Tweed Overdrive Special (continued)

[daveyajd]

Hello all,
It has been a couple of years and many miles of life since I have been here. But damn it's good to be back! My original project and all of the info I was gathering at that time can be found here.


http://el34world.com/Forum/index.php?topic=13510.msg126422#msg126422
 
I am back on the TOS build and hoping to establish some communication with those of you who have the answers. Just placed an order with Doug and the service is still OUTSTANDING! Thank you. Here are some pics.

[TIMBO]

NICE

[tubenit]

Impressive beginning!  Take it slowly and do great work particularly on the soldering.  We'll help you when and if that is needed.

I think you are going to end up with a great sounding amp!

with respect, Tubenit

[daveyajd]

Thank you for the kind words. I am pretty meticulous (see: Anal) :) so I plan on being very clean with all my wiring. I am wondering about one thing though. When I am connecting my pots to my 9 pin sockets should I use shielded cable across the chassis? And if yes do I just ground that shield at the nearest ground? Thanks.


Aaron

[sluckey]

When I am connecting my pots to my 9 pin sockets should I use shielded cable across the chassis? And if yes do I just ground that shield at the nearest ground?

I do. I ground the shield at the source of the signal only. That is the pot in most cases.

[daveyajd]

So no shield connection at the tube then? Great I will just ground it on my Buss bar. Thanks


Aaron

[daveyajd]

Ok, so I have a couple more questions. I am putting my parts list together for pots but I'm not sure what all of them are. I have moved things around on the faceplate a little and renamed them but heres what I have.


CLEAN: Gain(Vol) 1M audio, Tone 1M audio, Level 1M audio, Fatness(moved to the front) 250k audio


OVERDRIVE: Lean(Trim pot) 50k linear, Gain(drive) 250k linear, Tone 1M audio, Level 1M audio


Then of course the PPIMV Dual gang 250k audio.


Does this all seem right?


How does the Trim pot affect the OD channel?
Do you guys prefer CTS or Alpha? Can I mix and match?


Thanks,
Aaron

[tubenit]

Only thing I'd probably change is the OD  gain/drive 250kl to 250ka. I think you'll find 250ka to be more useful.

The rest seems fine.

Not sure what your trim pot question is?  The trim impacts how much clean signal is fed into the overdrive.

I don't have any preference at all.  I usually use Alpha and they've worked fine for me. I also mix and match which has been fine too.

with respect, Tubenit

[tubenit]

One other thing I'd change on the TOS is to add an "enhance" cap to the entrance plate resistor of the LTPI.

I find this smooths the high frequencies and actually has helped the "blooming" effect on my amps where the note opens up and blossoms more. I am thinking that somehow this may be eliminating some really high frequency oscillations which helps the tone?

I'd probably try a 220p but anything between 120p to 250p would work fine.

With respect, Tubenit 

[daveyajd]

That sounds good thanks for the help. I will try that cap.


 So I am getting my wiring straight for the PT using these diagrams. Couple of questions on connections.


I understand the yellow 5v to the rectifier pins 2 and 8, greens and green/white to heaters and center tap(ground) respectively.


Now I believe that the reds are high voltage and would go to rectifier pins 4 and 6. Which is my ground with those, the red/green 50v or the red/yellow 0v? What do I do with the other one?


I also believe that one black would go to my power outlet and one would go to a switch/standby lug. Is that correct? And then the orange (shield) is my ground?


Thanks,
Aaron

[tubenit]

Look at the Fender Super Reverb layout for some help:

http://el34world.com/charts/Schematics/files/fender/Fender_Super-Reverb-AA763-layout.pdf

http://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf

Red/green 50v is for bias.  Red/yellow CT means  center tap.

If you dig around on the internet and look at different schematics/layouts, you usually can find an example that helps answer the question.

With respect, Tubenit

[daveyajd]

Ok thanks. So I am correct in all of my thinking. The red/yellow 0v will go to ground but I'm still confused where the 50v bias will tie in. The Super Reverb I understand but that has a bias pot. This TOS will be cathode biased so where will it connect. Sorry if my ignorance is showing. 


Aaron

[Willabe]

but I'm still confused where the 50v bias will tie in. The Super Reverb I understand but that has a bias pot.

The red/green is a tap off the high voltage (B+) PT secondary wind and provides the acv that is sent to the -bias circuit where it is rectified through a reversed biased diode from +acv to -dcv and the -bias dcv is filtered with a revesed filter cap, + cap lead to ground. This is the raw -bias dcv.

Then the filtered raw bias goes to a voltage divider, 2xR's, 1. -bias set R (or pot) and 2. -bias range R. Some have a 2nd filter cap, some have an adjustable bias pot some don't. The bias pot just replaces an R in the -bias circuit to make it adjustable.

Here's link to Doug's tube library of information on how -bias circuits work;

http://el34world.com/charts/Biascircuits.htm


                      Brad     

[AZJimC]

If the amp is cathode biased, meaning the cathodes are not tied directly to ground, but rather through a resistor. This resistor allows voltage to rise on cathode, which means the grid is below cathode voltage, or negative in relation to the cathode. This accomplishes biasing the grid without a negative supply voltage, so your 50v bias winding is unused on a cathode biased amp. It is possible to prepare the bias supply and install a switch that allows switching between cathode biasing OR fixed bias via a negative voltage to the grids, but that would be a considerable modification to the amp as designed.

[Willabe]

This TOS will be cathode biased so where will it connect.

Doh! Missed this.

It doesn't, you just shrink wrap it off and stow.


                    Brad   

[daveyajd]

I'm confused by something else I am seeing on the layout and schematic. Why does the relay for the channel switching seem to connect to the effects loop? Shouldn't it just be wired up to the foot switch jack?


Thanks,
Aaron

[AZJimC]

The relay is connected to both channels and it's output then goes to the effects loop jack, which is bypassed when no jack is in, and then to phase inverter input. The coil of the relay would be connected to the footswitch jack. That part of the switch or relay, shows signal routing in the relay or switch.

[tubenit]

AZJimC gave a great explanation for you.

Put very simply ..................

Footswitch operates (switches) the relay sending either the clean or overdrive signal to the phase invertor.

One part of the relay connects to the footswitch.  Another part of the relay connects to the phase invertor.


There is a ton of info in ARCHIVES on relays which would be useful to you to read when you get around to wiring up the relays:   http://el34world.com/Forum/index.php?topic=14348.0

with respect, Tubenit

[daveyajd]

Ahhhh, that makes perfect sense. Thank you both.


Aaron

[daveyajd]

So I am fitting all my heater wires and I have a question on the 5879 tube. It only has one heater connection on pins 4 and 5. Do I just connect that from my PI 4 and 5 pin connection and leave my pin 9 connection untouched until the next 12a_7? Hope the wording of that makes sense.


Thanks,
Aaron

[AZJimC]

The 9 pin sockets are wired with 6.3vac heater power. The 12a*7 tubes are 12.6vac heaters. They are internally wired with a center tap on the heater, that connects to pin 9. This allows one leg of the 6.3vac to be on the ends of the 12.6v heater at pins 4-5, while the other leg is on pin 9, so 6.3v will heat a 12.6v tube.

The 5879 is a 6.3v tube, so it's heater connections are on pin 4 & 5, and the pin 9 center tap is not required. the short answer then is one wire from pin 4-5 of a 12a*7 socket goes to pin 4, and the wire from pin 9 of a 12a*7 would go to pin 5. This completes the circuit for that tube type.

[daveyajd]

Very good explanation. Thank you. Now, I know how the power light is connected using 6.3v heater wires but how will I power the LED that lights when I am on my OD channel? I have some very good info that tubenit sent me previously that explains how the wiring between the relay and manual switch/foot switch works but I don't see any connection to an LED or anything in what I have.


Thanks,
Aaron

[Willabe]

Here's the link for relay switching from Doug's tube amp library of information;

http://el34world.com/projects/relay_switch.htm


              Brad   

[daveyajd]

Thanks for the reply Willabe. It is pretty confusing to decipher that page.


I am using this prebuilt low power relay board from Weber if that changes anything.


https://taweber.powweb.com/store/chansword.htm


I understand the connections to and from this board that allow me to switch channels. But the LED is still unclear.


Thanks,
Aaron

[tubenit]

I am using this prebuilt low power relay board from Weber if that changes anything.

I have two Weber relay boards in a drawer that I removed from the amps I had them in because of the amount of noise they produced. One of the boards had a tolerable amount of noise. The other one was simply unusable for my standards because of excessive noise.

I did not have the same noise using Doug's boards.  Just an FYI.

I am not sure how to hook up LED lights on the foot switch using the Weber relay boards? 

With respect, Tubenit

[sluckey]

Do you need to operate a LED mounted on the amp chassis or in a footswitch?

[daveyajd]

The LED is mounted on the chassis not on a footswitch. I'm bummed to hear that these have been found to be noisy. Wish I would have known that years ago when they were recommended to me. I definetly would have gone with Doug's option.


Aaron

[sluckey]

There is a diode mounted near the relay. It is connected across the coil of the relay. Solder a wire to each lead of that diode. These two wires should be long enough to reach your LED. Connect the wire from the cathode of the relay board diode to the ANODE of the LED. Now solder a 470Ω resistor to the CATHODE of the LED. Finally, solder the other wire to the other end of the 470Ω resistor. The LED will now light up whenever the relay is energized.

Basically you have just put a LED and series current limiting resistor in parallel to the relay coil.

[daveyajd]

That is awesome! Thank you so much for that!


Aaron

[tubenit]

To be fair to the Weber relay, the one that was unusable for me was the 4 relay one.  The single relay noise was tolerable in my amp.  I used one relay (Weber) in an amp I built for my son and it seemed to work out OK in his amp. I don't remember his amp switching being noisy.  And, I've learned a lot about relays since I first used one.

Having said that, I've used Doug's relays in 3 amps now and his relay system is very quiet & I'm very happy with it.

With respect, Tubenit

[Willabe]

Wish I would have known that years ago when they were recommended to me. I definetly would have gone with Doug's option.

Doug's relay boards have only been around for a year or so?


                         Brad     

[daveyajd]

So a couple quick questions on my power supply wiring. Is it beneficial for me to twist my primary and secondary CT's together when I ground them? Not sure if that may cause weird issues.


Next, can I ground everything (CT's, PT ground, Mains) all to the main ground on the PT bolt? Or should I ground my main AC seperatly?


I know to run a buss bar along my pots to ground them but does that need to ground to that main PT bolt ground also?


Thanks,
Aaron

[daveyajd]

Here are a few new pics. Heater wiring is run, jumper wires on the 5879 and PI.


Aaron

[Willabe]

Looking good!   

So a couple quick questions on my power supply wiring. Is it beneficial for me to twist my primary and secondary CT's together when I ground them? Not sure if that may cause weird issues.

Some use twisted pairs when ever they can others don't. Sluckey just post a great build and he did not use twisted pairs on everything and the amp is very quite.

Next, can I ground everything (CT's, PT ground, Mains) all to the main ground on the PT bolt? Or should I ground my main AC separately?I know to run a buss bar along my pots to ground them but does that need to ground to that main PT bolt ground also?

Read this on grounding, if you haven't yet, it will help you.

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

Here's a pic of my GA77 build and I twisted pairs where ever I could.  It's very quite but it takes more time, not saying it's any more quite that Sluckey's. I look at it like preventive medicine, just in case.

Merlin likes to run the heater wires on 12 _ _ 7 type tubes over the middle of the tube socket and not loop them around the socket. You can see how I did this in 1 of the pics.


                        Brad   

[daveyajd]

Wow! Great link. Lots of good info in there. And I even understand some of it.    So I think I will plan on going back to my original plan of having a bus bar off of the pots. But the article mentions not connecting that third lug to the back of the pot, should I just connect it to that bus bar and not solder the back then? I guess I will ground my IEC, and CT's at one point, then just connect my filter capacitors ground to the bus bar. I am still a little ways from that point so I will continue my research.


Aaron

[daveyajd]

Alright. I just got my pots in. All except for the PPIMV dual gang. I will have to order that from AES I think. So I am going to start wiring the pots up a bit and I'm wondering a couple things. When looking at the layouts are they drawn from the viewpoint of lugs on top? In other words if I'm standing in front of the amp looking down the ground lug is on the right? What about the shorting jack for the input?


Also, after reading the link posted above it mentioned not grounding that lug to the back of the pot. I normally don't bend that to the back but I have always connected that to the lug with a jumper wire and then to ground. Is that correct?


One more thing, I found a dual gang pot at a military/government surplus store that measured about 180k on each pot. Will this be close enough to 250k to use?


Thanks,
Aaron

[TIMBO]

Take at look at Willabe's POTS (picture above)and this is how you will see them drawn on most layout plans (viewed from the back and ground lug on the left).

Willabe has opted to string all his ground lugs (preamp) together (green wire) some guys use a heavy gauge wire and make a BUSS and feed this through the loop in the lug. This works great till you have to replace a pot, so I snip a section out of the lug enough to slip the wire onto the lug.
You can ground your input jack to your pots and most ground this string at the input jack.

You can try use the 180k pot but you will have to change the parallel resistor to equal the grid leak resistor that the pot replaces.

[daveyajd]

Alright thank you. So I picked that dual gang up and I'm putting a meter on it. It measures about 185 or so and both pots are pretty close to each other which is good. Which resistor would I be replacing? Is it the 10K on the tube or the 2.2M on the pot going to ground?


Aaron

[TIMBO]

Where the dual pot is at the moment there would be two 220k resistors connected to ground, the dual pot replaces these resistors and therefore the pots needs to measure approx. 220k.
Your 180k is a bit shy of the 220k resistors and I'm not sure if you can use the pot without the parallel resistor, so hopefully one of the smarter guys will confirm. Doing great 

[tubenit]

You could consider doing this.  However, you wouldn't be able to turn the PPIMV volume to zero.  But when would you do that anyway?

With respect, Tubenit

[sluckey]

One more thing, I found a dual gang pot at a military/government surplus store that measured about 180k on each pot. Will this be close enough to 250k to use?

I'd use it.

[daveyajd]

-In regards to the info that Sluckey gave me above about putting an LED and limiting resistor parallel with the relay coil for my OD indicator.-


I have twisted the two wires that will come from the diode. Can I run these close to my heater wires that are running to the relay board or will this maybe cause some noise and hum problems?


Aaron

[sluckey]

I have twisted the two wires that will come from the diode. Can I run these close to my heater wires that are running to the relay board or will this maybe cause some noise and hum problems?

Not a problem.

[daveyajd]

I am ordering a few more parts soon and hopefully part of that order will be the electrolytics. The BOM calls for 150uf/50v. Seems to be pretty hard to find so the substitution is a 100uf/100v and a 47uf/100v in parallel. I have the 100uf/100v and the only 47uf/100v I find is an Illinois. Since the specs call for 150/50v I should be able to use a Sprague 50uf/50v right?


Also having a hard time finding the 10 watt resistor in 430R value. The closest I find is a 400R or 500R. Will either of these work. Or can I run the 400R and a 30R in parallel?


Thanks,
Aaron

[tubenit]

The 10w resistor can be 400R.   Honestly, you probably could use anything between 270R to 430R.  If you are ordering them, maybe get 270R, 330R, 360R, 400R.  Start with the higher value and work down listening to the tone.

Regarding the power tube cathode filter caps.  Anything 50v-100v should be fine.  Why don't you try the 100uf/100v you have and see how you like that.  You can later parallel a 47uf to 50uf  in the 50v-100v range if you want to.

With respect, Tubenit

[daveyajd]

That's great! Thank you. So the filtering will be fine with just 100uf? Interesting. What kind of changes to the tone are typical as I go down in resistor value?


Aaron

[tubenit]

What kind of changes to the tone are typical as I go down in resistor value?

Smaller resistor will increase the gain & sometimes warmth.  Larger increases headroom and sometimes clarity.

I have said it before ........... the Tweed Overdrive Special is a tweaker's build. Build it reasonably close to original and then adjust to personal taste in tone.  You can try different size filter caps for the cathode biasing.  Larger sized caps get it closer to sounding like
fixed biased tone, IMO.  Lower value caps can give a remarkable smooth tone but you do lose some bass tone as a result.

The area I would leave alone or tweak the least is the OD section.

With respect, Tubenit

[daveyajd]

Here is one of my questions of the night. Looking at pin 7 of V1. Do I need to run 2 wires from the it? One wire to the outer lug of FAT pot, the other to the middle lug of the volume pot? Or can I run 1 wire from the pin to the FAT pot and the on to the Volume pot? Thanks for all the help.


Aaron

[Glennjeff]

Hi daveyajd

I'm in Australia not USA but I think it is standard electrical practice to have a separate chassis ground bolt near you IEC mains socket and to run the IEC ground wire lug to that bolt.

The idea being that for electrical safety reasons the metal chassis has an easily inspectable / verifiable and very robust connection to earth.

Run all other grounds to a star point bolted securely to the metal chassis.

In my opinion, the heavier duty your ground wires and busses the better. I use 50 amp (copper) cable for ground wires and busses.

I may be in error on this.

[daveyajd]

Thank you Glennjeff. Yes, I did choose to ground my IEC at its own bolt. I just thought it would be the easiest and best way to go.


Aaron