Tweed Overdrive Special & Carolina Overdrive Special

[sluckey]

I don't get the theory behind the choke's DCR. Copper resistance yes, but as to the function of the inductor I thought that it would show the 3 H at double the mains AC: X = 2piFL.

Inductance is not dependent on frequency, so a 3H choke is 3H at 60Hz, 120Hz, or 1000Hz.

On the other hand, Inductive Reactance (X_L) is dependent on frequency. As frequency increases, so does Inductive Reactance. And that's the beauty of a choke used in a power supply. A perfect 3H choke would offer 2261Ω of resistance to a 120Hz ripple signal, but it would offer 0Ω resistance to DC. Ideally, we want the choke to offer a lot of resistance to the AC and no resistance to the DC. This is how a choke helps to 'smooth' the AC ripple into a relative 'flat line' DC. An inductor (choke) opposes any change in current (AC component) and a capacitor opposes any change in voltage (AC component). When a choke is used together with a couple capacitors in a PI arrangement, they work together to smooth the AC pulses to a usable DC.

But an inductor is not perfect. There will always be a DC resistance due to the resistance of the wire. This unavoidable resistance is in series with the choke and is responsible for any DC voltage drop across a choke.  And that DC voltage drop is what you are trying to mimic when you replace a choke with a resistor. By using a resistor whose resistance is equal to the DCR of a choke, you will maintain the same voltages downstream on the B+ rail. For sure, the filtered DC will not be as smooth as it would be with a choke, but that's not always necessary or desirable in a guitar amp.

[Ross]

If I understand Sluckey correctly, putting up only 6% of the choke's impedance (150/2.25K) at the offending frequency is what one gets for swapping out the choke. This effects the circuit's response, I would guess giving more sensitivity and a little more voltage but with the cost of some noise.

[sluckey]

Please keep in mind that I used 150Ω DCR purely as an example. I pulled that number straight out of my back pocket. It is a fairly realistic number though. Generally speaking, a 3H 200mA choke will have a smaller DCR than a 3H 100mA choke. More current capability usually means larger wire which usually means lower resistance.

[HotBluePlates]

If I understand Sluckey correctly ...

Are you needing to replace the choke with a resistor in your build, or is the question just academic?

[John]

Sluckey, great explanation. I knew that a choke was "better" but didn't know why. And of course, this is why a choke is better money spent on a SE amp rather than push-pull, because SE is naturally more noisy and the PP tends to "cancel out" the hum. <insert light bulb moment here>

[Ross]

Anyone out there with a TOS sized choke willing to put a VOM on it and let me know what its DC resistance is?

[tubenit]

Ross,

FWIW .......... I don't think it's a critical factor at all!  The COS I built had a choke. I later converted it to a TOS when I sold it.  Then TOS I built for myself had a resistor. The TBM has a resistor. The D'Mars ODS has a resistor. I think those are all 1k/2w?

I think anything from 220R/2w to 1k/2w would be fine.  I have switched between a resistor and choke before and didn't hear much difference. Choke maybe was a little smoother but not much.

With respect, Tubenit

[sluckey]

Anyone out there with a TOS sized choke willing to put a VOM on it and let me know what its DC resistance is?

I don't have one handy but the BOM lists the choke as 125C1 (probably should say 125C1A). Hammond 194B is a direct replacement and it's DCR is 108Ω +15%.

[Ross]

Thanks, Slu!

[stratguy222]

Finally getting around to this project, caught my attention a year ago and had it bookmarked since. Iv been studying the schematics posted for both the VVR and PPIMV versions, and I assume the PPIMV schemtic would be a good starting point, and I could always experiment with VVR later. Pretty excited to take on this project, planning to tweak the design to my tastes, and I have also been drawing up a quick visio layout that I can post here if anyone is interested.

[tubenit]

a quick visio layout that I can post here if anyone is interested.

That'd be great!  And I'd love to see one.

With respect, Tubenit

[Fictitiousfreedom]

Hopefully this is the correct place for this. I figure if my layout is correct it could someone in the future.

Im building the standard version TOS and have a few questions and also was hoping to share my layout. I do my layouts with affinity Design 2 on ipad, its easier for me than anything else i have tried.

The biggest problem I'm having is with the relay. First I made a layout of the TOS with just a dpdt switch for the channel switch like in the schematic. Then i made one using The Hoffman relay boards. My Transformer does have a center tap so i made sure to keep the power supply for the relay floating, the schematics and diagrams available show the bottom left terminal of the relay board connected to ground. In my layout i connected that terminal back to the “negative” terminal, as its been described, of the relay power supply board. Is this correct? I also didn’t include the LED or foot-switch jack but i understand how that works and I’m not sure if I’m going to use the relay yet.

I have a few other simple questions. The 5879 only has two heater connections, was i correct to assume that i would connect one heater to pin 4 and the other to pin 5?

Lastly, i made a board for my filter caps. Its quite big so i plan to use tall standoffs and mount it partially overtop of my power transformer cutout. Would it be better to cut the board into two separate parts? Im not sure if raising it up off the chassis too much could cause problems or if its just better to separate it regardless.

Thanks for the help, I've learned so much from this forum but i have a long way to go. This is my 5th amp and without these discussions i wouldn't have been able to do any of them.

[sluckey]

You have the negative side of the 5V power supply board connected to a relay contact. That's wrong! The negative side of the 5V PS should only connect to the switch and then to the relay coil.

[Fictitiousfreedom]

You have the negative side of the 5V power supply board connected to a relay contact. That's wrong! The negative side of the 5V PS should only connect to the switch and then to the relay coil.

Thanks for such a fast response!
I had a feeling that was incorrect but i cant figure out whee the bottom left terminal on the relay board goes. The schematics and diagrams all show ground. Which i understand that the signal needs to be switched to ground but because im using my transformers center tap i wasn’t sure if it was ok to connect the relay board to ground or if it was only the power supply that you never connect to ground. So i should connect the bottom left terminal to the ground bus on my pots? Thanks Sluckey, your an incredible source of information and i truly appreciate you and your time.

[sluckey]

So i should connect the bottom left terminal to the ground bus on my pots?

yes

[tubenit]

Lastly, i made a board for my filter caps. Its quite big so i plan to use tall standoffs and mount it partially overtop of my power transformer cutout. Would it be better to cut the board into two separate parts?

On all 3 of my amps I use turrets on two strips. This has worked well for me and I like being able to get at any wiring underneath IF needed.  And I use somewhat taller standoffs.


with respect, Tubenit

[Fictitiousfreedom]

So i should connect the bottom left terminal to the ground bus on my pots?

yes

Thank You!!!

Lastly, i made a board for my filter caps. Its quite big so i plan to use tall standoffs and mount it partially overtop of my power transformer cutout. Would it be better to cut the board into two separate parts?

On all 3 of my amps I use turrets on two strips. This has worked well for me and I like being able to get at any wiring underneath IF needed.  And I use somewhat taller standoffs.


with respect, Tubenit

Thank you as well!  You guys are the best! Ive been planning and preparing this build for a really long time and i think i am ready to start drilling my chassis.

Ill update my layout and post it just so that if anyone in the future can use it it'll be here.

Thanks again Guys your truly great.

[Fictitiousfreedom]

I haven't forgot to post my layouts and info, i’m trying to finish my faceplate still. Doing it after the build is not a great idea. I cut a piece of 16 gauge steel with a chisel, hammer, and a hacksaw. Not easy lol  Ive made 4 wooden templates and can finally drill it. I have a co worker 3d printing me some stencils i designed and if that works out it will finally be complete.

For anyone looking for info before i finish i have everything on my website www.fictitiousfuzz.com/tubeamps

[mebond11]

Hello, I am planning on building the TOS, and I had a question about the negative feedback.   Is it worthwhile to wire this as a switched NFB or a presence control?

[sluckey]

I would build IAW the schematic. Once the amp is up and running is a good time to play with NFB.

[mebond11]

Thank you!….another question about the Tweed Overdrive Special…What OT primary impedance has worked well with the 5881’s?   I am thinking 6600, which is the impedance of the 018343 OT, although many clones of this OT are 4200.  I don’t need any extra power…   

Also, the 022798 replacement PT is a little higher voltage than shown on the schematic (355-0-355, though Hammond does make a 325-0-325)…this would be around 460V B+.   The JJ 5881 data sheet says 500 plate volts max, and I think I can subtract the cathode voltage from the B+, but are there any other issues with the slightly higher voltage?

Thank you in advance for your help.    Really appreciate it and look forward to building this amp.

[tubenit]

I think of the Tweed Overdrive Special as a "tweakers amp" meaning it's one you build and then tweak/mod to best suit your taste and tone in playing. So, in my thinking the TOS doesn't require/need precise following the original schematic to get a great tone. However, I would lean towards following the schematic 1st and then make small changes at a time to mod it to your tone preference.


I think the 6600 OT is fine.  Dumble purposely mismatched OT impedance on some of his builds.


Over time, I have found myself preferring and enjoying lower voltages on output tubes. I think sometimes there is more warmth in tone playing at lower/softer volumes with lower voltages on output tubes.  Just my opinion and experience and it's subjective. Others may feel the opposite.


IF I were building another TOS,  I would probably use a Hammond PT with 300-0-300 and at least 140ma.  I think 325-0-325 is fine also.  Given the TOS is cathode biased, using 300-0-300 would comfortably allow the use of either 6V6's or 5881/6L6 using a GZ34 or even a solid state plug in rectifier.


I do think some specific brand resistors for plates and specific caps can play a factor in a smooth tone with this amp. Again that's just an opinion, personal preference and my experience. What I prefer may not suit others at all. You can check out The Amp Garage Dumble clone files to see about resistor and cap preferences with those amps.
(10) #102 Robben Ford Amp (High Plate Skyline) - The Amp Garage


with respect, Tubenit

[mebond11]

Thanks for the quick reply!   Hammond makes a PT with two taps, 355-0-355 and 310-0-310.   I may try that one, which has some flexibility for tweaking, or go with the 325-0-325.   I’m using a super reverb chassis, so I’m trying to fit the existing PT cutout, which limits the choices somewhat.

Thanks for the help!

[Tone Junkie]

Probably not the best place but tubenit .I have found I like lower voltages on some tubes also. For a non Marshall amp I like about 385 volts on my el34,s sounds really good. Bill