Question About Coupling Caps And Continuity Checking

[Gary_S]

This is a pretty basic question but i'd like to get some advice from the guys on the forum.

I have a Marshall 50W JCM800 4010 combo. I recently changed all the coupling caps and have had bad noise in the background. Like a general interference sort of sound, kind of crunchy and scratchy sounding. Anyway one of my EL34's also red plated. Initially i thought that i had overheated the caps i installed because i was using one of those unregulated irons with no temp control. I thought the dielectric in the caps had broken down with the heat and the caps were leaking DC onto the grids of the next stages.

So i removed those caps and fitted 7 new Xicon caps. Same result still a lot of noise. I had to find a work around in a few places becase the solder pads were lifted here and there. I fitted a bus wire to make the connections in those cases and just eliminated the trace.

I then pulled both output tubes and replaced them with a matched set of JJ's. No good, noise still there.

At that point i said that i'd need to go over all the connections going to and from the caps and check them for continuity to see there were no breaks in the circuit. I checked them out and i'm getting a continuity beep on all the connections and the meter is reading low ohms, down to around 0.3 ohms in most cases.

I have a couple of newbs questions about continuity checking: should i get a beep from the meter as soon as i touch the probe to the connection anywhere on the connection? or is it normal to have to move the leads around slightly before you make the connection?  I ask this because in the odd connection,  i have to move the leads around here and there to locate the best connection. Once i get it then the reading is going down after a while to a very low ohms reading.

I'm assuming the problem has been caused by the change in the coupling caps because it only happened after doing them. Is it possible that the changing of them might have caused some other related issue that has kicked this off?

Sorry about the beginners questions guys    I've got so many questions

[six el six]

Beginners questions are good. Always.

If your amp worked fine before you did the mod then at least you know where the problem began.

Your "reworked" connections must be corrected/made right. There's no half right in electronics.

Continuity between two points will cause your meter to beep immediately and the beep wll not abate until you break the connection. Sometimes there will be oxidation or gunk in between the probe and the  wire and this will break continuity. Learn to read the ohms setting as well as listening for the continuity beep on the continuity setting.

Changing coupling caps didn't do anything to your amp as long as the values are in the ballpark. Voltage ratings must be the same or higher on replacement caps.

Bad soldering causes a lot of problems for beginners. A good soldering iron like a weller wes51 or wtcp will solve a lot of those problems.

Hope that helps. Keep asking questions.

[Gary_S]

Thanks six el six. The caps i replaced were 0.02 uf and i replaced them with the same value. The only things that changed were the type of caps, i put in Xicon's as i didn't know what brand the originals in the Marshall were. They were small, rectangular shaped white one's, the originals.

I did away with the unregulated iron and bought a temp adjustable station so that i have more control over things and don't expose things to too much heat.

Someone told me that not wiping down the solder connections can give you noise because of flux buildup and stuff but i don't know about that. I thought if the connection was good then that wouldn't matter.

[Willabe]

that not wiping down the solder connections

What does that mean?   


               Brad     

[Gary_S]

that not wiping down the solder connections

What does that mean?   


               Brad     

Someone told me that it's best to use a small brush with some Isopropyl alcohol and brush any oxidation or excess flux off the connection.

[tubeswell]

You can still stuff-up soldering in new coupling caps, especially if the iron is too hot or on too long.  You can wreck new caps again and again, especially polystyrene (Mallory) caps. But apart from that, I've noticed that more and more batches of new mica caps have been bad lately. So check the treble cap in your tone stack for leakage, especially if it is a mica cap

Scratchiness can be a sign of DC where you don't want it. Red plating can be a sign of the wrong sort of DC on the output tube grids. Where does the 'wrong sort' of DC come from? Ask yourself and then look for it. If you don't find any, then it could be bad soldering

[Gary_S]

You can still stuff-up soldering in new coupling caps, especially if the iron is too hot or on too long.  You can wreck new caps again and again, especially polystyrene (Mallory) caps. But apart from that, I've noticed that more and more batches of new mica caps have been bad lately. So check the treble cap in your tone stack for leakage, especially if it is a mica cap

Scratchiness can be a sign of DC where you don't want it. Red plating can be a sign of the wrong sort of DC on the output tube grids. Where does the 'wrong sort' of DC come from? Ask yourself and then look for it. If you don't find any, then it could be bad soldering

Thanks for the reply tubeswell.

Yeah it's possible that the new caps were damaged, but i'm not sure. I used a new temp controlled station i have and i used a temp of 650 degrees. Maybe i kept the iron on too long but i don't think so as i was very aware that it can damage the dielectric of the cap, after the first time it happened! But i'm not ruling it out! maybe i did it again, or maybe the caps are bad.

To me it does sound like DC leakage so i just have to track it down to where it's coming from. Trouble is i'm in a bit of a quandary because all the cap connections to and from are testing ok for continuity and i'm getting low readings on the meter, down to 0.3ohms on most of the connections.

[tubeswell]

The only way to confirm DC leakage in a cap is to subject it to DC and measure whether its crossing the cap

[Gary_S]

The only way to confirm DC leakage in a cap is to subject it to DC and measure whether its crossing the cap

Yeah i took some measurements today paying most attention to what was getting onto the grids of the power tubes. I measured -31.9VDC on pin 5 on both EL's!  That's way too high i think, too much negative voltage getting onto the grids. That must be what's throwing the bias way out and red plating the tube.

So it's just what's causing it? It's got to be the caps and it's either they're damaged because i used too much heat OR they were bad from the start or it's bad solder connections. I didn't think i'd be getting a continuity beep and very low ohms reading though if the solder connections were bad?

So to check the DC leakage in the caps i'll need to unsolder the connection furthest from the plate end of the cap. That end should have no DC or less than 0.2V is supposed to be acceptable? anything over that is bad and i'd need to ditch the cap?

oh well! it's a learning process for me. I'd love it if i could study with someone who could show me in a second what it could take months or years to figure out on my own!!!

Recently had someone advise me to take it to a tech to get it looked over! That defeats the whole purpose of why i'm studying amps and electronics, i want to get the satisfaction of tracking it down and fixing any problems myself.

I'll carry on until i get it even if it takes forever

[punkykatt]

Gary_s,  when you go to solder heat sensitive  components, use a heat sink like a alligator clip or your needle nose pliers on the leads to absorb the damaging heat..

[six el six]

You can take your caps out and test them with meters if you want to. Nothing wrong with that. It just takes time.

An old magic eye tube cap checker can be used.

Or what tubeswell said.

[Gary_S]

Gary_s,  when you go to solder heat sensitive  components, use a heat sink like a alligator clip or your needle nose pliers on the leads to absorb the damaging heat..

Cheers punkykatt. i thought of doing that but maybe i'm being inexperienced here in that there was no room to attach a heatsink as the body of the cap is down flush to the board. They're Xicon's and they're radial caps as opposed to the normal axial one's with the leads coming out of each end. The only area to attach a heatsink on these caps is at the ends so the heatsink would not be between the soldered joint and the cap body. I don't know if that matters or not?

I don't know if it would work that way, maybe the heat would still disperse into the alligator clip even though it's not between the joint and the body. As i'm pretty inexperienced i don't know. Would placing it that way still protect the cap from damage?

I feel like a sponge trying to soak up information and i'm glad i came to this site. Man i love amps and want to learn how to do things properly

[SILVERGUN]

Yeah i took some measurements today paying most attention to what was getting onto the grids of the power tubes. I measured -31.9VDC on pin 5 on both EL's!  That's way too high i think, too much negative voltage getting onto the grids. That must be what's throwing the bias way out and red plating the tube.
[/quote]

Sorry I don't have a lot of time right now but someone will come along and help you more...let's start here:
Many fixed bias amps will have an adjustable bias supply with provision to make the bias more or less negative. Making the grids less negative will cause more current to flow through the tube. Making the grids more negative will cause less current to flow through the tube.

Read that carefully, because what it's saying is that MORE negative voltage on the grids will DECREASE the bias

So you would want to put MORE -VDC on the grids to stop them from red-plating....the NEGATIVE voltage is what slows the electron movement

Learning that alone today would be worth the price of admission

Sorry I gotta go...good luck and BE SAFE!

[Gary_S]

Hi SILVERGUN, yeah i pulled out one of my books and was reading it, it says anywhere from -20 to -60VDC depending on the amp in question. So that measurement wasn't that out of the ordinary of -31.9VDC.

I suspect DC voltage leaking on these as the problem again though causing the bad noise.

I'll go ahead and remove one side of the caps and check what reading i get to see how much DC, if any, is leaking through the cap.

They may have been overheated by me when installing because i never used a heatsink on them. Trouble is they're radial caps and the body needs to be down against the board surface, as i heard if there's a gap then when the amp heats up the board and components can expand and contract and the cap could end up failing by one of the leads ending up cracking because of the movement.

Someone told me to use some clear silicon sealant to stop any movement in that case but i've never heard of that before.

EDIT: Unsoldered the side of one of the caps to check for DC leakage and i got a measurement of around 6.8VDC!!!!!!!!!! over 0.2 is supposed to be bad. No wonder this amp's making crazy noises. The remaining 6 coupling caps will probably be the same. I'll have to ditch them and buy some more and solder them in without ruining them. How can you get a heatsink onto these radial caps though?  

[tubeswell]

EDIT: Unsoldered the side of one of the caps to check for DC leakage and i got a measurement of around 6.8VDC!!!!!!!!!! over 0.2 is supposed to be bad. No wonder this amp's making crazy noises. The remaining 6 coupling caps will probably be the same. I'll have to ditch them and buy some more and solder them in without ruining them. How can you get a heatsink onto these radial caps though?  

I'd just fix those known bad ones first and see if the problem persists. No point in getting carried away if the others are okay (or you may further needlessly-damage possible replacement caps)

[Ed_Chambley]

First, I will say I hate working on PBC, but they are a necessary evil today.  I never solder to a PCB until I test my heat on an scrap one.  Also, if you have room, there is no rule that says the leads have to be so short you cannot attach a heat sink and let the cap sit slightly above the board.  At least enough to get a clip on it.  Also, when I have to do PCB, like building pedals, I use steel wool and clean the leads, them I lightly tin them prior to placing them in the board.  This lets me get in and out with the heat very quickly.  My problem has never been ruining a cap with heat, but I have messed up my share boards.

[Gary_S]

I'd just fix those known bad ones first and see if the problem persists. No point in getting carried away if the others are okay (or you may further needlessly-damage possible replacement caps)

That's what i'll do tubeswell i'll test them one by one and see where they measure out at before i sling them out.

Also, if you have room, there is no rule that says the leads have to be so short you cannot attach a heat sink and let the cap sit slightly above the board. 

Ideally that's what i'd like to do Ed but i spoke over the net with a guy who is an amp manufacturer stateside who i won't name and he told me that having a gap between the PCB and the cap is bad because of the possible expansion and contraction of board components when the amp heats up. he said the leads of the cap can be stressed, when there's that potential for movement between cap and board, leading to the leads breaking. That's the reason i never used a heatsink in the first place really.

He advised me if i was gonna do that to use some clear silicone sealant to stop any potential fro movement in the cap.

[Willabe]

He advised me if i was gonna do that to use some clear silicone sealant to stop any potential fro movement in the cap.

I don't see how a little bit of silicone between the cap and PCB will stop the board from expanding/contracting from heat?

Wouldn't longer leads move slightly less than longer leads?



                   Brad     

[Gary_S]

I don't see how a little bit of silicone between the cap and PCB will stop the board from expanding/contracting from heat?

Wouldn't longer leads move slightly less than longer leads?                  

I didn't say it will stop the board contracting. I said that if there's a gap between the cap and the board an amp builder advised me to put some sealant to stop any excess movement which could result over time in the leads breaking. The guy who advised me is a really good amp builder so i see no reason to not believe what he says.

[six el six]

I forgot to mention solder wick. It's fantastic stuff for pcb work.

You may have pulled solder pads off the pcb when you pulling caps off. Learn how to not do that.

I've been using techspray brand solderwick. Good stuff.

A little flux on the wick for trouble spots.

Don't use cheap solder wick.

[Gary_S]

I forgot to mention solder wick. It's fantastic stuff for pcb work.

You may have pulled solder pads off the pcb when you pulling caps off. Learn how to not do that.

I've been using techspray brand solderwick. Good stuff.

A little flux on the wick for trouble spots.

Don't use cheap solder wick.

Thanks six el six. Yeah i like the wick stuff though the roll i had i've used it all up and need to get more. The roll i had the wick was really thin though, i think the next time i'll get someof the thicker wick. I saw some on Amazon and it was 4mm thick.

I was using one of those solder suckers; you know press the button and it sucks it up?  i don't think they're as good as the wick, they seem to leave some solder still sticking to the areas you're trying to clear.

You're right about the pads: in my keenness to get at it i pulled at some of the old caps when maybe some solder was still adhering it in position and pulling the cap out just ripped the pads out. Beginners mistake

[SILVERGUN]

Gary, back to your original post about one of your tubes glowing red....
Often, when a tube fails, it will take out the screen grid resistor, and any new tube you plug in will glow red, or won't work at all.

Check and replace the screen grid resistors if necessary...use the 5 watters EL34 sells here (or equivalent):
http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Resistors&ORDER_ID=!ORDERID!

This is a great article about bias, tube matching, etc., etc.: (read it a few times and let the wealth of knowledge sink in)
http://www.aikenamps.com/Biasing.html

[Gary_S]

Check and replace the screen grid resistors if necessary

Thanks SILVERGUN. What's the best way to check they are ok? I never thought of that although i've heard of it happening before.

But i'll have to test both screen resistors on both sockets.

Excuse my dumb questions about how to test them   it's just i know with caps they say you really need to take them out the circuit to test them or you won't get a reliable reading.

That schematic!!!!! i can't tell you the hours i've spent looking at that and the preamp one trying to work out what everything does!!!

[SILVERGUN]

According to the schematic you're just looking for 1000 ohms (1K) accross that resistor and you can test them with the power OFF....if the amp is back in the case, you could take the power tubes out and measure from pin 4 on the socket to pin 6

[Ed_Chambley]

Ideally that's what i'd like to do Ed but i spoke over the net with a guy who is an amp manufacturer stateside who i won't name and he told me that having a gap between the PCB and the cap is bad because of the possible expansion and contraction of board components when the amp heats up. he said the leads of the cap can be stressed, when there's that potential for movement between cap and board, leading to the leads breaking. That's the reason i never used a heatsink in the first place really.

He advised me if i was gonna do that to use some clear silicone sealant to stop any potential fro movement in the cap.
[/quote]
Never heard that.  Must be so if it is a reliable source.  I guess the idea is the cap will expand and contract at the same rate as the board since the cap and board temp would be the same.  Silicone will transfer the heat to the cap.  I have always thought the leads would be more flexible if left longer.  My thinking has always been to leave some lead on the cap as I have never seen a lead break in the middle, but I have seen them break right at the cap.  I may need to mend my ways.  Glad my pedals don't get hot.

[Gary_S]

According to the schematic you're just looking for 1000 ohms (1K) accross that resistor and you can test them with the power OFF....if the amp is back in the case, you could take the power tubes out and measure from pin 4 on the socket to pin 6

Thanks SILVERGUN it's much appreciated. All your help and advice is invaluable.

  Gary

Edit:  Just measured both of those screen grid resistors: one was at 975 ohms and the other 1008 ohms. Are those readings ok? seems like the higher one is but i don't know about the 975. What kind of reading are you looking at if it's no good and needs replacement?

[SILVERGUN]

Resistors are rated with a plus/ minus tolerance that is shown in the specs as a percentage.... those resistors are probably either the  5 or 10 percent type, so those measurements are both within spec.

When you put the new JJ tubes in there, did the same tube position red plate?

[Gary_S]

When you put the new JJ tubes in there, did the same tube position red plate?

I didn't keep them in long enough to find out because i just tried the new tubes as an elimination type thing to see if it helped the noise or not.

So i put the new tubes in and turned the amp on and the noise was still there so i switched it off straight away as i didn't want to fry a new set of JJ tubes

Anyway having my amp opened up i was measuring the resistance on some of the resistors on the board. This is nothing to do with the issues i'm having with the amp, this is just another question!!! Most were ok, in fact all the one's i measured were fine but there was one resistor that was mega hard to get a reading from; you know the two 1Meg resistors round about the PI?  well the right hand one was a bugger to get any reading out of. I eventually did get it but i don't know if the leads were a bit oxidised or contaminated and that was blocking me getting a clear reading right away? the one on the left i got the reading straight away.

What do those two resistors do?  I studied the schematic and i've tried to work out what those two are for and what job they're doing. I'm trying to find out and understand every component in the schematic and what they do.

Anyway i ordered some new Xicon caps and will pull theold ones out that are damaged. When i solder the new one's in i'll use a heatsink on the leads and hopefully i won't damage these this time

Man it's terrible but we've got to learn by our mistakes!!

Thanks SILVERGUN.

[tubeswell]

... the two 1Meg resistors round about the PI?  ...What do those two resistors do?  I studied the schematic and i've tried to work out what those two are for and what job they're doing. I'm trying to find out and understand every component in the schematic and what they do

for an LTP, the 1M (between each grid and the junction of the tail resistor and the cathode resistor) are the grid leak resistors. These allow surplus +ve charge that otherwise would accumulate at the grids to be neutralised by being replenished with electrons from the ground return. In the case of an LTP, the grid leak resistors are bootstrapped to the tail resistor which helps with improving the stage's input impedance. You don't need to use 1M in an LTP. You can get away with 470k and still have really high input impedance and good bandwidth, and with 470k instead of 1M, you get the bonus of reduced hiss.

[SILVERGUN]

Gary, I would like to share this with you because I too have been learning a lot in the last couple of months and this has been invaluable...
He starts out slow with simple electronics definitions and slowly goes through and explains each step in the circuit and what it does....this one DVD did more for me than the first 2 books I bought combined (there's just something about having something taught to you by a live person)...I have had to break it down into sections, BUT, it has been well worth the time to digest...if you can afford it, it may save you lots of time wondering :

http://www.amazon.com/Understanding-Tube-Amps-Gerald-Weber/dp/B002B8BU7Q/ref=sr_1_1?ie=UTF8&qid=1350478350&sr=8-1&keywords=gerald+weber+dvd

[SILVERGUN]

... the two 1Meg resistors round about the PI?  ...What do those two resistors do?  I studied the schematic and i've tried to work out what those two are for and what job they're doing. I'm trying to find out and understand every component in the schematic and what they do

for an LTP, the 1M (between each grid and the junction of the tail resistor and the cathode resistor) are the grid leak resistors. These allow surplus +ve charge that otherwise would accumulate at the grids to be neutralised by being replenished with electrons from the ground return. In the case of an LTP, the grid leak resistors are bootstrapped to the tail resistor which helps with improving the stage's input impedance. You don't need to use 1M in an LTP. You can get away with 470k and still have really high input impedance and good bandwidth, and with 470k instead of 1M, you get the bonus of reduced hiss.

Got all that Gary?  .....go back and start from "this is what a resistor, capacitor, tube, etc. does" and you'll be much better off than trying to piece all of these single questions together.....you'll have to understand whats happening to the signal throughout the circuit in order to truly wrap your head around all of this.

Good luck and PLEASE don't take this suggestion personally....I just know you will learn more from Gerald Weber than you can from asking us 100 questions 1 at a time 

[SILVERGUN]

Sorry about the beginners questions guys    I've got so many questions

I'm referring to your quote from your initial post.......there's nothing wrong with questions

[Gary_S]

Got all that Gary?  .....go back and start from "this is what a resistor, capacitor, tube, etc. does" and you'll be much better off than trying to piece all of these single questions together.....you'll have to understand whats happening to the signal throughout the circuit in order to truly wrap your head around all of this.

Good luck and PLEASE don't take this suggestion personally....I just know you will learn more from Gerald Weber than you can from asking us 100 questions 1 at a time  

That's the thing i know what resistors and caps are for and what they do but the problem is understanding what they do in each circuit as there are a lot of variations.

I like Gerald Weber's books; i have the first three along with The Tube Amp Book by Aspen Pittman and Dave Hunters book on tube amps. They all explain bits and pieces and do it well but there are still a lot of gray areas that need a light shone on them.

Thinking of the Weber video. There's something about being shown something that's just much easier than reading it and trying to digest it from the book. However the problem is i'm from the UK and the only version i've seen for sale UK side is for American DVD systems which which is not playable on our systems unless you have a special player. I might just have to do what i'm doing and muddle on on my own.

I'll have to look into whether the DVD is available for our system. I've not seen one so far.    I sent Gerald an email about it to ask whether they make them in a format compatible with our regional players.

Cheers SILVERGUN.

  Gary

[SILVERGUN]

That's the thing i know what resistors and caps are for and what they do but the problem is understanding what they do in each circuit as there are a lot of variations.

Then you NEED this video...he does a great job of walking you through the signal path and explaining what components do and why, AND a couple quick examples of mods or different substitution component values to achive different results..

Just order a USA dvd player with that right from amazon
http://www.amazon.com/Philips-Region-1080p-Upconverting-Player/dp/B004BI6MVS/ref=sr_1_12?ie=UTF8&qid=1350490235&sr=8-12&keywords=dvd+player

Do you have a computer that plays dvds (widows media player)?

[SILVERGUN]

Do you own that 4010?
If so, what would you like to get out of it?.....what type of music are you into?....who's tone do you like?...did you use it before this problem with good results?......pedals?

 

[Gary_S]

That's the thing i know what resistors and caps are for and what they do but the problem is understanding what they do in each circuit as there are a lot of variations.

Then you NEED this video...he does a great job of walking you through the signal path and explaining what components do and why, AND a couple quick examples of mods or different substitution component values to achive different results..

Just order a USA dvd player with that right from amazon
http://www.amazon.com/Philips-Region-1080p-Upconverting-Player/dp/B004BI6MVS/ref=sr_1_12?ie=UTF8&qid=1350490235&sr=8-12&keywords=dvd+player

Do you have a computer that plays dvds (widows media player)?

I have a laptop and it plays DVD's but i had a dvd from the US made for that system and it wouldn't play through my pc.

That DVD is what i'm looking for.

Do you own that 4010?
If so, what would you like to get out of it?.....what type of music are you into?....who's tone do you like?...did you use it before this problem with good results?......pedals?

 

Yeah i do SILVERGUN. I like rock and heavy rock with some blues thrown in as well. Never had any problem with it and i think i should be able to fix this as it doesn't sound very complicated. it's a matter of getting the caps in without ruining the dielectric.

I've used a lot of pedals before with this amp big muff's, distortion pedal from Boss the DS1 delay pedal the DL4 from Line 6.

[SILVERGUN]

I have a laptop and it plays DVD's but i had a dvd from the US made for that system and it wouldn't play through my pc.

What operating system are you running and what DVD decoder?

[Gary_S]

What operating system are you running and what DVD decoder?

Well the only DVD's i watch i play them on this Toshiba laptop so i'll have to check up on it SILVERGUN. I rarely ever use a conventional dvd player these days.

I emailed Gerald Weber at Kendrick and hope someone mails me back about whether these are available on our PAL system.

  Gary

[tubeswell]

Hi Gary S - I gather you've seen this website?

http://www.valvewizard.co.uk/

It contains a number of useful articles on tube amp design, including one explaining the fundamentals of a triode gain stage (downloadable for free) as the 1st Chapter of the 'designing pre-amps' book FWIW

[Gary_S]

Hi Gary S - I gather you've seen this website?

http://www.valvewizard.co.uk/

It contains a number of useful articles on tube amp design, including one explaining the fundamentals of a triode gain stage (downloadable for free) as the 1st Chapter of the 'designing pre-amps' book FWIW

Thanks tubeswell, i've never seen that site before. It looks good and i'll read up on the stuff there.

  Gary