the CTH (Compact Tube Hybrid) Rev A thread... - Page 36

So I got some spare time to put the amp into case. The buzz is a bit more silent but still there whenever I touch the volume knob.
When you said ground the pot, does that mean I should connect it to the case too? It's isolated now.
Edited by Lil' Knight - 1/27/13 at 3:36am

If the case is metal I'd connect it to SG (which = IG input ground).  And if the volume pot is not mounted to grounded metal then you should run a small ground wire to it.

On the RK27s I loosen one of the 2 little screws on back, slip a small/thin wire under it, screw it back in & run that to IG/SG/case ground.  One way or another grounding that knob.

I'm using the Hammond case with plastic panel so the pot is totally isolated. Also, the pot used is TKD 601 which does not have the screws like those of the RK27. Looks like I should just connect the ground pin to SG then.

The need to ground the pot when using a metal knob is not uncommon and more a standard practice.  Don't want it being an antenna for noise or a path for static shocks to go directly to your input stage.

It's no big deal to do it either, if there's no easy way just solder (file, flux, solder) a ground wire to a thin big washer or sheetmetal cutout & thread it onto inner side of pot shaft.

Edited by cfcubed - 1/28/13 at 4:39am

I picked up a set of new headphones recently and it turns out that I have a 60Hz or 120Hz hum (don't really have a way of figuring out what it is, exactly - can't find any good comparison sounds on youtube) in my CTH that I could not hear with the K501 (due to the headphones' low sensitivity). I've ensured that it is not my source introducing this hum. The case of my volume pot isn't grounded to anything and nothing changes when I touch the volume pot so I don't think that's anything. I'm using the Rev. A version of the BoM so C3H is an updated 330uF. I have noticed that the buzz gets a little more noticeable at 6.3V heater supply (6CG7). My other tubes are a 12AU7 and a 12BH7. I have tried using a surge protector and running it straight out of the wall. Disconnecting inputs doesn't change anything. I'm pretty clueless when it comes to troubleshooting so that's about as far as I got. Any help?

Edited by Ikarios - 1/30/13 at 8:02pm

>  a set of new headphones recently and it turns out that I have a 60Hz or 120Hz hum

 

It's been a long time since we've had a report of this with CTH...  Hum using newer BoM parts and possibly not related to grounding.

 

Exactly what/which are your new headphones?

You say touching the vol knob has no impact, does touching its (metal?) case have any?

About where is the vol knob in its rotation when you notice the hum & do you only notice the hum between music passages/during the quietest parts (or is it more noticeable than that)?

 

If this is not a ground issue then you seem to be on the right track WRT C3H, which was raised to suppress such noise.  

R18 values also have some impact here - I wonder what value yours are & if they are std BoM/100R.

 

I believe I adhered to the CTH Rev. A BoM for all resistor/cap values, apart from the LED. The headphones I hear the hum on are HD700s, which are, from the specs, much more sensitive than my K501. I hear the hum at all rotations of the volume knob, even at the lowest, although it's pretty faint, and difficult to pick out if music is playing. It's similar to the faint drone of an air conditioner in another room, or a buzzing fluorescent light. If I get to about 12 o'clock in the rotation the amp starts to get noisy, with some DAC noise, but that's at almost unlistenably loud levels. I listen at around 9/10 o'clock at most. I've tried touching pretty much every exposed piece of metal on the outside of the case - RCA grounds, headphone jack, volume knob, exposed area of metal case, all to no avail. The only thing that changes the hum a bit is fiddling with the AC jack's nut (loosening and tightening) but my back panel is pretty tight back there and I don't want anything accidentally bridging. I suppose I'll have to take it apart and take pictures anyway, though.

Edited by Ikarios - 2/1/13 at 10:43am

How do I wire up an LED under the Tube?  What pads do I use for it?

 

Thanks

You can use PCB's embedded e12 green/on/latch LED pads.  If you're already using it think the circuit will still behave if you parallel in another LED (test this).

Think I had to enlarge the PCB hole under the tube socket (e.g. w/a reamer), shrink-wrap LED leads & glued LED in socket's hole.

Thanks.  Still waiting on a part from ups, and then I can finish it up