FYI, this is a PCB, not point-to-point wiring. I've been looking at the PCB diagram pretty closely, and it does not appear that I can change what goes to the OPT primaries easily. The pick-off point is a pad where the C15A (currently 75uF) cap positive comes from.
So it seems that if I change the 75uF to a 22uF, then I'm sending a lot more ripple to the OPT primaries. Unless I insert something between the OPT pick-off point on the PCB and the OPT primaries, maybe another cap and resistor?
pcb by Wigwam Jones, on Flickr
In the above PCB diagram, I'm looking at "TO T3 PRI" and "TO T2 PRI".
The power supply network is C15A, R14, C15B, and C15C.
I am also confused on the original schematic. I am not sure if R14 is 5.6 Ohms or 5.6K Ohms. The scan is kind of bad right there. 5.6 Ohms makes a bit more sense to PSU II, but I can't tell for sure. I'll have to measure it when I have a moment.
Any thoughts on how I can reduce ripple before it gets to T2 and T3 primaries, assuming I drop C15A from 75uF to 22uF? I have no problems inserting a choke as recommended where R14 used to be. That would be nice.
Thanks!
So it seems that if I change the 75uF to a 22uF, then I'm sending a lot more ripple to the OPT primaries. Unless I insert something between the OPT pick-off point on the PCB and the OPT primaries, maybe another cap and resistor?
pcb by Wigwam Jones, on FlickrIn the above PCB diagram, I'm looking at "TO T3 PRI" and "TO T2 PRI".
The power supply network is C15A, R14, C15B, and C15C.
I am also confused on the original schematic. I am not sure if R14 is 5.6 Ohms or 5.6K Ohms. The scan is kind of bad right there. 5.6 Ohms makes a bit more sense to PSU II, but I can't tell for sure. I'll have to measure it when I have a moment.
Any thoughts on how I can reduce ripple before it gets to T2 and T3 primaries, assuming I drop C15A from 75uF to 22uF? I have no problems inserting a choke as recommended where R14 used to be. That would be nice.
Thanks!
The TRUE limit for the rectifier is the peak repetitive current rating. Until computer simulation, this was hard to even estimate - now it's a few seconds work. The maximum capacitance value was given for the worst-case design (full 125 mA load, maximum voltage, minimum transformer resistance). Relax any of these and more capacitance can be used without exceeding the peak current limit.
Thanks, Tom. If I'm understanding you correctly, perhaps I'm not going to have a problem with the current 1st capacitor rating? FYI, I had trouble finding a 75uF 450V electrolytic so I ordered some 68uF 450V caps instead. Thought I'd sub one of those in when I replace the cardboard multi-cap, until I got the feedback in this thread about using a much smaller value.
Peak repetive current rating for 5Y3GT is 440 mA. I used the following parameters: AC voltage 250, transformer resistance 200 (adding some for primary), input capacitor 75 uF, 2 Ohms, load 80 mA constant current. PSUD says the peak current is 250 mA (after it stabilizes), so it's within ratings. Increasing voltage to 275 and current to 125 mA (more than the transformer is good for...) results in 350 mA peak, still OK. Decrease the transformer to 50 Ohms, and it's 410 mA, getting close...
Peak current above the rating can result in cathode damage and arcing. Basically, the cathode can only emit SO much for a given cathode design and temperature.
Oh... and Otto Schade was an RCA designer who is well known for his work on rectifiers, but also wrote about the "new" 6L6 as it was introduced. He showed a simple plate-to-grid feedback system that gave the usual benefits of negative feedback and required just one resistor in its simplest form. Google "RH84" for an example.
Peak current above the rating can result in cathode damage and arcing. Basically, the cathode can only emit SO much for a given cathode design and temperature.
Oh... and Otto Schade was an RCA designer who is well known for his work on rectifiers, but also wrote about the "new" 6L6 as it was introduced. He showed a simple plate-to-grid feedback system that gave the usual benefits of negative feedback and required just one resistor in its simplest form. Google "RH84" for an example.
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a fb version better than UL
you can make even mosftet look like triode
OK, thanks for the explanation!
The amp already has standard GNF as far as I can tell from the schematic and PCB layout. From the OPT secondary, R7 & C2 (left) and R8 & C3 (right) seem to provide the GNF. I suppose I could disable that easily enough and then try to implement some other form of feedback such as what you're describing, but to be honest, I think I will leave that for another time.
I sincerely appreciate the advice, though!
I think, res. in PSU is 5.6 kohm, not 5.6 ohm, because this res 5.6 kohm separate PS between output stage and pre stage and must be in kohm and decrease +U for pre stage, depending of current of pre tube. /+U = I x 5.6 kohm/.
IMO, 3D wired is better.....vectors of magnetic fields around elements are in different directions and are almost some self compensated. This is important for buzzzing, hummmming....
Thought I'd let you all know I got this finished. The wiring isn't the neatest, mainly due to the fact that the top still goes up and down, but it works. Hum is nearly non-existent. Amp sounds pretty good so far. I added a 100K stereo volume pot and a set of .01 caps to the input, and I used the inexpensive output transformers I had on hand as discussed.
IMGP0995 by Wigwam Jones, on Flickr
IMGP0991 by Wigwam Jones, on Flickr
IMGP0992 by Wigwam Jones, on Flickr
IMGP0985 by Wigwam Jones, on Flickr
Thanks for all your help and suggestions!
IMGP0995 by Wigwam Jones, on FlickrIMGP0991 by Wigwam Jones, on FlickrIMGP0992 by Wigwam Jones, on FlickrIMGP0985 by Wigwam Jones, on FlickrThanks for all your help and suggestions!
well done Wigwam, how you can reduced hum? I have similar amp but can't figure out. Changed all supply cap and new rectifier tube also pulled 12AX7 out but still hum. Do anyone have any idea? please.
I am far from an expert, but I did the following things:
1) Grounded the earth connection on my 3-prong electrical mains supply to the chassis.
2) Grounded the negative secondaries of the output transformers to the chassis.
3) Grounded the negative inputs of the RCA jacks to the chassis.
4) Used a tinned copper buss wire to make sure all grounded components, including the PC board, were grounded to the chassis.
5) I tightly twisted the 6.3V and 5V leads from the PT to the PCB, to try to cancel out any hum that the AC might cause.
I did not experience any buzz at that point. If I turn the amplifier up to maximum (I added a 100K pot to the RCA input) with no input, I can hear the faintest hum if I put my ear up to the speaker, but this is far less than I am used to experiencing with even good commercial solid-state amplifiers.
I have to believe that good grounding is the basis for reducing hum in this case. I sincerely hope this is helpful to you, and that others chime in if they have superior knowledge (as I said, I am just a beginner).
I can only say that it has been my experience that hum can usually be traced down to a component that is not grounded. Often in my case it has been the output transformer frame itself, or the output transformer secondary negative wire. So now I just ground the heck out of everything.
Best of luck!
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Thank you very much Wigwam. I will try and will report later. I'm a beginner and love to learn about tube stuff.
I'd like advice You to make above schem., instead 50 ohm connect chock 10-15 Hn about 100-150 ma and hummmmm,thanks chock will decrease.
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