The two channel amp 5C2+6SJ7 optional grid leak/cathode biased circuit

Jerry garrcia

Tele-Meister
Joined
Nov 14, 2021
Posts
353
Age
48
Location
Sweden
Or 6SL7 with separate cathodes?
That’s the easy way 😀. The thing is that I’ve got a nice stash of octal tubes but no 6SL7’s. Might use them in another build. They do have the same amplification factor as the 6SC7 but some how not as fun since they are much easier to find and feels like a more “dull” tube🤔. Can’t explain why. Just a feeling. But the 6J7! That’s a tube to be used in a build.
It would have been fun to have the possibility to use the 6SC7’s other half for a extra gain stage but that doesn’t seem possible with my lack of experience or the laws of physics. Why a common cathode?
In that case the 6SL7 might be superior. So many tubes, to much daytime work and shores and looking forward to retirement, in about 17 years. We’ll see if there is any country to retire in after this NATO application. Not much choice now due to the “strategically” placed county I live in with the far from friendly neighbour country.
Might have to go for a hunt for more tubes. One could never have enough of them.

Also interested, a part from help with this circuit design of this build, to get a easy build cabinet design. Doesn’t have a lot of advanced wood tools and planning to cover it all with tolex. Shouldn’t it be possible to use wood screws and sink them in to the wood and use some filling material to smooth things out? Any help with that would really help as well.

For this build I’ll stick with the 6Sc7 as in the 5C2 circuit and a 6SJ7 with switchable grid leak and cathode bias and separate tone controls for each preamp tube.
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
8,417
Location
Canada
I was planning of attach the 68K grid leak/grid stopping resistor directly on pin 4 on the 6SC7 socket and the 0.047uF + 68K on pin 4 on the 6SJ7 socket. Read that it might decrease hum (and don't have much space on the board). The original GA-50 schematic has a 47K grid leak resistor. When reading about it, it seems like it doesn't matter?

I was thinking about that. The original schematic does state a 100K in that position. But I have also read that there are some concerns with the amp producing a "dull" sound. Also I only have 100K 1W resistors and they are large and ugly. Have some nice looking 470K and 1M resistors. Should I change it?

I did also read some stuff that people have concerns with the plate current and its effect on the primary winding of the impedance of the OT in that circuit. Don't really understand what they mean with that.

A 2.2K it will be.

Hey, I just copied what is already produced. As you know I don't really care if it is a true clone/exact copy. I just want to have a nice sounding amp. If anybody has a better idea I'm all ears. I might produce this and use a "better" triode design for the next build or change the components and design after a while? Time and money is all I got, not :)

Thank you! Strange that they used this design if this problem might occur. I just ordered 10, 16 and 20V capacitors with 220 and 470uF on my phone during a board meeting. Quite similar in size. Will a 470uF be better and will a 10V work?

Thanks for all the inputs! Thinking of putting this thing in an old tube radio.
I just was wondering what you will have for a resistor from the grid to ground. I think you had 1M on the other amp.

You could put the nice looking ones in, I am sure the amp will like you for it.

Not really sure how the triode operation will effect the OT, may need some more reading.

2.2k is a classic.

No, the circuit will work. It is not a configuration I have used yet, Marshall has put it in many amps though.

The grid probably will be biased anywhere from around 1-2V so it will be plenty. I have a few tube radios that I want to use for an amp. The last one I picked up for $10, the thing looks like new and the guy had it plugged in and playing while I was there. I don't have the heart to molest it so it is just going to sit on the shelf for now.
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
8,417
Location
Canada
That’s the easy way 😀. The thing is that I’ve got a nice stash of octal tubes but no 6SL7’s. Might use them in another build. They do have the same amplification factor as the 6SC7 but some how not as fun since they are much easier to find and feels like a more “dull” tube🤔. Can’t explain why. Just a feeling. But the 6J7! That’s a tube to be used in a build.
It would have been fun to have the possibility to use the 6SC7’s other half for a extra gain stage but that doesn’t seem possible with my lack of experience or the laws of physics. Why a common cathode?
In that case the 6SL7 might be superior. So many tubes, to much daytime work and shores and looking forward to retirement, in about 17 years. We’ll see if there is any country to retire in after this NATO application. Not much choice now due to the “strategically” placed county I live in with the far from friendly neighbour country.
Might have to go for a hunt for more tubes. One could never have enough of them.

Also interested, a part from help with this circuit design of this build, to get a easy build cabinet design. Doesn’t have a lot of advanced wood tools and planning to cover it all with tolex. Shouldn’t it be possible to use wood screws and sink them in to the wood and use some filling material to smooth things out? Any help with that would really help as well.

For this build I’ll stick with the 6Sc7 as in the 5C2 circuit and a 6SJ7 with switchable grid leak and cathode bias and separate tone controls for each preamp tube.
Right, forgot to congratulate you at becoming a NATOite, (I had a better workd made up but can't remember it now. The little big guy said that it won't make too much difference, probably bit off more than he can chew as it is. (that is as far as I am going in that direction). Screws won't give you away if you cover them up. We'll see how the amp fairs. We have a long history of tweaking things until they are right around here.
 

Jerry garrcia

Tele-Meister
Joined
Nov 14, 2021
Posts
353
Age
48
Location
Sweden
I just was wondering what you will have for a resistor from the grid to ground. I think you had 1M on the other amp.
Is it this you mean?
CF9337D8-4DAA-43F0-8517-9D57B5955A68.jpeg

Copied this from the GA-50 circuit that uses the 100K. Some circuits use 1M like this that someone produced.
bias switch samlad.png

Am I totally wrong?
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
8,417
Location
Canada
The top channel does not have a resistor in that place. The grid is floating and there is no bias voltage developed due to the cathode resistor. Since there is no negative voltage on the grid to control the current through the triode it is acting as a diode rectifier, just a cathode and a plate. So it is passing the full current that it can. The triode will not burn up due to the 270k resistor in the plate that limits how much current goes through the circuit but it is not a good way of doing things.
 

Jerry garrcia

Tele-Meister
Joined
Nov 14, 2021
Posts
353
Age
48
Location
Sweden
The top channel does not have a resistor in that place.
Aha, I hear you. Thought that Leo would have thought about that. Just copied the design from the original schematic.
1652798327462.png

The 5F2a has that 1M resistor. Was that a development of the circuit? I don't know. Maybe it was.
1652798474002.png


1) So should I add a 1M resistor to ground on the jack in your opinion? If I do that will I need to change the plate resistors? Quite complicated in this over worked brain of mine.
2) Should I increase the 100K grid to ground in the cathode biased pentode channel do you think? If so is there a theoretical benefit between a 270K, 470K or 1M resistor?

Thank you for that finding. Easier to fix it now than later. I learn as I move on. A couple of month ago i didn't even know what a cathode was.
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
8,417
Location
Canada
The difference is that the Fender has two inputs. The second 75k resistor acts as the grid leak resistor. With what you have it relies on the guitar's volume control to provide the bias voltage. Pull the cord out of the guitar and there is no path to ground. Not really a good choice to save a resistor in the design, subsequent designs have a 1M resistor in place to give a path. Yes it would be a good idea to put in a 1M.
 

Jerry garrcia

Tele-Meister
Joined
Nov 14, 2021
Posts
353
Age
48
Location
Sweden
Finally. Got a couple of hours and soldered it all together. Checked NOS tubes.
Started up with variac and no smoke or accidents.
PT CAX290 and OT 1760C. Using a LED as pilot light on the 5 VAC secondaries with a rectifier diode and a 680 ohm on one leg.
Wired as 240 input voltage and using the 275 VAC windings. Filter caps 47, 22 and 22 uF.
1) The main problem are the B+.
470R before B+2 node and a 22K before B+3 node. Checked the resistors before I started up the amp.
Tested with tubes in.
B+1 = 361 VDC
B+2 = 357 VDC
B+3 = 303 VDC
Up the resistors? Both or just one? Suggestions to what values?

2) wired two output jacks. One 4 and one 8 ohms. Since only one is plugged in the other is empty. Is that ok? The reason I asked is that you shouldn’t run an amp without the output jack plugged in and now only one is plugged in. Newbie.

Didn’t play but really quiet.
F40F7AC9-8D9C-47FD-A9C2-F95227EA8B7B.jpeg
D1029C79-36E2-4EC8-89A6-45AE9DEFD85C.jpeg
3D9F339A-2135-4513-8B07-35500ED1CA8B.jpeg
BAF6E093-A913-4A2B-9289-7A3C4DE39B9B.jpeg
F48F0FB2-AA04-4ABB-BEA5-49DFF6EDC405.jpeg
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
8,417
Location
Canada
Finally. Got a couple of hours and soldered it all together. Checked NOS tubes.
Started up with variac and no smoke or accidents.
PT CAX290 and OT 1760C. Using a LED as pilot light on the 5 VAC secondaries with a rectifier diode and a 680 ohm on one leg.
Wired as 240 input voltage and using the 275 VAC windings. Filter caps 47, 22 and 22 uF.
1) The main problem are the B+.
470R before B+2 node and a 22K before B+3 node. Checked the resistors before I started up the amp.
Tested with tubes in.
B+1 = 361 VDC
B+2 = 357 VDC
B+3 = 303 VDC
Up the resistors? Both or just one? Suggestions to what values?

2) wired two output jacks. One 4 and one 8 ohms. Since only one is plugged in the other is empty. Is that ok? The reason I asked is that you shouldn’t run an amp without the output jack plugged in and now only one is plugged in. Newbie.

Didn’t play but really quiet.
OK, it is time to break out Ohm's Law. Let us start with the 470R resistor. You have a voltage drop of 4V. V = I R. Rearrange the terms and we get V / R = I, to get the current used in the preamp. 4 / 470 = 0.0085 A, or 8.5 mA. You have to do some reasoning now. The preamp tubes have a pair of 270k resistors for the triode and the pentode has a 100k. there is 300V feeding the trio. Say the tubes need 50V across them (pretty conservative) so the plate resistors have 250V across them (you could measure what the voltage is across each resistor and do the math with the real numbers but this is a back of the napkin exercise). You could do some math to get the parallel resistance of the three ( 1 / R-total = 1 / 270k + 1 / 270k + 1 / 100k) or be lazy and pull up a online parallel resistance calculator to do it for you.


270 - 270 - 100

The calculator says 57, since we did not put in the thousand zeros we just tack on a k to the number, 57k. Does it seem reasonable? Two 270k's in parallel is a little above 100k, if we have two 100k's in parallel we would have 50k. So we are on the right track. Now to use Ohm's law. V = I R, or V / R = I. 250V / 57,000 = 0.0043A or 4.3 mA. A little on the high side as we probably estimated high on the voltage but it gives us a number to work with.

The previous calculation with the 470R resistor we calculated 8.5 mA. So since there are two ways for the current to go, to the 6V6 screen and to the preamp section, that means the screens are getting 8.5mA - 4.3mA = 4 mA.Let us look up the 6v6 datasheet to see what the tube might want.


Class A amplifier at 315V, zero signal screen current, 2.2mA. I would say that is a touch on the low side and 3mA might be more realistic, not too far off from our 4 mA.

That was all fun, but what was the goal? To see if things might be operating properly and if the current we calculated across the 470R resistor is accurate. So now that we think it is, how do we drop more voltage so the preamp section is closer to 250V? We need to lose 50V. Just for giggles let us say we have 8.5 ma with the 50V dropped as we want. So with Ohm's Law we can calculate the resistance we have to add. So 50V / 8.5mA = 5,880 ohms.


So if we add 5.9k in between the first filter cap and the preamp filter cap we should be in the ballpark. It is not exact due to the errors we have in our assumptions and the first two sections have some AC ripple but this is going with the information we have. So how do we want to divide up the resistance? If we want a little saggy 6V6 then some resistance before the screens will add some. If we want a stiffer sound we can just add the resistance before the preamp filter capacitor. The added benefit with before the screens is that there is a little less hum from the 6V6. I would look in my spare parts and see if I have a 1-1,5k resistor to put before the screen and maybe a 27k for the preamp. If I had other values I would see what they add up with and calculate what I think might work and then try with something that seems reasonable. After firing up the amp again I would measure voltages and see where things lie. If I am happy, well the world is happy. If not then use the measurements and see what math will give me what I want.

Electronics is all about the math. If you do not know Ohm's law, how voltages add up in a loop to equal your supply voltage, that currents are equal in a loop or that when current splits in parallel branches that they have to add up to your supply value, it gets a little tough to know where to go.
 

Jerry garrcia

Tele-Meister
Joined
Nov 14, 2021
Posts
353
Age
48
Location
Sweden
I would look in my spare parts and see if I have a 1-1,5k resistor to put before the screen and maybe a 27k for the preamp.
Thanks. Woke up at 04 am and started to flip around the resistors. Found a 1.2K but don't have any 27K. Only 68K, 22K, 47K and smaller/larger values 1-3 W resistors. I hate to have to wait.....
 

Jerry garrcia

Tele-Meister
Joined
Nov 14, 2021
Posts
353
Age
48
Location
Sweden
Or I can add a 47K and a 68K in parallell to get 27.8K and maybe use a 1K resistor between first an second filter cap?
 

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
8,417
Location
Canada
Or I can add a 47K and a 68K in parallell to get 27.8K and maybe use a 1K resistor between first an second filter cap?
Just tack them in there with a little solder. You do not need to make a sturdy connection or cut the leads just to experiment with.
 

Jerry garrcia

Tele-Meister
Joined
Nov 14, 2021
Posts
353
Age
48
Location
Sweden
Now replaced resistors between the filter caps

B+/B+2 =1.0k
B+2/B+3 =27,8K

Still high values but dropping.
B+ = 364 VDC
B+2 = 356 VDC
B+3.= 290 VDC
Lower more?
Pentode 195 VDC on plate
Triode 169 VDC on plate.
The 6sc7 sounds fine and tone works. The pentode has a low volume and no change on output regarding switch position. Miswired?

The 6sc7 circuit sounds fine
 
Last edited:

printer2

Poster Extraordinaire
Joined
May 24, 2010
Posts
8,417
Location
Canada
What is the screen, grid and cathode voltage of the 6SJ7 in both switched positions? Also the measured resistance measurements of the grid in both positions?
 
Last edited:

Jerry garrcia

Tele-Meister
Joined
Nov 14, 2021
Posts
353
Age
48
Location
Sweden
6v6
Plate 340 V
Screen 354 v
Grid 83 mV
Cathode 18.4 V

6SJ7
Plate 192 V
Control grid 298 mV/-
Grid 2.52v/-
Screen 27.2V/21.7V
Cathode 21.7V

6SC7
Plate 166/169.5V
Grid 1.0 mV
Cathode 2.19 V

Values I wrote down on a napkin last night. Will do more accurate readings including resistance when I get home from work tonight.
 




Top