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.Or 6SL7 with separate cathodes?
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.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.
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.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.
Is it this you mean?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.
Aha, I hear you. Thought that Leo would have thought about that. Just copied the design from the original schematic.The top channel does not have a resistor in that place.
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.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.
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.....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.