[fz1jmp]

Hello all;

Searching for that sound for years and years, stumbled upon someone selling a Hogan 6sn7/76/300B, went to listen, Audio nirvana.

Last week I turn it on and I hear a hum. It's low and almost inaudible, but loud enough when you turn it off you know it. I do everything I can do.

Change power cords, sockets, turn everything else off, change IC's, reset all tubes, swap sides of each tubes, switch out 76 and 6sn7's (don't have 300b's to switch). The sound started on the right, when I switched the speaker cables from the right to the left, bingo it moved. I thought it was my speakers, I even swapped them. It hum's with nothing on, no load.

I contacted 2 places recommended here, but I'm not sure I can part with it for any length of time. Any thoughts as to what this could be? Should I pick up a pair of 300b's?

Thanks!

[sfox7076]

Anything new in the room that could be putting out interference?  Speaker wire is generally unshielded, so it can pick up stuff (like if it goes over a power cord).

[JakeJ]

Whoa, whoa, whoa!  Before you start throwing money at it let's do some more testing.  You say it moved when you swapped speaker cables, please clarify.  You swapped the cables at both amp and speakers ends, just the amp end, or just the speaker end?  If you swapped the cables at both ends then I am suspicious of the cables themselves.  If only one end then further isolation testing is needed.  If you tried just the amp end then swapped the speakers and it didn't move then that takes us back to the amp, if not then possibly the speakers.   Just a bit more detail on this problem will help us help you.

[Ericus Rex]

Just to be sure, you've got a hum, not a 'SHHHHHH'  Correct?  To me, a hum implies burnt resistor, bad solder joint or bad tube socket.  A 'SHHHH" usually means tube gone bad.

Agree with Jake.  Buying more 300b's should be your absolute last resort.


Concerning isolating the noise;  I would expect the noise to go to different speaker when you changed the wires at the amp.  Does it change when you change the L & R of your source?  If so, the hum is coming from your source, preamp or interconnects.  (Another thing to do is to remove all connections except speaker and power.  If hum is gone, your amp is probably fine and the noise is elsewhere).  If noise is definitely in amp and not source try these FREE tips:

-With amp on, tap each tube with your fingernail or pencil eraser.  If noise changes while tapping you've found your bad tube or socket.

-Amp off.  Rock each tube within its socket.  Pins can oxidize and corrupt the connection within the socket.  Rocking the tubes should scrape some fresh surface.  Turn on amp and listen.  If this fixes problem then remove all tubes, clean the pins with fine sandpaper and the sockets with small wire brush (looks like pipe cleaner).

- Switch tubes one type at a time.  You mentioned this in your OP but let's make sure you did it correctly.  Swap your 6SN7s only.  Turn on amp.  If one tube is bad the sound should go to other channel.  No change?  Move on to 76s.  Depending on your bias scheme, you may want to check bias after each swap just to be sure.  Auto-bias?  Don't worry about it.

-Amp off and unplugged.  Remove bottom plate and inspect each internal component and solder joint (you may need to remove tubes.  Don't rest upside-down amp on tubes).  Anything look burned out?  Any breaks in any other the solder joints?  Keep your hands out as caps can still contain a charge.

Don't freak out.  It's probably something simple.  May even be something else plugged into the same circuit (unlikely if only in one channel) or interference as sfox suggested.

Best of luck!

[fz1jmp]

Let me start out with a Big Thank you!!!

OK, now thats out of the way!

I did change the caps in the speakers a few weeks back, I immediately thought it was a poor solder on my part. I ruled out the speakers 2 ways;
I physically swapped out the speakers with another pair, still there.
I changed the amp ends only, the hum moved from right to left speaker, that tells me its not the speakers.

I physically pulled all tubes and used cleaner on all the ends. Then swapped them with each other. No change with hum. I will do each pair tomorrow morning, its late and I dont like turning on the amp and not using it.

I use a PS audio duet and I actually installed hospital sockets years ago. I pulled everything but my amp, still hum. I then pulled the duet and plugged in direct. Hum. But I did not switch outlets. I will also do that tomorrow.

I did look at the bottom, which is open and all I can say is wow! Lots of connections! I will post a pic. I did not see anything offhand but I will slowly and methodically inspect the connections.

The hum is low down, not a shhhhhhhh. You can hear it regardless of vol level.

If none of the above works, I saw some recommended repair folks in the forum. Would leaving it harm anything in your opinions?

I've been having a heck of a time with my transport. I've had a Cal Audio Delta for what seems like forever, but its seen its better days. When it gets warm it has a tendency to stutter and skip. I found a super deal on a MVP861 Mcintosh and just bought it. I figure it will be a pretty sweet match for the Hogan.

Anyway thanks to all!!!

Jack

[JakeJ]

Excellent work so far, Jack.  Try not to get discouraged and keep at it.  You are being methodical about your approach and that's good.  If it isn't the tubes and it isn't the speakers or cables then it may be a simple ground loop hum.  Are all powered components in the system plugged into the same outlet?  If not then are all outlets (mains receptacles) on the same circuit breaker?  If not this may be the problem, it has bitten me in the past.

Keep at it and please keep posting with what you have done and what you plan to try next.

[ltr317]

I agree with Jake, sounds like a ground loop hum.  If you have a cheater plug, connect that to your amp power cord plug and see if the hum goes away. 

[fz1jmp]

Again thanks for all suggestions and thoughts. I swapped each tube with its opposite, changed left 300b with right, no change. I changed electric sockets and no change. I ruled out the speakers based on changing the IC's at the amp end only. It follows the right RCA plug, not speaker. I tried a cheater plug, the amp did not like that!

I have a Radio Shack sound level meter that I pulled out. I sat down next to the speaker and used it. It is definitely emanating from the woofer. Now on the Hogan, there are 'hum' knobs. When I slowly turn them, I actually hit a sweet spot where the hum is lowest I put the meter on 60 and can get it to register 0, but that does not mean its noiseless. I'm not 100% sure what the 60 or numbers relate but I will look that up. The hum knobs are around 11:00, anything more or less brings more hum. What exactly do the hum knobs do? When I play music I don't seem to hear it. But when its quiet I do. I can live with it, but I sure want to know what it could be.

Thanks!!!

[JakeJ]

OK, Jack, I went back to your first post and was reminded that you said it moved, from one channel to the other, correct?  If it was a ground loop issue it would be from both channels and equal in loudness.  Next having re-read your last post that it has something to do with the hum pots I am back to being suspicious of the amp or the tubes. But...since it didn't move when you swapped the 300B's that kinda points to the amp.  You are on the east coast so you might try to find someone who can test your tubes for you (if you don't have one of your own).

Prior to this I am guessing the amp was quiet and you had no hum issues.  So, not a ground loop problem, moves from one channel to the other when you swap the speaker cables at the amp, but it does not move when you swap the 300B or any other tubes, hmmm...I think it may be the amp and you should get it checked soon, before there is a catastrophic failure.  I could be wrong so I hope some one can chime in with some further info or offer to test the tubes for you.

[fz1jmp]

Jake;

I kind of agree with you, I certainly think its the amp. Now I kind of shuddered when you said, "catastrophic failure". That would be heartbreaking! Is that a possibility? Should I not turn it on? The hum has not gotten worse and when I listen I don't hear it. I would not want to damage it any worse, should I stop using it?

I sincerely appreciate the discussion, I would rather be safe then sorry. I sent emails to 2 recommended repair places, but its the weekend, so I did not expect any responses. If anyone knows an excellent repair person, east coast preferred, please let me know.

Thanks!

[JakeJ]

Sorry to cause you any panic but you did say this piece brings you Audio Nirvana so I would err on the side of caution.  At this point I think it is quite possible the 300B's need replacing if they have a lot of hours on them.  And I kinda hope that's it because, while it's expensive to replace tubes, it ususally more expensive for repairs.

The other possibility is that some resistor in or around the signal path has drifted out of spec and is causing the hum.  If it is, then that's where I am worried about the failure, catastrophic or otherwise.

[BobRex]

Does the hum volume increase as the volume goes up?  I'm betting the answer is no.  If that's the case, then is the amp AC or DC biased?  More than likely it's AC biased - the presence of a "hum pot" (or hum knob) indicates that.  Well, almost all AC biased SET amps hum to some degree.  The hum pot is there to balance the 2 phases and reduce the hum to as close to nil as possible, but there is almost always some residual hum.  More than likely it has always been there and you are just becoming more aware of it.

There are 2 ways to eliminate the hum, the first is to go to a DC biased amp.  The second is to forgo full range operation and only use the SET above, say, 100Hz.

[fz1jmp]

The hum does not increase as the column goes up. Ahhh, I do understand what you are saying. I guess that knowing full well these amps hum, Hogan actually incorporated a hum pot to help. The hum has gotten more noticeable in the last 2 weeks or so. I know when I 1st got it home and when I auditioned it, the hum was less.

JakeJ; Thank you again for the advice, I was not panicked like "Fire" in the middle of the night panic, but you're dead on with the loss of Audio Nirvana panic! The 300b's are Valve Art's. I would guess they are the original ones. Should moving the left to right and right to left moved the hum?

[avahifi]

Lets simplify and try again please.

First, please again let me know if this is a single channel or two channel problem.

Next, turn system off, disconnect interconnect cables from preamp at the power amp.  This leaves power amp connected only to the speakers.  Now turn power amp on.  Is hum still there?  If it is, then it is a power amp problem.  If the hum is now gone, it is being sent to the power amp by something ahead of it in the system - or possibly by poorly shielded interconnect cables.

Let me know the results of these simple tests and I will then ask you to make a few more simple tests.

Regards,

Frank Van Alstine

[JakeJ]

Jack,

I would think that if the 300B's are going south then the hum would have moved channels as it is not able to bias correctly and hold bias.  However, since I am not familiar with the Hogan circuit, my advice should be taken with a grain or three of salt. 

Please conduct the test(s) Frank Van Alstine suggests as he is very experienced and is trying to isolate the true source of the problem.  You must find the true source first so that you fix it right the first time and don't waste money on things that don't correct the problem.

Thank you, Frank, for stepping in as our troubleshooting thus far has been a bit of a hodge-podge.

[fz1jmp]

Lets simplify and try again please.

First, please again let me know if this is a single channel or two channel problem.

Next, turn system off, disconnect interconnect cables from preamp at the power amp.  This leaves power amp connected only to the speakers.  Now turn power amp on.  Is hum still there?  If it is, then it is a power amp problem.  If the hum is now gone, it is being sent to the power amp by something ahead of it in the system - or possibly by poorly shielded interconnect cables.

Let me know the results of these simple tests and I will then ask you to make a few more simple tests.

Regards,

Frank Van Alstine

Thank you Frank for your time and questions;

It's a single channel issue, its the right side. While there is slight hum in left, its barely noticeable.

After disconnecting the IC's and I turn on amp, same exact hum, no difference.

Again my gratitude to all answering and thinking about my issue!

Jack

[avahifi]

Well, now we have learned something.  The hum issue is definitely originating in the power amp so it is not a ground loop issue or something being affected by other components. It is doubtful that your AC wiring or other external things are pertinent here.  I also doubt if it is a power supply issue unless there are independent power supplies for each channel.

I assume swapping tubes from one channel to the other makes no difference, the hum always stays in the right channel, correct?

Do you have a pair of shorting RCA plugs?  If so plug them into both amp inputs and turn it on again.  Does this change the hum issue at all?  Make all connections with the amp turned off of course.

Frank

[fz1jmp]

Frank;

You are 100% correct, swapping the tubes make no difference at all.

I do not have shorting plugs, I actually had to look up what they were. Now that I know what they are, I have been looking for a pair for the last 30 minutes! Seems besides England, they are somewhat tough to find. I will persevere.

Will that show the hum is coming form outside the amp?

When I look at the all the wiring, all I have to do is look underneath, it has no bottom cover, could I see anything if I knew what I was looking for? I tried that last week, but did not see anything notable.

Jack

[JakeJ]

If you are handy with a soldering iron you can make a pair with Radio Shack parts.

[avahifi]

No bottom cover?  Hummm!

I wonder if that possibly is the issue.  I can note that when we bench test an old or rebuild Dyna PAS-3 preamp without the bottom cover installed we then see excess hum from its outputs with the scope.  This hum goes away as soon as the metal bottom cover is installed.

Where is the amp located in relation to near by AC power cords or other equipment that might be radiating electromagnetic fields into the unit, made much easier to happen due to the lack of a bottom cover?

Meanwhile, the use of shorting plugs on the inputs will help tell us if the hum is being picked up at the inputs or being generated internally.

Regards,

Frank Van Alstine