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Weird ground loop problem

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dammit I'm stuck with this. It's probably simple, and solution is all over the web but I'm a moron and can't find it . Power amp is quiet without source , preamp connected .With one channel of a preamp connected it's still quiet but when I plug the second channel it starts to hum . Cheater plugs on pre , amp and both doesn't help . Changing preamp for TVC passive doesn't help either -same problem . What a heck...:eek:
 
As this happens with two different preamps, it appears to be a problem BETWEEN the ground connections of the amp inputs. Do you have a schematic of the amp? A physical wiring diagram, especially of the input stage and the grounding "scheme" may help also.

These ground/hum problems are only "simple" after they've been solved.
 
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"With one channel of a preamp connected it's still quiet but when I plug the second channel it starts to hum .."

Does this happen regardless of which input is the 2nd channel? IOW, is it always the same input that starts the hum, or is either as long as it is the 2nd?
 
It's either as long as it's second . Input RCA's are isolated from the chassis and microphone shielded cable goes to input tube (both ends of shields are insulated , one lead carries signal the other ground) It's a pcb board so it's hard to change ground arrangement but PSU ground connects trough the screw to chassis near Power transformer and "signal" ground near output transformers and common speaker terminals.
 
dammit I'm stuck with this. It's probably simple, and solution is all over the web but I'm a moron and can't find it . Power amp is quiet without source , preamp connected .With one channel of a preamp connected it's still quiet but when I plug the second channel it starts to hum . Cheater plugs on pre , amp and both doesn't help . Changing preamp for TVC passive doesn't help either -same problem . What a heck...:eek:

repeat your experiment but connect the amp as normal with a normal ground if it uses one.......put a cheater plug only on preamp.
 
well, lets see. I'm assuming the amp is diy or modded old tube audio.

if this is true, then it sounds like you need to either change your coupling method on the amp from dc coupled to capacitor coupling or modify the DC coupling circuit to handle the input impedance properly. To experiment, put a 1uf cap in the signal + line after the amp's input jack before all input grid resistors. this will AC couple the signal line.

If your into the DC coupling, increase or decrease the grid to ground resistor to match input impedance and/or increase the series grid resistance.

the main problem is most likely a lack of impedance stability which leads to a small amount of DC voltage being exerted onto the source's output. This causes the amplifier's power supply noise to impress on the feedback network of the source's line amp causing a slight positive feedback.

common pitfalls with diy amps: the 100k grid to ground resistors. In actual reality, I don't know why I see 100k to ground in a lot of DIY circuits. this resistor value is calculated, not defined. there are very very few circuits/tube combinations where this actually works. 330K is a better arbitrary valued resistor. Most newer devices don't like the 1M either. Its too high of an input impedance to load the source's output.

And what I mean about "source" is its CD/Computer/Tuner/Phono device.

If you have schematics of the amp, it would be helpful to me and the rest of the guys.
 
it's an old foe of mine Antique Sound USA (Slightly different cosmetic version of old Acoustic Sound Lab PP KT88 amp ) which I revisit from time to time (and bother good people on board ;)because it doesn't sound half bad in triode mode and my girl likes the gold /chrome cheesy look:D . It is nicely dimensional but little wooly on the bass. I converted 12au7 driver section to 6CG7 . I think I will work on different grounding scheme . Originally it has main ground attached to chassis near IEC socket. Psu ground near Power transformer and main filter caps and signal ground near the middle of chassis. The amp was repaired many times by former owners and and few times I had to take off the hood too . Blown screen resistor and blown series reg. circuit . This hum in one channel is not very loud but with 100dB speakers it is noticeable.
I thought of improving PSU section a little but with space restriction inside I'm not sure what could be done really .
TIA , L
 

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with no source hooked up to amp, and cheap speakers as load,

is R27 really 470 ohms?

theoretically it should be 470K.

remove negative feedback network (c3,r14) to troubleshoot. if you have horrendous noise this means the design needs some bugs worked out. yes its true that NFB cancels noise, but it should be used to stabilize gain only.

resistors on top tubes should match identically to the bottom tubes in the channel.

active bias on input tubes is a little touchy and injects noise into the circuit. Battery bias would be optimal too.
 
it's an old foe of mine Antique Sound USA (Slightly different cosmetic version of old Acoustic Sound Lab PP KT88 amp ) which I revisit from time to time (and bother good people on board ;)because it doesn't sound half bad in triode mode and my girl likes the gold /chrome cheesy look:D . It is nicely dimensional but little wooly on the bass. I converted 12au7 driver section to 6CG7 . I think I will work on different grounding scheme . Originally it has main ground attached to chassis near IEC socket. Psu ground near Power transformer and main filter caps and signal ground near the middle of chassis. The amp was repaired many times by former owners and and few times I had to take off the hood too . Blown screen resistor and blown series reg. circuit . This hum in one channel is not very loud but with 100dB speakers it is noticeable.
I thought of improving PSU section a little but with space restriction inside I'm not sure what could be done really .
TIA , L

ha....I have one of those...they are very picky on tube Ip . Mine runs dead quiet using 6l6GC tubes, and the amps starts to protest with hum as I move up to and thru el-34's, kt-90s, 6550's and kt-88's. I have to use low Ip tubes in mine ....tubes with an Ip value in the area of 25 sound quieter than the average power tube which has Ip values in the area of 28 to 32. Since the hum is power supply related...running higher impedeance speakers helps as well.
 
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