Another 5F1-ish build with hum

[TheGimp]

I decided to build a Champ like amp using 6P1P and 6N2P tubes as a small practice amp.

OT is CE P-T31 8W, 5K SE opt with 8 ohm out only.

B+ is 325V.

Hum is induced in first stage as it is dependent on volume setting. (I added a Moonlight single control tone control).

It looks to me like the hum is possibly induced by the heater supply as one side is grounded instead of doing two resistors from each side to ground. This is necessary as the power on indicator lamp I'm using has one side to ground.

Hum isn't terribly bad, but is more than I'd like.

I tried a ground break without help.

Any ideas?

[tubeswell]

Re-wire your lamp and your heaters

[marcoloco961]

It looks like your heater wires are directly under your two shielded cables running parallel and almost touching. I think the first thing I would do is get those heater wires completely away from the shielded lines. you should try and cross all wires at a 90 degree angles.

Something more along this line. Also, what is the twisted white wires coming from the one switch. It looks like one is just floating and attached to nothing?

[TheGimp]

I hate to rip out the heater ground and re-do it along with disconnecting the lamp, but may have to if only to test it.

The two coax wires are near the bottom plate when it is installed. The heater wires are against the top of the chassis so there is 2" of spacing between them. The two coax to the switch are the GNFB that is switch selectable. Disconnecting it has no effect so the proximity to the heater wires has no effect.

I replaced the white wire with a longer one. The twisted pair allow me to bypass (short) the tone control. I disconnected both wires with no effect.

Thanks for the recommendations. I'll keep trying.

[marcoloco961]

I hate to rip out the heater ground and re-do it along with disconnecting the lamp, but may have to if only to test it.

The two coax wires are near the bottom plate when it is installed. The heater wires are against the top of the chassis so there is 2" of spacing between them. The two coax to the switch are the GNFB that is switch selectable. Disconnecting it has no effect so the proximity to the heater wires has no effect.

I replaced the white wire with a longer one. The twisted pair allow me to bypass (short) the tone control. I disconnected both wires with no effect.

Thanks for the recommendations. I'll keep trying.

Two more things to try, I guess. In the pic below I have two things circled. One is a resistor that is 22K on the schematic. Maybe it's just late but it doesn't look like a 22K banding. The other is the ground for the third filter cap, is it grounded below the board?

Or you might just have to re-wire the heater and the lamp as suggested.

[KT66]

Whenever an amp comes across my bench with the one-side-of-the-heaters-to-ground arrangement, I always remove it -it's noisy. I often elevate the heaters also by using the DC from the power tube cathode trick - you can do this even if you have no CT on the heater winding by connecting the 100R resistors for the false center tap to the power tube cathode.

Here is the layout, schematic and a pic of a tweed Princeton clone I did for a friend that shows this setup :

[Structo]

Yes, that heater scheme is obsolete and very noisy.

Run both heater wires above ground.
Run a 100 ohm resistor from each side of the heater wiring to ground for your artificial center tap.

Double check all your color codes and values.

[TheGimp]

I rewired the heater supply to use two 220R resistors to ground, this eliminated most of the hum/buzz.

I'll have to get an isolated indicator lamp assembly.

I increased the first cap value to 47uF and that reduced the remaining hum to an acceptable level.

I recommend anyone building a 5F1 to use 47uF instead of the original 22uF. It makes the amp a lot quieter.

All color codes are fine. The camera does not capture the bands correctly.

[rp]

Just a quick side question on heaters: What is better a ct or floating w/ 100 ohm resistors? Or is it half dozen of one half of an other?

[Structo]

I think the general thought is that two low tolerance 100 ohm resistors will give you better AC balance than a center tap that may or may not be in the center of the winding.

But I think most guys would use the center tap if the PT has one.

It would be interesting to try each on an amp to see which was quieter.

[M Fowler]

I have tried it both ways on a TW Rocket and found no difference in mine.

But I do believe the HI-FI guys like going with no CT and artifical tap using the two 100R resistors to ground.

[TheGimp]

Another option is a hum balance pot. Generally a 250 Ohm pot across the heater supply with the wiper to ground or even better to an elevated supply.

A resistor divider from B+ to ground can be used to generate a 50-75V reference point as the elevated supply. Or as mentioned by KT66 tie it to the elevated cathode bias to the output tubes.

Now I need to build the head cab for the amp.

[dynaman]

Looks like that big old loop of shielded cable could be shortened by a few inches and that would get it away from the AC wires.

I noticed you used two different brands of resistors. Gotta be careful when mixing 4 and 5 band color coded resistors.

[rp]

I increased the first cap value to 47uF and that reduced the remaining hum to an acceptable level.

Isn't that a bad idea for the 5Y3? From what I understand it won't last long. On the other hand a class A amp doesn't sag so you can just plug a SS thingy in it's place if your voltages are still in bounds ie less than 370V.

My champ is dead silent - for anyone else planing on a build go ptp in a 13x5x2.5 alu Bud box, it's such a simple circuit that you might as well. You gotta plan for it - draw it out, and you need NOS ccs that are longer. Mine was a combo so I had to run one piece of shielded. Had it been a head I could've built it with no wires except the heaters and power string which would be maybe 3". Much harder to work on but it's just 3 tubes.

Edit: oops just noticed no tube rectifier. Just 2 tubes - definitely go ptp.

[M Fowler]

I like PTP and if I had used a smaller chassis I could have eliminated some wire in my Champ build.