hello all, im wondering if one of you with more experience might be able to help me out.
I built the optimised version of Morgan Jones Mini Tube Headphone Amplifier: http://headwize2.powerpill.org/projects/showproj.php?file=cmoy5_prj.htm
(figure 10)
But i am having a problem with it.
The DC offset of the output likes to jump around alot. it looks like someone is playing around with the verttical pos. on the scope.
I made the following changes to the amp, so i could use parts i had.
b+ = 226 vdc
the .47uf cap was changed to 2x 1uf in series (for .5uf)
the output cap was increased from 470uf to 680uf
carbon resistors
The tubes are 6bk7b which i was told was a sub for the 6dj8
The amp sounds very good, i only noticed the dc offset problem when i looked at my woofers and saw them pulsate in and out slowly (roughly once or twice a second) when music was playing, and when it was silent.
I am using this amp as a preamp and i dont wanna fry anything in my reciever. (I added 2 rca jacks connected switch poles of the headphone jack. )
I do notice one other thing. The point where you are supposed to measure 111v reads 172v and 182v for the l and R channels.
Thanks Tom
I built the optimised version of Morgan Jones Mini Tube Headphone Amplifier: http://headwize2.powerpill.org/projects/showproj.php?file=cmoy5_prj.htm
(figure 10)
But i am having a problem with it.
The DC offset of the output likes to jump around alot. it looks like someone is playing around with the verttical pos. on the scope.
I made the following changes to the amp, so i could use parts i had.
b+ = 226 vdc
the .47uf cap was changed to 2x 1uf in series (for .5uf)
the output cap was increased from 470uf to 680uf
carbon resistors
The tubes are 6bk7b which i was told was a sub for the 6dj8
The amp sounds very good, i only noticed the dc offset problem when i looked at my woofers and saw them pulsate in and out slowly (roughly once or twice a second) when music was playing, and when it was silent.
I am using this amp as a preamp and i dont wanna fry anything in my reciever. (I added 2 rca jacks connected switch poles of the headphone jack. )
I do notice one other thing. The point where you are supposed to measure 111v reads 172v and 182v for the l and R channels.
Thanks Tom
ok, the only caps lower than the 680uf caps i have are 1uf, so i tried them. The dc ofset problem is somewhat solved there is a small dc offset, still jumps, but only a little. The bass is a little weak, so i think im gonna have to buy bigger caps.
Could the dc offset be caused by not having the 2 triodes in the output stage perfectly matched?
Could the dc offset be caused by not having the 2 triodes in the output stage perfectly matched?
Hi,
No, the D.C. is stopped by the coupling cap.
Don't make the value of that cap too big or your woofers will die on you.
Do you have DVM so you can measure the input impedance of the receiver?
Is it a tube unit?
Or, if you don't, do you have a manual for it?
I really don't see how you can have DC on the output, this looks more like AC instability from the PS or a faulty tube.
Cheers,
No, the D.C. is stopped by the coupling cap.
Don't make the value of that cap too big or your woofers will die on you.
Do you have DVM so you can measure the input impedance of the receiver?
Is it a tube unit?
Or, if you don't, do you have a manual for it?
I really don't see how you can have DC on the output, this looks more like AC instability from the PS or a faulty tube.
Cheers,
fdegrove said:Hi,
No, the D.C. is stopped by the coupling cap.
Don't make the value of that cap too big or your woofers will die on you.
Do you have DVM so you can measure the input impedance of the receiver?
Is it a tube unit?
Or, if you don't, do you have a manual for it?
I really don't see how you can have DC on the output, this looks more like AC instability from the PS or a faulty tube.
Cheers,
To measure input impedance I just connect the dvm to the audio input and set it to resistance measurement?
If so 52kR.
Its a sony, i have a 120w p/p el509 tube amp in the works
Tube_Dude said:
ok will try it
Grid stoppers
(Hi, I'm new here. Nice to meet you all. )
You could try changing the grid stopper R at V1 from 100R to 1k, and also put another 1k at the input grid of V2 (yes, even though it is DC coupled). Putting these 'grid stoppers' directly at the tube socket eyelets tend to kill oscillations.
I have built a couple of incarnations of this amp, and it does tend to oscillate easily. I think it has to do with using these particular tubes (6922 family, frame grid, great for RF) and how the overall wiring and grounding is done - the shorter the better.
The powersupply could also need some work, come to think of it, some amps motorboat if you run the pre and power stages from the same HT source. That would cause DC-like slow oscillations like what you describe. The remedy: Try decoupling B+ with larger R and C's between the stages.
Good luck!
Btw, can circuit simulations (which I know next to nothing about) take things like motorboating into account or is it just trial and error depending on the chassis topology?
(Hi, I'm new here. Nice to meet you all.
)You could try changing the grid stopper R at V1 from 100R to 1k, and also put another 1k at the input grid of V2 (yes, even though it is DC coupled). Putting these 'grid stoppers' directly at the tube socket eyelets tend to kill oscillations.
I have built a couple of incarnations of this amp, and it does tend to oscillate easily. I think it has to do with using these particular tubes (6922 family, frame grid, great for RF) and how the overall wiring and grounding is done - the shorter the better.
The powersupply could also need some work, come to think of it, some amps motorboat if you run the pre and power stages from the same HT source. That would cause DC-like slow oscillations like what you describe. The remedy: Try decoupling B+ with larger R and C's between the stages.
Good luck!
Btw, can circuit simulations (which I know next to nothing about) take things like motorboating into account or is it just trial and error depending on the chassis topology?
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