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

andrewRneumann

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Good catch. I was wrong. The reading should be around 500K regardless of the volume control position. I’m not sure why your reading is changing when you move the controls. The fact that you are getting >400K is a good sign.
 

andrewRneumann

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Did you install 4.7M anti-pops on the upper sides of C3 and C5? That’s probably why we are getting less than 500K and seeing the change when you move the right volume control.
 

Jerry garrcia

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Did you install 4.7M anti-pops on the upper sides of C3 and C5? That’s probably why we are getting less than 500K and seeing the change when you move the right volume control.
Yes I did. I used @printer2 preamp section layout for it was easier with the R16-18. Then combined it with your layout for the PT, sag resistor and CT. Since I used 4,7M resistors on c3 and c5 I didn’t put in the 4,7M R32 from your layout.
Unfortunately the Swedish postal service is not up to a normal standard so the 2W resistors for R25 and R26 hasn’t arrived yet. So now I just have the 1/4w resistors as a temporary solution.
Good to give it a short power up? Just for see if any power goes to B+ and the plates?
86CFF5DC-B5A3-4AA9-BB4B-D542FBDC3F06.jpeg
image.jpg
 

andrewRneumann

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I’d advise against it. When you start the amp, a huge rush of current is going to go through those resistors and I can’t guarantee they will hold. In addition to that, they may not be designed for 450V which is what they will see the instant you turn the switch on. (The same reason we didn’t use 1/4W on the plate load resistors.)

Safety tip for when you do eventually turn this on. I would always advise wearing safety goggles the first time powering up. If you have a variac, use it to slowly bring up the voltage. If not, just pray nothing blows on the first power up. Sometimes we have a hidden gremlin and we don’t want to be in the line of fire when something goes “pop”. Electrolytic capacitors can go “pop” in an extremely violent manner.
 

Jerry garrcia

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I’d advise against it. When you start the amp, a huge rush of current is going to go through those resistors and I can’t guarantee they will hold. In addition to that, they may not be designed for 450V which is what they will see the instant you turn the switch on. (The same reason we didn’t use 1/4W on the plate load resistors.)

Safety tip for when you do eventually turn this on. I would always advise wearing safety goggles the first time powering up. If you have a variac, use it to slowly bring up the voltage. If not, just pray nothing blows on the first power up. Sometimes we have a hidden gremlin and we don’t want to be in the line of fire when something goes “pop”. Electrolytic capacitors can go “pop” in an extremely violent manner.
OK :(
So when I'll put in the resistors (have 5,6K and 22K, hope it isn't to much?) the VDC will be around 450 after the diodes at B+1(because no rectifier tube?) and the drop to around 250 VDC at B+3? I have been reading today about Paraphase phase inverter and Push-pull. still a bit harder to understand than a class A amp circuit. Then measure B+ on plates and screens. Then put in the preamp tubes and then the power tubes and then, hopefully smooth jazzy tones? Correct?
Last question for tonight. It is quite cranked in there right now. Is it ok just to twist the R25 and R26 around the caps bottom end just touching the eyelet and have a good physical connection and be placing a good covering solder joint? Or do I have to remove all the solder and place it all in the eyelet? The reason I ask is that every time there is a risk that I accidentally touch a wire with the iron and need to resolder the whole wire. Not lazy!!!
67C691F3-6AF3-4942-9745-49577F756EB3.jpeg
 
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printer2

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OK :(
So when I'll put in the resistors (have 5,6K and 22K, hope it isn't to much?) the VDC will be around 450 after the diodes at B+1(because no rectifier tube?) and the drop to around 250 VDC at B+3? I have been reading today about Paraphase phase inverter and Push-pull. still a bit harder to understand than a class A amp circuit. Then measure B+ on plates and screens. Then put in the preamp tubes and then the power tubes and then, hopefully smooth jazzy tones? Correct?
Without any significant current flowing the filter capacitors down the line will have the same voltage on them than the first capacitor. You only get a voltage drop from the I x R = V, if the current is not flowing (as the capacitors eventually charge up) you do not get a voltage drop across the resistors.

I am betting you will get those jazzy sounds, for what it is worth. Sorry fol letting Andrew carry the load lately, been busy with other things.
 

andrewRneumann

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OK :(
So when I'll put in the resistors (have 5,6K and 22K, hope it isn't to much?) the VDC will be around 450 after the diodes at B+1(because no rectifier tube?) and the drop to around 250 VDC at B+3? I have been reading today about Paraphase phase inverter and Push-pull. still a bit harder to understand than a class A amp circuit. Then measure B+ on plates and screens. Then put in the preamp tubes and then the power tubes and then, hopefully smooth jazzy tones? Correct?

With no tubes it will be 450V to chassis on all three power supply nodes (B+1/2/3). Check 450V to chassis on all plates and screens. Check 3.2vac to chassis on all heaters—it will run a little higher with no tubes. And yes, you are right at the limit of your power supply capacitors (450V!) so let’s keep this check short and sweet. Wear your safety goggles. I believe in a previous post I mentioned that I would have preferred 500V caps with this particular PT. If you lose a cap because too much voltage, better now than later.

With tubes our targets were 370, 320, and 250 right? They won’t be exactly on target unless we are extremely lucky. We can assess and decide if they are good enough or make an adjustment to R25, R26, or R30 (sag). There is a lot of guesswork because it is hard to know the exact current the tubes will draw. We’ve come up with educated guesses, but be prepared to do a little adjusting once it’s running.
 

Jerry garrcia

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Without any significant current flowing the filter capacitors down the line will have the same voltage on them than the first capacitor. You only get a voltage drop from the I x R = V, if the current is not flowing (as the capacitors eventually charge up) you do not get a voltage drop across the resistors.

I am betting you will get those jazzy sounds, for what it is worth. Sorry fol letting Andrew carry the load lately, been busy with other things.
Stupid of me. Of course. That Ohms law. Andrew has been for me like a Sherpa dragging a 400 pounds smoker up Mt Everest. Without him and you I would been acting like Michael Douglas in that movie when he gets quite angry. Once again thanks!
 

Jerry garrcia

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With no tubes it will be 450V to chassis on all three power supply nodes (B+1/2/3). Check 450V to chassis on all plates and screens. Check 3.2vac to chassis on all heaters—it will run a little higher with no tubes. And yes, you are right at the limit of your power supply capacitors (450V!) so let’s keep this check short and sweet. Wear your safety goggles. I believe in a previous post I mentioned that I would have preferred 500V caps with this particular PT. If you lose a cap because too much voltage, better now than later.

With tubes our targets were 370, 320, and 250 right? They won’t be exactly on target unless we are extremely lucky. We can assess and decide if they are good enough or make an adjustment to R25, R26, or R30 (sag). There is a lot of guesswork because it is hard to know the exact current the tubes will draw. We’ve come up with educated guesses, but be prepared to do a little adjusting once it’s running.
As I have mentioned before I’m out of words. Your struggle with me and the patience is remarkable. I’m wondering if you are a teacher by profession?
If you and @printer2 ever passes Stockholm Sweden prepare for a treat!
I will follow your every guidence as I have done during this whole process. If you hadn’t called out I would had a burned out PT and probably even more trouble. I’ll keep you posted and hopefully I can share a CC pickup equipped 1952 Levin archtop beautiful sounds through the speaker some day.
Can I take the short cut with the resistors?
 

andrewRneumann

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As I have mentioned before I’m out of words. Your struggle with me and the patience is remarkable. I’m wondering if you are a teacher by profession?
If you and @printer2 ever passes Stockholm Sweden prepare for a treat!
I will follow your every guidence as I have done during this whole process. If you hadn’t called out I would had a burned out PT and probably even more trouble. I’ll keep you posted and hopefully I can share a CC pickup equipped 1952 Levin archtop beautiful sounds through the speaker some day.
Can I take the short cut with the resistors?

If you insist on powering it up, I would be more comfortable if you just removed R25 and R26. They are the wrong size! There is nothing to prove by leaving them in there. We already know voltage is going to get where it needs to go because you measured all those resistances. You can power it up and check B+1 is 450V and that it reaches the power tubes pins 3. You can also check the heater AC voltage.

Wear goggles. Wear insulating shoes.
Steady hand on voltage probe. Other hand well clear of chassis.

Once you shut down the B+1 cap will retain it’s voltage! Do not do anything inside that amp until you’ve unplugged it and have verified with your voltmeter there is less than 30VDC on the power supply caps.
 

Jerry garrcia

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If you insist on powering it up, I would be more comfortable if you just removed R25 and R26. They are the wrong size! There is nothing to prove by leaving them in there. We already know voltage is going to get where it needs to go because you measured all those resistances. You can power it up and check B+1 is 450V and that it reaches the power tubes pins 3. You can also check the heater AC voltage.

Wear goggles. Wear insulating shoes.
Steady hand on voltage probe. Other hand well clear of chassis.

Once you shut down the B+1 cap will retain it’s voltage! Do not do anything inside that amp until you’ve unplugged it and have verified with your voltmeter there is less than 30VDC on the power supply caps.
I’m sorry. The language barriers sometimes make room for misunderstandings. Look what happened between Clinton and Jeltsin (maybe some alcohol was also a part of that?). I didn’t suggest to power it up with the R 1/4W. The shortcut I meant was when the 2W 5,6K and 22K arrives, if I could twist them around the caps and wire at P1-P3, like on a turret board and secure with solder?This is because I have been soldering and desoldering that area and are afraid of touching any wires with the soldering iron.
 

andrewRneumann

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I’m sorry. The language barriers sometimes make room for misunderstandings. Look what happened between Clinton and Jeltsin (maybe some alcohol was also a part of that?). I didn’t suggest to power it up with the R 1/4W. The shortcut I meant was when the 2W 5,6K and 22K arrives, if I could twist them around the caps and wire at P1-P3, like on a turret board and secure with solder?This is because I have been soldering and desoldering that area and are afraid of touching any wires with the soldering iron.

Whatever you need to do. Just remember we may be changing those resistors if we don't hit our target voltages. It would be wise to put them in an easy way to remove. (Don't wrap the leads at first.) If you decide to keep the values, then go back and make them really secure. If you use an alligator clip on the leads of the caps, that will help keep the caps from overheating while you solder.

This is also a potential "shortcut" that needs a really close look.

67C691F3-6AF3-4942-9745-49577F756EB3.jpeg


You have B+ very close to ground here. You have measured resistance with from B+1 to chassis (it should be O.L. or >>100K after the caps charge) and determined there isn't a short. Still, make sure there isn't a solder bridge and clean out that little gap as best as you can. Would you say that gap is about 1mm? That's probably 30 times more than 450V could jump, so as long as it's clean, it should be good to go. I suppose some current could leak across the board, but I don't know enough about that to be concerned.
 

Jerry garrcia

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Whatever you need to do. Just remember we may be changing those resistors if we don't hit our target voltages. It would be wise to put them in an easy way to remove. (Don't wrap the leads at first.) If you decide to keep the values, then go back and make them really secure. If you use an alligator clip on the leads of the caps, that will help keep the caps from overheating while you solder.

This is also a potential "shortcut" that needs a really close look.

View attachment 945308

You have B+ very close to ground here. You have measured resistance with from B+1 to chassis (it should be O.L. or >>100K after the caps charge) and determined there isn't a short. Still, make sure there isn't a solder bridge and clean out that little gap as best as you can. Would you say that gap is about 1mm? That's probably 30 times more than 450V could jump, so as long as it's clean, it should be good to go. I suppose some current could leak across the board, but I don't know enough about that to be concerned.
Back from work and have put in two new resistors. Both 2W. R25 5,6 kOhm and R26 22 kOhm (closest to 22K I could find). Cleaned up the area between sag resistor and B+1 with alcohol. Will vacuum the board and brush it. Then walk the dog. Then open a guiness beer (no sipping) and power it up in about an hour… 😬
How will I know if a cap is broken? Can I trust a digital multimeter?
 
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andrewRneumann

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Back from work and have put in two new resistors. Both 2W. R25 5,6 kOhm and R26 22 kOhm (closest to 22K I could find). Cleaned up the area between sag resistor and B+1 with alcohol. Will vacuum the board and brush it. Then walk the dog. Then open a guiness beer (no sipping) and power it up in about an hour… 😬
How will I know if a cap is broken? Can I trust a digital multimeter?

One final check of your work. Resistance to chassis from B+1/2/3 are all >>100K or O.L. Resistance between B+1/2/3 reflects the 5.6k and 22k you installed.

Caps can explode, but you may notice the cap “bulging” before it goes. If B+1 is over 450V, we should stop and consider our options.
 

Jerry garrcia

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One final check of your work. Resistance to chassis from B+1/2/3 are all >>100K or O.L. Resistance between B+1/2/3 reflects the 5.6k and 22k you installed.

Caps can explode, but you may notice the cap “bulging” before it goes. If B+1 is over 450V, we should stop and consider our options.
B+1 - B+2 = 5,56K
B+2 - B+3 = 21,9K
B+1/2/3 = O.L
Bought new fuses…
 




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