Closed loop stability help needed.

[TheGimp]

I'm using Edcor transformers, GXSE10-8-2.5K, GXSE15-8-3.5K, XSE10-8-4K, and GXSE10-8-5K.

The GXSE10-8-5K gives the lowest overall distortion.

The GXSE15-8-3.5K giving the lowest distortion at 20Hz due to it's larger core.

I'll play with bias current and change test frequency for the FFT and see what shows up.

[DavesNotHere]

float you heater supply, take a .1uf cap and go from signal ground to the + of your dc supply. I think the heater is throwing trash into the tube elements. I haven't found a documented name for it but I call this electromagnetic impression interference. To test your tube circuit remove all voltages except the heater. take an oscilloscope and look at the cathodes of all the tubes. Ideally, there should be nothing but a strait line on your scope. If not do the cap trick. This does wonders for finding the hum in a damage/ill configured heater circuit.

[DavesNotHere]

btw computer switch mode PS operate around 19.5Khz

[TheGimp]

I used a resistor divider from B+ to ground to set a 60V reference for biasing the heaters. I also set up a separate divider for the bottom of the Mu stage when I was testing it, and biased it to be 60V above it's cathode (on a different heater transformer).

However, I did not bypass them with caps, which appearantly does help a great deal.

A 1uF in parallel with a 0.033uF cap reduces it considerably.

[DavesNotHere]

what are you using for heaters power supply? ac with a transformer or a dc supply out of a computer tower? if your trying to use a laptop power supply, you have to filter that pulsed dc that comes out of those types of power supplies.
you can troubleshoot if it is the heater circuit by substituting it with a motorcycle battery with its neg lead tied to the B+ reference.

[TheGimp]

I tried 220, 440, 1K, 2K,3K,and 4K as I could see enough harmonics to be useful.

In all cases above 440Hz, the fft looked identical at 65mA with all harmonics shifted. If I decreased or increase bias current , third harmonic decreases at the expense of 5th harmonic for a bit, then turned up and both third and 5th increase along with all other harmonics.

at 220 and 440Hz, third is very dependent on bias current, but 5th is not effected near as much. As bias current decreases, third harmonic decreases.

So, I am guessing that at low frequencies (below 1KHz) harmonic distortion is more of a function of magnetic flux, and at frequencies (1KHz and above) it becomes more dependent on coupling capacitance/Leakage inductance.

[TheGimp]

DNH, I am not using the computer to provide power to anything. It is there to run the audiotester program for FFT analysis.

The amp is powered by two transformers for heaters, and one for B+/B-. The power is CLC filtered and then regulated to +/- 250V.

Was "Dave's not here" on the Big Bamboo album? Or "Up in smoke"? Cheech and Chong were great.

[DavesNotHere]

ok, I can picture the heater circuit now that you should have. two 1W 120 ohm resistors across the heater winding. this will make a virtual ground point for your heater winding. at that center tap, a wire is placed to the voltage divider network. two .1 uf caps, one side of the cap is tied on each end of the heater windings, the other side star ground. the B+ and B- has a .1 uf cap tied to them and the star ground. now any noise that is happening on your powers should be gone. also, are you using a capacitive probe? I suspect a possible ground loop with your diagnostic equipment but since the unit is malfunctioning, I'm not going that direction. A .1 cap is a good cap to use in between signal injection diagnostic devices and .01 in rf circuits. O-scopes can couple AC by selecting a switch.
I got that nick name in the music world, along with: "Little Davey","Davey", "Drtechno", "Dr.T", and "Mr. Wizzard "

[DavesNotHere]

oh yea the metal musicians call me "the mad scientist" lol