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My '79 2-input DR-103 has developed a hum (I believe it's 60hz AC noise). Symptoms:

-it's there whether I have anything in the input or not.
-it doesn't change with either of the preamp volumes.
-it gets louder or softer with the master volume.

I should say, it amplifies and works just fine, but the background hum is annoying.

I tried pulling 2 of the poweramp tubes in pairs, it does it with all 4, with the 2 outers only, and with the 2 inners only - no change.

The amp was re-capped with F&T's and rebiased with new JJ EL34's by a meticulous professional tech about a year ago and only played at low to moderate volumes in my basement since then.

I did the following work myself:

-removed the Canadian Importer Mod
-converted V2 to the Biacrown OL version (added a capacitor and 2 resistors, a minor "tweak" that I reviewed with Mark H via email before I undertook to be sure I had everything correct)
-after I realized I didn't like the OL V2 mod that much, I removed the extra flying cap and resistors and returned it to the resistive mixer with the tag-strip and 2-470k resistors.
-replaced V1 and V2 with current production Tungsol 12AX7's

I'm not really sure exactly when it started humming, as when I got it back from being recapped, and I did the OL mod on V2, I was running the preamp controls very high and the master very low, and in that mode the hum is not really noticeable.

Since I've removed the OL mod and gone back to the resistive mixer, I'm now running the preamp very low and the master very high with a gain pedal infront instead, so now I really notice the hum, but like I said, I don't know when exactly it started.

Any ideas before I take it to the tech to have it looked at and cost myself some $ ?

thanks
TRJB

Does it go away when you pull the PI tube?

I haven't tried that - you think maybe the PI tube is shot? It still has the original '79 Mullards in V3 and V4 - I replaced V1 and V2 as they went microphonic a few months back.

I'll give it a try and see if it solves the issue or not.

thanks Mark
TRJB

Hi Mark,

I finally found some time to troubleshoot some more. Here's what I found:

1) With the PI tube pulled out and everything else as normal, there is no hum, the amp is dead silent when turned on and standby is disengaged.

2) With a brand new PI tube vs the original one that was in there, there is no significant difference, the hum persists.

3) If I turn the bright and normal preamp volumes both to zero, and turn the master volume up to max, the hum is what I would consider moderate - IE, it's not "silent", but the hum is not huge either, but still, not normal. When I do the same thing with my Marshall, it does not hum like the Hiwatt does.

4) When I have the master volume at max like in #3, if I increase the Bright volume from zero to max, the hum doesn't really change (I get a bit of hiss, which I think is normal for the amp's volume to be fully dimed like that?)

5) When I have the master volume at max like in #3, if I increase the Normal volume from zero to max, the hum starts to increase a lot more as I increase the Normal preamp volume - it gets to be about 3-4x as loud as in #2 when fully dimed.

Not sure where to go from here.

Thanks for any additional help you might be able to provide.

Cheers
TRJB

Sorry, I've been busy the the last few weeks...

Did you try swapping out V1?

Is the coax going from the volume controls to V2 only grounded on the control end?

--m

Thanks Mark.

Yes, the coax cables are going to both V1 and V2 are grounded at only one end - the ones to V1 are grounded at the input jacks; and the ones at V2 are grounded at the pots. At V1 and V2 just the center wires are connected, the shielding is trimmed and wrapped as it came from the factory.

I replaced V1 and V2 with known good tubes from my Marshall, and it made only a very small change - the "very minor" hum that I said was present when the master was cranked but both input volumes were at zero is now gone, with it like that, it's pretty darned silent. So there was a minor issue with one of the tubes causing that.

HOWEVER - the other more prominent issue still persisted - that is, with the master cranked, when I increase the Normal volume and nothing is plugged in, a much louder hum is introduced and gets louder the more I raise the Normal volume. This does NOT occur with the Bright volume - when I increase that there is no hum whatsoever.

I swapped the V1 tube again with another known good one, no change.

Then I took a plastic stick, and tapped around inside the preamp section with the Normal volume set at about 1/2, and when tapping on the pin that that the normal coax is connected to at V1 I heard a "thump...thump" through the speaker. I thought that was odd, because it did that with 3 different known good tubes that are not microphonic in V1!

So I continued tapping, and found that if I tapped along the Normal coax cable that goes from the input jack to V1, I got the same "thump thump" as I tapped the cable, but a little quieter than if I tapped the pin at V1.

Then when I got to the input jack, if I tapped the resistor that is soldered across the jack I got a fair bit louder thump thump.

I get no other sounds when tapping anywhere in the amp, only along the Normal input coax wire and input jack. Bright and everything else is silent.

Very strange to me to have a microphonic input jack and coax cable...

[EDIT] So the remaining oddity is that when I plug in a Y-cord to bridge the 2 channels, the hum on the normal channel disappears - I can now crank the master, and raise the normal volume and it remains silent. But if I unplug, then the hum returns to the normal channel, and not the bright.

Do you think the jack is bad, or the cable, or just need to refresh the solder joints at the jack? Or have any other idea?

Many thanks
TRJB

It sounds like the grounding on the input jack where you experience the loudest tapping noise is not as good as it should be.
I would re-solder that.

Mark is the authority, so he may have something to add.

good luck!

I agree, it certainly seems like a grounding problem around the Normal input jacks. Did you ever get it sorted?

I haven't done much with it lately. With summer here, I'm busy running around with the family, and out to the lake for weekends, so haven't had much time to play guitar or look at any equipment maintenance over the past few weeks. I'm just heading out to the lake for a few weeks extended vacation so won't get another chance till mid- or late-August, but will definitely try resoldering the ground at the input jack when I get back.

Thanks for the help so far, I'll report back when I get it sorted.

Cheers
TRJB