Just had Different Thoughts about external Bias Check Points and adjustment pot

[Platefire]

I've put them on several of my past amps and are pretty cool to find out what your bias/mA is running and adjust without pulling the chassis.

As I consider putting them on the current amp I'm working on, it seems kind of useless to me being able to read and adjust the bias externally without knowing at least the plate voltage! The only way you can get at that is pull the chassis. It's making more sense to me just to put your one Ohm resistors inside along with bias pot inside because you need access to plate voltage inside???  The only way external checkers and adjustment make sense to me is if you just assume your power tubes plates are running approximatly what the schematic says and go by that??? but from my past experiance different brand tubes of the same type pull slightly different current. So is external check points and adjustment kinda useless in a way? Platefire  

[sluckey]

Sounds reasonable to me.

[terminalgs]

I've contemplated the same thing.  for the reason you pointed out, a single bias voltage test point isn't useful for setting bias  (where does it end, do you add banana jack test points for 2 plates, 2 screens, and a B+ ? that's 6 test points).  sorta overkill.  In the end, you'll probably pop the hood anyhow..

I guess the usefulness of just one or two test points is for performing a general health check (but then again, don't we use our ears for that???.

Someone talk me and Platefire into external test points!!

[Colas LeGrippa]

I use a fuse box for that particular purpose, with no fuse ( evident ). I have to insert my meter red lead  far in to get the HV reading . It is safe. After my reading, I screw back the plastic fuse box cap on. I have put a red sticker on the cap, to make sure it is different from the real fuse box.

Colas

[Platefire]

Colas

That's sounds like a possability with a fuse box but you must be using a different fuse box than standard fender type. From my experiance the spring & fuse tension is what holds the fuse cap on and without the fuse/spring, it would easily fall off. Guess you could tape it on? The main problem with that is the extra real estate it would require to install.

My solution would to use my US Army tube adapters with test points. Install the adapters, then the tubes in the adapters, fire up amp, set bias, remove adapters and re-install tubes. I haven't actually put that into practice but think that will work. Platefire 

[jazbo8]

I am sorry but this is not a good idea, i.e., it is dangerous to have HV where it could be reached by the average user. Even if you do not plan to sell the amp, and it is only for personal use - you still have no idea who will get their hands on the amp whether you are or are not around. YOU may know exactly how to connect the terminals/leads properly to adjust the bias, but the others are unlikely to know or even care to learn, so that's just asking for trouble IMO.

[HotBluePlates]

I've put them on several of my past amps and are pretty cool to find out what your bias/mA is running and adjust without pulling the chassis.

As I consider putting them on the current amp I'm working on, it seems kind of useless to me being able to read and adjust the bias externally without knowing at least the plate voltage!

Meter Tip Jacks. Hoffman has em.

You're gonna have a single ground jack to use for your meter's black lead. You might have another tip jack per tube for the other side of the 1Ω resistor (so 2 of these for a 2-output-tube amp). You could add one more jack for the B+ at the OT center-tap, if you just had to have plate voltage.

Yes, the actual plate will be a few volts less due to drop in the OT. No, that won't make any real difference in your bias setting. Yes, these jacks are safe for having B+ like that, because the contact is well shrouded by the body of the jack, and only something small & narrow (like your meter's probe) will fit where the contact is.

[jazbo8]

A follow-on to my post above - it is not how well the HV wires/jacks are installed or insulated, but rather you are not going to be around forever to supervise/watch over the amp during its life, and someone will use the HV jacks for unintended purposes - this is a deadly situation and should be avoided.

[Platefire]

jazbo8- In otherwords, you don't it would pass "UL" certification. I see your point. How about labeling the jack "Danger" High voltage DCV Test Jack."

HBP-the extra jack(that Doug has) your talking about is the "Bias tip jack", same as used for the 1 Ohm resistor contacts? I just wonder if there is some kind of specialty jack made for this purpose that is rubber sealed that would open up for the prob and close when removed. That way it would maybe prevent any surface moisture from entering in from a spill or something. Platefire   

[SoundmasterG]

Colas

That's sounds like a possability with a fuse box but you must be using a different fuse box than standard fender type. From my experiance the spring & fuse tension is what holds the fuse cap on and without the fuse/spring, it would easily fall off. Guess you could tape it on? The main problem with that is the extra real estate it would require to install.

My solution would to use my US Army tube adapters with test points. Install the adapters, then the tubes in the adapters, fire up amp, set bias, remove adapters and re-install tubes. I haven't actually put that into practice but think that will work. Platefire 

The Army thing that you are talking about is essentially what the Weber Bias Rite is....you put them in the tube socket, and then put the tubes into them. They read the cathode current and give you a reading on a meter, and you can flip a switch and then it shows the plate voltage. I use it on most amps when I set the bias. For my own amps I put the tip pin jacks in....one for ground, another each for the cathodes, and one for the B+, but I use a divider to divide it down to safe levels. So 450V would become 4.5V, and if you size the resistors right in the divider it ends up reading quite accurately at the tip jack...close enough to set the bias to anyway.

Greg

[Platefire]

I have a Ruby Bias-master but best I can tell it has no provision for getting a plate reading. Platefire

[HotBluePlates]

A follow-on to my post above - it is not how well the HV wires/jacks are installed or insulated, but rather you are not going to be around forever to supervise/watch over the amp during its life, and someone will use the HV jacks for unintended purposes - this is a deadly situation and should be avoided.

HBP-the extra jack(that Doug has) your talking about is the "Bias tip jack", same as used for the 1 Ohm resistor contacts? I just wonder if there is some kind of specialty jack made for this purpose that is rubber sealed that would open up for the prob and close when removed.

Guys... Have you seen/used these jacks before? The opening is recessed and nothing can make contact unless it is as-small as the tip of your meter probe. I have these on one of my amps and a 2-year-old in the house, and never even a glimmer of an issue. See the picture below, and know the shaft that goes through the chassis panel is 1/4" diameter.

If you have to cover it, you need a small hinged door. I've seen military gear with a small circular door the size of a dime, a spring hinge and gasket material covering the side that contacts the rest of the chassis when closed.

That way it would maybe prevent any surface moisture from entering in from a spill or something. Platefire   

No amp is safe from a spill. If you're transporting and worried about rain, you need a cover, and again no amp is completely safe from the moisture.

Again, there are military weather-proof electronics, but every part is gasketed (and horrendously expensive) and a moisture/fungus-proof coating goes over the entire innards, making servicing a pain (all coatings have to be removed and/or entire boards thrown out to fix a single component).

[Colas LeGrippa]

excellent

[Ed_Chambley]

I would not try to convince anyone of using external test points to the one ohm resistor for correct bias settings.  It is very handy to let you know in advance is one of your power tubes are failing if you do not have a pair or quad handy.  I use them for that reason.

I prefer external adjustable pots, but all I do is start out on the cold bias side.  Turn them up till I like the sound.  If no plates get red, I have completed what I consider the perfect bias for me.

If I am working on a repair/retube for someone else I would not feel right not pulling the chassis and getting between 60 to 65% unless they request hotter.

[Platefire]

HBP. Sounds Good, Thanks! Another question, you seem trying to avoid running the plate voltage directly from a plate tube socket pin which in most cases would be a closer run to the back of the chassis. Can you explain why you would prefer the tap location being the CT voltage prior to the OT.

Thanks for all the input. This sure helps me decide how I feel about it. The Weber Bias calculator uses setting levels they consider Hot, Average & cool. I personally like to set mine between average and hot--slightly above average. What Ed said as using them as
health check and to stay below red plating makes sense. For those that don't feel comfortable running a hot jack to the back of your amp, I understand. Not sure how I feel about it because I know that my children and grandchildren will end up with a lot of my amps. I would feel perfectly comfortable with it as long as it is in my possession--but that will come to an end. Platefire

[Willabe]

There's another reason that tip jacks are useful if you have separate bias pots.

If you put a new set of tubes in and bias them up by % by pulling the chassis then as the tubes age they wear differently and become more and more unbalanced. As this happens the amp will lose bass and become noisier. So if you have the external tip jacks and have dual bias pots you can quickly bring them back in balance without pulling the chassis.


          Brad         

[sluckey]

I don't think HBP was trying to avoid running the plates to two test points. I bet he was just running the CT B+ to a single test point for simplicity. The voltage at the CT will not be very different than the plate voltages.

If you really want to run a wire from each plate to a test point there's a pretty safe way to do it. Just connect a 100KΩ resistor between each plate and a test point. If you use a meter with 10MΩ input resistance, you'll still have a correct plate voltage reading. But if someone should stick a paper clip in the test point they'll quickly get out of it but they won't require defibrillation.

I'm not recommending putting easily accessible high voltage test points on any consumer product. I've worked on plenty of electronics equipment that has lot's of high voltage test points available for use by QUALIFIED PERSONNEL ONLY!

[jim]

Here is my DR I use every weekend.  Power tubes fail. I carry an extra set of tubes and a small  $2.00 DVM from Harbor Freight to gigs.  Its a snap with TP's but
the bias meter is a bit more cumbersome at gigs for me. I put the bias pot where the useless ground switch was.  Jim

[HotBluePlates]

I don't think HBP was trying to avoid running the plates to two test points. I bet he was just running the CT B+ to a single test point for simplicity. The voltage at the CT will not be very different than the plate voltages.

Exactly. Why pay for 2 test points when you only need 1 (just like why you might only use a single ground test point and not one for each tube).

"But I have voltage drop across my OT half-winding..."

Measure it sometime with the chassis pulled. Maybe drop 5-10v from 450v B+ at the OT center-tap. That's a 1-2% error, and your meter will likely have as much error built-in.

... plate to a test point there's a pretty safe way to do it. Just connect a 100KΩ resistor between each plate and a test point. If you use a meter with 10MΩ input resistance, you'll still have a correct plate voltage reading. But if someone should stick a paper clip in the test point they'll quickly get out of it but they won't require defibrillation. ...

I really like this idea as a method of current-limiting!

450v / 100kΩ = 4.5mA  --> hurts but won't kill.    450v * 4.5mA = 2w

450v / 10.1MΩ = ~0.045mA         0.045mA * 100kΩ = 4.5v       0.045mA * 4.5v = ~0.2mW

You could make that 100kΩ resistor 1/8w or smaller (the smallest you can find anywhere would be good; perhaps this 1/20w one at Mouser). Then normal use is alsways fine while a fault (or kid jamming a pin in the test point contact) burns the resistor open. I can tell you from touching 450vdc myself that the kid will learn a valuable lesson but won't be hurt.

[Platefire]

I wasn't really thinking running a test point from each power tube plate. I was just thinking a single run from closest tube socket that would be closer/shorter than the CT feed in most cases--not always. It's just a matter of maybe running a little more wire from CT feed--no big deal.

You lost me on the math with a 100K in line resistor to test point using a meter with a input impedance of 10M for DCV. I'm having a hard time understanding how a 100K wouldn't reduce the voltage reading and the plate voltage would still be correct. I like the idea, just don't understand. You got 450 plate voltage, you insert a 100K resistor between plate and test point and it still reads 450--I must have missed something? Platefire   

[sluckey]

You lost me on the math with a 100K in line resistor to test point using a meter with a input impedance of 10M for DCV. I'm having a hard time understanding how a 100K wouldn't reduce the voltage reading and the plate voltage would still be correct. I like the idea, just don't understand. You got 450 plate voltage, you insert a 100K resistor between plate and test point and it still reads 450--I must have missed something? Platefire

Well, it wont be 450v. It will be 445.5v. 100KΩ in series with 10MΩ gives R_T of 10.1MΩ. So, 450v applied to 10.1MΩ causes a current flow of 44.5µA. And finally, 44.5µA thru your 10MΩ causes a voltage drop of 445.5V. Only 4.5V is dropped across the 100KΩ. That's only 1% error!

And on HBP's 412 meter that has a 200MΩ input resistance the error would be totally negligible.

[Platefire]

OK. Got ya! That's certainly the best/safest way to do it. Thanks, Platefire

[HotBluePlates]

It also occurred to me that you weren't totally off the mark with the idea of some kind of cap or plug to block access to the jack. Something broadly along the lines of the plastic plug you put in an outlet to childproof your house.

The drawback is you'll need to fashion it yourself, in all likelihood. But I think you'll worry less if you see these jacks and hold one in your hand.

[PRR]

Tube current varies a lot.

At given current, chassis B+ does not vary so much.

The main error would be wall-voltage, which you can check apart from the amp.

And this isn't precision time-keeping. A 10% change of wall suggests maybe a 10% change of tube current, but a 30% change in the brightness of your room lamps and a 3:1 change in their average Life. You probably "know" your wall-voltage within 10%, otherwise you are whining about dim lamps or constant lamp replacement.

A current-tap is mostly what you need.

Tip-jacks go back before the earliest radios, and were generally accepted as "safe" for decades. I notice that in the last 20 years meters have switched over to a more-safe design which eliminates the bare probe in the hole (and possible finger-touch). That does not change the issue of electricity coming OUT of the jack, but tip-jack requires the kid to find a paper-clip, and once the kid starts putting paperclips in holes he'll find a lot more trouble than one amplifier. (Note that modern US wall-outlets have shutters, supposedly for less dead children.)

It is NOT necessary to bring full B+ to a test point. In fact it's probably reckless. Aside from shock, a careless meter connection could put a dead-short on the amplifier power supply, ouch.

Use a Voltage-Divider. 1Meg (0.5W) and 1K is 1000:1. The worst-case voltage in any sane guitar-amp will be under 1V at the test point. Put your DMM on "mV" and then overlook the "m" when taking a reading. Meter-loading with a 10Meg (even 1Meg) DMM is negligible (0.1%). (The cheapest sort of $9 needle-meters will not work well this way.)

> I've worked on plenty of electronics equipment that has lot's of high voltage test points available for use by QUALIFIED PERSONNEL ONLY!

Yes; but also behind the airport fence inside a nominally locked or check-pointed building. A G-amp may be in a room full of drunks or a house with kids.

[SoundmasterG]

It is NOT necessary to bring full B+ to a test point. In fact it's probably reckless. Aside from shock, a careless meter connection could put a dead-short on the amplifier power supply, ouch.

Use a Voltage-Divider. 1Meg (0.5W) and 1K is 1000:1. The worst-case voltage in any sane guitar-amp will be under 1V at the test point. Put your DMM on "mV" and then overlook the "m" when taking a reading. Meter-loading with a 10Meg (even 1Meg) DMM is negligible (0.1%). (The cheapest sort of $9 needle-meters will not work well this way.)

This is what I do, though I use different value resistors to drop the voltage 100 times. It works well and is quite accurate to 100 times less. On the first amp I put the B+ on the back through a tip pin jack, I was warned by you PRR that was possibly dangerous...and after you said it I realized why...and how...so I put the divider in since.

I have a Ruby Bias-master but best I can tell it has no provision for getting a plate reading. Platefire

Platefire, I am not aware of any others besides the Weber that tests the cathode current AND the plate voltage...the Webers do it and it is super handy...but just make sure you don't play a lot while the meter is in line or you will blow the 1 ohm resistors inside the meter. Other than that one caveat, they are super worth it.

Greg

[Stankfut]

The drawback is you'll need to fashion it yourself, in all likelihood. But I think you'll worry less if you see these jacks and hold one in your hand.

Would this work?  http://www.mouser.com/ProductDetail/Switchcraft/515X/?qs=sGAEpiMZZMsUzhEcHltCuazO2ABDwL91

[Platefire]

On the Switchcraft cover--you would just about have to have it in hand to tell rather it would work. It's made for the 1/4 Jack threaded shaft to fit through the hole and the nut installed between the base and the flip cap. It just might work if the tip jack outside larger diameter is about the same as 1/4 jack outside nut diameter. Attached is a drawing of the cover and also the tip jack. Platefire

[PRR]

> blow the 1 ohm resistors

Have you done this? It is very hard for a vacuum tube to blow a 1 Ohm resistor.

[dscottguitars]

I'm curious about this issue too.  I have put test points on my amps but used a solder lug with 3 points, bent it down to about a 1/4" off the chassis and then put a good size glob of solder on.  I drilled a hole through the chassis and put rubber grommets as the insert points for the meter leads.  I did this for the two 1 ohm resistors and the B+.  I've not had any problem this way but I do like the tip jack idea.

Concerning the 'safety' of the high voltage point.  What's the difference in checking the high voltage with a meter if the chassis is open and you touch the lead to the tube socket or checking it with the tip jack?  Putting a voltage divider seems like overkill for something you would do if the amp is opened up.  

[Willabe]

I have put test points on my amps but used a solder lug with 3 points, bent it down to about a 1/4" off the chassis and then put a good size glob of solder on.  I drilled a hole through the chassis and put rubber grommets as the insert points for the meter leads.  I did this for the two 1 ohm resistors and the B+.

Cleaver thinking but that's a lot of work and parts when you can use tip jacks.

Concerning the 'safety' of the high voltage point.  What's the difference in checking the high voltage with a meter if the chassis is open and you touch the lead to the tube socket or checking it with the tip jack?  Putting a voltage divider seems like overkill for something you would do if the amp is opened up.

Look back at what the guys said about this, it's clear it's about possible danger to others and not to techs who know what they're doing.

      
                  Brad    

[dscottguitars]

Yes it was a lot of work.  Now that I know about the tip jacks I'm going to use those.

[PRR]

> if the chassis is open

If the chassis is open you just probe your points.

I thought the idea was to check things with-out opening the chassis?

[SoundmasterG]

> if the chassis is open

If the chassis is open you just probe your points.

I thought the idea was to check things with-out opening the chassis?

That is the idea and is why I do it. Once I build an amp I'd rather not have to take it out of the box and open it all up again, especially if it is every time a power tube needs changed so I can set the bias.

The voltage divider is just to make things safe for anyone besides a tech who might look at the amp. 4.5 V is a lot safer than 450V to the unaware.

Greg

[dscottguitars]

What is unsafe about using a multimeter and putting it in a tip jack?  That's what I was referring to.  Since it's safe to do with the chassis open, it should be safe in a tip jack.  It's pretty hard to get a shock if you're using the lead from a meter, and it's not a difficult thing to do.  If a person is that ignorant about how to use one, they shouldn't be using it anyway.  Using a voltage divider just invites more inaccuracy due to the quality of the resistors, drift with age and such.

On every amp I've built the filter caps have always bled out their voltage faster than I can grab the meter to check if its safe, so anyone 'playing' around with the amp, which shouldn't be, isn't in any harm anyway.  My stereo and TV have holes in the unit for heat escape that a kid could put a wire in there and get a shock too but they still pass safety from the manufacturer.  That's why I think it's overkill to drop the voltage in a safe test point.  A tech or not, it's not a dangerous thing to use a meter and check the voltage.

[Willabe]

But if someone should stick a paper clip in the test point they'll quickly get out of it but they won't require defibrillation.

I'm not recommending putting easily accessible high voltage test points on any consumer product. I've worked on plenty of electronics equipment that has lot's of high voltage test points available for use by QUALIFIED PERSONNEL ONLY!

[Willabe]

I am sorry but this is not a good idea, i.e., it is dangerous to have HV where it could be reached by the average user. Even if you do not plan to sell the amp, and it is only for personal use - you still have no idea who will get their hands on the amp whether you are or are not around. YOU may know exactly how to connect the terminals/leads properly to adjust the bias, but the others are unlikely to know or even care to learn, so that's just asking for trouble IMO.

But if someone should stick a paper clip in the test point they'll quickly get out of it but they won't require defibrillation.

I'm not recommending putting easily accessible high voltage test points on any consumer product. I've worked on plenty of electronics equipment that has lot's of high voltage test points available for use by QUALIFIED PERSONNEL ONLY!

[HotBluePlates]

...  If a person is that ignorant about how to use one, they shouldn't be using it anyway.  ...  That's why I think it's overkill to drop the voltage in a safe test point.  A tech or not, it's not a dangerous thing to use a meter and check the voltage.

All the worry is not for the tech who might check the amp. It's for everyone else who might make accidental contact with a high voltage point made accessible from outside the chassis.

If you're not worried, cool, don't include the feature on your amp. But don't add test points where high voltage can be probed, and sell the amps. You might be asking for a lawsuit later that can wreck you.