Tweed Overdrive Special (continued)

[daveyajd]

Hey all,
Got a new dose of parts in so I'm working on my filter cap layout. Will you all look at these pics if you get a chance to make sure it looks ok? Initially I was going to put them all on their own PS board and run wires but I like the way they fit on the main board. Plus this keeps the run to each stage as short as possible so I think I will add turrets where I need them and do it like this. Thanks for all your help.


Aaron

http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/CAPS%202_zpsgmexqr7d.jpeg


http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/CAPS%201_zpsw8eycbxk.jpeg

[tubenit]

That looks fine to me to have the filter caps on the main turret board.  I think you are going to have a very nice sounding build.

with respect, Tubenit

[daveyajd]

Thank you. I am so excited to finally finish this amp! Another question on running heater wires. I'm planning on running my PT heaters to a terminal strip and from there it will "branch out" and run to my lamp assembly, my relay and my tubes. I just want to make sure this is correct and that I don't have to connect everything one to the other to the other and so on. Here is a photo to show you what I mean.


Thanks,
Aaron

[sluckey]

I'm planning on running my PT heaters to a terminal strip and from there it will "branch out" and run to my lamp assembly, my relay and my tubes.

I like that plan.

[daveyajd]

Then that is what I will do. I'm sorry if these questions seem simple. I've been pretty sure about my layouts and wiring throughout this build and I've usually been on the right track. Largely thanks to all the great info and people on this board. Sometimes I just need to hear a confirmation.


That being said one more question tonight. The black wires from the PT are not polarized right? Either wire can go to the neutral from the wall or the switch, right?


Aaron

[sluckey]

That's correct.

[daveyajd]

In my last parts order I got some Mogami shielded cable. This stuff seems weird. The core wire is mixed with a fiber of some sort. Mogami says it adds tensile strength. Maybe so but I find it almost impossible to work with. I can't twist it like normal stranded and it won't seem to tin at all. Anybody have any experience with this stuff?


So I am re-evaluating my shielded runs and I'm wondering how effective it is to twist two wires together, just like heaters, and use one as the signal and the other grounded at one end for the shield. Does that work? The wire I'm using would be 20ga solid.


Aaron

[tubenit]

So I am re-evaluating my shielded runs and I'm wondering how effective it is to twist two wires together, just like heaters, and use one as the signal and the other grounded at one end for the shield. Does that work? The wire I'm using would be 20ga solid.

I would not do that. I'd use shielded cable.

With respect, Tubenit

[Willabe]

Can you pull the fiber strands to side of the of wire strands and cut them?


                Brad     

[daveyajd]

Yeah Brad I can separate them but I still don't like the way they work out. I may fool with them a bit more but I may just move on with something else.
 
Another question on how to connect the trim pot. I moved it to the front so will I just run my cables like I show in the pics? The trim pot doesn't appear to have a lug that's grounded so can I still use shielded cable and ground the shield to my bus? Is shielded needed in this run?


Aaron

[g-man]

Deleted

[Willabe]

Good idea.


            Brad   

[daveyajd]

Great news. I will try that.


Aaron

[tubenit]

Another question on running heater wires. I'm planning on running my PT heaters to a terminal strip and from there it will "branch out" and run to my lamp assembly, my relay and my tubes

That is what I do on my builds.

Another question on how to connect the trim pot. I moved it to the front so will I just run my cables like I show in the pics? The trim pot doesn't appear to have a lug that's grounded so can I still use shielded cable and ground the shield to my bus? Is shielded needed in this run?

I just use shielded wire for long runs & wiring runs that are close to something that I think might induce noise. And yes you can ground the shield of shielded wiring on the buss wire. I do that all the time with no issues & have quiet amps.

IF I am looking at your photo correctly,  you have a short run from the relay to the trim pot. I would not use shielded wiring for that. You have a long run from the trim pot wiper to V2-7, and I would use shielded wiring for that.

with respect, Tubenit

[daveyajd]

Thank you for the reply Tubenit. I apologize for the confusion in my pics. I will use a shielded cable from the wiper to the tube, but the other runs that you thought went to the relay actually go back across to the turrets where the trim pot is drawn on the scheme. (I think) Picture 2 tries to show that. I'm thinking I won't use shielded since it just connects to the board.


Thanks,
Aaron

[Willabe]

Most of the time it's the longer grid wires that need to be shielded.

Especially the low signal grid wires, ie, from the input jacks and from a reverb tanks output/return to the grid of the reverb recovery tube, which has less of a signal than from a guitars PUP's.

Some guys like to put the 1st input tube directly under/next to the input jacks and just run the 68K input tube grid stopper from the hot of the jack to the input tubes grid. So no need for shielded cable. But if you have a pre punched chassis you have to work with that.

I've built a few scratch amps that I sent the signal from the 1st preamp tubes plate to the 2nd stage but I had the coupling caps hanging off the volume pots with very little lead wire on the pot side and did the same with a PI but in a twisted pair, since the 2 are out of phase with each other, to the power tube coupling caps but I put those caps very close, right to the power tube grid stoppers. Worked fine.


                     Brad   

[Willabe]

Like this;

Purple/black in 1st pic, preamp plate to coupling caps.

Purple/yellow in 2nd pic, PI to power tube grid stops.

[daveyajd]

Just want to make sure I have wired this up right. 5v from the PT to rect tube pins 2 and 8. From pin 8 to the top of the Off-Stby-On switch. The bottom of that switch side will go to my reservoir filter cap. The other side of the switch has the AC on top and the PT primary on the bottom.


Aaron

[sluckey]

Just want to make sure I have wired this up right.

That's correct.

[daveyajd]

Thank you. Got a little more done today. Here are some new pics. Feedback about possible issues you see is always welcome.



Thanks,
Aaron

[tubenit]

I think you are doing a great job and that it will be a very fine sounding amp!

With respect, Tubenit

[daveyajd]

Is it better to have the 470R resistors as close as possible to the power tube pin? I've considered mounting them on the terminal strip I have pictured above and running wires to and from. Wondering if this is poor form?

Aaron

[sluckey]

It's not critical where you put them. Some people put them on the board. I like to put them directly on the socket between pins 4 and 6. I mount them standing about half inch above the socket pins. Like this...

     http://home.comcast.net/~seluckey/amps/6v6plexi/P-6V6_05_big.jpg

[daveyajd]

Thank you you for that. How are you able to use pin 6? My layout shows a 110k between pins 5 and 6. The PPIMV connects to pin 6. And it looks like pin 4 connects to the filter stage through the 470.

[Willabe]

Different power tubes have different pin outs. 6L6's and 6V6's allowed Fender to do this;

http://el34world.com/charts/Schematics/files/fender/Fender_Deluxe-Reverb-AB763-layout.pdf


                Brad   

[daveyajd]

This layout is drawn for 5881, 6L6, 6V6. So I'm not sure that I could wire it between those pins. If it's not a big deal I will probably just wire them on the terminal strip.

Aaron

[sluckey]

Look at my pic again. You'll see two resistors on each socket. The little one is the control grid stopper. It's mounted between pins 1 and 5. The big resistor is the 470Ω screen resistor. It's mounted between pins 4 and 6.

Pin 1 and pin 6 are not used in the 5881, 6L6, 6V6 tubes. You can use them as you wish. I copied my layout with those two resistors from Fender. Anyhow, I was just showing you how I mount that 470Ω resistor for your consideration.

Which layout are you using that has a 110K resistor mounted between pins 4 and 5?

[daveyajd]

Ahh. Now I'm seeing it. The layout I'm using shows a 10k resistor from pin 5 to 6. Then my shielded cable goes from PPIMV to pin 6 so that pin is simply a landing spot for one leg of that resistor and my PPIMV. In other words I can use 1 or 6 as landing pins for filter caps or PPIMV as long as the cap wire goes through the 470 to pin 4 and the grid stopper (10k) is between pin 5 and the master volume. Thank you all so much.

Aaron

[daveyajd]

Alright, so my LED for the OD is a blue light using 5 volts at 30mA which should be a limiting resistor of about 48R. I am fine using something like an 82 1/4 watt, right. Is the brightness affected at all with different values.


Also, I'm sure I've seen it posted somewhere before but can someone direct me to an explaination of how this type of cliff type jack is wired up as the footswitch jack. 


Thanks,
Aaron

 

[sluckey]

Alright, so my LED for the OD is a blue light using 5 volts at 30mA which should be a limiting resistor of about 48R. I am fine using something like an 82 1/4 watt, right. Is the brightness affected at all with different values.

I've never heard of a 5 volt LED before. That sounds like a REVERSE VOLTAGE rating. We need to know the FORWARD VOLTAGE rating. The forward voltage would be something like 1.7v, or 2.1v, or even 3.5v for a super bright blue LED such as RadioShack sells.

Once you know the forward voltage rating and the forward current rating it's easy to calculate the value for the current limiting resistor (CLR). But you still need one more piece of info... What is the supply voltage?

Here's an example of how to calculate the value of the resistor for a 3.5v @ 30mA LED when the supply voltage is a 9v battery. The LED will be max bright when 30mA flows thru it. This 30mA must also flow thru the CLR. Since the LED will have 3.5v across it, the CLR must drop 9v-3.5v = 5.5v. And since there will be 30mA flowing thru the CLR, the resistance will be the voltage across the resistor divided by the current thru the resistor which is 5.5v/.03mA = 183.3Ω. So, the CLR should be 183.3Ω with this LED used on a 9v supply voltage to allow full/safe brightness. In reality you don't need maximum brightness (max forward current). Even half the current will usually give plenty of brightness, so just increase the value of the CLR to reduce the brightness, but never go lower than the calculated 183.3Ω or you will exceed the rating of this LED. In this example, I'd probably just use a 470Ω CLR.

So now, what wattage should the CLR be? That's also easy to calculate. The power dissipated by the CLR will equal to the square of the voltage across it divided by the value of the resistor. For the 183.3Ω CLR that would be 5.5v x 5.5v / 183.3Ω = 0.165W. Double that power for a safety margin so use a .33W resistor. Just get the next large wattage which is half watt. Applying the same math to the 470Ω CLR will show that a quarter watt would be sufficient.

In summary, R_CLR = (V_supply - V_LED)/I_LED. Oh, and it's important to remember that the LED forward voltage will remain constant (within reason!). It does not depend on the current flowing thru the LED.

[sluckey]

Also, I'm sure I've seen it posted somewhere before but can someone direct me to an explaination of how this type of cliff type jack is wired up as the footswitch jack. 

The cliff jack in the pic is a two circuit (stereo) jack. It is capable of being used with a double footswitch to control two circuits.

We need to see the schematic you are working from to tell you specifically how to wire it. We also need to see the footswitch or a schematic of the footswitch.

See pic for terminal identification...

[daveyajd]

Thanks Sluckey. I attached a couple of pics of the LED package. It is from Radio Shack and the forward voltage rating is 5v. I have done the calculations that you were talking about and that's how I got to a resistor of about 48. (43.33 actually)
My assumption in that calc was the supply voltage. I was using 6.3v in my equation because that's my heater voltage. This LED is connected to the diode on the relay as learned from your post above. Would that still be the 6.3v? As for the wattage of the resistor. If I follow your calculations (using the numbers I have been using) the wattage is so small it seems that a 1/8 watt would be fine.


6.3-5 =1.3


1.3 x 1.3 =1.69


1.69/48 =.035


.035x2 =.07


Of course if my supply voltage is something else than this all falls apart.


As for the cliff jack. Here is the diagram that I got from Tubenit so long ago. How would a cliff jack fit in with the wiring it shows? I don't know for sure what foot switch I will use. I may build my own or I may use the single button style that Doug sells. I think the original BOM called for this type of jack so it was isolated from ground but I'm not sure.


Thanks,
Aaron

[sluckey]

You will not use 6.3 as your supply voltage. Leave the LED disconnected until you get the relay board working. Then, with the relay energized, measure the dc voltage across that diode on the relay board. That is the voltage you will use for the supply voltage to the LED. Recalculate based on that number, then connect the LED.

[daveyajd]

Well, shite. Ok, I will wait until the end to dial that in. Thank you for that.


I am still trying to figure out the wiring for channel switching. Can't seem to quite wrap my head around it.
I found another diagram showing a cliff jack so maybe you can help me get it. I am going to open Inkscape and sketch it out a bit.

[sluckey]

You only have one relay, right?

Will you also have a front panel switch to control the relay?

[daveyajd]

Yes, one relay. A manual switch on the front. (SPDT right?) and the stereo cliff jack on the back. This is what I came up with.


Aaron

[sluckey]

That will work but it is a little more complicated. It relies on two sets of contacts to work. I would suggest a simpler drawing using only one set of contacts. Less moving stuff to fail.

Your drawing is "A". My suggestion is "B". Use the one you like better.

EDIT.... And you only need a SPST switch on the panel and in the footswitch.

[daveyajd]

Ok. Yours does seem more straight forward. And I do not have to ground that sleeve on the switched side,right? And these should not need to be shielded at all as it is only switching the relay, right?


Aaron

[sluckey]

Don't ground anything in those pics. Shielded cable is not necessary.

[daveyajd]

I didn't mean "ground" as in to the bus or chassis. I just meant connecting it to the common on the relay.


Aaron

[daveyajd]

Alright, I asked this question earlier in this thread and didn't really get an answer to it. I just want to make sure I wire up the Fatness pot correctly. I'm wondering if I run two wires from V1 pin 7, one to the wiper on the Gain (vol) and the other to lug 1 on the fatness. Or run one wire from V1 pin 7 to lug 1 on the fat pot and then connect that lug to the wiper on the Gain(vol) pot. I attached a picture highlighting the areas.


Aaron

[sluckey]

Depends on where the fat pot is physically located. Isn't it on the front panel? If so, I'd do it like this...

Connect a wire between the vol pot wiper and the hot side of the fat pot. Now connect one shielded cable between the wiper of the vol pot and V1-7. Connect another shielded cable between the wiper of the fat pot and V2-2. Ground both shields at the pots (or pot buss) only.

[daveyajd]

So I think I may have gotten the wrong jacks for my input and effects send. These look like TRS jacks instead of the 12A jacks they should be. Will these still work for either my input of effects sends. I am about to order a few more parts so I can include the right jacks if I need to.


Thanks,
Aaron

[sluckey]

So I think I may have gotten the wrong jacks for my input and effects send.

You need the switch function on the 12A.

[daveyajd]

Ok. Thanks. I will add those to my upcoming order from Doug.


Is this how I will wire up my impedance selector switch?


Thanks,
Aaron

[sluckey]

That's basically correct. I'd move the NFB wire from the switch wiper to the 16Ω (or 8, or 4) lug so the NFB will remain the same regardless of which impedance selection you make.

[daveyajd]

So how does that work? Wouldn't you only get signal on the NFB when that impedance is selected?

Aaron

[Willabe]

No, because the output signal is present at/on all 3 OT secondary taps all the time. 


                   Brad   

[daveyajd]

Ok. Ii get it. Also, think I made a mistake on my selector switch picture above. I drew my arrows as if the red wire from the OT went to the selector switch. After looking at Hammonds wiring diagrams I see that the primary side red, blue, brown go to filter cap A, and my output tubes respectively and the secondary taps all go to the selector , the ground to the output jack. Then a wire to connect the selector  switch to the tip of the jack.

Do I just ground the output jack  at my main ground with my PT  and heaters?

Aaron

[Willabe]

Do I just ground the output jack  at my main ground with my PT  and heaters?

If your using a Switchcraft jack then it is already grounded to the chassis through it's metal frame. If your using a plastic body Cliff type jack (Marshall jack) then you have to run a dedicated ground wire to some ground point.

With amps that have -FB loops, like yours, they often ground the OT at the phase inverter (PI) filter cap star ground point, because the -FB loop is fed to the PI, so the PI is part of the power amp output loop.

Here's a link to grounding schemes that might help you;

http://www.valvewizard.co.uk/Grounding.html


                            Brad