Phonoland

Now that the weather is a little warmer and I no longer have to run the heater in the garage, I've noticed that there is some hum present in the amp of my Seeburg 100B, and I can't seem to get rid of it. It isn't super loud, but it is noticeable when no music is playing.

The hum is still present if only the rectifier tube is installed, and is slightly amplified when the output tubes are in. All the capacitors in the amp have been replaced. Here's what I have tried to date with no success:

-Checked and reflowed all ground connections.
-Cleaned the speaker plug and jack connectors. Jumping the center tap directly to ground also did not help.
-Swapped rectifier tubes.
-Disconnected fluorescent lamp ballast.

Anybody have any thoughts on what may be causing this and how I can get rid of it?

Hi Odog,
Warm--in Boston ?? Oh, well, I HAVE been there when it's been warm, but also very cold (to me !).
Anyway--is your phono on "free play", or is the latch bar continuously energized ? If so, this can cause this hummmmmm--
Are you sure the noise is coming from the speaker--?? There should be no ability for the amp to "make noise", of any kind without the play control relay energized ( amp/motor run relay). as when open, the rectifier tube is off ??
Ron Rich

I guess warm is a relative term. Right now I have the amp power bypassing the play control relay so I can connect my ipod to it and listen while I'm working. The hum is definitely coming from the speaker and the amp.

Hi Odog,
Yep--"warm" is a relative term--was 80 here yesterday afternoon--had to turn on my AC, in my car--
Anyway--there are several things that could be causing this ( I can't hear it, so, I'm guessing)--
1. First and foremost, you are attempting to do something that amp was never intended to do !
2. Depending on how your external source is connected, and "cable type/routing", it can cause hum. For certain, you will hear "noise" in between selections, if you have "dead time", as you have no mute (squelch, if AVC equipped) function.
3. Depending on which model amp is installed, the AVC may be causing this--
4. If the "hum prevention cap" is missing from the top of the first tube (typical !), or if that upper connection is "poor", you'll get more "hum".
5. If you have only the cabinet speaker connected, and the "wattage switch" is set anywhere, other then on "Max", you'll get hum.
6. You may have a "noisy tube", or two--somewhere in that amp. ( Tube type amps are somewhat "noisy", by design).
Ron Rich

Thanks Ron,

2. Depending on how your external source is connected, and "cable type/routing", it can cause hum. For certain, you will hear "noise" in between selections, if you have "dead time", as you have no mute (squelch, if AVC equipped) function.

The hum is still present with no source connected, so I don't think it is being generated by the cables.

3. Depending on which model amp is installed, the AVC may be causing this--
4. If the "hum prevention cap" is missing from the top of the first tube (typical !), or if that upper connection is "poor", you'll get more "hum".

It is an MRA1-L6, but in the past it was modified to include an AVC circuit. The hum is still present when all but the rectifier tube are pulled.

5. If you have only the cabinet speaker connected, and the "wattage switch" is set anywhere, other then on "Max", you'll get hum.

Switch is set to Max.

Would it be worthwhile to try increasing the filter cap values?

Since tube equipment operates with very high impedences, they're prone to hum issues if not properly repaired.

If the filters have been replaced, proper grounding is mandatory.
As for the other signal-carrying parts, proper dress is to be followed to minimize hum pickup.

A lot of so-called "new" techs simply don't have a clue as to following good component/lead dress procedures, and wind up with a slop-job that they can't figure out for crap.

It is remotely possible that the field coil has enough shorted turns to be causing an excessive load on the B+ and this exceeds the filtering (time constant) of the filter caps.

The field coil is around 5600 ohms.

Generally, a constant 120 cycle hum on these machines is caused by inadequate grounding of the center tap on the B+ transformer or failing filters.

If the hum is 60 cycle, look for poor shields or leakage of heater to cathode in one of the tubes. Does it vary with the volume control, or not? That will tell you what section to look into.

This quote:

The hum is still present if only the rectifier tube is installed, and is slightly amplified when the output tubes are in. All the capacitors in the amp have been replaced.

-tells me that the problem is inadequate grounding of the center tap.

On the few of of those old machines that I've dealt with, I always removed the center tap lead from the speaker plug and connected directly to chassis ground.

BTW: If you are feeding an Ipod or other line level source into the same input where the pickup goes, you will likely be overdriving it and also have excessive bass due to the RIAA eq in that section.

I've noticed that in recent years cable manufacturers have, in some cases, stopped making proper braided shields on audio cables and just used a sort of loose spiral wrap (cheaper). This results in inadequate shielding at some points in the cable and allows hum to occur in low-signal applications. Check the cable that you are usinf to connect the device to the amp.

Rob/NYC

Odog,
Since I can't hear it, and since you have now told me that the amp was equipped with the /avc kit, I strongly suspect the reason for your hum is the AVC circuit--Try the following--jump the avc switch on the mech while playing your I-pod--see if that makes any difference. Also--the routing of the "AC bi-pass" to the amp, may be some of the problem--??
Ron Rich

Thanks for all the suggestions guys.

So if I'm getting hum with only the rectifier tube in, I should start by looking at the B+? And the possible causes are:

Bad ground on the center tap of the B+
Bad filter caps.
Lower resistance on the field coil.

Anything else I should look at for this scenario? Would an issue with the filter choke cause this? I'm not sure how to test that.

Any, all of that, plus what I wrote above--
Filter choke?--read resistance--
Ron Rich

OK, I finally had a chance to try some things. I moved the center tap from the speaker plug and connected it directly to ground, which didn't help. I also tried to increase the first two filter caps from 15uf to 22uf, which also didn't help. All tests were done with only the rectifier tube installed.

Field coil resistance measured at 5200 Ohms (hot), and the choke resistance was 315 ohms.

If anyone can think of anything else to try, I'd be happy to. Otherwise I can certainly live with this, as it is not really that noticeable.

As always, thanks for your help guys.

We do seem to be running out of ideas here. On thing to try is to connect an external speaker directly to the speaker switch's highest wattage point and turn the internal speaker to it's lowest setting. the idea is to see if the hum is actually originating in the amp or at the speaker itself via some sort of leakage from the field coil.

Also, on push-pull circuits hum that is generated in the output stage is supposed to be suppressed by the inherent out of phase design. The does require reasonably balanced output tubes. Since this amp has only one cathode resistor and bypass cap a simple method of determining which tubes have adequately similar emission is to connect a DC meter across the cathode resistor. Then place only one output tube at a time in a socket and measure/note the cathode voltage. Place tubes with the closest cathode current in phase-opposite sockets. In the absence of accurate test equipment this will gave a decent indication of emission.

Again, is the hum 60Hz or 120 Hz?

Rob/NYC

Sounds like 120hz to me. I guess I could post an audio sample if it would help.

I attached an external speaker as you suggested, and the hum is present there as well.

The output tubes are new and were supposedly a matched set, so they should be OK. The hum is also present even with no output tubes installed.

Could this be a transformer issue of some type? I feel like I have swapped out everything else.

Odog,
As I'm sure you realize, not hearing the hum, puts everyone at a dis-advantage--
This last post suggests that the hum is not from the speaker ? I am wondering if you are "making mountains out of molehills" here--SOME "hum" is inherent, in anything that has power, flowing in it. Tube type amps are always "humming" to some degree. You might try to determine the source of this hum, with your fingers, by gently touching components.
I know I asked this before, -but got no answer, so 'll do it once more before giving up-----
Is the "hum cap" installed over the connector (which must be "tight") on the first tube--or is it, like many, "missing" ?
Are you SURE that the hum is not coming from an energized, latch bar solenoid ?
Ron Rich

Thanks Ron, I didn't mean to suggest that the hum was not coming from the speaker. Rob had suggested earlier that I try connecting another speaker to see if the hum was coming from the amplifier or perhaps the speaker itself. As the hum was also present in the second speaker, I'm assuming it is being created in the amplifier.

There is a hum cap on the 6J7, but I don't believe that the source of the hum is that early in the circuit, as it is present with only the rectifier tube installed, and does not increase when the volume is turned up.

It is not from the latch bar solenoid, as that is not connected right now. There is a slight hum from the transformers, but what I am trying to eliminate is coming out of the speaker. It also seems slightly louder than what I have heard in other jukeboxes or tube amps.