setup1
Super Member
Thanks!
-
Feeling lucky? Check out the AK 24th Anniversary Giveaway thread and join in the fun!
Komuro 300b DC coupled amplifier.
- Thread starter ianthesecond
- Start date
Thanks!
The power supply capacitors do more than suppress hum. They react constantly to the load on the power supply for demands of current. You want high quality capacitors with low ESR to meet those instantaneous current demands.
ianthesecond
Active Member
Ok I've got the hum cancelling pot ready to go in, im just wondering what i do now with the centre tap of the 5v winding. Do i leave it disconnected and let the pot act as the centre tap or do i connect it to the centre of the hum pot along with the cathode resistor? I dont want to short anything and i don't want to fry the pot ...
ianthesecond
Active Member
Im guessing the centre tap on the 5v winding is now redundant but i'd better check ...
That's correct. Leave the center tap of the 5V filament transformer floating. The center tap of the hum pot goes to the top of the bias resistor (triangle). Clear?
ianthesecond
Active Member
Thanks! I actually drew a load of little sketches and worked it out last night. I tried the hum pot, it hasnt helped. I've taken Paul's other advice and ordered some wima caps for the filter caps - a 30uf in place of the 1uf and a 100uf instead of the 60uf. If it affects the B+ voltage I'll correct it with a 5r4
ianthesecond
Active Member
... And i could actually put my 25H/80ma choke in the power supply because it's not being used in the other position
Yes, I agree that many problems in building your own gear turn out to be relatively simple issues once solved, and in many cases hum can be attributed to bad grounds or ground loops. However, I find that I continue to refine my own builds, sometimes over years, as I better understand better building practices and technical details and find new tubes to try.
So with Ian adding the hum pot, his overall build is more refined and in the end once the hum problem is overcome it will aid in the overall sound. So I would definitely add 30uF of capacitance to the first cap off of the rectifier and that may solve the hum problem. An electrolytic will be fine for that position in parallel with the oil cap, but the capacitors that feeds the output tubes should at least be paralleled with a high quality MKP.
ianthesecond
Active Member
I was planning of putting the 30uf wima mkp cap in parallel with the big 1uf oil cap
ianthesecond
Active Member
And then the 100uf wima cap instead of the 60uf. They're on their way so I'll swap those and report back.
Also I'm pretty sure the ground scheme is ok- it's the same as my other amps, when i lift the ground connection the signal circuitry isn't grounded anywhere else... However the point where ive grounded anything isn't tight against the chassis so i think it might be possibe that some mechanical vibration could be resonating somehow and generating current interfering with the fields from the power transformer. I dunno... Ive still got things to try
Also I'm pretty sure the ground scheme is ok- it's the same as my other amps, when i lift the ground connection the signal circuitry isn't grounded anywhere else... However the point where ive grounded anything isn't tight against the chassis so i think it might be possibe that some mechanical vibration could be resonating somehow and generating current interfering with the fields from the power transformer. I dunno... Ive still got things to try
Generally, one grounds the green wire from the power cable to the chassis using a ring terminal soldered to the wire. The most common ground point is a power transformer nut and bolt right near the power inlet. The ground bus for the rest of the amplifier would then be connected to the same point. Ensure that the RCA input jacks are insulated from the chassis and connected to the "quiet" end of the bus along with the first tube's grid and cathode resistors. The power supply caps along with the transformer secondary ground go to the "noisy" end of the bus.
ianthesecond
Active Member
That's exactly how I've grounded all of my amplifiers. The sensitive stuff upstream of the noisy stuff. I actually lifted the connection and tested it, there's no second connection to the chassis. I'll make sure the contact is clean and not interrupted by a layer of paint....
I think better filtering will sort it
I think better filtering will sort it
ianthesecond
Active Member
Actually, thinking about it, I always let the ground return to the same point as I earth the chassis so there's just one connection. It's never been a problem before but could it be an issue here?
ianthesecond
Active Member
Sorry that's exactly what you described... I think it's 100hz hum, I don't think it's a grounding issue
ianthesecond
Active Member
I'll know later, my new caps have just arrived!
ianthesecond
Active Member
I've put the 30uf cap in parallel with the 1uf. Ive also put a 150uf electrolytic cap in parallel with each of the 80 uf caps in the voltage doubler circuit and these mods have brought the hum down to an acceptable level.
However on an unrelated topic I've checked the voltages in the amp and noticed the 7193 is running at 70v and not 100v like the operating conditions the specified 6j5 should have. I'm going to increase the cathode resistor by the difference between those two valves' internal resistances to decrease plate current
However on an unrelated topic I've checked the voltages in the amp and noticed the 7193 is running at 70v and not 100v like the operating conditions the specified 6j5 should have. I'm going to increase the cathode resistor by the difference between those two valves' internal resistances to decrease plate current
Last edited:
I'm happy that the hum problem is now under control and you can now fine tune the amp to ensure it is operating properly.
One thing I have noticed when following other people's schematics is that they sometimes, or even frequently, don't actually make sense on paper. That is to say that the operating points of tubes don't conform to the actual datasheet. And with this amp we find that is the case. I used the Triode Pentode load line calculator to plot the operating point of the 6J5 to see how close it is to the actual load lines. While it is close, there are some discrepancies. So for a 47kr load resistor and the necessary 100V plate voltage that will properly bias the 6V6, the calculator gives a plate current of 3.5mA and a grid voltage of -3V. These do not agree with Komuro's schematic. He predicts a plate current of 4.78mA and a grid voltage of -2.25.
This calculator suggests a cathode resistor of 855r. 820r and 910r are standard values, so you could start with one of these resistors to determine if this brings the plate voltage up to where it will properly bias the 6V6. Alternatively, you could temporarily install a 1kr pot and adjust it until you get 100V on the plate. Then measure the resistance and use the next closest value resistor.
One thing I have noticed when following other people's schematics is that they sometimes, or even frequently, don't actually make sense on paper. That is to say that the operating points of tubes don't conform to the actual datasheet. And with this amp we find that is the case. I used the Triode Pentode load line calculator to plot the operating point of the 6J5 to see how close it is to the actual load lines. While it is close, there are some discrepancies. So for a 47kr load resistor and the necessary 100V plate voltage that will properly bias the 6V6, the calculator gives a plate current of 3.5mA and a grid voltage of -3V. These do not agree with Komuro's schematic. He predicts a plate current of 4.78mA and a grid voltage of -2.25.
This calculator suggests a cathode resistor of 855r. 820r and 910r are standard values, so you could start with one of these resistors to determine if this brings the plate voltage up to where it will properly bias the 6V6. Alternatively, you could temporarily install a 1kr pot and adjust it until you get 100V on the plate. Then measure the resistance and use the next closest value resistor.
ianthesecond
Active Member
I didnt get a chance to play with the resistor values last night but im definitely getting 70v on the anode of the 7193. Today I collected a 6j5 and tomorrow i will wire that up and see what the voltage is. Then ill start by doubling up the 470ohm cathode resistor to 940 and see what that does.
I also found some 470uf/430v electrolytics today, im going to put then in the voltage doubler circuit and see if they improve things further.
Thanks for all your help, the amp sounds great now, and the subtle low level detail is there. It's been more interesting than the amps which have sounded great from the moment they were finished...
I also found some 470uf/430v electrolytics today, im going to put then in the voltage doubler circuit and see if they improve things further.
Thanks for all your help, the amp sounds great now, and the subtle low level detail is there. It's been more interesting than the amps which have sounded great from the moment they were finished...