Bypass cap and resistor jig

[Mike_J]

Want to create a jig for determining best bypass cap and resistor values for new builds. Thinking of using metal film resistor values of 820R, 1.5K, 1.8K, 2.2K, 2.7K and 3.3K and electrolytic cap values of 1, 5, 10, and 22uF. The attachment represents how I see it being constructed.

Using two eight position non-shorting rotary switches rated for 300mA of current and 125VDC. Place a 1.5K carbon comp and 1.5K carbon film resistor in open positions so all positions have a resistor on them and there will be a path to ground for the cathode? Will also allow for comparing the tone difference, if any, of the three different types of resistors commonly used in guitar amps.


Does anyone see a problem with this design or have any improvements to suggest?


Thanks
Mike

[JB]

I made something very similar a few years back.  I went with a pot instead of switched resistors, and made a paper scale with most common Rk values marked, e.g 820, 1K, 1K5, 2K2, 2K7, 3K3, 3K9, 10K etc.

I used a rotary switch for the caps though, with a range of values from 0.47, 0.68 etc. on up to 22uf. 

I haven't played with different resistor types particularly, not sure you'd hear much difference in a cathode resistor with only 1 or 2V across it and a couple of mA's flowing.

[Mike_J]

I made something very similar a few years back.  I went with a pot instead of switched resistors, and made a paper scale with most common Rk values marked, e.g 820, 1K, 1K5, 2K2, 2K7, 3K3, 3K9, 10K etc.

I used a rotary switch for the caps though, with a range of values from 0.47, 0.68 etc. on up to 22uf. 

I haven't played with different resistor types particularly, not sure you'd hear much difference in a cathode resistor with only 1 or 2V across it and a couple of mA's flowing.

The pot is not a bad idea. Could put a couple of lower value caps in the open positions on the switch. Had to place something in the open positions so there is always a resistance to ground on the cathode. The resistor type came to mind versus using two more metal films although I think you are right there probably won't be much difference in tone between a metal film and carbon comp at two volts.


Thanks,
Mike

[tubenit]

2.2uf is a pretty common value in my builds. 

With respect, Tubenit

[Mike_J]

2.2uf is a pretty common value in my builds. 

With respect, Tubenit

Thanks Tubenit, will add it to the board. Think the board needs to have two levels so there is no risk of getting shocked while turning knobs. Then needs rails so it can be safely placed across an open chassis.


Probably should design something for coupling caps as well. Would need to be a higher voltage switch though.


Thanks
Mike

[PRR]

> getting shocked

This is for tuning 12A_7 and similar?

Should be no shock. Cut the resistor out of the nearest stage and measure cathode voltage. 12AX7 with even 400V supply won't be 8V. 12AU7 could be 36V. Both at very low current. A very sweaty finger will bring these down below 12V.

I don't see a real need for a board or even a PDF. It's not a road-gig tool. It won't impress the groupies. Put one end of caps/resistors on switch terminals. Twist the other ends together. Run leads off. Put it in a plastic box, perhaps TupperWare.

[FranciscoPerez]

Why not going lower in capacitance? You wouldn't probably hear much of a difference between 10uf and 22uf, and 470nf - 100nf could come in handy.


Edit: Oh, didn't see JB's post. I'd include some lower values as well.


Maybe you could get a substitution box from eBay or similar. It might save some money and it's housed inside an enclosure etc... I bought a Calrad unit some time ago for about $10, and works great for this kind of things.

[Mike_J]

> getting shocked

This is for tuning 12A_7 and similar?

Should be no shock. Cut the resistor out of the nearest stage and measure cathode voltage. 12AX7 with even 400V supply won't be 8V. 12AU7 could be 36V. Both at very low current. A very sweaty finger will bring these down below 12V.

I don't see a real need for a board or even a PDF. It's not a road-gig tool. It won't impress the groupies. Put one end of caps/resistors on switch terminals. Twist the other ends together. Run leads off. Put it in a plastic box, perhaps TupperWare.

Thanks PRR, like the twist the other ends together idea. This in conjunction with the TupperWare idea would certainly work. Just cut two 3/8" holes for the switches and small holes for the wires and it is done. Have plenty of those TupperWare like containers that the meat companies put there sandwich meat in. That would be easier to build and store than the elaborate contraption I was planning.


Thanks,
Mike

[Mike_J]

Why not going lower in capacitance? You wouldn't probably hear much of a difference between 10uf and 22uf, and 470nf - 100nf could come in handy.


Edit: Oh, didn't see JB's post. I'd include some lower values as well.


Maybe you could get a substitution box from eBay or similar. It might save some money and it's housed inside an enclosure etc... I bought a Calrad unit some time ago for about $10, and works great for this kind of things.

Thanks Francisco, have a resistor and capacitor substitution box but the components are pretty poor, the cap values are small, and resistor values too high. For bypass resistors and caps think it would be best for my application to have everything in one box made specifically for that purpose. Pretty sure this unit can be made for less than $15.00 especially with the PRR suggestion in the previous post.


Thanks,
Mike

[HotBluePlates]

Maybe you could get a substitution box from eBay or similar. ...

Thanks Francisco, have a resistor and capacitor substitution box but the components are pretty poor, the cap values are small, and resistor values too high. ...

You can't remove the old components and solder in new ones of the values you want? Or are you saving it for plate load resistor and coupling cap duty?

[hesamadman]

I have made multiple boxes out of small project enclosures from radio shack. My resistor boxes have a pot mounted in the center with wire soldered to the pot and sticking out the side with alligator clips. I have on that's 10k. One that's 1m. Maybe a 25k also not sure. Those are mounted in very small boxes.

I also have a 10 position rotary switch mounted in a project box with alligator clips the same way. Only it has different cap ratings. I would include smaller cap values like mentioned above. I use .68 and 2.2 often.

[p2pAmps]

I've never used these so who knows...

http://www.tedweber.com/csub-ecc-ll

http://www.tedweber.com/csub-ecc-lh

http://www.tedweber.com/csub-ppc

http://www.tedweber.com/csub-ccc

[sluckey]

Either of those that have the banana binding posts would be ideal for Mike_J's purpose. Just attach the resistor of your choice under the binding posts. Or use without the resistor.

[Mike_J]

Maybe you could get a substitution box from eBay or similar. ...

Thanks Francisco, have a resistor and capacitor substitution box but the components are pretty poor, the cap values are small, and resistor values too high. ...

You can't remove the old components and solder in new ones of the values you want? Or are you saving it for plate load resistor and coupling cap duty?

The caps were 100 volt and resistors were undersized as well. Put silver mica's in the cap box and as you mentioned 1/2 watt plate load value resistors in the resistor box. Not terribly impressed with the switches that came with the boxes. Doubt that I should even be using them to test plate load resistors. Seem undersized to me.


Quality is the main reason for a project like this. Can make sure the switches will withstand the current and voltage running through them. Will probably build a box or boxes for plate load resistors and coupling caps with switches that can safely handle the voltages applied to them.


Thanks
Mike

[Mike_J]

I have made multiple boxes out of small project enclosures from radio shack. My resistor boxes have a pot mounted in the center with wire soldered to the pot and sticking out the side with alligator clips. I have on that's 10k. One that's 1m. Maybe a 25k also not sure. Those are mounted in very small boxes.

I also have a 10 position rotary switch mounted in a project box with alligator clips the same way. Only it has different cap ratings. I would include smaller cap values like mentioned above. I use .68 and 2.2 often.

Did the same thing you did but use a long narrow strip of G10. Drill a 3/8" hole in the G10 to mount the pot. Like you I use alligator clips with pretty short leads. Just suspend the pot above the area that I am measuring. Have a plastic knob on the pot. Works well and uses up scrap G10.


Have attached a modified board with more values. I have eight position switches on hand that I planned on using. Having more positions would probably be better. Could then use values of .1, .22, .47, .68, 1, 2.2, 3.3, 4.7, 10 and 22uF as well as a space open for no cap across the resistor. That would be 11 positions. So a 12 position switch would be needed to incorporate all those caps.


Thought more about the suggestion by JB to use a pot instead of a multi-position switch for the resistors. Think an 820R resistor in series with a 5KL pot with jacks to connect a multi-meter to would be pretty useful. Will make a layout to see if it would work.


Thanks
Mike

[Mike_J]

I've never used these so who knows...

http://www.tedweber.com/csub-ecc-ll

http://www.tedweber.com/csub-ecc-lh

http://www.tedweber.com/csub-ppc

http://www.tedweber.com/csub-ccc

The .001 - .22 @ 630VDC and .1 - 47uF @ 50VDC boxes would work for guitar amp work. However, think it would be advantageous to have the resistors and caps in one box. Then only two alligator clips would be needed as connectors to the amp. This box would be used exclusively for bypass caps and resistors.


In addition to having control over the quality of the components being used, pretty sure these boxes can be constructed for less than $20.00 in parts.


Thanks
Mike

[JB]

Just pulled my little cathode jig apart to see what I did.  10K Lin pot, no minimum resistor (that's a nice upgrade idea).  12 way rotary switch with values from 0.47 upto 22u then a big jump upto a 220u.  Couple of empty positions too.

Little plastic box with a pair of flying leads that I just tack solder in.  Croc clips would be luxury!  I stuck a paper label on the front with the cap values/postions marked on it and measured/marked the most useful resistor values. 

I was lucky enough to be given an old decade capacitance box that runs from 10pF upto 0.1uF, rated 600V DC.  That does me for coupling caps.  Must be any number of those out there cheap.
 

[Mike_J]

Just pulled my little cathode jig apart to see what I did.  10K Lin pot, no minimum resistor (that's a nice upgrade idea).  12 way rotary switch with values from 0.47 upto 22u then a big jump upto a 220u.  Couple of empty positions too.

Little plastic box with a pair of flying leads that I just tack solder in.  Croc clips would be luxury!  I stuck a paper label on the front with the cap values/postions marked on it and measured/marked the most useful resistor values. 

I was lucky enough to be given an old decade capacitance box that runs from 10pF upto 0.1uF, rated 600V DC.  That does me for coupling caps.  Must be any number of those out there cheap.

Like your idea about using a pot instead of resistors on a multi-position switch. Minimum resistor seemed like a good idea because then there is always a path to ground for the cathode. Could get a precision resistor to match measured value if it mattered. That 600V decade box would be nice to have. All components need to be able to handle the voltage.


The attached layout is very similar to what you are describing. Please advise if you see any problems.


Thanks
Mike

[Mike_J]

Either of those that have the banana binding posts would be ideal for Mike_J's purpose. Just attach the resistor of your choice under the binding posts. Or use without the resistor.

Stole your idea for banana binding posts on the latest version.


Thanks
Mike

[Mike_J]

Anyone know where I can get an adapter to go from the normal ends on a multi-meter to a banana jack tip. Tested my meter wires and they are too small for connection in the banana jacks I have. Better option would be to find jacks that match the meter leads.


Thanks
Mike

[Willabe]

Anyone know where I can get an adapter to go from the normal ends on a multi-meter to a banana jack tip.

They make/sell test cables like that, try Mouser and Digikey.

Better option would be to find jacks that match the meter leads.

Doug sells those.     Their listed as bias tip jacks, link below;

http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=JacksPlugs&ORDER_ID=631293670

[Mike_J]

Anyone know where I can get an adapter to go from the normal ends on a multi-meter to a banana jack tip.

They make/sell test cables like that, try Mouser and Digikey.

Better option would be to find jacks that match the meter leads.

Doug sells those.     Their listed as bias tip jacks, link below;

http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=JacksPlugs&ORDER_ID=631293670

Thanks Willabe, got the test cables from Mouser and the bias tip jacks from Doug. Had trouble with a plastic 12-position switch purchased from Mouser. It melted when I was soldering the caps to it. Got a switch that held up to the heat and was less than half the price from another source. Placed a 470R resistor from the wiper of the 5K pot to the negative tip jack. Therefore, there is always at least 470R of resistance from the cathode to ground.


Have attached a picture of the completed box. Stole most of the ideas from JB. Used a project box from Radio Shack as suggested by Hesamadman (found one in the area that is still open). Still room left in the box for one more jig idea. Thanks to all for your help.


Thanks
Mike

[Willabe]

Nice. 

[JB]

Nice! 

Mine has trailing leads that I tack solder in, yours is the Deluxe version!

[Mike_J]

Have room for more in the box so I want to add a plate load resistor selector and a coupling cap selector to the jig. Don't think the plate load resistor selector will be a problem. Should be able to get a 250KL one-half watt pot and a few more bias tip jacks from Doug. Mark ohm ratings of 100K, 150K, 180K and 220K on the faceplate and the plate load resistor selector is done.


The coupling cap selector is more difficult. Can't find a multi-position rotary switch that is rated for the voltages the selector would be used.for. An alternative is to use bias tip jacks and just move the wires around. Not sure about the voltage rating on a bias tip jack. Probably not the safest thing I would have ever done to more the wire from jack to jack to change cap values. (See attached layout) Any suggestions would be appreciated. The three position switch Doug sells looks pretty stout. No idea if it is rated for this purpose but it looks like it could be.


Thanks
Mike

[HotBluePlates]

... I want to add a plate load resistor selector and a coupling cap selector to the jig. Don't think the plate load resistor selector will be a problem. Should be able to get a 250KL one-half watt pot and a few more bias tip jacks from Doug. Mark ohm ratings of 100K, 150K, 180K and 220K on the faceplate and the plate load resistor selector is done. ...

I don't think you'll see much difference going from 100kΩ to 220kΩ (you could try a quick resistor swap in an existing amp to hear for yourself). Looking forward to other uses, would something like 47kΩ-500kΩ be more useful?

And since you're not going below 100kΩ (or 47kΩ in my suggestion, or lower), sounds like having a series resistor for the lowest desired resistance is worthwhile.

BTW, is the pot you're looking at rated for the full supply voltage of whatever amp stage you'll connect to? The body of the pot might be ground, yet a part of the pot is connected to the power supply at as much as 300v or more?

... The coupling cap selector is more difficult. Can't find a multi-position rotary switch that is rated for the voltages the selector would be used for. ...

No 300v+ rated switch, but you'll be comfortable grabbing a pot with 200-300v present?

I don't know the voltage ratings of the commonly-used rotary switches, but I know other have used them to have selectable coupling caps. I'm using a switch "rated" for 120vdc, but detailed reading of the data sheet shows voltage breakdown is between 500-100vac (500vac is a peak of 707v) and I'm using very much less current than the switch's rating. Every Fender amp you've ever seen has a Standby switch only rated for 120vac, even though it's switching 400v+.

So it can be hard to determine valid switch selections.

[Mike_J]

I don't think you'll see much difference going from 100kΩ to 220kΩ (you could try a quick resistor swap in an existing amp to hear for yourself). Looking forward to other uses, would something like 47kΩ-500kΩ be more useful?

And since you're not going below 100kΩ (or 47kΩ in my suggestion, or lower), sounds like having a series resistor for the lowest desired resistance is worthwhile.

BTW, is the pot you're looking at rated for the full supply voltage of whatever amp stage you'll connect to? The body of the pot might be ground, yet a part of the pot is connected to the power supply at as much as 300v or more?

No 300v+ rated switch, but you'll be comfortable grabbing a pot with 200-300v present?

So it can be hard to determine valid switch selections.

Thought a one-half watt rated pot would work as Fender used one-half watt plate load resistors. Have a 350K - 2 watt pot that measures around 420K. If I added a 47K resistor in series with the wiper of that pot then it would get close to the 500K (about 470K) you mentioned. Will use a plastic chicken head knob. Hopefully that will provide some protection from electrocution.


So far it has been very hard to determine a valid switch selection. Still thinking Doug's three position impedance switch is a good option. Would probably use a few of them. For example .1, .047 and .022 on one switch and .01, .047 and .002 or .001 on the other. Turn the standby switch off when moving between between the two switches. Doubt that there is any more danger to this method versus risk normally associated with using a MM.


HBP, you and many other members on the forum know much more about this than I do. One of the main reasons I post things like this is to benefit from your wisdom. Trying to build a jig that is useful, will last and is not dangerous to myself or anyone else.


Thanks
Mike

[HotBluePlates]

... Thought a one-half watt rated pot would work as Fender used one-half watt plate load resistors. Have a 350K - 2 watt pot that measures around 420K. ...

You thinking is correct as far as avoiding the pot's resistive path from burning open. My concern was the ability of the pot to avoid shorting your B+ supply to ground (which is what would happen if the pot arcs to its case/chassis).

However that 2W pot you mentioned: I don't recall exactly, but think it carries a rating for 350vdc. So that would be good. The 2w pots are often used in VVR or powerscaling circuits, not because that resistance needs a 2w rating but because the pot is rated for the voltage present in that part of the circuit.

... So far it has been very hard to determine a valid switch selection. Still thinking Doug's three position impedance switch is a good option. ... Doubt that there is any more danger to this method versus risk normally associated with using a MM.

Wait for Willabe to chime in. I think he used Alpha rotary switches. I overspent when I got my switches. I have a got feeling the impedance selector switch is way more than you need, especially because the current through the switch will be very small.

[Willabe]

Yes I am using Alpha rotary switches for my K bypass caps. 

They are working fine. 

[Mike_J]

Yes I am using Alpha rotary switches for my K bypass caps. 

They are working fine. 

Used a 125VAC, .3A switch for the K bypass caps. Think that will work fine. My current concern is what switch to use for a coupling cap selector switch. It will have as much as 300VDC on it. Don't know if a 2 pole, 6 positions switch that is also 125VAC, .3A will be sufficient for that purpose.


Thanks
Mike

[HotBluePlates]

Yes I am using Alpha rotary switches for my K bypass caps.  ...

I thought you had Alpha for your coupling caps on the breadboard?

[Willabe]

Yes, sorry, for coupling caps too.

[Mike_J]

Yes, sorry, for coupling caps too.

Looked on Mouser for Alpha rotary switches. They are 125VDC, .3A. Thank you for the help.


Thanks
Mike

[Mike_J]

Here is what is hopefully the final layout for this project. It has selectors for plate load resistors, coupling capacitors, and bypass capacitors/resistors. Hopefully it will be useful in removing the blattyness and honkiness I have when the one-channel is set at max volume.


Thanks to all who helped.
Mike

[Mike_J]

Figured out a way to eliminate two alligator clips by using the attached layout.


Thanks
Mike