mauiboy81 wrote:the 6.3v center tap is connected with the power tube cathodes to the + of the cathode bypass cap. I moved it to ground with the ht center tap last night but I didn't notice a difference so I moved it back to the cathode.
Try a proper elevated supply for the heaters - I never got on with using the cathode bias resistor for heater elevation (YMMV, obviously). You've probably only got ~ 10V there anyway with the EL84s cooking, and the heater supply is ~9V peak, so little real 'elevation'.
(On my Express, I used 150k + 47k in series across B+ with a 10µf cap from the junction to ground. 47k resistor goes to ground. This will give to about 25% of your B+ at the junction to connect to your heater centre tap. )
Also try using 2 x 100R resistors to create an artificial centre tap on the heaters and connecting this to the elevated voltage, rather than the genuine centre tap (may work better, may not).
Where on the b+ rail do you take it from? Can I take it from right after the diodes or does it need to be further down?
What do you mean by 150k+47k in series across b+ with 47k to ground?
Edit: Do you mean from b+ to ground?
I probably want to put it before the standby switch right?
What is max voltage for elevating the heaters? Should i shoot for something like 55v
Will adding a .01uf ceramic cap across the outer filament wires do anything for noise? Maybe across the pilot light would be convenient?
Would snubbing the filament secondary be in addition to elevating them?
Do the elevation first. The filament snubber is more a hi fi thing and for snobs with high efficiency speakers
What do you mean by 150k+47k in series across b+ with 47k to ground?
If you have 300V B+, this is a simple voltage divider to ground putting ~75V at the junction between the two Rs with the 47k connected on the 0V potential side. Reversed you would have ~225V (too much). The cap is there to smooth the ripple for the voltage elevation.[/i]
The CT is more about humming than the hissy noise. The 100/100 R is to provide a balanced reference to ground rather than use the filament CT of PT which might not be as well balanced. I am a total noob compared to most here but now I always use the 100Rs - I buy 20 and measure and match them.
I'm not sure where your hum is coming from though I was just mentioning the heater voltage elevation because you might have some noise from the voltage difference heater/cathode on the CF tube which you said was the problem. I don't think you need to use snubbers either unless you were using maybe DC heaters and were anal about noise.
I'd take it from the 2nd B+ node, after the choke - not the first B+ node where the diodes are, but I don't believe it is especially critical. (It can be after the standby switch).
May not be the cause of your problem, but worth ruling out.
Ok i did it. I took it from the screen node. 150k, 45k, 22uf 100v. So now when I use my meter to test the heaters it reads ~75v dc and I can't get it to read AC. Is that right? With the amp in standby I can get AC.
It didnt really do much but I will leave it because I don't want to undo it.
I am probably one of those anal about noise people you spoke of. I think that I may be chasing something that isn't really a problem.
mauiboy81 wrote:Ok i did it. I took it from the screen node. 150k, 45k, 22uf 100v. So now when I use my meter to test the heaters it reads ~75v dc and I can't get it to read AC. Is that right? With the amp in standby I can get AC.
It didnt really do much but I will leave it because I don't want to undo it.
It sounds like you did right. I guess you must be checking the heater voltage to ground. You should still get AC if you check from one side of the heater supply to the other BUT both connections to you meter will be at about 75V so be careful! With the amp in standby, there is no supply to the elevation circuit.
If it made no difference, then I don't think this is your problem (there are other things to try if it made some difference).
Grounds are Hv ct by itself. Plate and screen caps and power tube cathodes together with now with the elevated heater supply. Another star point with pi filter cap, pi and master volume, preamp filter caps. Tonestack, volume, preamp cathodes to buss grounded by input. Input jacks grounded where they attach to chassis. OT grounded at speaker jack.
mauiboy81 wrote:Grounds are Hv ct by itself. Plate and screen caps and power tube cathodes together with now with the elevated heater supply. Another star point with pi filter cap, pi and master volume, preamp filter caps. Tonestack, volume, preamp cathodes to buss grounded by input. Input jacks grounded where they attach to chassis. OT grounded at speaker jack.
OK, I think I can visualise what you have...
I'm wondering if you may be picking up some buzz from the rectifier. I think you will get more switching noise with the SS rectifier, so you might want to connect the PT CT (!) directly to the first filter cap ground, and to separate this from the rest of the filter cap grounds.
Did you see Merlin's paper on grounding? (pdf file)
It's worth following those principles if you want to minimise noise.
Could still be something else - maybe a recording of the noise and some gut shots would help someone to spot something?
