18 Watt Lite HUM when on standby!

[rockybottom16]

I recently upgraded the PT in my 18 Watt Lite build (100mA to 150mA), and the hum when switched on seems to have increased noticably. This is while the amp is still on standby. Switching it to "play" increases the hum just a slight amount, and the amp sounds as good as it did before the PT change. No significant voltage changes either. The hum is coming through the speaker. The new tranny is mounted vertically, outside the chassis, whereas the old one was horizontal in the chassis cutout, so the wires are necessarily a couple inches longer coming from the PT now. The OT is pretty close to the PT, but the laminants are at 90 degrees.

Any ideas as to what's going on here? Help!

[big_teee]

Can you include any pictures?
T

[rockybottom16]

Pictures of what in particular? I'm a bit reluctant because this is not my prettiest work. I built it in a tweed princeton chassis using princeton trannies and a slightly modified princeton eyelet board; and it's a little cramped to say the least. I used a terminal strip to mount my diodes on because there was just no other way. The whole thing was just an experiment, but the dang thing sounds so good it's now my favorite amp. Whoda thunk.

Anyway, I just moved wires around for a while with my chopstick with no perceptible change whatsoever. I'm beginning to suspect that I'll need to move the OT away from the PT. It's only about an inch away from it now. I'm hoping I can do it without taking the board out. I also tried adding another 8uf filter cap in parallel with the first filter (from 32uf to 40uf total) with no audible change in hum. One thing I noticed, whether usual or not, is that the plates of the power tubes develop around -300mVDC on them over the course of several minutes after the amp is switched on. Should I assume that is coming from the OT's proximity to the PT? The hum isn't really noticable when your playing, you only can hear it when you stop playing and it's real quiet. Kind of like the hum you'd expect to hear from an SE amp. Certainly not an issue when jamming or gigging. Just one of those little things that will bug me 'til I get it sorted out.

[Mick Bailey]

If you pull the output tubes does it still hum? If it does then you probably have a serious coupling issue between your transformers.

[rockybottom16]

Power tubes out, still has hum. Guess I'm gonna have to move the OT away from the PT. How far away? Half the way across the chassis? All the way to the input side?

[Chuck H]

Do you have the transformers oriented with the laminates in different directions from on another?

[rockybottom16]

Yes. The laminates are at 90 degrees to each other. But the two trannies are only about an inch away from each other.

[Chuck H]

Aluminum chassis or steel?

[Mick Bailey]

You need to use the headphone trick to relocate your transformer. If your transformers are way too close then just reorienting them won't work, you need separation. Pull the power tubes and remove the OT. Connect a earphone/headphone to the secondary (no other connections) and turn the amp on to standby. Now you can listen to where the best location is for your OT.

[soundguruman]

I recently upgraded the PT in my 18 Watt Lite build (100mA to 150mA), and the hum when switched on seems to have increased noticably. This is while the amp is still on standby. Switching it to "play" increases the hum just a slight amount, and the amp sounds as good as it did before the PT change. No significant voltage changes either. The hum is coming through the speaker. The new tranny is mounted vertically, outside the chassis, whereas the old one was horizontal in the chassis cutout, so the wires are necessarily a couple inches longer coming from the PT now. The OT is pretty close to the PT, but the laminants are at 90 degrees.

Any ideas as to what's going on here? Help!

When an amp has HUM in standby...
USUALLY: The speaker voice coil is inducting the magnetic field from the power transformer.

Install a GROUND wire between the amp chassis and the speaker frame.
The speaker frame must be grounded to the amp.

When I say GROUND, it's not to the speaker (+ -) terminals.
It's just to the metal frame of the speaker.

The wire can be attached with a ring terminal, to the metal speaker mounting studs, in the cabinet.
scrap the paint off the speaker frame, where the wire is attached, to make sure you have a good surface contact.

Then the other end of the wire should be permanently attached to the amp chassis, in a similar way.

Once the speaker frame(s) are grounded to the amp chassis, the (standby) hum should stop.
If you have 2 speakers, you have to ground both frames.

OTHER amps that have the same problems: Music Man, or a speaker with a large magnet...
Like an EVM speaker.

NOW if you have HUM when the standby is ON: (when prior you did not have hum...)
The position of the transformers needs to be changed.
TURNING one of the transformers 90 degrees usually helps stop the problem.
Traditionally, the output transformer is rotated 90 degrees, and remounted.
BUT the same thing can be accomplished by turning the power transformer 90 degrees.
(which ever produces less noise, or is easiest...)

[Chuck H]

You need to use the headphone trick to relocate your transformer. If your transformers are way too close then just reorienting them won't work, you need separation. Pull the power tubes and remove the OT. Connect a earphone/headphone to the secondary (no other connections) and turn the amp on to standby. Now you can listen to where the best location is for your OT.

Yup. I asked aftr the chassis material because it's often possible to simply relocate the OT to the other side of the chassis, but you need a steel chassis for effective shielding and the leads must be carefully routed. Anyway...

The headphone trick is great. Using it, it's often possible to mount the OT closer than you might think because there are inconsistencies and phase anomalies in the distribution of EMF. It's sometimes possible to find a quiet spot between the gaps, so to speak. I've done this when locating reverb pans in head cabinets. One inch the wrong way and HUMMMMMM. But dead silent when just right.

[Mick Bailey]

Flipping the OT to the other side of the chassis is something I do quite a bit with Hi-Fi builds, generally because I can organise the space better than with a guitar amp (no gain issues to deal with, lower parts count and I don't mind a greater build-depth). It can work wonders even with an aluminium chassis and mostly gives a more compact design overall. If you have the space and depth it's worth a try, you may even be able to use the same mounting holes.

The headphone trick reveals some strange optimised locations, many of them quiet, but not practical - like tilted as well as rotated transformers.

[big_teee]

Please tell us again why you replaced the 100ma transformer?
I use a 150ma on my Marshall 1987 50 Watters with 2 EL34s.
100ma HT, should be more than adequate for a 18 watt amp.
Maybe I'm missing something here?
T

[soundguruman]

Flipping the OT to the other side of the chassis is something I do quite a bit with Hi-Fi builds, generally because I can organise the space better than with a guitar amp (no gain issues to deal with, lower parts count and I don't mind a greater build-depth). It can work wonders even with an aluminium chassis and mostly gives a more compact design overall. If you have the space and depth it's worth a try, you may even be able to use the same mounting holes.

The headphone trick reveals some strange optimised locations, many of them quiet, but not practical - like tilted as well as rotated transformers.

What ever works is what should be done.

[rockybottom16]

Thanks for all the tips, guys! The chassis is steel. The reason I changed transformers is because after running the amp for an hour or so the PT was pretty warm. I could only hold my hand on it for 2 or 3 seconds. Of course it's a tweed-style chassis and the trannies are kind of in a little oven tucked up in there, so tube heat may very well have accounted for some of it. The other reason is it seemed that whenever someone in the forums was recommending a PT for an 18 watter, they specified at least 120mA. Now I'm beginning to think I should have just installed a small cooling fan instead of changing trannies. I may still. It would probably be less work than relocating the OT, since I may have to remove the board. I'm not sure what to do at this point. I think I'll try the speaker grounding idea first, and then decide.

Is it general consensus that 100Ma HT should be sufficient for an 18watt Lite?