

Ok, nailed it... and it was really not what I wanted to find -- a bad tube socket.  Everything reads perfectly right up to the bottomof the tube socket, but that pin from the top doesn't have any clips left, and only a bit of metal down pretty far in the tube socket.  Feels very different from all the other pinholes, which all grab the piece of wire/scribe I use to poke around and "see" thins.

Of course I don't have one of these hole-shrinkersockets on hand, and I tried, but I cannot get the metal that's left to make contact with the tube pin.

Jim, yes, it's more 120 yhz than 60 as far as I can tell.

-- Jim

[2wo]

Has the pin perhaps been pushed out of the bottom of the socket. Maybe it can be reseated like a connector terminal...John

[jrebman]

Hi John,

Sorry, no -- I checked that and there is just no metal springs left on that pin socket.  As a final test, I pulled the tube while the amp was on, and the sound was exactly the same -- no pops, clicks, change in the hum or level.  Plugged it back in, wiggled it around and not any sign of contact even trhying to be made.

I called BHQ and spoke to the good doctor who is sending a new socket an a manual.

It's ok, I have plenty of things I can do with the system down for a little while.

-- Jim

[Natural Sound]

If it's only 3 to 5 seconds, it may be the 2A3 grid that is floating, so I'm suspecting the grid stoppers - check both of them. Perhaps measure resistance while poking or wiggling them.

Hmmm... My Paramour II's have always hummed for 3 to 5 seconds when first turned on. Are you saying that they shouldn't do that?

[Paul Joppa]

If it's only 3 to 5 seconds, it may be the 2A3 grid that is floating, so I'm suspecting the grid stoppers - check both of them. Perhaps measure resistance while poking or wiggling them.

Hmmm... My Paramour II's have always hummed for 3 to 5 seconds when first turned on. Are you saying that they shouldn't do that?

Wow - sorry, I was not very clear at all!

It's normal to have a bit of hum and some pops as the 2A3 filament warms up - which takes a few seconds. The original post was worried about a much larger hum that appears after that, but before the 12AT7 comes up to temperature.

[Natural Sound]

If it's only 3 to 5 seconds, it may be the 2A3 grid that is floating, so I'm suspecting the grid stoppers - check both of them. Perhaps measure resistance while poking or wiggling them.

Hmmm... My Paramour II's have always hummed for 3 to 5 seconds when first turned on. Are you saying that they shouldn't do that?

Wow - sorry, I was not very clear at all!

It's normal to have a bit of hum and some pops as the 2A3 filament warms up - which takes a few seconds. The original post was worried about a much larger hum that appears after that, but before the 12AT7 comes up to temperature.

Thanks for clearing that up. I was ready to fire up the soldering iron.

My Paramour II's sit in a rack with glass shelves so I can see the bottom of the amp. The hum seems to stop right around the time the LED on the constant current board illuminates.

I still have a slight 120hZ buzz/hum in my right channel. Its not cabling, I've tried all different combinations and it's still there (barely) and the hum moves to the other channel when I switch cables from left to right. I suspect that its the selector switch in my FPIII. I have had problems with the selector switch and sweetest whispers from day one. I've checked everything else 50 times. I replaced the whispers with goldpoints and that helped a lot. I'll go after the selector switch next. Knowing that the switch switches signal and ground makes me highly suspect of something on the ground side. Perhaps this discussion would be better served in the FPIII section of the forum.