Two different preamps (one octal and one regular) in a Tweed Delux "clone"

andrewRneumann

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I think 280V might be a little low for the screens, 300V would be better with the plates at 350V. That will leave 50V to drop yet, 10-12k would then work.

Maybe we ought target 5E3 voltages of 370-320-250. 5B3 voltages do seem kind of low. @Jerry garrcia you are looking for a more clean sounding amp right?

I’m going to keep giving my own sketches on the layout just so you have options. @printer2 has come up with some good ideas that I will incorporate in mine. My goals again are to get an amp the works using the constraints of a board that’s already built and installed in the chassis.
 

Jerry garrcia

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Maybe we ought target 5E3 voltages of 370-320-250. 5B3 voltages do seem kind of low. @Jerry garrcia you are looking for a more clean sounding amp right?

I’m going to keep giving my own sketches on the layout just so you have options. @printer2 has come up with some good ideas that I will incorporate in mine. My goals again are to get an amp the works using the constraints of a board that’s already built and installed in the chassis.
Hi and that’s correct. A clean sounding amp with the possibility to get som “character” from the 6SJ7 preamp tube selection. What I would dream about is the possibly to have the choice between the sounds of a clean GA-20 or a tweed delux with a 12ay7 tube.
 

andrewRneumann

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Here’s my final submission for now:

671EDE8F-0A25-4946-AF6F-4BE6F19DAC42.jpeg


I’m targeting B+ voltages of 370/320/250.

Changes from last revision:
1. Added value to sag resistor R30 (270R/10W). I left it in the HT C/T under the assumption you won’t be adding any eyelets to your board. This will get warm, so keep it as far away from capacitors as possible. Even have it off the side of the board for more airflow around it if you can.
2. Changed dropping resistor (R25/R26) values to 5.1K and 18K.
3. Added anti-pop resistor R32 (4.7M) across terminals of channel switch.
4. Connected shield pin (1) of 6SJ7 to chassis instead of ground bus.
5. Moved ground bus connection to power supply capacitors to negative terminal of C11.
6. Moved grid leak resistor R5 to a more convenient place.
7. Added “grid leak resistor” R31 (1M) from input of 6SJ7 (C6) so that Hi/Lo input network works as advertised when 6SJ7 active channel.

Good luck, let me know if you have any questions. I’d like to think I thought of everything, but probably not.
 

Jerry garrcia

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Here’s my final submission for now:

View attachment 938825

I’m targeting B+ voltages of 370/320/250.

Changes from last revision:
1. Added value to sag resistor R30 (270R/10W). I left it in the HT C/T under the assumption you won’t be adding any eyelets to your board. This will get warm, so keep it as far away from capacitors as possible. Even have it off the side of the board for more airflow around it if you can.
2. Changed dropping resistor (R25/R26) values to 5.1K and 18K.
3. Added anti-pop resistor R32 (4.7M) across terminals of channel switch.
4. Connected shield pin (1) of 6SJ7 to chassis instead of ground bus.
5. Moved ground bus connection to power supply capacitors to negative terminal of C11.
6. Moved grid leak resistor R5 to a more convenient place.
7. Added “grid leak resistor” R31 (1M) from input of 6SJ7 (C6) so that Hi/Lo input network works as advertised when 6SJ7 active channel.

Good luck, let me know if you have any questions. I’d like to think I thought of everything, but probably not.
Tears in my eyes. Thnx from me and my son. It will be a nice thing to do this evening instead of putting patients in ventilators and connecting them to dialysis machines. Now just trying to get of early enough to find a store that is open so I can acquire a sag resistor R30.
Have some questions.
1) how can I tell the difference if a resistor is 2w?
2) haven’t found a 18k resistor. Does a 20k work?
3) one guy questioned the pin location on the valves. Do you think they are ok now?
4) a friend of mine just realised that he did soldered all wires, in a 5F1 circuit, to the valves, on the backside of the board. No joints touches the chassi. Will that make any difference humm wise? They are supposed to be on the side not facing the chassi.
Thanx and I Hope it will be done this weekend and I’ll keep you posted.
 

printer2

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The difference in sizes is usually printed on the larger sizes. Usually you would ask for the right size or it would be on the package if the parts are in small quantities.

resistor-wattage.jpg


18k to 20 k is the same in this application. The 5.1 or 5k is the same, the electronic industry has both sizes at times just to mess with you. The soldering on the back side of the board does not matter, just easier to change things easily if on the exposed surface. Circuit boards have the parts going through the holes and soldered on the back. If you need to change something you have to unmount the board to get to solder it. That is the nice thing about the old style construction of these amps, you can modify or fix it without pulling stuff.

Personally I would not bother with the sag resistor yet, you may not find you need it. The transformer may give enough sag as it has resistance in the winding. I would want to get it up and running without too many changes and then go from there. I am not impressed with the top view of the layout drawing. You are suppose to be able to look down at the parts and compare it to the drawing in order to see where they are to go. To do it right would be to redraw it. I redrew the 6SJ7 for the part side view, I did not do it for the other tubes. I will do a little more work, "I'll be back."
 

andrewRneumann

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1) how can I tell the difference if a resistor is 2w?

Size is the big factor, but there is no real easy way. Some 2W are smaller than other 2W. When I have a stash of resistors, I have to keep them in labelled bags or find some way of remembering the wattage.

2) haven’t found a 18k resistor. Does a 20k work?

Sure.

3) one guy questioned the pin location on the valves. Do you think they are ok now?

I have double checked them, but I'm not immune from error. I suggest double checking them yourself and making sure they make sense. I did not change the numbering--so remember to follow the numbers--not the layout (which is reversed).

4) a friend of mine just realised that he did soldered all wires, in a 5F1 circuit, to the valves, on the backside of the board. No joints touches the chassi. Will that make any difference humm wise? They are supposed to be on the side not facing the chassi.

Shouldn't make a difference for hum. It's a really pain to troubleshoot and modify when things are soldered on the back of the board. I don't recommend it if possible.

Personally I would not bother with the sag resistor yet, you may not find you need it. The transformer may give enough sag as it has resistance in the winding.

Just for the record, I did do calculations based on the 290BEX datasheet secondary resistance with 230vac mains voltage. It's not just a guess, but I agree--these things may need adjustment because there are just too many unknowns until it's fired up.

Please do us a favor... before you power on, send a full set of gut-shots of the chassis. We need clear, well lit, photographs of the whole board. We also need to see the edges of the chassis--clear shots of all the valve sockets and controls / jacks on the other side too. Please cut and paste them "inline" to a post here.

Before we power on, we should leave all the tubes out and spend some time checking proper resistance all over the place. The first power on will be without tubes and we will check voltages all over the place. Rob Robinette has a start up sequence somewhere on his site and I would recommend it. It's easy to get excited and just power it on to "see what happens." Unfortunately, we all make mistakes, and that's asking for trouble. (I have already made the suggestion for a "light bulb limiter" so I won't belabor the point--you've been warned!)
 

printer2

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A sucker for punishment, actually do not want to see you fry anything, I am modifying the layout for the proper number direction. Still a little way to go. I am starting to think a sag resistor might be needed. I looked up the power transformer and it says it has a Red to Red secondary resistance of 120 ohms which is fairly low, means it will not sag all that much when current is drawn. Also what is the voltage rating of the filter capacitors? Unloaded the power supply might see 450V. That will drop when current is drawn but with no tubes in it can get up there.

On the 2W resistors, some manufacturers have made smaller resistors and kept the same rating. They just use materials that can take a higher temperature. At least that is the theory. My brother used a resistor that was smaller but was rated for the same wattage, then found they were burning up in their products. You know, you need a certain surface area to dissipate a given amount of heat. The manufacturer was a little optimistic. Oh right, my brother designs electronic equipment.
 

printer2

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I'll be back to list the changes, need a break. But I think this is workable. Changed all the pin numbers the right way and reconnected stuff.

5E3 with Pentode.jpg
 

Jerry garrcia

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I'll be back to list the changes, need a break. But I think this is workable. Changed all the pin numbers the right way and reconnected stuff.

@printer2 and @andrewRneumann
Amazing. There are some differences though with your two layouts regarding resistor values and placements. Especially by C 3 and 5 and the c14 is supposed to be by the 270R/10w? An other question I have it’s that I have been chasing down a 270 ohm/10 w (quite difficult to find nearby) resistor that is added on Andrews layout. Yours has an 270/5w. Does that matter? Now found a electronic store I our outpost Sweden that specify the wattage in the regular resistors so it will be possible to get 2 and 5w.
A light bulb limiter?
Of course I will send pictures before I start up. Hopefully I’m allowed building time this weekend…
 

printer2

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Yes, it was a lot of fun going between the original drawing, my drawing, Andrew's drawing 😣. Then redrawing the pin-outs and rewiring. I just left the sockets as they were as I did not know how yours are oriented.

"Especially by C 3 and 5 and the c14 is supposed to be by the 270R/10w?"

I wanted to do the least amount of changes to your wiring. The resistors on C3 and C5 are there to not leave the one side of the capacitor hanging out in thin air when the switch is on the other position. This reduces the tendency to pop if you are switching the position when the amp is on. It is just convenient going from the eyelet the cap is attached to with the resistor to the ground bus. I am all about easy. The resistors can be anything from 1M and up, 4.7M is a good choice in it not loading the signal, I could live with any between that and 1M. In this case you just do not want to load the circuit much and provide a high resistance path to ground.

C14 is the cathode bypass capacitor by R27, I just flipped it on the other side of R27 in order to make some more room. Andrew has it moved in order to make room on the board for the sag resistor. I would like to see them all on the board if it is possible. I do not like the sag resistor being on the negative side of the power supply, just a personal thing.

At one time I could calculate out how much voltage would be dropped by the resistor, not a simple thing as it is not dc or a sine wave. But it has been a while since I learned it. The values are a rough guess, it also depend on where the voltage ends up and what your ears like.

R16, R17, R18 are fine how they are positioned. Remove the wire going to the R27, C14, that was an error when I simplified from two preamps to one. Andrew has R17 and R18 going to the ground buss, a wire from the eyelet R17, R18 to the ground buss is the same electrically. The other end of R18 goes to the 6CS7 either way.

Looking at the board I just noticed the jumper from R27/C14 to C13. If you needed to make room for the sag resistor you could take out the jumper wire there and replace it with R27/C14, then take the C13 eyelet back to the centertap of the power transformer. Anything to make stuff fit and convenient to you.

I hope the renumbering of the sockets associated wiring is right. Another set of eyes would be great. It should match up what you wired up, should be easier for you than messing around with the top view. The 'not connected' is for where the two wires crossed. Probably obvious but since the wires are the same color I did not want to take a chance.

We used to have an electronic distributor in town when I was a kid, could get almost anything. Now it is a struggle to find anything short of mailing it in. A 270 or 330 ohm 5W will be fine, will have to see where the voltage ends up in the end anyway.

I think that is it. Oh right,


 

andrewRneumann

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I'll be back to list the changes, need a break. But I think this is workable. Changed all the pin numbers the right way and reconnected stuff.

View attachment 939277

Don’t forget HT C/T needs a 0V reference somewhere. On my layout I just connected -C13 to -C12. Some people connect it to the chassis but I’m not a fan because that sends all the screen and preamp DC current through the chassis. Whatever way, the HT C/T needs to be held at 0V somehow. (In my case, 0V upstream of the sag resistor.)
 

printer2

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Don’t forget HT C/T needs a 0V reference somewhere. On my layout I just connected -C13 to -C12. Some people connect it to the chassis but I’m not a fan because that sends all the screen and preamp DC current through the chassis. Whatever way, the HT C/T needs to be held at 0V somehow. (In my case, 0V upstream of the sag resistor.)
I thought that was what the dashed wire did beside C13. If not, yes it needs to be.
 

andrewRneumann

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An other question I have it’s that I have been chasing down a 270 ohm/10 w (quite difficult to find nearby) resistor that is added on Andrews layout. Yours has an 270/5w. Does that matter? Now found a electronic store I our outpost Sweden that specify the wattage in the regular resistors so it will be possible to get 2 and 5w.

There are two 270 ohm resistors in my layout. One is for the cathodes of the 6V6 power tubes (5W). The other is a sag resistor (10W) for the power supply to bring the voltages down where they should be and also add some "dynamics" to the response of the amp when driven hard.

@printer2 thinks 5W is enough for a sag resistor. He's probably right. I tend to engineer for the absolute worst case and then add some safety beyond that. This resistor would only be in danger of overheating if you were driving the amp for a long time at or near full volume. If you want 10W effective power handling, you can use two 500-ohm 5W resistors in parallel or two 130-Ohm 5W resistors in series.
 

printer2

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Or two 150R. If you think 270R and 0.1A that is 27V x 0.1 = 2.7W. We usually double the wattage of a resistor compared to the actual wattage. So 5W is on the border for that current. I would stand it off of the board a little though. Or to conserve space, a 470R 5W with a 2W 820R piggybacked on top of it would give a little more capacity with standard parts. Mind you, two rectangular resistors would easily sit one on top of each other. Would cut down on dissipation of each but still give more than enough. Two 680R's? In that case I would have them mounted firmly on the board. Just depends on how you want to go about it.
 
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Jerry garrcia

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There are two 270 ohm resistors in my layout. One is for the cathodes of the 6V6 power tubes (5W). The other is a sag resistor (10W) for the power supply to bring the voltages down where they should be and also add some "dynamics" to the response of the amp when driven hard.

@printer2 thinks 5W is enough for a sag resistor. He's probably right. I tend to engineer for the absolute worst case and then add some safety beyond that. This resistor would only be in danger of overheating if you were driving the amp for a long time at or near full volume. If you want 10W effective power handling, you can use two 500-ohm 5W resistors in parallel or two 130-Ohm 5W resistors in series.
I just converted resting a medtech company I have, to also include electronics. So I was able to start a distribution account in a German electronic manufacturer 😀💪🏽. Du to that I was able to order the resistors needed. They’ll probably arrive next week.
Hard to know which one of the layouts I should build from… they have some differences. Have done the soldering where they are the same except for that I grounded pin 1 on V3 and the pin numbering on V3 from @printer2 layout. They will be the same I guess.
Now just deciding if I shoul use the sag resistor and the 4,7M resistor on S2 as @andrewRneumann. Or the 3,3 nF by the diodes, or where to attach the red/yellow wire from the PT, or which schematic to use as template for grounding R16/R17/R18? So many decisions…
Hopefully it will turn out great in the end. Now back to flipping a intubated Covid patient to supine position.
Have a nice end to the weekend.
 

printer2

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Now just deciding if I should use the sag resistor and the 4,7M resistor on S2 as @andrewRneumann.
Sag resistor can always be put in later as long as you have room and a spot to solder it to.

C5 and C3 are the outputs from the pentode and the triode to the switch which goes to the volume control. The reason for the resistor to be in this position is to reduce the voltage at the side that is hanging out in the air when the switch is in the other position. With the 4.7M across the S4 and S6 pins it is combining the output of the pentode and triode. While there is a voltage divider effect from the tube not being used through the 4.7M to the output of the other tube any noise that is being generated from the unused tube will end up on the active tube. Will it be a problem? Only way is to find out (the input from the unused tube is open circuit which means it can catch stray garbage. Shielded cable on both inputs can reduce the possibility. The sag

Or the 3,3 nF by the diodes, or where to attach the red/yellow wire from the PT, or which schematic to use as template for grounding R16/R17/R18? So many decisions…
Hopefully it will turn out great in the end. Now back to flipping a intubated Covid patient to supine position.
Have a nice end to the weekend.
I have yet to use a cap like the 3.3nF cap in that position, you could always put one in later if you have a problem. The two different placements of the resistors just put them in different places. Electrically they are the same.

Wish you luck with your patient.
 

Jerry garrcia

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Hopefully I’ll get the parts today. A good thing is that the creator of the fist wrong build schematic feels bad for his mess up and will create a eyelet board and send me new components for free in a couple of weeks. The only problem with that is that the most parts, chassi and hand made cabinet is made for this construction. If it turns out good how would one do to create an optimal component build schematic of your two’s creation? @printer2 and @andrewRneumann
 

andrewRneumann

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Have done the soldering where they are the same except for that I grounded pin 1 on V3 and the pin numbering on V3 from @printer2 layout.

That makes sense. I didn’t even notice that shield pin.

Now just deciding if I shoul use the sag resistor and the 4,7M resistor on S2 as @andrewRneumann.

Both of these resistors are optional. The 4.7M is just to equalize voltage potential between the two throws of the switch to minimize popping when you move the switch. I would try it without and then if you get popping, install it and see if it helps.

The big sag resistor is also not required. If you leave it out, just wire the HT C/T (0V) to the negative terminal of the first filter capacitor. My opinion is that you will want this though… it’s going to give you the response of a tube rectifier which is distinctly vintage sounding.

Hopefully I’ll get the parts today. A good thing is that the creator of the fist wrong build schematic feels bad for his mess up and will create a eyelet board and send me new components for free in a couple of weeks. The only problem with that is that the most parts, chassi and hand made cabinet is made for this construction. If it turns out good how would one do to create an optimal component build schematic of your two’s creation? @printer2 and @andrewRneumann

That’s nice he is willing to help you get what you paid for. As far as generating a new layout… I’m sorry, but I just don’t have the time or energy to devote to that. If the schematic is good, there are 1,000 different ways to lay it out and many can be considered optimal. Or maybe none of them are optimal.

Still, if you post the new layout when you get it, I will happily look it over and give you my thoughts. It might be a good idea to get this from your fabricator before he fabricates it.
 




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