Hum problem

[rutledj]

I build a clone of the 6gl6 vibroberb and have a slight hum problem. I have 200 ohm resistors across the 6.3v to ground. I have my ac wires up in the air. The hum intensity stays the same regardless of volume which leads me to think it might be transformer coupled hum.

Any suggestions would be appreciated.

Thanks,
Rut

[Luthierwnc]

You usually see 100R resistors from the leads to ground with the CT taped-off.

This is not an original idea but attach a series .1/600v cap to a test lead with a clip on the other end. Insulate it really well with shrink tube. Then ground the clip and start touching places in the signal chain starting with the entrance to the PI. If it kills the hum, work back into the preamp. If it doesn't, work forward into the power amp.

If/when you find a spot where there is a big difference in how much noise you get, check your lead dress there.

Skip

[Andy Le Blanc]

you might want try a hum pot in the fil. supply.... tap it to ground or a positive voltage source..... up to say 50v.... just had a conversation with an
avid ampeg user..... hed never in 50 years bought a fender cause it didnt have a hum pot...

[rutledj]

you might want try a hum pot in the fil. supply

Exactly where would you connect that in the filter ckt?

Or do you mean in the 6.3v heater ckt?

Rut

[Luthierwnc]

There was a thread within a couple of weeks on elevating the heater supply. One of the guys uses a 2 watt 10 or 15 turn pot. Do a search.

On the subject of searches, do the cap/ground thing too. The noise you described could also be a filter. sh

[Andy Le Blanc]

sorry.... old term....... filament...... if only Edison had a more open mind

they used to call it a humdinger....... theres many ways to dress one into
your apparatus......

[rutledj]

Well, I've tried raising the artificial center-tap to around 40vdc and it didn't sound any quieter than when it was connected to ground.

Also, I have a problem in the reverb ckt. If I turn the level any way up at all I get a solid tone in the amp. The reverb works but this constant tone appears, different than the hum problem, higher pitched.

I've built several amps now and never had this much problem getting rid of hum!!!!

Any other suggestions would be appreciated.

Rut

[Firestorm]

Hum that doesn't change with volume is induced after the the volume control, so you need to look at the second stage of both channels and V4. Check the shielding on your reverb cables AND on the footswitch lead (I assume you have a footswitch, you did say "clone") -- that footswitch is directly connected to the reverb recovery tube's grid; a BIG antenna if it isn't grounded well. Also make sure the heater filaments on your output tubes aren't cross wired -- you have to have the same wire going to pin 2 on both tubes (and the same wire on pin 7).

Then, there's ground loop problems --- but we'll think about that if you can't track it down otherwise.

[rutledj]

I have narrowed the source down to the reverb ckt. If I remove the tubes and bypass the ckt it if much quieter. I've tried rerouting the grounds for the reverb ckt and isolating the cable jacks for the tank to no avail. It appears that it's either a quiet amp with no reverb or a hum induced one without - aaaargh!

[Phil_S]

Have you got a picture showing placement of all transformers? I'm thinking maybe it is how they are positioned.

[Karl]

I am having the exact same problem with my Vibroverb clone. When the level is belove 3 it has no hum and above 3 the hum increases with the level.

I have checked everything in the reverb circuit and I can't find what is causing this hum.

[rutledj]

Here are some pics. You can see where I've been playing around with some components but you get the idea of how stuff is layed out.

[Andy Le Blanc]

check the reverb tank #....... you might have a tank that creates a ground
loop through its connection..... you might need a tank where either the input
or output has an isolated ground....

should be the third letter of the tank code..... you may need a "C" or "D" type
connector.... types "A" and "B" have both in and out grounded to the tank

this is for accutronics...... check the tank visually or for conntinuity any how
may be try lifting the ground of the driving trannys out off the chassis ground
some one chime in if this is a bogus lead

[rutledj]

I've tried 2 different tanks. One out of another mesa mark 1 clone I built. Still could be I guess.

[Firestorm]

You could try unbolting the reverb tranny and moving it around a little. Transformers mostly couple through the bell ends and you've got your OT aimed at the reverb tranny. Fender usually had the reverb transformer at an angle.