Dean Markley CD-60 (not a reissue)

[skunqesh]

Hey all, long time lurker, first time posting.

I've got an older Dean Markley CD-60 that makes an annoying very low frequency hum when powered up - no guitar or patch cord plugged in.
It starts humming a few minutes soon after warming up.

It only happens if the amp chasis is flat or even with the ground surface.

If I tilt the amp ~45deg forward or back - the hum disappears.

I've removed the reverb tank, swapped power tubes around, and tried a different speaker.
It's worse with the OEM speaker, but can be reproduced with any speaker.
Grounding does not appear to be an issue.

This doesn't seem to be motorboating, but my experience is limited.

Tapping on the (6L6GC) power tube closest to the transformer gives a similar low frequency 'thud' like noise, but doesn't persist like when the amp is even/flat with the floor.
Wondering if it's a loose socket? It looks good to the eye, but again - hard to tell just by looks.

Any infor or hints are much appreciated!

[g1]

When you say 'swapped power tubes around' do you mean you have tried other power tubes in the amp?
If you are just swapping their positions, you need to try others.

[skunqesh]

DM CD-60

When you say 'swapped power tubes around' do you mean you have tried other power tubes in the amp?
If you are just swapping their positions, you need to try others.

Thanks for the reply,
Yes - I tried other tubes, in addition to swapping the positions - caveat - the extra (aka used) tubes I have lying around might be suspect, as well?
They (the used tubes) sound okay in another amp with 6l6gc specs for it's power.

I'm going to try plugging in some brand new ones from a recently updated Dean Markley CD-120 amp w/ fresh J&J 6L6GCs

It's just weird that the humming goes away if I tilt the amp forward or back. I've never heard/seen anything like this.


Also - What's the skinny on powering up a 60 watt amp with only one tube at a time?
From what I've read it should be okay for briefly testing the sockets, but I'm no tube/electronics specialist.

Any advice on how to begin checking the sockets for continuity issues? assuming it's not a tube issue.

fwiw, I have read up on how to avoid electrocution when testing elements within the amp, but again - no expert here, and am aware of some of the more immediate dangers.
(I did once grab a live AC 110v line while remodeling a basement - that experience was pretty tame compared to what I understand the filter/power DC caps are packing)

regards

[The Dude]

I'm wondering if there's an intermittent connection that makes and/or breaks as you rotate the chassis. I might try the "chop stick test" and lightly tap around the amp to see if you can narrow down the issue. It might also be a good idea to re-tension the tube sockets. Be careful! There will be high voltage on the socket pins until the caps discharge.

[skunqesh]

I'm wondering if there's an intermittent connection that makes and/or breaks as you rotate the chassis. I might try the "chop stick test" and lightly tap around the amp to see if you can narrow down the issue. It might also be a good idea to re-tension the tube sockets. Be careful! There will be high voltage on the socket pins until the caps discharge.

I’m going to trouble shoot a few more basic things this weekend , then try re tension the pun sockets . I’ll make sure to check/discharge the caps.

[Chuck H]

Tilting the amp forward OR back making the hum go away seems suspect. Does the amp hum like that with no guitar plugged in?

[skunqesh]

Tilting the amp forward OR back making the hum go away seems suspect. Does the amp hum like that with no guitar plugged in?

Yes, it has a low freq hum without anything patched/plugged in. And the reverb tank removed.
Originally I thought the tank was a source.
It may well be the speaker, at this point. But I’ll investigate more this weekend.

[glebert]

Are you sure the hum is coming from the speaker? I've got a Fender solid state that we swore had an electrical hum but it was actually a mechanical vibration because the chassis wasn't securely screwed to the cabinet properly.

[skunqesh]

Are you sure the hum is coming from the speaker? I've got a Fender solid state that we swore had an electrical hum but it was actually a mechanical vibration because the chassis wasn't securely screwed to the cabinet properly.

I did tighten up the chasis mounts and speaker mounts. Nothing seemed loose in particular.
The hum will only present if a speaker is plugged in.

[Chuck H]

Those originals have notoriously thin chassis. Rather than lilt the amp, try reaching in and grabbing the power transformer to see if that stops the hum. Just to eliminate the possibility of acoustic vibration.

[skunqesh]

Looks like it was a socket issue.

After swapping in brand new tubes the hum never came back - but -
after trying to replicate the hum with the original tubes - it's still gone.

Perhaps trialing all those different tubes cleaned/scoured out an oxidized pin socket??

anyway - next up - replacing the SPDT button (boost?) EQ switches on the CD-60 - they're all cashed out. anyone recommend a good switch to replace them with?

thanks again for all the advice.