'Grid Stoppers' for Drip Electronics LA-2A REV10 PCB

[matta]

Hey Folks,

Since this is being discussed at length and I know that parts of this info is scattered across various threads I thought I'd try put in all in one for future reference.

I'm sure there are quite a few people using the Drip Electronics LA-2A REV10 PCB for their LA-2A project, myself included, along with the Sowter Irons.

It was brought to my attention that this MAY cause oscillation and that 'grid stoppers' may need to be added and since these were not accounted for in the Drip Rev 10 boards, need to be added. (The newer boards have corrected this I belive).

Now what/how do we go about doing this, or where do they belong blah blah etc.

Quoting AnalogPackrat

Are you using a Sowter output transformer? Those are known to sometimes cause HF oscillations in the WCF (12BH7 part of the circuit). To eliminate this, simply put 1k grid stopper resistors in series with the grids on both 12BH7 triodes. Desolder your current grid connections at the V2 socket, solder 1k resistors to the tube socket (pins 2 and 7) and then solder your original grid wires to the free ends of each 1k resistor.

Now this is easier said and done with a P2P version but how do we do with the REV10 PCB's?

Firstly lets look at the circuit diagram and see what needs to happen.

As you can see we need to add 2 x 1K resistors in series with the Pin 2 and 7.

Looking at the Drip PCB here is what the silk side looks like.

And the track side looks like.

Now I know what needs to be done but to be honest since I've not done this myself and I'm wondering what the easiest/cleanest way based the PCB above would be to do this 'mod'?

Cheers

Matt

 

[Gus]

What I might do

pin one cut the trace and solder the resistor from pin one to a solder pad on the bottom of the board.

Pin two it looks like you will need to cut pin two pad away from both traces going to C 3 and R 19. You will have three seperate pads. Jumper C3 to R19 and add the resistor on the bottom.

 

[matta]

[quote author="Gus"]What I might do

pin one cut the trace and solder the resistor from pin one to a solder pad on the bottom of the board.

Pin two it looks like you will need to cut pin two pad away from both traces going to C 3 and R 19. You will have three seperate pads. Jumper C3 to R19 and add the resistor on the bottom.[/quote]

Thanks Gus,

I get the Pin 2 option I think. I have come up with another solution that may work for Pin 7, based on your description and my deduction.

What do you think of the following?

EDIT: See updated pic below....

Thanks again for the input.

Cheers

Matt

 

[NewYorkDave]

How about cut the traces at pins 2 and 7, gently scrape away the coating on both sides of the cut, tin the traces and tack on surface-mount resistors?

Anyway, in your drawing, you have one of the grid lines crossing the plate, which is something you should avoid doing.

 

[matta]

[quote author="NewYorkDave"]How about cut the traces at pins 2 and 7, gently scrape away the coating on both sides of the cut, tin the traces and tack on surface-mount resistors?

Anyway, in your drawing, you have one of the grid lines crossing the plate, which is something you should avoid doing.[/quote]

Thanks for the advice Dave.

This is my first ever Tube/Valve project and I'm taking the easy route on a PCB, high voltage scares me, Hah hah!

Thanks for the info and advice on NOT the crossing the plate.

It is easy to do as you say with Pin 7, but harder with Pin 2 as the junction meets at the socket leg and not much trace work with.

I had another idea that may be neater and cleaner....

How about cutting off the PCB mount leg of the tube sockets on PIN 2 & 7 on the component side of the board and then putting the resistors in series with the PIN 2 and PIN 7 solder pads and solder it directly to a part of the cut socket pins?

Matt

 

[Gus]

I read pin 7 as a pin 1 My part with pin 1 is no good.

 

[NewYorkDave]

Cut the socket pins? Nah, I wouldn't do that. It'd weaken the mechanical stability of the socket.

Looking at your photo more closely, I'd cut the trace to pin 7 and put the resistor between the pads of pin 7 and R15. Cut the traces between C3 and pin 2 and between R19 and pin 2. Run a jumper directly from C3 to R19, and the resistor from R19 to pin 2. There's no significant current in the grid circuit so you could use "wire wrap" wire for the jumper.

 

[Rob Flinn]

Thing to remember with grid stoppers is to make the body of the resistor as close to the valve base as possible. Or at least that is what is says in valve books of old like Mullard etc

 

[matta]

[quote author="NewYorkDave"]Cut the socket pins? Nah, I wouldn't do that. It'd weaken the mechanical stability of the socket.

Looking at your photo more closely, I'd cut the trace to pin 7 and put the resistor between the pads of pin 7 and R15. Cut the traces between C3 and pin 2 and between R19 and pin 2. Run a jumper directly from C3 to R19, and the resistor from R19 to pin 2. There's no significant current in the grid circuit so you could use "wire wrap" wire for the jumper.[/quote]

Thanks Dave,

I think what you are saying is very similar to what I had in my orginal drawing, the only difference being the PIN 7 hook-Up, here is what I THINK you are getting at.

Thing to remember with grid stoppers is to make the body of the resistor as close to the valve base as possible. Or at least that is what is says in valve books of old like Mullard etc

Thanks Rob, always good to know and will implement it as such.

Cheers

Matt

 

[matta]

Well here you have it!

I took a photo so others can now see how the mod is done on this PCB. I think it will save confusion in future.

I see that Sonic Warrior has already added it to the META (Thanks!).

Thanks to everyone who chimed in, I and I'm sure others to come, thank you.

A couple more bits and bobs to add and tomorrow I should be able to fire her up!

Cheers

Matt

 

[Purusha]

Hey guyz, can I use the smaller (blue coloured) 1K resistors for this grid thing? I see Matt
used bigger 1W resistors... but I don't have any other 1K at my hand at the moment.

 

[JdJ]

Thanks Matta!
:thumb:

I have to do this at some point. Please let us know how it works out.

-Josh

 

[Purusha]

I am almost done. Will report if it helped me.

 

[NewYorkDave]

Purusha, teensy resistors are fine. As I mentioned above, there's no appreciable current flowing in the grid circuits.

 

[Purusha]

[quote author="NewYorkDave"]Purusha, teensy resistors are fine. As I mentioned above, there's no appreciable current flowing in the grid circuits.[/quote]

Thanks, although I took apart one of my cap discharge cables and used
0.5W resistors at the end. Too bad I didn't see your post above in time

BUT HEY, it works great now!!! NO OSCILLATIONS ANYMORE.

Now I just have to find out why I have hum if I insert the LA-2A into my
Midas which has unbalanced inserts. The hum is gone when connecting
it through normal balanced chain. Any idea why?

 

[dripelectronics]

holy bejeezers.

i think ive been asleep for ages .

way nicework , matta.


yeah im glad i added them to the v2.

your great. !

g.

 

[matta]

Please let us know how it works out.

As Purusha said it works great and while I didn't check to see IF I would have problem I thought there was no harm in adding them and my unit sounds great, no problems whatsoever.

way nicework , matta.

Hardly! All the hard work and research was done by you! Rather let me thank YOU for all your hard work and for a great project.

Cheers

Matt

 

[wkbdgeorge]

Is this grid stopper idea only needed in this pcb?  What about a point to point?  What about other tube compressor designs?

 

[kazper]

based on transformer used... and is in the quoted text on the first post of this very thread...

 

[wkbdgeorge]

Sorry, that answered half of my question. But, Why would the transformer change the need for that??  If everything else in the schematic is the same, why would the transformer change anything if its ratio is the same?