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VTL hum on both channels

I have a VTL 75/75 which I purchased recently. Both channels have a slight 60Hz hum. I tried the cheater plug, isolating the amp from the rest of my system, and using a RCA shorting plug to see if it was coming from somewhere else but it seems the amp is noisy.
I even connected a Sola power conditioner for a try.
I was able to download a schematic from the internet for a 50/50 which is similar to the 75/75 and got out my scope. I could see a 60Hz signal on the output at the speaker terminals. Checked all the power caps and had hardly any ripple. The only place where I saw the same looking signal was on the bias power supply. The DC voltage is around –50 volts with around 1 volt of ripple. Being new to tubes I do not know if this could be a cause of the problem. The capacitor in the circuit is 47uF at 100 VDC. I was thinking of increasing the value to see if the hum went away.
Any help would be appreciated.
fixer53
3 responses Add your response
herman
2,212 posts
 
That 1V of ripple on the bias seems high. No current is being drawn so that should be pretty smooth. That bias is on the grids of the tubes so it is being amplified and it is no suprise you can hear it.

You may not need to increase it the value of the cap, just replace it. How old is the amp? You might want to think about replacing all of the electrolytics while your at it. If you have room get some of these. http://www.alliedelec.com/catalog/pf.asp?FN=1433.pdf
caoauto's avatar
caoauto
7 posts
 
Hi, since I have the same VTL amps and need to lower the power from 300W/chanel down to about 75W/chanel for second system, can I just pull out the power tubes and leave couple power tubes to match 75w/chanel? does it hurt to the amps or ruin any things? Thanks all.
fixer53 OP
1 posts
 
I put in new caps. I replaced the 47uF with 500uF and added a 250uF right after the rectifier. What I have is a 4 diode rectifier that had no filter cap, then a 90 ohm resister going to the bias pots. Thats where I increased the capacitance. The ripple went from around 1Vp-p to about 20mV p-p. Still hear noise. Can't hear it when music is playing but I know I will obsess over it.
On the schematic for the 50/50 there are caps (100uF @ 100VDC) going across all 10 ohm resisters for bais adjustments. Should I add? Will it help? The worst thing I could do is chase my tail. I do beleive I'm on the right trail because I put one probe right across the speaker terminals and one on the bais supply. They match perfecly.
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