Another Tweedle Dee

[mkslwsk]

Hi Everyone.

I'm new to TAG but have had a few experience with tube amps in the last few years.
I build amps and fix some friends gear as a hobby and to learn more about.

I recently read Charlie Wilson topic on the Tweedle-D and had an itch to build one.

Last weekend I decided to build it and from Friday to Sunday did all the work required.
I made my own chassis from 1.5mm aluminum just because I have the tools to properly work this material at this size.
I also decided to make my board with a technique I recently saw on a Hamradio build.

Wired it all up and it sounds amazing!!

I shot a p.m. to Charlie to thank him for postings this project but then decided that it was worth posting here to thank everyone else that contributes to the vast amount of information that is available here.

There is no head-shell or cabinet because I still don't have the tools and knowledge required for a proper woodwork.

For some reason I can't attach pictures but you can check them out here in a GooglePhotos album: https://photos.app.goo.gl/KFVJ2LW9RozrWWs9A

Thanks again to Charlie and to TAG!

Mikhail

[R,i,c,k]

[quote=mkslwsk post_id=415824 time=1597338444 user_id=21131]
Hi Everyone.

I'm new to TAG but have had a few experience with tube amps in the last few years.
I build amps and fix some friends gear as a hobby and to learn more about.

I recently read Charlie Wilson topic on the Tweedle-D and had an itch to build one.

Last weekend I decided to build it and from Friday to Sunday did all the work required.
I made my own chassis from 1.5mm aluminum just because I have the tools to properly work this material at this size.
I also decided to make my board with a technique I recently saw on a Hamradio build.

Wired it all up and it sounds amazing!!

I shot a p.m. to Charlie to thank him for postings this project but then decided that it was worth posting here to thank everyone else that contributes to the vast amount of information that is available here.

There is no head-shell or cabinet because I still don't have the tools and knowledge required for a proper woodwork.

For some reason I can't attach pictures but you can check them out here in a GooglePhotos album: https://photos.app.goo.gl/KFVJ2LW9RozrWWs9A

Thanks again to Charlie and to TAG!

Mikhail
[/quote]

Mikhail-
Looks good. It's good to see, when you say you have the tools to work the aluminum you mean a straight strong piece of wood and two c-clamps. I think it's much more impressive to accomplish things without throwing a pile of money at it.

If you want to detail the method you used to make the boards, I'd like to hear about it.

-ric

[pompeiisneaks]

Cool build... question, is that copper clad board? I'd think having the uncovered leads of hte components that close to the large copper area is quite dangerous. It could cause shorting and all kinds of other havoc.

It might be worth it to remove any copper that's not where it's needed?

Otherwise, cool build!

~Phil

[mkslwsk]

when you say you have the tools to work the aluminum you mean a straight strong piece of wood and two c-clamps. I think it's much more impressive to accomplish things without throwing a pile of money at it.

I tried to bend the aluminum sheet using wood and clamps, but gave up after trying really hard to get a nice edge.
The C-clamp and wood that you can see in the picture is actually to guide the cut, not to bend.
I got a metal brake made by a local shop here, it can handle steel up to .5mm and aluminum up to 2mm, and the max-width of 50cm regardless of the material. It is not expensive and works great!
I tried to make the brake myself but I don't have the proper tools and technique to deal with steel bars and the like.

If you want to detail the method you used to make the boards, I'd like to hear about it.

https://www.youtube.com/watch?v=RKdsXU5iIF0

The only difference is that I used a diamond drill (12mm if I remember) like those: https://www.amazon.com/Diamond-Baban-Co ... B01L92YM1A

Cool build... question, is that copper clad board? I'd think having the uncovered leads of hte components that close to the large copper area is quite dangerous. It could cause shorting and all kinds of other havoc.
It might be worth it to remove any copper that's not where it's needed?

It is indeed a copper clad board. I just broke and drilled it.
Clearance between any pad is at least 1~2mm, I'm not sure if that could be a problem.
I had a short near the PSU because one cut was shallow, just made it again with a box-cutter.
And I also accidentally shorted it once while plugging an alligator clip to measure voltages.

I connected the large copper area to ground, to act as a ground plane. But I'm also not really sure if that has any impact on the circuit.


Now you got me thinking. I had one issue with oscillations with all volumes and tones maxed.
Could this possibly be because of this ground plane?

[drew]

I shot a p.m. to Charlie to thank him for postings this project but then decided that it was worth posting here to thank everyone else that contributes to the vast amount of information that is available here.

One of the quirks of this site is that you can send a PM, but the recipient does not receive an email alert that he has a PM waiting. So if you send a PM to someone who posts regularly, that person will probably see the "new message" notification on the upper right corner of the screen; but a PM to a less frequent visitor like Charlie may never be seen, or only be seen months later.

[pompeiisneaks]

Not sure about oscillations, but you could cause all kinds of intermittent shorts to ground from B+ if not careful, the leads of any of the resistors/capacitors could touch the 'ground' area and cause a dead short. you could avoid this by covering the leads with some small shrink tube etc, or put some kind of conformal coating under the leads?

~Phil

[mkslwsk]

I tried to be really careful with all soldering, and after mounting all components I tested all pads/leads for continuity to make sure none was touching ground.
I was thinking that it would be possible to have a short like a discharge/spark because of the small clearance/creepage.
But you are right Phil, The short I created at the power supply while building the amp could have happened in any other spot and be a bit hard to find. Maybe working on a blank board adding small copper pads (like eyelets) would have been a better approach.
I believe I can cut round copper pads with the same drill bit, just a matter of running it through the whole board.

Regarding the oscillation I changed the wires that goes from PI plates to power tube grids for shielded wires grounded at the PI side and also added a 100p cap between plate and grid on the stage right after the volume and tone pots.
According to my calculations and simulations this cap should create a low pass filter with cutoff near 10kHz.
Not sure if the math is right but that solved the issue with no significant changes in tone.

I also made another change to the original circuit. Added a master volume right before the cathodyne. This means more wires and may have contributed to the oscillation.

[mkslwsk]

I did not realised that by adding that 100pF capacitor I would cut lots of frequencies when volume was not maxed.

Started chasing the oscillation, it should some how be related to the MV I added because no one ever reported this problem with this build.
It was indeed. My MV was routed to the 4th input hole.. plate and grid wires, a capacitor and the pot all having signal in phase with the input! when the volume was up enough started oscillating!
Rotated everything 180deg to have the lugs far away from the input jack and it is now fine.

[alkuz1961]

I strongly recommend replacing the board with something more isolated. During operation, this board will experience vibrations and other dynamic loads. Capacitors have a large mass and thin leads. So the inevitability of a short circuit just a matter of time.

And it also seems to me that the resistors have too small power rating.

IMG-20200808-WA0015.jpeg

What I see in the photo is most likely 0.125 W. For plate it is better to use 1W resistors.

[mkslwsk]

Thanks alkuz,
I understand the reason for the cap to short againts the ground plane and that would be bad..

Resistors are all .5W or higher. I do have 1W that could have used for the plates though.

I do have a hard time getting good components at a reasonable price.. and for some reason stores are short on some components here, incluing higher than 1M resistors and 1 to 2W ratings.

This board was an experiment for this technique and while chasing the oscillation one pad already unglued and was ripped, I'm planning a replacement board made of fiber with eyelets or turrets and holes to guide and hold the wires.

[drew]

What I see in the photo is most likely 0.125 W. For plate it is better to use 1W resistors.

Check out the 1/2 watt metal films that Mouser just sent me:

BF5F6B80-FF06-4CA4-9AE4-92E67C595E42.jpeg

[alkuz1961]

I have had real experience using small-sized resistors with a power of 0.6 W in a compact preamp. And they were punctured by high voltage after a short period of time. I believe there are similar small resistors that have not only a power rating, but also a guaranteed voltage protection, but most likely this does not apply to this case