Building "the perfect Champ"

[ScarTube]

Hello people,


i´me building the Champ 5C1 schematic. Its my fifth DIY-amp project and I really love it. Everything works and sound above expectations except that one f**** problem every single ended amp builder gets to struggle with. Yes it is.... HUM... I want to get rid of it as much as it is possible.

What I did so far:

1. Connecting the CT from the heater windings (6,3V)
2. Making stargrounds according the article of R.G. Keen

These actions did make the amps a lot more silent

What I want to do:

3. Installing a brute force filter with a choke

I have found a schematic and a lot of information about the 5F1 Champ schematic. I want to build it in by a 5C1 schematic.

This is what I want to build:



According my studie of information on this and other forums it has to work but i´me not shure of it. Can you please check the values of the components and give me some feedback...

I was also thinking to separate the B+ line from the 6V6 and the 6SJ7. Than you would get something like this:



The reason I want to separate those B+ connections is because my logical brain tells me that it would reduce signal contamination from each other. However, this reasoning is not based on a good knowledge of electronics


It would be nice if someone with the right experience and knowledge could help me out with the right schematic and with the right values of the components.


Regards, ScarTube.

(Sorry for my bad englisch, I blame google translate :-) )

[Andy]

if you connect it as shown in the second drawing, that 500-ohm resistor will drop a lot of voltage. if that's what you want, that's fine, but normally, the power amp plate voltage comes from before the choke.

[ScarTube]

New try

if you connect it as shown in the second drawing, that 500-ohm resistor will drop a lot of voltage. if that's what you want, that's fine, but normally, the power amp plate voltage comes from before the choke.

Oops made a mistake by drawing the second schematic

This is the right one... right?



And about that voltage drop. I don´t really now, I just want a good working and good sounding amp. If the voltage drop is necessary the create this. Than the responce is YES... otherwise NO...

[Tom Phillips]

The circuit you show in post #3 will work to reduce the hum significantly compared to the stock Champ power supply circuit.
Connect the OT AFTER the choke.

You did not post the voltage reading. Can you measure it? It is often high enough that you can allow some voltage drop and you may just be able to use a 500 Ohm resistor in place of the choke. It's easy to give it a try. Especially if you do not have the choke yet.

Regards,
Tom

[Enzo]

The 5C1 (with resistor) and the 5E1 (with choke) both have two stages of filtration before the plate node. All later Champs take plate supply off the first node. Thyose earlier formats will have less hum. Later designs were considered "good enough" even though they had increased hum. I'd prefer the 5E1 power supply for your situation


Hum comes from a variety of places, and each source has its own cure. Adding more filter caps will have zero effect on hum from grounding issues, and shielding tubes or grounding the 6vAC will do nothing for insufficiant filter caps. SO it is important to find out WHERE you hum is coming from before trying to cure it.

First, is your hum 60Hz or 120Hz? 120Hz is power supply ripple.

Isolate the problem, does the hum go away if you turn the volume control to zero? Does the hum go away if you remove the 6SJ7?

You mention the 6SJ7. The 5C1 uses grid leak bias for that tube - note the grounded cathode and huge 5 meg grid resistor. That system is going to be noisier that more modern systems. Look at the 5E1, it gets similar gain from the more common 12AX7.

APparently your power transformer has a center tapped 6v winding - good. Grounding the center tap will be better than just grounding one side like your schematic. Now try this: instead of grounding that center tap, connect it instead to the cathode of the 6V6 - pin 8. That will elevate the heaters by 15-20 volts and could reduce any heater radiated hum.

But I must stress that doing heater hum things will have no effect on power supply ripple, amnd adding filter caps will do nothing to help heater hum. Hum is NOT generic.

[Bruce / Mission Amps]

Oops made a mistake by drawing the second schematic

This is the right one... right?

[ATTACH=CONFIG]19875[/ATTACH]

And about that voltage drop. I don´t really now, I just want a good working and good sounding amp. If the voltage drop is necessary the create this. Than the responce is YES... otherwise NO...

Yes, take the B+ to the output transformer after the choke.

This is very very similar the B+ rail I used in the 5F2H harmonica amp, a few years ago. And the brute force PI filter idea works perfectly.

Only I used a KT66 power tube.
Mine has a big 5Hy@200ma choke and the first three filter caps are either 16uF or 22uF.
With your amp, what you will want to do is experiment with the 500 ohm resistor, so that when the power tube is fully drawing all it's current from the OT, the voltage drop across the OT is slightly less then the voltage drop across the 500 Ohm resistor which feeds the screen node and the preamp node.
That was wordy....
In other words, tweak that resistor value so that the measured screen voltage is very slightly less then the measured plate voltage of the power tube. It really is OK if the screen voltage is a little higher then the power tube plate, but I always strive for it to be slightly less... just a few volts.

[ScarTube]

The 5C1 (with resistor) and the 5E1 (with choke) both have two stages of filtration before the plate node. All later Champs take plate supply off the first node. Thyose earlier formats will have less hum. Later designs were considered "good enough" even though they had increased hum. I'd prefer the 5E1 power supply for your situation


Hum comes from a variety of places, and each source has its own cure. Adding more filter caps will have zero effect on hum from grounding issues, and shielding tubes or grounding the 6vAC will do nothing for insufficiant filter caps. SO it is important to find out WHERE you hum is coming from before trying to cure it.

First, is your hum 60Hz or 120Hz? 120Hz is power supply ripple.

Isolate the problem, does the hum go away if you turn the volume control to zero? Does the hum go away if you remove the 6SJ7?

You mention the 6SJ7. The 5C1 uses grid leak bias for that tube - note the grounded cathode and huge 5 meg grid resistor. That system is going to be noisier that more modern systems. Look at the 5E1, it gets similar gain from the more common 12AX7.

APparently your power transformer has a center tapped 6v winding - good. Grounding the center tap will be better than just grounding one side like your schematic. Now try this: instead of grounding that center tap, connect it instead to the cathode of the 6V6 - pin 8. That will elevate the heaters by 15-20 volts and could reduce any heater radiated hum.

But I must stress that doing heater hum things will have no effect on power supply ripple, amnd adding filter caps will do nothing to help heater hum. Hum is NOT generic.

Enzo, first thanks for your reply.

The reason you would choose for the 5E1 schematic is because you assumes that the installation of a brute force filter has too much influence on the uniqueness sound and rapid response which the 5E1 and 5C1 are known for. I am aware that I want to install the filter that may affect the sound. Therefore I will first do the less radically test using the choke as drawn in the 5E1 schematic.

The hum in my projects comes partially from the heater windings and partly from the PT. By connecting the CT of the 6,3AC to pin 8 of the 6V6 I get indeed the 60Hz hum off. This really gives a good improvement. The 120Hz hum that remains is louder when I turn the volume further open.

In the future I´me mabe going to build the 5E1. Butt now I first want to go very far in building the 5C1 schematic. My goal is to do this the best I can. A wise man once told me: "build one amp at a time". However I write down you´re information for future builds.

Can you explane what it means if:

1. the hum go away if you turn the volume control to zero

2. the hum go away if you remove the 6SJ7 = PT hum (120Hz), right?

[printer2]

I did a SE Champ type amp and found that increasing the capacitance on the screen gave me the most drop in hum as compared to the other filter locations. My two cents.

[Chuck H]

Is it possible there is induced hum due to transformer proximity??? I haven't noticed objectionable 120Hz in Champ-ish builds that use increased filtering. If you REALLY want 120Hz tanked you could abandon the choke idea, go back to the resistor type circuit and use a Pi filter instead of the choke. Then you could offer the rectifier tube a nice safe 20uf, 22uf cap or some such small-ish value, then a 100R resistor and then a 100uf cap as the main B+ filter to the OT. 120Hz will magically dissapear from the HV rail. Subsequent filters would be 20uf or 22uf.

[ScarTube]

To Bruce/mission amps.

First also thanks for your reply Bruce.

I will experiment in the following week. First i´me gonna do the easy tweek out of the 5E1 schematic like Enzo said. Than I will build the schematic with the brute force filter.

You said you used 16uF or 22uF. So you didn´t used the 40uF before the choke. I have read somewhere that the 40uF is the biggest vallue the 5Y3GT can have. If I lower the 40uF also to 16 or 22 it will probably give a less deformation of the original sound. Otherwise it will be less effective to flatten the DC ripple from the power reciever. Am I right there?

I have ordered a 4H @ 90 mA and a 4H @ 50mA choke by Tubedepot. Both chokes would do the job. Would I get a better PT-hum-reduce result if I use the 90mA choke. Or doesn´t the amperage capacity have influence on the filter result?

"In other words, tweak that resistor value so that the measured screen voltage is very slightly less then the measured plate voltage of the power tube. It really is OK if the screen voltage is a little higher then the power tube plate, but I always strive for it to be slightly less... just a few volts."
Ok, I´lle have to adapt the schematic first so I can do some measurements. Could I use a pot to find it out? Which are the ultimate resistance values that I must keep in mind. Can I get a short circuit if I will reduce the value to 100 or 200 Ohm? And should I use a 2 or 5W resistor if I have determine the right value and change the pot by a fixed type?

[Bruce / Mission Amps]

I am not a magnetics expert.... but I guess what happens with too much current in the choke is that the core "saturates" and the inductance drops.... that is why you would want to use a choke that is rated higher then the max amount of current you will be drawing from the PI filter.
If it saturates I imagine that the DC resistance of the wire in the choke will just cause the two capacitors of the PI filter to "get closer" together, but still be separated by the DC resistance.

Speaking of capacitors, with a rectifier tube who's ratings are capacitance limited, you should probably use the smaller filter cap value FIRST in the PI circuit. The rectifier tube will be held away from the second filter cap by the DC resistance of the choke any how.... look at the tube's spec sheet and sometimes it will tell you what the min resistance the tube can take.
So, with a 5Y3, I'd use a 16uF-22uF cap followed by a minimum of a 3Hy-5Hy 90ma choke, and then you can use pretty much use what ever high voltage filter cap you want you want.... 16uF-22uF-33uF-47uF. There will be VERY very little AC ripple at that point!!
With respect to the capacitor limit set by the rectifier tube's spec's, I don't think it really much matter anymore after the choke.
The first resistor after the end of the choke doesn't have to be too big... but don't use a potentiometer for fine tuning.
It isn't rocket size and you'd need a pot with a minimum of a 1 to 2 watt rating anyhow.
I've used many 2 watt 390 to 560 resistors in lots of setups like this.

In most amps, this resistor is limited to a smaller amount of current compared to plate node.
As a wild haired guess... using 30ma of screen and preamp tube current flowing through the node....
.030a^2 *470 ohms = +.4 watts... less then a half watt...so I'd use a 2 watt resistor.

And, for good measure, you can always put another 270-470 ohm 1-2 watt resistor on the actual tube socket from lug 6 to 4 and feed the screen voltage into lug 6.

[ScarTube]

Thanks folks,

I will start building tomorrow and try out differnt setups. I will try to capture the results on video and will post it in a you-tube video. Release-date (I hope before ) 14/09/2012

[imaradiostar]

Just to throw a wrench in the works- how about a mosfet or bipolar based capacitor multiplier? It could achieve much lower ripple levels with minimal voltage drop. it could be built on a small eyelet board and bolt to the chassis discretely. Most times people have excess voltage on a champ. With this idea you could throw a little voltage away but make a really quiet plate supply.

Jamie

[Steve Conner]

If turning the volume control down makes the hum go away, then it's certainly not coming from the power tube's plate or screen supplies. So, it seems to me there is no point in smoothing them any more, you should look elsewhere for further hum reduction.

In my last single-ended amp, I ended up with a pi filter on the plate supply, plus a capacitance multiplier on the screen supply. I wanted it super-quiet because it was to be used as a driver for a larger power amp.

[Chuck H]

Good point on the volume control setting.

Someone did mention just using a Pi filter... As long as the rectifier is happy with the uf it sees you can bump the main filter for more uf's. More uf's = less ripple.

plus a capacitance multiplier on the screen supply.

I'm interested.