Can hum be caused by transformer?

[dbishopbliss]

David,

I see a 1.0uF 50V Z5U cap in the parts list for the FP III.  I think that is what is across the voltage divider.  I think you can go to Radio Shack and find a 0.1 or 1.0 uF cap there for a few dollars to give it a quick try.  Since you have 70V across the bottom of the voltage divider you want a 100V cap.  It doesn't have to be soldered just crimp it at first to check then solder if it lessens the hum.

You are supposed to be on vacation.  Why are you still checking the boards?  :-)  I have a bin full of 1.0 and 0.1 caps, I will try one of those tonight. 

A couple of things:  Have you ever tried just grounding the center tap of your heater winding?  If you did, what were the results?

Second, I presume your Voltage divider is across an already-filtered supply, correct?  So, I would use a smaller cap as bypass in order to use something other than an electrolytic.  Of course, that generally sets the practical limit to less than 1 uF, but I don't think you are necessarily looking to filter out anymore ripple with the bypass.

Oh, I guess I have one other question: What are you using for heater wiring?

I haven't tried grounding because I know that I need to float the ground because of Vhk limits (100V). 

The voltage divider is coming off the last capacitor in the CRCRC filter, so yes its filtered. 

My heater wiring is Laquered Cotton Braid Solid Core Wire.  Now that I think about it... the insulator on that is pretty thick so it doesn't allow for as tight of a twist as some other wire I've used.  However, I'm not so sure it is the wire dressing that is the problem.  The level of hum doesn't change regardless of where I move the wires.

By the way... the heaters are AC.

[Grainger49]

I am on vacation in the N. Ga. mountains but see the boards 3 times a day.

[JC]

OK, regarding grounding the center tap, what would the Voltage between Heater  and cathode be if you were to ground the center tap?  I guess I am unclear as to why your cathode would be more than 100V different than your ground.  Do you have some direct coupling going on somewhere?

[dbishopbliss]

OK, regarding grounding the center tap, what would the Voltage between Heater  and cathode be if you were to ground the center tap?  I guess I am unclear as to why your cathode would be more than 100V different than your ground.  Do you have some direct coupling going on somewhere?

My circuit is essentially a Foreplay I circuit, but I didn't realize it until after I assembled my project.  Therefore, its a grounded cathode direct coupled to a cathode follower inside the same tube.  The cathode voltage of the cathode follower is around 150V.  I have raised the potential of the heaters by 70V.  I chose that value because it is close to half the difference between the cathode voltages of the grounded cathode and the cathode follower.  I'm not sure if that is really a good reason to choose that value, but it sounds good.

That said, I bypassed the bottom half of the voltage divider with a 2.2uF cap (DME2W2P2K-F) I had in my drawer last night and the hum went away (at least the really bad hum).  I will try a 0.1uF cap tonight to see if there is a difference.  I have a whole bunch of the 0.1uF caps and only 2 of the 2.2uF.  If those don't work I will probably get a 100V version (DME1W2P2K-F) of the same cap because I'm building this for the Bottleneck Linestage Competition and the 250V cost double.

[JC]

If it doesn't mess with your parts total too much, you may also want to try just clipping a .1 across the 2.2 to start with.

I'm not aware of any "preferred" heater bias Voltage.  You needed to get your Heater:Cathode Voltage to a safe region for the tube, and you have accomplished that.

[dbishopbliss]

I played with a bunch of different bypass capacitors tonight; 0.1uF polyester film, 2.2uF polyester film, 2.0uF film obbligato, 10uF film obbligato.  I couldn't hear a difference between any of them so I'm going to go with the cheapest 0.1uF polyester film cap I have. 

I have also re-mounted the power transformer on hard rubber gaskets using screws and bolts that go through the wooden chassis.  I was using wood screws before and because of the chassis shape, I could only get two of them into the wood (and one of those wasn't really very tight).  I haven't had a chance to plug things in again because of other changes I'm making, but I assume the mechanical hum will be reduced significantly.  Now that I think about it, the wooden enclosure may have even been acting as a resonator amplifying the vibrations.  I'll let you know how things go tomorrow evening.