New SB220 meters. Finally.

[nomadradio]

Progress on the SB-220 meter front.

This is the guinea-pig amplifier that gets the first pair of these substitutes. This should be a "beauty shot", but the bright white meter faces get overexposed.

The red wire on the plate meter no longer goes to point "A" on the meter board, but has to get moved here, to point "F".

This bypasses the original 3600 ohm resistor in series with the winding of the stock meter. The resistors on the Plate meter's pc board add up to 1000 ohms. They take the place of the resistor that gets bypassed.


The wires to the grid meter are a bit short.

I'll have to come up with a fix for that, if I'm going to market them as a "DIY" solution for a blown meter.
I still have no setup to illuminate these meters. The two brown wires that fed 5 Volts AC to the lights on the old meters should get taped up, at least for now.

For now, the right-hand meter reads grid current only. I'm going to try making it read the high voltage by changing the three 1.5-meg 2-Watt resistors on the amplifier's meter board to a series string of six 510k 2 Watt.

Should work.

More to come.

Later.
73

 

[BlowinSmoke]

What is the specs on your new meters? FS current and internal resistance? You should be able to change the original resistors on the board and add one perhaps and you won't have to go nuts. What are those values on the new meters?

 

[nomadradio]

New meters have a 1 mA full-scale sensitivity, compared to the stock meter's 200 microAmps.

Coil resistance is 18 ohms, compare to the nominal 1300 ohms of the original meter.
The two series resistors on the board allow us to draw 1 mA from the original grid-current shunt resistor. The original meter simply connected directly in parallel with the shunt resistor. The new meter needs the voltage-dropping resistance in series with it.

The stock plate meter has a 3600-ohm series resistor. The new meter needs to show full scale with the 1-Volt drop across the 1-ohm factory shunt resistor. The resistors on the plate meter's adapter board add up to the 1000 ohms (including the coil's resistance) needed to read full-scale at 1 Amp of plate current. If we used the original 3600-ohm dropping resistor it would never read more than about one-quarter scale at 1 Amp of plate current.

Part of the "go wild" appearance of the adapter board has to do with the protection diodes. You want to hold the fault current through the meter coil to around 200 percent of full scale to prevent coil damage. I use 1N5819 schottky diodes back-to-back. They conduct at around 2/10 of a Volt. The nominal 980-ohm dropping resistance gets split into two series resistors. The point where they meet will have about 1/10 of a Volt across it at normal full-scale reading. The diodes will kick in and conduct at around twice that. Could have used a single dropping resistor if we just skipped the protection diodes.

Used the same math for the grid meter, but at the lower voltage drop that appears across the 0,82-ohm grid-current shunt resistor.

The intent is to minimize the installation complexity. Changing a resistor on the stock meter board would be easier for me, but not necessarily for a weekend DIY.

73

 

[nomadradio]

Time sure flies when you're having fun.

Took me a while to track down a source for the backing plates to fit this meter. Had them made by a local "wholesale" sign shop. Never knew there was such a thing as a wholesale sign shop. The guy who makes my outdoor signs referred me to them. They take that "wholesale" thing seriously. The name isn't even on the outside of the building, just their logo and street number.

But they have a monster computer-operated router. They found the right thickness aluminum sheet for this piece and cut out 130 of them for me. I guess a quantity that size qualified as wholesale.

He even found some sheet stock that's white on one side.

They fit perfectly. For now I'm printing the scale on plain copy paper. The "Meter" program from Tonne Software does the job, but I'd like to tweak the scale art a bit more. Got some fancier paper that's a bit brighter, but I still haven't broken open the package.

Next thing I need to figure out is a way to cover the bottom half of the thing. Used some black electrical tape with a satin finish for the first couple of these. The black-plastic piece that comes with the factory version of this meter snaps on, but where I would buy that piece by itself is anyone's guess. Sounds like a job for a 3D printer, but getting the surface finish to look nice becomes an issue.

It's still not a product ready for prime time, but it's a big step closer.

Gettin' there.

Slowly.

73

 

[Shadetree Mechanic]

Looking good! Maybe the sign shop could cut some blanks for it with tabs for the 3 sides? Then bend the sides down in a soft jaw vice. Glue them on with two dabs of silicone sealant.

 

[nomadradio]

That's an idea I didn't have. Might be the best solution.

Thanks for the suggestion.

73

 

[nomadradio]

Even elephants don't have a gestation period this long.

Progress is slow, but continues.

Gotta come up with a better scale than copy paper. Turns out it has a tendency to warp. This one curled up just enough to make the pointer hang up.

Bought some plastic 8.5x11 sheets that are supposed be laser-printer friendly. Boy Howdy that stuff is expensive!

Still haven't tried printing a scale on that stuff. Sure wish the software would put more than one on a single printed sheet.

One bright spot, though. Finally ordered fifty sets of the circuit boards. Just gotta get them cut apart, get the parts on them.

Just makes me wonder if we'll have a quantity to offer before the very last SB220 gets dumped at the local e-cycle depot.

73

 

[kopcicle]

 

[nomadradio]

Talk about a back burner.

Used a couple of these for repairs here, but didn't get all that far as a resale product.

I discovered spray contact adhesive. MUCH better than double-sided tape.

Made a jig to get the scale lined up squarely with the backplate.

Turns out a sharp knife works best to trim the scale to the backplate.

Still better than using scissors.

The flat black electrical tape that covers the lower third of the face is kinda lame, but does the job.

Did finally track down a part number for the factory piece that just snaps over the bottom part of the face. They call it a 'mask', number 5203K18P0001.

Fat lotta good that did, though. Apparently nobody imports that piece.

Closing the shop's door for the plague has reshuffled some priorities.

Ugly or not, they'll get ready for prime time soon.

We always say that.

73

 

[kb7uxe]

Hello from Dan kb7uxe ( good on QRZ )
when you get them about done,
I would like enough for my 2 amps..
the original meters are getting a bit ""cranky""....
But them so am i .... ;-)
old age does that to you....
Thank you
Dan...

 

[walterjn]

Just an idea, why not tape off the clear meter cover & spray paint the flat black mask on? A few extra steps I know, however it might be easier than the electrical tape.

 

[GlocknSpiel]

Very cool! Work the kinks out and mass-produce these meters as a set on Feepay, and you'll sell gajilions!
I know this is an old thread, but what can I say? I did a search for "Grid+Plate+sb220" on 'duckduckgo', and this old thread popped up first.
They look great too!

 

[AG7K]

I know this is an old thread but did this project die??

 

[GlocknSpiel]

I found a possible printable solution to meter scales using an app called "METER" available from tonnesoftware.com. Measure the original scale, design the scale using the "wizard" feature, then print out the meter nomenclature onto Koala's "Waterslide Decal Paper", and there ya are. The remaining problem is to find actual meters that are the same physical size, with the same movements as the originals. I don't even know what the movements are. Ebay has a lot of meters that can be re-labled for amplifiers, but most are chinesium.
I've been playing around in "METER" and designed one for a 5KV DC meter I had laying around.
The pic shows a rough idea of what the app can do. I printed it out on regular paper to save the decal paper for actual meters. ALSO, this decal paper is great for labeling on homebrew projects.
But this meter pictured is Yuge! And Chinese. So it' no good for SB220's ,or other real American amplifiers!

 

[nomadradio]

Yeah, that's the program I'm using for meter scales. This project stalled while I came up with a decent-looking way to cover the bottom part of the meter. The original design had a black plastic piece that clipped over the transparent lower part of the meter face, concealing the zero lever and pivot stuff. Can't buy that piece by itself. Figured it was a candidate for a 3D-printed part, but that part of the project just stalled and stalled. Really ought to get that part drafted in 3D CAD and try it on a borrowed printer.

73