Reducing filament hum in a Tubelab SE

[boywonder]

I posted this question in an existing/old thread over in the tubes forum, but it appears that I'll get more action over here.............

When playing the Tubelab SE on my Half-Changs (95db), I have a little hum that I would like to reduce. It's not bad at all, but if I could knock it down a bit that would be great.

So.......I'm running 300B's, and an Edcor XPWR-131 power transformer.

C1, C3 and the other stuff around the filament regulator is per the schematic. I'm measuring 1.25V PP at C1, and about 28mv PP at C3, and 5.93V dc at C1 Are these reasonable values?

Per Georges calcs from another related thread, it appears that I have less than .5V of headroom for the regulator. I'm measuring 4.9V at the filament tube socket.

5.93-1.25/2=5.3v.

I've paralleled 34000 uf with C1, lowering the Vpp to about 540mv. This, at least on paper, is enough headroom for the regulator, but the ripple at C3 is still about 30mv PP with 15000+34000 uf capacitance at C1.

I've also experimented with a separate (6A current rating) filament transformer that increases the dc volts at C1 to about 6.3V, which should be plenty of headroom for the regulator. This had no effect on the 28-30mv ripple at C3. Adding another 10000 uf to C3 also had very little effect on the ripple at C3...

Any ideas?

[Evenharmonics]

Have you measured AC at the speaker terminal? My TSE is 1 mV and no audible hum from my 97db speaker. I use choke in place of R4.

[boywonder]

Originally Posted by Evenharmonics

Have you measured AC at the speaker terminal? My TSE is 1 mV and no audible hum from my 97db speaker. I use choke in place of R4.

I'm using a 5H choke in place of R4 and I have 15mv PP at the speakers.

[gootee]

Do you have some photos that show your layout and wiring? Maybe you left some "loop area" in some wire pairs (instead of twisting or shielding them), which might be radiating and/or picking up AC like antennas. Or maybe an input ground reference point shares some portion of a ground return conductor with some AC.

[boywonder]

Originally Posted by gootee

Do you have some photos that show your layout and wiring? Maybe you left some "loop area" in some wire pairs (instead of twisting or shielding them), which might be radiating and/or picking up AC like antennas. Or maybe an input ground reference point shares some portion of a ground return conductor with some AC.

Here it is. Power transformer is on a separate chassis with an umbilical. RCA signal cables are small coax........I suppose that I could twist the heater wires where they enter the chassis over to the PCB.

[Evenharmonics]

Originally Posted by boywonder

I have 15mv PP at the speakers.

That's very high. Something doesn't seem right.

1. Are you using volume pot at the input and how far is it turned up (or down)?
2. What value are you using for R31 and 32? This changed the hum level for mine. Tubelab said something about oscillation when lower value was used.

[boywonder]

Originally Posted by Evenharmonics

That's very high. Something doesn't seem right.

1. Are you using volume pot at the input and how far is it turned up (or down)?
2. What value are you using for R31 and 32? This changed the hum level for mine. Tubelab said something about oscillation when lower value was used.

No volume pot, power amp only.

I'm using 36K R's for R14 & R25 (Mosfet sources) since my B+ is 350-400 per George's notes.

[rknize]

Have you tried shorting the inputs to ground?

[boywonder]

Originally Posted by rknize

Have you tried shorting the inputs to ground?

The 15ma is with the inputs shorted to ground. Doesn't change much with my preamp plugged in either.

15ma PP/(2xsqrt2)=5.3 ma RMS

[rknize]

Do you have 100x probes so that you can look at C5 and C7?