Hum after output transformer replacement

[mwickett]

I'm looking for some help as I get a little further into the hobby of modifying amplifiers. I've made some successful small modifications to a Crate V18, consisting of replacing resistors, reverb tank, and speaker. My question now concerns an Epiphone Galaxie 10 to which I've already added a speaker out jack and installed a new speaker. The Galaxie is a 10 watt single-ended 6L6 amp with a 10" speaker.

Now I have attempted to replace the very puny output transformer of the Galaxie with a Hammond 125DSE as has been reported by others in the past. The replacement was rather straightforward, though it involved the relocation of the transformer from inside the chassis to the top (the side facing the speaker), i.e., the same side the power transformer is on. It just fits in the cabinet in the new location, with very little clearance from the Eminence Rajin Cajun speaker.

The new transformer has really opened up the sound of this little amp, and I like it very much. However, the problem is there is a strong 60-cycle hum now. I tried moving the transformer and its wires around with it unmounted from the chassis and the chassis out of the amp, but neither its position nor orientation seems to affect the hum. The hum does not exist on standby nor with the tubes removed. I have replaced both the preamp and power tube with others I have on hand to no effect.

Is there something I could have missed that should have been done when replacing the transformer? Could I have inadvertently messed up something else?

[David Root]

IIRC the 125DSE is an open transformer, ie no bell covers. This could be your problem. Sure the original OT was probably the same design in that respect, but it was enclosed in the grounded chassis.

Now you have that OT on top of the chassis right close to the speaker coil, spewing EM radiation all over the place and maybe coupling with the speaker coil. Also close to the power tubes too I'm sure.

Whether or not that is what's happening, you are coupling the AC frequency into the signal somewhere. Does it change anything if you put shields on the preamp tubes? Would need to be steel I think.

For comparison, I have a 50W ODS clone with the OT up really close to the speaker frame and the speaker coil, but it's a BF Bassman OT which has bell covers on it. Zero AC hum. No preamp shields.

[mwickett]

When I moved the chassis away from the cabinet and speaker, there was no change at all in the hum, so I don't think the coupling is directly to the speaker. I first thought the coupling must be between the power transformer and the output transformer, which is why I tried moving the output transformer around, away from the power transformer, and rotating it different ways, again with no change to the hum. The preamp tube cover is only aluminum, I think.

[paulster]

Pull the power tubes and see if you still get hum through the speaker. That will tell you whether you're getting magnetic coupling between the power transformer and the output transformer, and then take it from there.

I think it's likely, but it helps to be sure before you start chasing your tail.

[selloutrr]

I'd tighten all your bolts screws and touch up and new solder points you made, if you had to force anything during install resolder those as well. make sure your transformer's center tap is solidly grounded. A cold solder joint can give you hum.

if they are the same tubes you where running before i'd assume they are ok.

Did you over heat any components during the installation?

[mwickett]

Thanks for all the replies. No sound at all with the power tube out. It's single-ended, so no center tap.

I'm thinking I'll pull it out and resolder everything I touched, since I'm not the best at it. What components should I check that might have been damaged? The board traces for these connections are not too near other components.

[quayhog]

Try rotating the speaker 90 degrees.

[jjman]

Are you sure it's 60hz and not 120? Can't be 60hz EMI if it was generated merely from the OPT. And any 120hz in the OPT, and there will be at least a hair, will already be heard thru the speaker since it's SE and there's no hum/ripple canceling.

60hz not related to the PT could be heater wiring or ground referencing, or a bad tube. 120hz related to the filter caps, grounds and lead dress.

Negative side of the speaker is grounded? No changes to tubes or heater wiring?

[Drumslinger]

same scenario when you remove the preamp tube? Filter cap going bad or drifted?

[mwickett]

It is 120 Hz, not 60 Hz. I hadn't paid close enough attention.

I resoldered the connections and the hum is still just the same. I changed no other connections in performing this modification.

The hum IS still there with only the preamp tube removed. It is NOT still there with the power tube out.

Do those 2 things narrow down enough more for me to be able to check something specific? How do I check the filter caps, for example?

I appreciate the advice here. I realize I am pretty novice. Replacing the OT didn't seem like an expert operation.

[selloutrr]

with out a tester it's not easy to test the filter caps.
are you setup to discharge the filter caps? if so after you remove the voltage you can do a basic good bad test with an ohm meter. if they test of 15Kohms they are probably good.

IF you do not know how to discharge filter caps DO NOT DO THIS TEST!!! the voltage can KILL YOU!!! and destroy your meter.

No noise to your speaker with the power tube removed points to it not being the transformer, except the transformer is not being excited so it's still questionable. the simpliest test without a test jig would be to reinstall the old transformer and if all the problems are gone it's the new transformer.

Do you have other power tubes to try in the circuit?

[mwickett]

I discharge everything before I work on the circuit at all, but thank you for the extra warning. I will do a basic check of the caps.

I did try another 6L6 which gave the same results.

If the filter caps test reasonably, I'm thinking I agree that I will have to install the old transformer to check things. Ugg.

[selloutrr]

it's not fun to redue what you just did but it does answer the question.

sorry I wasn't trying to come off rude. I just have mixed feeling about giving strangers advice to poke around inside hi voltage amps. I'd hate for anyone to get hurt. I put the warnings so that people who aren't forum members or newbies don't get hurt.

[M Fowler]

How about that added speaker out jack can you be inducing hum through the OT at that point?

[mwickett]

Well, I put the old transformer back in, and lo and behold it quiets down. Put the new one back in, hum's there again. I again moved the output transformer all around the chassis to see if there was any effect, but there is no effect from its position.

The hum does not change with the amp volume, and I won't be using it for recording. Other than the hum, it sounds much better with the bigger transformer, so I don't know what to do besides just turn it up louder and play!

Thanks to everyone for their comments. It would seem there's nothing "wrong" with any components. If anyone has any further ideas about how to reduce this hum, or hum in a single-ended class A amps in general, I'd love to hear 'em.