Bias Reading/Adjustment Questions

[Platefire]

I've been reading the Trainwreck pages. Their method of reading bias I tried today. The one for placing your MM positive to the OT center tap and the negative to power tube plates. On my Bogen/Bassman
I got a reading for both tubes pretty close to 30 mA. On the other hand using my exterior test points hooked up to 1 ohm resistors to ground and power tube cathodes, my reading for both tube are close to
35 mV. I know Doug said he sets his 6L6's at about 35. Is that mV or mA??? So I'm trying to figure where I'm actually at and am I biased kind of cold and need to raise it to 35 mA? Kind of confused about the two different readings. Trainwreck pages didn't put much stock in the voltage readings. Platefire

[bigsbybender]

I know Doug said he sets his 6L6's at about 35. Is that mV or mA???

mA

Fixed Bias voltage will usually range somewhere between -20 to -50v depending on the amp.

We use the meter set on voltage and measure over the resistor (Ohms law V/R=I (current).... otherwise to measure current, you'd have to put the leads of your meter in series with the circuit (what the trainwreck page is more or less doing)

I'm really surprised at the discrepancy.

j.

[stratele52]

Your cathode mesuments on the 1 ohms resistor include some grid current. Substract fews milliamp from your 35 ma and you have same as the plate.
Mesuring the plate current is a very good way but VERY  dangerous. I work only with a 1 ohms resistor or bias probe.

[HotBluePlates]

I've been reading the Trainwreck pages. Their method of reading bias I tried today. The one for placing your MM positive to the OT center tap and the negative to power tube plates. On my Bogen/Bassman I got a reading for both tubes pretty close to 30 mA. On the other hand using my exterior test points hooked up to 1 ohm resistors to ground and power tube cathodes, my reading for both tube are close to 35 mV.

You can then assume that 4-5mA (accounting for possible rounding) of extra cathode current (1 ohm resistor method) represents screen current.

I know Doug said he sets his 6L6's at about 35. Is that mV or mA???

I suggest you never trust a recommended plate current when the actual plate-to-cathode voltage is not also stated.

Let's say I have an amp and I say I like to set the tube current at 107mA per tube (!). That number probably won't help you unless I also say I'm using 6L6GC's (30w dissipation rating), have 300v B+, and ~187 ohms of cathode resistor. It would then help you to know that I didn't subtract screen current from my tube current suggestion, and that the "107mA" really represents cathode current. Then, when you run the numbers, you'll see that 107mA through 187 ohms drops ~20v, and that the plate dissipation number checks.

Yes, I'm really running the plate cooler than 30w because the plate current is lower than cathode current by the amount of screen current. But will the 5-7 extra mA's matter in the face of 107mA's?

[Platefire]

Just reading what all of you are saying--or reading between the lines so to speak, what I hear you saying is you really need to balance your readings with calculations to know where you're at?

The reason I'm trying to nail this down is I played over my Accomplice several years cold bias with the original TAD tubes. When I realized it was cold biased around 25mV on the 1 ohm test method, I ramped it up to 35---then my tubes started red plating. I bought me a new set of JJ 6L6's and adjusted to 35 and man what a difference in the sound and response of that amp--like diffence of night to day. So when I realized what a diffence it can make in an amps performance to be in the right bias range as I learned with the Accomplice, I'm re-looking at all my amps, even cathode resistor biased amps to see where I'm at and what adjustments I may need to make. I have a bias Master I've been using to check the cathod biased amps but not sure how accurate that is. Platefire

[HotBluePlates]

Just reading what all of you are saying--or reading between the lines so to speak, what I hear you saying is you really need to balance your readings with calculations to know where you're at?

Exactly.

You also have to think ahead enough to second-guess your readings sensibly. For example, the OT shunt method that you recently learned and tried measures only the actual plate current through that half of the push-pull output stage (so it's really 2 tubes' plate current in something like a Twin). The 1 ohm cathode resistor measures cathode current, which includes screen and plate current. So if you calculated an "ideal" plate current, then set it using the 1 ohm resistor, you will actually land at slightly lower plate current than you expected.

But there's no such thing as one ideal plate current value; it can be anything, depending on the characteristics of the specific amp you're looking at.

[PRR]

> test points hooked up to 1 ohm ... cathodes
> about 35. Is that mV or mA???

If it is 1 Ohm, then mV and mA are the same. (35mA times 1 Ohm makes 35mV.)

As others say: 30mA in _plate_ (OT) plus 5mA in screen is 35mA in cathode. No great discrepancy. On 6L6 at 30mA plate I would expect 2mA-3mA screen so there is a small discrepancy, not worth exploring.

As was said: "there's no such thing as one ideal plate current value".

[stratele52]

Platefire,
You discover how much bias is important in tone.

Under biased ( Hot bias ) ; amp will lack of punch and tubes run noticibly hotter.
Over biased  ( cold bias ) ; amp will sound thin and brittle, with tubes running too cold for proper performance.

A correctly biased amp will sound clean and tight at moderate volume, than at higher volumes break up and distort musically

Do you know Weber bias calculator ?

http://www.webervst.com/tubes1/calcbias.htm

[Platefire]

  You pay extra to get matched tubes and I have yet to get a set that was reading the same thing when measuring bias. So with me shooting at 35 with one running about 32 and the other 35, I have been setting my bias to the highest reading tube=35 leaving the other at 32 and now subtracting 5mA for cathode and screen that would put the lower one at approx. 27 mA. Kind of over biased/cold.
  So on the Bogen/Bassman amp I'm currently working on, I used the the shunt method and adjusted the 32 one up to 35mA and the higher one is now about 38mA. That put the mV/1 Ohm reading up to about 39 and 42.
  So the purpose of the external test points voltage measuring system is to save you from opening up the amp when you need to bias--so I'm thinking running the mV up an exta 5 to compensate for the extra cathode and screen voltage would be a safe enough thing to do to get closer to a happy medium between cold and hot. That's kind of the drift I got from PRR.
  I copied the bias calculator to my favoites, Thanks!  I put my plate voltage of 466 in with 5881 tubes and it said my bias should be 39 mA. I think I'm getting more in the ball park now. Sure makes a difference in performance when it's on the money! Platefire   

[HotBluePlates]

  You pay extra to get matched tubes and I have yet to get a set that was reading the same thing ...  

So stop paying extra!

If exact match/balance is important to you, change over your bias circuit to include more than 1 bias pot (at least 2 ways to do this; assumes you're using a fixed-bias amp).

I'm to the point that I don't care about bias anymore unless a tube is redplating.

[Platefire]

I've never biased an amp with 4 PP power tubes before. So entering in to dealing with the bias on the 1975 Twin Reverb, I want to make sure my understanding is correct.

I've been doing my biasing using the shunt/trainwreck method using mA setting on my MM putting the negative to plate and positive to OT center tap.

So in reading the trainwreck page 27, paragraph 4 it says "You should adjust the bias until your meter reads 60 milliamperes". So the way I'm interpreting this is if I want the four tubes to be set at 35mA each (140mA total), when I check one tube using the shunt method, I should adjust it to 70mA? because the 50%/1/2 of the PP tubes are tied together on pin 3 plate. So when I measure the other side of the PP side it should be close to 70mA also---correct? for a total of 140mA altogether?

Please verify if my understanding is correct or not. Thanks, Platefire

[birt]

as stated before, your numbers always have to correspond to the plate voltage.
that being said, if you shunt the OT primary you measure the current of all the tubes on that side of the OT. so in a deluxe amp your result will be the current of one tube's plate. in a twin it will be 2 tubes. in an SVT it's 3 tubes. this method works but i wouldn't recommend it for bigger amps.
-because when you measure 2 tubes plate's at the same time you can't just divide by 2. the tubes can be very different and one can be very cold and one close to redplating. you might as well don't measure it and just make sure no tube is redplating.
-because it is very easy to slip with your probe and short plate voltage to ground or other connections. also all current is going through your dmm.

if i have an amp with 4 or more power tubes and i notice one of the tubes is not matched (within reason) with the rest i just swap it with other ones to see if i have one that is more suitable. clients don't always like different brands or different looking power tubes combined but i used to work for a rental company a lot and their main goal is to have the amps working well and sounding good for a long time. not internal esthetics.

[Platefire]

OK, if the shunt method is not the best method to read bias for the health of your amp or your MM, what is the best method for any amp that don't have exterior test points. I've already blew up a couple of MM using this method--so my ears & eyes are open!!!

I just need a good method/proceedure to check bias on any amp. A proceedure & formula---it's important to me to nail this down! Guess I'll look through Doug info and see what he's got. Thanks, Platefire 

[sluckey]

http://aga.rru.com/TechTips/disp.cgi?file=LV/BiasAdj.tt

[birt]

from the outside it's best to take individual readings for tubes is with a bias probe that has a 1ohm resistor in series with the cathode. but a lot of amps you still have to open up to change the bias. if you don't use a bias probe or install 1ohm resistors you can also bias using a scope. it takes a bit more time but if you do it well it's quite accurate.

[Platefire]

Well I'm starting to repair amps and I need to be able to check anything that comes in the door to find out where it's at and if it needs to be corrected. I'm just going to have to study the subject a while until I come up with the best methods for me. I just downloaded a document that is attached pdf file I will study a while. The problem seems to be the best way to measure idle current off any amp that come down the pike and then off course how to set it. 

Note: I like the alternative shunt method in attached artical, reading resistance from OT center tap to secondary with amp off and then reading voltage drop accross same with amp on---then applying I=U/R. Only problem, you can't make real time adjustments using this--it would have to be incremental on/off adjustments until it is right.

[Ed_Chambley]

Thus guy sells truly matched tubes. http://analogtubes.com/tubeshop/
I know, everyone knows the best tube supplier and I am just another one.
I have been in his shop many times and he has he equipment to do it correctly.  He is in the Atlanta area, but is moving to NC soon.
The problem with matching tubes is there are many in each lot purchased which will not match.   Guess what they do with those?  they sell them as singles which means if you do not get matched, you will get the worse of the matching.  If the matched set you are buying do not match whoever you got them from should replace them.  The problem with this is time and shipping costs.  Some tube sellers will even tell you if the bases have a scratch, they cannot be returned.  This is the same as saying we are going to sell the un-matched set to someone else.
My last purchase from Analog Tubes were 4 winged c's.  2 sets of matched pairs.  I have one set in my Super Reverb.  They measure 38.6mv and 38.4mv.  The other set is just as good.  He also has matched PI tubes.  The better matched sets someone sells means the more discarded tubes they have.  You, you can individually bias tubes in an amp by adding an additional circuit and pot and is a good idea if you are building an amp, but if you are repairing for others it is best to get matched sets.  Even with individual bias circuits, soon as you add a signal and the tubes begin to heat up, they will not stay matched until they return back to idle.

[bluesbear]

It's easy to match your own tubes if you have external tip jacks with 1 ohm resistors. Just stick in a set of tubes and bias them. Pull one and write down the number it was biased at. Replace it with another tube. ***DON'T touch the bias control again!*** Pull the tube you just put in and write the number on it. Repeat for all your tubes. Always pull from the same position... in case of slight discrepencies between the 1 ohm resistors. When you're done, just match the numbers. Of course, all this depends on having a bunch of unmatch tubes laying around.
Dave

[Platefire]

Thanks for the link sluckey, I just now saw it and read it. I'm a firm believer in the 1 Ohm cathode to ground with exterior test points and ground lug. After reading sluckey's link and the artical I attached I'm still favoring the OT shunt method for vintage amps without any of the more modern cathode resistor test points.

I did see where they recomended not to use this method from some Marshalls, so I'm learning that on hindsight after the fact. Something I tried and think is pretty safe using this method that you would start the test with your amp turned off using two insulated alligator test leads and conducted the test as follows:

1-Attach one end of the alligator clip lead to pin 3 and run the other end of the lead to the outside of the amp chassis to a non-conductive surface/pad where your MM is located, about 1 foot away. Pull the insulated convering over your alligator clip down on pin 3 as far as it will go to protect it from any contact to adjacent pins/wires.
2-Attach the other insulated alligator test lead to your OT center tap and run the other end outside the chassis to non-conductive pad and MM. Also pull the insulation down on your alligator clip to protect it from contact to adjacent pins/wires.
3-As an additional safty test, do a continuity check through MM/pad end of test leads to adjacent pins wires inside chassis to be sure no short is created.
4-Position your MM on the insulated pad outside of chassis to where you can read it and adjust bias at the same time. Connect pin 3 test lead to MM com probe and connect OT center tap test lead to MM positive probe. I actually suspended the connection of the test leads to MM probe in air so they wasn't even toutching the insulated pad. Set your MM to 200 mA range and turn you amp on standby.
5-Get your bias adjusting screwdriver in hand, turn amp off stanby and when mA reading stabelize on your MM screen adjust the bias pot to desired mA. This would have to be repeated for the other PP side.

[Platefire]

How bout the Fluke 113 for OT shunt method. Info says the DMM needs to have almost "0" resistance when shunting OT meauring mA. So far I haven't found a spec that tells me that on the 113. Platefire

http://www.multimeterreviews.com/fluke-113-review/

[PRR]

> I need to be able to check anything that comes in the door

Get an infrared thermometer. Surface of the glass near the plate should not be much over 400 degrees F, but power tubes usually should not be much cooler.

If it's in that ballpark, and you need to move on, let it be.

If it's running 450 deg F, or 290 deg F, you ought to drop and do a bias-check.

(<300 deg F is normal for small tubes; it would be near-impossible to get a 12AX7 very hot.)

You'll see a lot of amps. Keep mental notes about what range "most" tubes run. There's always the oddballs: new Champ 600 with 17 W Pdiss must be awful hot. Traynor BassMate 7199 was shipped running stone-cool (not a problem).

If you read the datasheets, some tubes are "OK" with 425 deg F on the glass. I've seen higher. But this is a "pushing it" rating. Fine for cheap under-ventilated TV sets with 30 day warranty. Not wise for you to send out a tube running that hot.

[Platefire]

PRR

That's the first time I've ever heard of checking a tube by heat. Never ever come accross where it was even discussed before. It just so happens I have a model RG101 EXTECH Hygro-Thermometer and InfraRed Thermometer from my Government training in Mold School. Main thing I used it for before I retired was to check the tempature of air coming out of the HVAC cieling vent over my cubical. So maybe now I can put it  back to work for a higher purpose.
  So if I'm understanding you right and of course it would help to read the data sheet on the paticular tube your checking but for power tubes in general you could consider 300D maybe on the cold side and 400D on the hotter side. Platefire

[PRR]

> help to read the data sheet on the paticular tube

Who has time?

Anyway all tubes are made with the same glass and seals; or maybe a "good" and a "better" grade. Peas in a pot.

6550 and 6LQ6 data shows 250 degrees _C_ "at hottest point" on bulb. These are surely the "better" tubes: both were introduced to replace lesser tubes in severe service. However heat flow at these temps goes exponential, so less heat is not a lot lower temperature.

Aim at various "happy" power tubes and note the ranges.

BTW it takes 5-10 minutes for the glass surface to stabiize.

[Platefire]

Thanks, PRR.

Next time I get an amp out and fire it up I will give it a shot. I like an amp where I can look in the back and see all the power tubes. The Marshall JMP 50 I  just worked on, the back plate covers all visablity of the tubes but all my personal amps are open to where you could at least get a line of sight at some angle. It would be a good way to do a quick check on the tubes to see where your at to determine if I need to investigate further. I assume you need to aim at anywhere on the plate? Platefire

[Platefire]

I checked my Champ this morning while I was drinking coffee and after 20 Minuet warm up I got 6L6=223D right in the middle of the plate that seemed to be the hottest point. At top or bottom of plate below the hot spot it was less than 200. Also checked the 12AX7=81D and 5Y3=140.

On the power tube that's a little cold. I might try my JJ 6V6 and see if it warms up a bit. Platefire

[sluckey]

Checking a receiving tube's temperature is a neat thing to do, if you ain't got anything better to do. And if you spent hours and hours of experimenting you could probably come up with a chart that roughly relates tube temp to plate dissipation. Rube would be proud.

But it ain't an accurate way to check bias. I don't think PRR meant it as an alternative to checking bias either. It was just a quick check to see if you 'may' need to check the bias. There are several well established and documented methods for reliably checking bias. Learn them and know how to adapt. And know the pros and cons about the different methods. You'll be able to check bias on any amp.

IR temp meters are great. I have one at home. And I use one at work all the time. They're great for scanning power distribution panels looking for hot breakers or lugs. They're nice on the HVAC system too. And I even have to use one to check the battery temp on my 160KW UPS. I also use one to check the temp of my radar modulator components and amplitron tube. Lot's of good practical uses. But they ain't needed in a guitar amp shop. Other than the "Gee, that's nice" factor. Practically speaking, your eyes and fingers can tell you all you need to know about a receiving tube's temp. Don't touch those EL84s! They will steal your fingerprints!    

[Platefire]

Oh, I was just playing with it. Like you say, if you suspect something going on with a power tube, that would be a quick check to see if your might in the danger zone. I have studied bias a whole lot since starting this thread and the best to me hands down is the 1 Ohm resistor from cathode to ground method. And if they don't exist on an amp, it might be a good idea to install them temporarly on a vintage amp to check and then remove them. So if I was working on an amp with exterior test points & bias pot, I wouldn't look any further.

I understand on the OT shunt method there is a lot of opprotunity for mistakes with the leads touching ground or another wire plus your DMM needs to be a good one with nearly "0" resistance in the meter. I do think it can be done safely with the proper DMM and using insulated Alligator test wires carefully placed with the amp turned off, double checking test leads for continuity to any other adjacent points other than the plate pin, to ensure your not creating a short. Also carefully connecting test leads to DMM probe in a manner there is no opprotunity for a short there. This would take a little time and effort but on a vintage amp would be the second best thing to temporarly installing 1 Ohm resistors cathode to ground.
 
The alternative shunt method by resistance measuring with power off calculated with live amp plate voltage readings and OT center tap reading is the safest shunt method.

I have a bias meter that plugs into the tube sockets but based on the last usage of it, I don't trust it anymore. It don't agree with my DMM readings. I have two DMM's and sometime check them against each other to see if they agree as a double check. I am presently looking at the Fluke 113, a rather low end Fluke but never the less a Fluke. That would be an upgrade from what I have.

So Yeah, tempature checking may be a good tool for an amp without exterior test points or bias pot to see if you need to check closer. Platefire

[sluckey]

FYI, Fluke sells a good test lead pack that includes clip on probes. No need to use gator clips connected between the amp and meter leads. Very easy and safe to use.

[PRR]

> I got 6L6=223D

6L6GC (I assume) is a strong tube, but not a 6550. 250 C might be too hot. 200 C might be cool for a self-bias amp (fixed-bias are often idled cooler because they suck more with signal). I think 223 splits the difference, seems entirely "safe to send out the door" after a $5 jack repair.

If I did a $5 repair then found the finals running 270 deg C, I would stop and do actual bias-checks so it did not come back the next day melted-down.

Your 140 on the 5Y3 seems low. This is also a power tube passing the same current as the 6L6. Voltage-drop is lower, but old 5Y3 is a much cruder tube than 6L6GC. Yes, it was proportioned in days when 250 deg C glass was not available. And new-made "5Y3" is different. Be interesting to know how other rectos are cooking.

It's like Sluckey's batts or power panel. If all the lugs run 30C-50C, then one day one lug is showing 80C, ask why. If he just installed the electric hot-tub and it is coming up to "hot", this "may" be OK. If the load is two 'scopes and a PC, then suspect bad-connection which will get worse fast. It's only a hasty-check for unsuspected trouble. It's justified now the IR thermos are under $20 and very handy; at that cost/effort it makes sense to quick-skim every outgoing job for hot-spots.

[Platefire]

I've been talking about the Fluke 113 DMM but just found out the resistance measurement only goes up to 6K. So I started checking on the 114 and it goes up to 40M resistance reading but no mA reading.
Now I'm looking at the 115 that seems to have everything I need for amp building and repair. I'm still looking at what part of the spec tells me the resistance generated by the meter for mA readings. I've attached a pdf file with the specs, maybe you can find it. Platefire

BTW-I've presently got an Etek 10709 that I picked up for only $20.00 New a couple of years ago that has a lot of features and works surprisingly well for what it is. 

[mresistor]

PRR - as you are in the know about a good low cost IR thermometer, can you suggest/recommend one?

[Rev D]

 Not to be a complete dolt, but as long as your not red platting, can't you just adjust an amp to where it sounds good (thats the objective anyway isn't it)? I mean I checked my high powered princeton reverb that I built (somewhere about 35 watts or so, typical Fender sort of output for twin 6l6's), but I would twist the bias up a tad and check to see if the trem worked right (a concern on a princeton due to its trem circuit) and then tweak a bit more, checking for red platting. I finally found a sweet spot and I accept that if its running a bit hotter I'll sacrifice tube life for a good sound (not a concern for me as I don't use NOS tubes, just to expensive for a poor non-working musician at the moment). I know that's a rough neck way of doing it, but I've read some do bias that way, it worked on that amp anyway.   (of course on a new build, I do check to see if its in some sort of limit and calculate the MA as above just to know where is at and then tweak from there)

Regards,

Don

[Platefire]

  No problem on running the bias up hot on one of your own amps at the sacrifice of your tubes. When you start repairing other people's amps you don't want to create a potental problem that would be related to your repair work and have them knocking back at your door with a Crispy Critter of a smoking amp in hand. My thoughts on a customers amp is to bias on the safe side of hot for instance if your safe usable range based on the type tubes and plate voltage is 30 to 40 mA maybe make it 35 to 40mA. If your tubes on a fixed bias PP A/B amp setup is a little out of balance like they usually are, I usually set the hottest one right at the highest end of the safest known limit or over the limit a bit just to get the lower one up a bit. Don't know if that's good practice but that's what I've been doing with no big problems so far. Platefire

[Rev D]

 Yeah I understand what you mean. You do what the customer wants and as long as the amp sounds like he remembers it, a bit cooler bias is probably a good rule of thumb. Thanks for pointing that out. I'm not into doing to much work on other peoples stuff, although the spare change would be good about now for sure.

Good on you having the huevo's to do that stuff!  

Regards,

Don

[Platefire]

Oh I love trouble shooting/repairing old style tube amps, it's just that sometimes you get one that there is no easy way out and a total mess but that make you appreciate the gooduns.

BTW: On another note, I was doing some recording today using my Accomplice and decided to measure the temepature on those tubes. Was about 298 on the hottest spot on the 6L6GC and the Rectifier

[PRR]

One guy says to to poke a current meter at the amp.

Another guy says if it aint redplating he plays.

Both are right, for specific situations.

I "love" to play with bias current BUT there's times the amp HAS to go on-stage very fast. If there's no complaint, you would not tear-open or pull-tube to check bias. But it would be poor form to touch an amp and let it back to a paying customer "pink-plating". Not so over-hot you'd notice might, after an hour, become a melt-down.

Harbor Freight had a cheap $20 IR thermo. I think they got even cheaper. OTOH car-parts stores have nice ones, though not cheap.

Either way, if you can "see" the power tubes, you can get a reading in 3 seconds. 15 seconds to look for possible trouble might save a bad morning-after.

It's also good for finding thin-spots in wall insulation. If you are pouring oil on the fire to keep the room to 68, and most of the wall surface is say 63, but there's areas at top or around outlets at 52, suspect the insulation is sagged or was disturbed by wiring. (Some heat-leak at window-edges and wallboard openings is normal, but there's normal and "Yow!".)

The Fluke 115 sale-sheet and Owner Manual does not specify the burden on Amps. It would be reasonable to suspect the max voltage drop is similar to the low-range Volts scale, in this case 0.6V; but that may be wrong. Why do you care?