One meg resistor on input jack not reading the same

[Mike_J]

I wired the input jacks on my 5f6a Bassman today. Measured the 1/2 watt metal film resistors before installation and they both measured one meg. After installing everything on the jacks I tested the resistor values by plugging a guitar cable into the jacks and they both measured plus or minus 500K, Anyone can destroy a resistor but it takes special skills to only destroy one-half of a resistor and do it twice. Can anyone please clear up my confusion?

[sluckey]

Once you put a resistor in a circuit there are lots of other things that may be in parallel with that resistor. And that will change the "in circuit" measured resistance. A good brain exercise would be to look at the SCHEMATIC and figure out why you only have a reading of 500K.

[Mike_J]

I haven't installed them in the amp yet. Wired them on the outside of the amp because I am using 18 gauge stranded wire to ground the shielding for the RG174. It had to be installed in the same hole on the jack ground with an 18 gauge solid core wire to tie the ground from the buss bar to v1 grounds. I haven't even started doing the circuit board. Just got my order of teflon wire in. Mainly was doing the input jacks now so I could check for interference with the circuit board before installing it. Will also install the tube sockets for the same purpose.


Two one meg resistors in parallel yield a 500K resistor rating. What has me confused is the only side of the two jacks that l am sure are connected are the grounds. Wouldn't the tip side of the one meg resistors have to be connected for the resistors to be in parallel. Looked at the schematic and I am still confused. Additional guidance would be greatly appreciated. Appreciate you help.

[sluckey]

I don't know what you are trying to describe. Show us the exact schematic/layout you are using. And show us some hi-rez pics of your jacks.

[Mike_J]

I am using the original Fender 5f6a schematic and layout (at least for the input jacks) except that I am using the Hoffman 5f6a layout for the input jack grounds. My chassis has the four holes for input jacks as per the 59 Bassman. Will forward high rez pictures tomorrow.



Thanks
Mike

[PRR]

I'm gonna need a picture. The right answer looks like "zero": the jack switching shorts the 1meg.

[Mike_J]

Here are three pictures of the jacks as wired.

[Mike_J]

Here is another picture I couldn't see on the previous post.

[sluckey]

Your jacks are wired properly. Cut the black wire indicated so you have two sets of hi/lo jacks. That black wire is not needed since you are running two ground wires to the preamp ground. Put one set of jacks aside and check the resistor the same way you did before. Now there's no way the two resistors can be in parallel, but I bet the resistor still measures approx 500K. Report back.

I had a whole batch of those switchcraft jacks that were bad. You could measure about 500K to 700K between tip and sleeve even with nothing connected. The cheap Mexican phenolic insulator separators were conductive. I tried cleaning them with chemicals. Nothing helped. If you have another new jack check resistance between tip and sleeve. Report back.

The good news is that even if your jacks are conductive, they will still work just fine for input jacks or speaker jacks.

[dude]

The cheap Mexican phenolic insulator separators were conductive.

I thought switchcraft jacks were made in USA..., parts are made in Mexico...?

[Mike_J]

Your jacks are wired properly. Cut the black wire indicated so you have two sets of hi/lo jacks. That black wire is not needed since you are running two ground wires to the preamp ground.

I misspoke when I said the two ground wires are going to the buss bar and the V! grounds. The second wire is going to be grounded to a lug on the chassis near the input jacks as you suggested in another thread earlier this week or last. Do you still want me to cut the wire? I suppose if I cut it in the right place I could ground both sets of jacks to the same ground lug with a little planning.

[sluckey]

Do you still want me to cut the wire?

Yes. You can put it back after you solve the mystery.

[Mike_J]

Do you still want me to cut the wire?

Yes. You can put it back after you solve the mystery.

Cut the wire and as you expected the resistance is still +/- 500K. I found four 12a jacks that I have probably had for a long time. Measured them from tip to sleeve. All measured .4 ohms. Don't have any more 1/2 watt one meg metal films left but do have some 3 watt metal films. Do I need to start over again with the old jacks and 3 watt resistors?

[PRR]

> 12a jacks .... Measured them from tip to sleeve. All measured .4 ohms.

You should also measure with the switch "open". Either insert a perfectly insulating "plug" or gently slip thin insulation between the switch contacts.
https://robrobinette.com/images/Guitar/5E3P_Build/Switched_Jack.jpg

<1Meg on a 1/4" jack sure seems wrong. But they ARE derived from Telco parts and sold for headphones. Not guitars. Not my 11Meg Heath VTVM. With current market price pressure, I guess leaky wafer may be part of the price we pay.

[sluckey]

Cut the wire and as you expected the resistance is still +/- 500K.

Guess that rules out any idea that the two resistors are somehow connected in parallel. Do both resistors measure the same?

I found four 12a jacks that I have probably had for a long time. Measured them from tip to sleeve. All measured .4 ohms.

That's not good! They should read open. Unless you have a jumper between sleeve and switch. Or are measuring between tip and switch!

BTW, I just measured 3 new, never used 12A jacks and they all measure open circuit between tip and sleeve, just like a good jack should.

Do I need to start over again with the old jacks and 3 watt resistors?

I would solve the mystery. Carefully remove the resistor from the jack. Now measure the resistor. Then insert a plug into the jack and measure between tip and sleeve.

[Mike_J]

> 12a jacks .... Measured them from tip to sleeve. All measured .4 ohms.

You should also measure with the switch "open". Either insert a perfectly insulating "plug" or gently slip thin insulation between the switch contacts.
https://robrobinette.com/images/Guitar/5E3P_Build/Switched_Jack.jpg

<1Meg on a 1/4" jack sure seems wrong. But they ARE derived from Telco parts and sold for headphones. Not guitars. Not my 11Meg Heath VTVM. With current market price pressure, I guess leaky wafer may be part of the price we pay.

Placed a thin piece of cardboard between the sleeve and the tip and it read OL. Bounced around 30+ megs before settling on OL. My thin cardboard may not be an ideal insulator. Is there a way to test jacks to see if they are leaky? Would be nice to reject them if possible before doing any work on them. If it is a consistent design flaw we have to live with that is less than ideal. Is there another company that makes a 12a that isn't leaky?

[Mike_J]

Cut the wire and as you expected the resistance is still +/- 500K.

Guess that rules out any idea that the two resistors are somehow connected in parallel. Do both resistors measure the same?

I found four 12a jacks that I have probably had for a long time. Measured them from tip to sleeve. All measured .4 ohms.

That's not good! They should read open. Unless you have a jumper between tip and switch. Or are measuring between tip and switch!

BTW, I just measured 3 new, never used 12A jacks and they all measure open circuit between tip and sleeve, just like a good jack should.

Do I need to start over again with the old jacks and 3 watt resistors?

I would solve the mystery. Carefully remove the resistor from the jack. Now measure the resistor. Then insert a plug into the jack and measure between tip and sleeve.

Resistors still measure about one meg. Inserting a plug into the jack gives me a reading of OL. I seem to recall having a problem with 12a jacks some time ago. Should have thrown them in the garbage can. What can I do to get good jacks. Rather not use leaky jacks. If I remember correctly when I had my problem Doug decided he would check his jacks although I did not buy the faulty jacks from him. If he is still doing it the answer would be to buy good tested jacks from him. They have to exist since you have three of them. So if I measure resistance from the tip to the sleeve it should read open or OL on my meter correct?

[sluckey]

Your jacks and resistors are good. You have boogered your RG-174 cable. You have a high resistance (approx 1M) between the center conductor and shield. It's easy to melt the center conductor insulation when soldering a wire to the shield, especially when that wire is a huge 18AWG. Replace your cable and be careful when soldering a wire to the shield. Better yet, get some RG-316. It's teflon and will never melt. That's all I ever use anymore.

[Mike_J]

Thank you for the RG-316 suggestion. I like to use teflon wire. Makes it harder to make a mistake. Speaking of a mistake, I measured the jacks I wired 22 years ago. The number two jacks both measured 68K as they should. However, the number one jacks measured a varying rating in the approximately three meg range. Shouldn't the number one jacks measure approximately 1.068 meg from jack to tube socket?


I was probably confused about what you were asking for when you asked me to measure from tip to sleeve. Since there was nothing plugged into the jack it would measure the same as if I put the positive and negative leads of the MM together which also yields a .4 ohm reading since the tip and sleeve are touching on the jack. Can't see how that would be any fault of the jack.


It is critical that I develop a method of wiring these jacks that works properly. First thing is the one meg resistors on each jack one. Somewhere long ago I read that it is very important to keep the one meg resistor leads short and very close to the jack. To do so makes it impossible to put a heat sink on the resistor to protect it. Don't know if the resistors are getting damaged because they all still measured one meg when removed from the jacks. They measure either much less or much more than one meg when measuring the end of the wire going to the tube socket and ground when wired as a group of four per the earlier pictures.


Your explanation about the shielded cable may be valid for both. I originally used a cable that had two conductors in the same cable when I first made the amp. Think the dielectric for both wires in the cable was teflon but the exposed wires couldn't have had 1/8" of distance between them. This was an idea much better in theory than practice. Pretty sure individual RG-316 cables would be better soldered to the outer shield and heat wrap protected as your picture showed.


Real problem getting a good build if I can't even get the input jacks installed correctly. Unbelievably frustrating.

[sluckey]

If the 68K resistors are on the board like the original, then you don't even need shielded wire. Fender never used any shielded cable in their old layouts.

[PRR]

> Somewhere long ago I read that it is very important to keep the one meg resistor leads short and very close to the jack.

No.

The 68k *maybe* should be close to the *tube* but many different ways seem to work well.

I'm not sure why your jacks are so frustrating.

[Mike_J]

If the 68K resistors are on the board like the original, then you don't even need shielded wire. Fender never used any shielded cable in their old layouts.

I put the 68K resistors on the tube socket. Even so I don't think the grid wire would be longer than three inches going straight to the socket over the board. Certainly worth trying while waiting on RG-316 cable.


Going to use a layout based on Hoffman's board. Like it better for running the grounds and the power rail versus the Fender layout. Have already drilled the holes in some G10 so I will see how it works. Laid it out so I could use the Xicon or larger caps if wanted. Should have room on the board to attach vertical standoffs to attach the grid wires to the 68K resistors. Hopefully would make it possible to keep the grid wires high enough off the board to make it work.


It seems like any cable would have some impact on higher frequencies because the shielding carries a resistance. I remember at one time measuring different cables to see which had the lowest pF resistance rating. George L's had the lowest by a long shot. Something like 19 pF per foot where others measured around 100pF. Pretty sure I used some George L cable in some of my builds. Might depend on how bright the guitar is that is used with the amp. A bright guitar could use some taming I would think.


When it gets finished I will take pictures. I am sure many posts are ahead before that day comes.

[Mike_J]

I'm not sure why your jacks are so frustrating.

Thank you for your response. Glad to know I can allow for some lead length on the one meg resistors. Can use three watt metal films that I have on hand. The jacks frustrate me because jack 1 which has the one meg resistor on it and the 68K resistor to keep radio interference out of the amp should be what I read, approximately 1.068 megs, when I measure from the tip of a guitar cable plugged into the jack to the end of the wire that would be connected to the grid of the preamp tube. Not sure why this is not happening other than the strong possibility I overheated the RG174 cable. Another possibility is I don't understand what reading I should be getting when doing the measurement explained above. I am going to try a straight wire and see what happens.


Basically, I want to have an objective way of determining whether my wiring is correct, in this case from the jack to the end of the wire or cable feeding a grid of V1 measured in ohms. If it shouldn't be approximately 1.068 megs then it is just a misunderstanding on my part which is certainly possible.

[sluckey]

The jacks frustrate me because jack 1 which has the one meg resistor on it and the 68K resistor to keep radio interference out of the amp should be what I read, approximately 1.068 megs, when I measure from the tip of a guitar cable plugged into the jack to the end of the wire that would be connected to the grid of the preamp tube.

First off, there ain't any 68Ks connected right now, so forget about them. With your meter connected as stated, you should read ZERO ohms. And you should measure 1M between tip and sleeve.

Not sure why this is not happening other than the strong possibility I overheated the RG174 cable.

Probably. Look, your jacks are fine and your resistors are fine and you have everything connected correctly. Gotta be the cable. Remove the shielded cables from the jacks. Then measure resistance between the cable shield and center conductor. If you have any resistance reading, you messed up the soldering job. Easy to do, especially if soldering a big ole 18AWG wire to the cable shield.

I recommend you wire the jacks ***EXACTLY*** as shown on this page...  https://el34world.com/charts/CommonHookups.htm

If the distance from the jacks to the tube socket is less than 4 inches, just use a regular single conductor wire. This is bulletproof for any low gain amp such as a 5F6A.

Another possibility is I don't understand what reading I should be getting when doing the measurement explained above.

BINGO! Take a look at page 11 of this pdf...  http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

Should give you enough understanding of the circuit to know what meter reading to expect regardless of where you connect the probes. If you still don't get it, just wire the jacks as per the pic in Hoffman's common hookups info. Then turn the page.

[thetragichero]

that pdf has come in handy many a time wiring input jacks

[Mike_J]

The jacks frustrate me because jack 1 which has the one meg resistor on it and the 68K resistor to keep radio interference out of the amp should be what I read, approximately 1.068 megs, when I measure from the tip of a guitar cable plugged into the jack to the end of the wire that would be connected to the grid of the preamp tube.

First off, there ain't any 68Ks connected right now, so forget about them. With your meter connected as stated, you should read ZERO ohms. And you should measure 1M between tip and sleeve.

Not sure why this is not happening other than the strong possibility I overheated the RG174 cable.

Probably. Look, your jacks are fine and your resistors are fine and you have everything connected correctly. Gotta be the cable. Remove the shielded cables from the jacks. Then measure resistance between the cable shield and center conductor. If you have any resistance reading, you messed up the soldering job. Easy to do, especially if soldering a big ole 18AWG wire to the cable shield.

I recommend you wire the jacks ***EXACTLY*** as shown on this page...  https://el34world.com/charts/CommonHookups.htm

If the distance from the jacks to the tube socket is less than 4 inches, just use a regular single conductor wire. This is bulletproof for any low gain amp such as a 5F6A.

Another possibility is I don't understand what reading I should be getting when doing the measurement explained above.

BINGO! Take a look at page 11 of this pdf...  http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

Should give you enough understanding of the circuit to know what meter reading to expect regardless of where you connect the probes. If you still don't get it, just wire the jacks as per the pic in Hoffman's common hookups info. Then turn the page.

Problem number one is you were talking about a sleeve. It starts with an s so it has to be the same thing as the shunt right? Wrong. The resistance between the RG174 shield and conductor was over one meg. I did a continuity test from the shield to the conductor believing if I overheated the dielectric (PVC between the shield and conductor) then I would have continuity. Wrong again apparently. How is a resistance created when you overheat like I did? By the way thank you so much for having the patience to explain this to me. It had to be painful.

[sluckey]

The resistance between the RG174 shield and conductor was over one meg.

Mystery solved!
It should be infinity, open circuit. Some meters display OL for infinity. Any other reading is bad. A 1M shield/conductor reading in parallel with the 1M resistor is 500K. And that brings us full circle.

[Mike_J]

The resistance between the RG174 shield and conductor was over one meg.

Mystery solved!
It should be infinity, open circuit. Some meters display OL for infinity. Any other reading is bad. A 1M shield/conductor reading in parallel with the 1M resistor is 500K. And that brings us full circle.

Plus this gives me the quality control method that I was sorely missing and will keep this from happening to me again. The continuity test I thought was doing something was not helpful. Thank you so much for your help.

[Mike_J]

So I am out in our Florida room (really a big screened porch) the breeze is blowing gently from the west. Have some RG316 and some 22awg wire for grounding the shield. Wire up my input jack and what happens. I get a reading of 567K ohms.


At the time some woman from New York City from down in the bowels of our neighborhood is shouting like a wild woman. My immediate reaction is I need to go find their house and have a talk with her husband to make her shut up.


Realizing my reaction may be attributable to the reading I am getting on my multimeter I ask myself how can this be happening again. Measure the ohms across the cable I am using and it is some ridiculous number so I go get another cable and one meg. Immediately I am happy because now I know I didn’t screw up before with the exception of having a piece of garbage cable and I will be able to use my RG174 when  I finish using my Rg316.


The lady hasn’t made a peep since I got the one Meg reading and all is right with the world.

[Mike_J]

Below is a picture of the offending piece of garbage cable next to the redeeming longer cable. I post this as a warning to all who decide to try what I have done.


Wanted to create a powered pedal board. Measured different cables for pF and found George Ls were by far the best. Problem was those little end things were going to cost me as much as a new Harley.


Well I was looking for an alternative and went all the way to the other extreme. Bought these cheap little piece of Chinese garbage end things and cut up some old guitar cords that no longer worked properly and made a horrible powered pedal board which was never used until I decided to use a cable from it to test my input jacks.


I am sure you already have learned the lesson from this fiasco. Forgo that new bike and build your powered pedal board out of George L cables and end things.

[acheld]

Spot on regarding George L cables.  Heh, they cost the earth -- but, they are reliable and easy to use.

I've had good luck with Monoprice cables.   They are cheap imports, but the quality is just fine.   I have not had a bad one yet.

[Mike_J]

Spot on regarding George L cables.  Heh, they cost the earth -- but, they are reliable and easy to use.

I've had good luck with Monoprice cables.   They are cheap imports, but the quality is just fine.   I have not had a bad one yet.

Very useful tip.


Thanks
Mike

[thetragichero]

i like rockboard flat cables. decent price and allow more space on the board. i use whatever instrument cables, not fussy about them

[Mike_J]

i like rockboard flat cables. decent price and allow more space on the board. i use whatever instrument cables, not fussy about them

I am ridiculously fussy about them which should surprise nobody. I think my favorite starts with an m.

[Mike_J]

Mogami is what I was looking for. Used Monster Cables. Had a lifetime warranty until something went wrong with them which was frequent. I bet that Mogami is a Japanese company. Oh no, I am a cork sniffer.

[PRR]

..I bet that Mogami is a Japanese company.

WAS!

Mogami   MIT INC.
MIT INC. was dissolved on June 30, 2020 and closed its office

Yes, very Japanese:
http://www.mogami.com/e/history.html

While the company may have closed, we can bet someone will buy the rights to the name.

[Mike_J]

Very good thing to know. Wish I had purchased more from them. Cables are very quiet.


My parents bought me a Vox guitar around 1970. Not the same Vox as the AC30 Vox. My guitar teacher told me the day I got it that it was a piece of junk. Quit playing for five or six years because of that. I think it was Thomas Organ Company that bought the name. Then made garbage guitars. It still upsets me to this day.

[glass54]

...and if you want real junk, Vox AC30 made in the early 90's. PCB was a nightmare, phenolic resin board with tube sockets soldered directly with minimum PCB track width and poor ventilation and burn marks everywhere. Looking back, the only sensible solution was a Hoffman Turret board  .
Where were you Doug, could have saved me pulling out a lot of hair
Today, I consider that particular vintage AC30 (early pcb model) as donor amps for Doug's Boards.
In regards to cables, I use the Mogami 3080 as (Balanced) test leads and they made a quality product. Sad to hear PRR as they made good products and were great to deal with in the Aussie Broadcast hardware supply chain.
I like Canare (yes Japanese again) L-4e6s for guitar leads. I have managed to get lots of offcuts up to 3m long over the decades and L-4e6s is 6mm in diameter, great for Plug entry. Yes, its a little over the top StarQuad configuration with excellent dual woven shield but you only buy it once. Downside is time consuming preparation for lead dressing. Excellent in my location with temp variation and the cable "has no memory effect" therefore rolls out nicely. (I also use it for microphone leads)
Kind regards,
Mirek

[thetragichero]

monster cables were horrible, but it was cool that you could send the drummer to any guitar center with an arm full of em and he'd come back with new ones. they've stopped that so for instrument cables i get cheap proco from antique electronic supply/ce dist. they're like ten bucks for a 20 footer

[Mike_J]

monster cables were horrible, but it was cool that you could send the drummer to any guitar center with an arm full of em and he'd come back with new ones. they've stopped that so for instrument cables i get cheap proco from antique electronic supply/ce dist. they're like ten bucks for a 20 footer

Guess I’ll be doing the same thing. Can buy five 20 footers for the price of one Mogami 10 footer. One of the five is bound to be worth having.

[acheld]

I do not know the details, but I think Marshall Electronics owns Mogami Cables.   The bulk cable (I believe still made in Japan) is available from Markertek.  Redco is cheaper, and I'm sure there are others.  You can not beat W2524 as a guitar cable, with W2319 a smaller alternative.   Not cheap, but in this case, you get what you pay for.   

Their pre-made cables are quite expensive, but seem bullet proof.   I generally prefer to make my own with Canare connectors (and Neutrik SilentPlug).    Roll your own!

[thetragichero]

monster cables were horrible, but it was cool that you could send the drummer to any guitar center with an arm full of em and he'd come back with new ones. they've stopped that so for instrument cables i get cheap proco from antique electronic supply/ce dist. they're like ten bucks for a 20 footer

Guess I’ll be doing the same thing. Can buy five 20 footers for the price of one Mogami 10 footer. One of the five is bound to be worth having.

and that's how i treat em: they're a consumable like strings. sure I'll cut off a few inches and try to repair but ultimately it goes into the pile of "works sometimes" cables

[Mike_J]

I do not know the details, but I think Marshall Electronics owns Mogami Cables.   The bulk cable (I believe still made in Japan) is available from Markertek.  Redco is cheaper, and I'm sure there are others.  You can not beat W2524 as a guitar cable, with W2319 a smaller alternative.   Not cheap, but in this case, you get what you pay for.   

Their pre-made cables are quite expensive, but seem bullet proof.   I generally prefer to make my own with Canare connectors (and Neutrik SilentPlug).    Roll your own!

Marshall Electronics should give us each five 15 foot cables to compare to them to the Magami’s I liked so much. Would be a good marketing plan for them. Think I will contact them to propose the plan.


Rolling my own has not worked well for me. Am pretty sure I made the most terrible cable in the history of man. Better off begging.

[thetragichero]

in high school i didn't have the money for a second cable but somehow had two 1/4” jacks so i made one out of rg59 coax... it worked although wasn't very flexible. oh the golden days...

[shooter]

in high school i didn't have the money

In the Navy, We couldn't have "un-authorized" cables in the runs, so we used; amp to BNC to speaker.   
necessity ......

[acheld]

I was unfortunate enough to take an old USN repair ship through an overhaul, and the amount of clever (but unauthorized) wiring we found in my department alone (sick bay!) was astonishing.   Of course, the damn thing was built in WWII and the overhaul we're talking about took place in the 80's, but still . . . a lot of hidden copper was removed, with a lot of BNC connectors heading back to several cabinets marked as "medical" with red crosses and locks, but which contained old record players and radios.

I could not save any of it.  The XO got it all.

There were a lot of bright guys on that ship over the years!