Hum from DR-103 (60 hz?)

HIWATT amps from the Hylight Electronics era

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therealjoeblow
Posts: 27
Joined: Mon Nov 10, 2008 6:39 pm

Hum from DR-103 (60 hz?)

Postby therealjoeblow » Fri May 26, 2017 7:53 pm

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

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mhuss
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Re: Hum from DR-103 (60 hz?)

Postby mhuss » Sat May 27, 2017 12:52 am

Does it go away when you pull the PI tube?

therealjoeblow
Posts: 27
Joined: Mon Nov 10, 2008 6:39 pm

Re: Hum from DR-103 (60 hz?)

Postby therealjoeblow » Sun May 28, 2017 1:42 am

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

therealjoeblow
Posts: 27
Joined: Mon Nov 10, 2008 6:39 pm

Re: Hum from DR-103 (60 hz?)

Postby therealjoeblow » Tue Jun 20, 2017 5:11 pm

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


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