Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Madison on July 27, 2012, 01:59:50 am

Title: Boost foot switch hum problems
Post by: Madison on July 27, 2012, 01:59:50 am

When I turn on the boost this hums like crazy.
I have tried grounding it different ways; on the input jack, via the foot switch jack.
Nothing seems to help.
About the only thing I haven;t tried is an insulated input jack.
Where have I gone wrong?

Title: Re: Boost foot switch hum problems
Post by: tubeswell on July 27, 2012, 02:39:13 am

Does the boost hum when you engage it from the amp's control panel switch (as opposed to using the footswitch)?

Title: Re: Boost foot switch hum problems
Post by: jeff on July 27, 2012, 05:01:51 am

What is this switch actually doing? I don't understand the circuit. Could you show how it's connected to the cathode bp cap(rest of the cicruit)?

Title: Re: Boost foot switch hum problems
Post by: Madison on July 27, 2012, 07:50:33 am

Oh, this does have a CT on the 6.3 taps from the PT.
Sorry forgot to add it.

Title: Re: Boost foot switch hum problems
Post by: sluckey on July 27, 2012, 08:24:48 am

Quote

When I turn on the boost this hums like crazy.

Put a filter cap on the rectifier output (470-1000uF). Even better, use a double pole switch. One pole switches the bypass cap, the other pole switches the LEDs. Or even more better, drop the idea of the LEDs altogether.

Title: Re: Boost foot switch hum problems
Post by: Madison on July 27, 2012, 11:14:23 pm

Unfortunately, I have already drilled the chassis for these LEDs so kind of no turning back.

I am liking this double pole idea.
Excellent!
Actually the panel switch already is.

I am going to have to mod the foot switch, wire, jack.

Thanks a lot!

Title: Re: Boost foot switch hum problems
Post by: sluckey on July 28, 2012, 12:19:55 am

You may not need double pole switches if you put a filter cap on the rectifier output. As it is, you're unfiltered dc (actually 120Hz pulses) is being connected to the cathodes. It's gonna hum. Very much like a short between heater and cathode.