One or two bias test points?

[Andy Le Blanc]

thank you...... it great to to data presented like that
Im old enough that a computor is still a new tool in many ways
my wife just got a copy of the excel .... its a little differnt
my old bench computor bit it a while back but seeing a program used
shows its value

[Phil_S]

Andy:I have been working with electronic spreadsheet programs since about 1980. This came very natural to me. Once I have the printed sheet in front of me on the bench, it's really very easy to see where the tube falls on it. How much precision do we really need here? I think that chart is about as good as it needs to get. The only catch is that line voltage varies, so tubes evaluated on different days with similar results aren't necessarily matched. I imagine that a pro uses a regulated power supply to eliminate this problem. Enjoy. --Phil

[Andy Le Blanc]

the line voltage where Im at is somtimes as high as, 128 ..... I dont have the means to stabi lize it.......
most venues here in maine have old wiring to boot so things are all over the place.......
A amp sounds and respondes differntly from place to place, time to time.....
as a result I have to be a generalist...... Its nice to have external test points
for bias and balance if the amp utilizes a fixed bias..... It creates a need for
an owner to worry .... and creates a need for them to come to you for service
a well balanced amp will make a bit more watts and demonstrate more clean
power and headroom and all that........
last year I scrounged up some old tube bases and pulled some PCB mounted
sockets out of a blown fender to make my own bias probe.....
the sockets had inch and a quarter long leads and fit perfectly with the bases....
I put it together with a box that had test points and 1 ohm resistors....
plug it into the output tube sockets... plug in the tubes and your in business
it showed B+ and draw and could be used for matching .... the box
providing the test points
but as you say it is dependant on several things like variance of AC
my house is so old that the quality of the neutral ground changes with rain
and not everyone has reliable test measurement so you have to question
just how valid a "closely matched" pair is... is it 250 ma. or .25A

[skyboltone]

my house is so old that the quality of the neutral ground changes with rain

That's illegal according to the National Electrical Code. The utility company establishes the neutral by grounding (with a ground rod at the base of the pole) the appropriate neutral terminal on the service transformer mounted at the pole or on the ground. That terminal is then hardwired via the service drop to your service where (at any convenient point between the service entrance and the first disconnecting means) it is bonded to the service neutral and the premises grounding electrode system (ground rod and grounding system wiring). There is never any reason for current to flow between the utility grounding electrode and the premises grounding electrode unless there is an accidental disconnect of the utility neutral drop. In that case the premises wiring system doesn't work at all with very weird voltages across the board. If you have fluctuating neutral to hot voltages depending on ground wetness or conductivity I would engage a competent wireman to investigate the entire system and find the loose neutral connection. Probably in the service equipment.

[Andy Le Blanc]

you bet.... its an eternity getting the power delivery company to do anything
we'll be taking the opportunity to get it straightend out this year.....
should be upgrading to a larger amp service too..... but you have to wait
and wait and wait......
there are some places that still got the knob and tube.... old edison connections
and now that the power company is a subsiduary of some foreign owned outfit
you have to wait....and wait

[FUCHSAUDIO]

For the cost of a 1-Ohm 3-W resistor, you should do what I do on every Fender, Marshall, or tube amp worth a crap: Put in a 1-ohm on each power tube, then you can check the bias for each tube. While you may only have a single pot, being able to monitor each tube (and select a fair mid point for all tubes or select a set from a bulk buy), is worth doing.

[skyboltone]

I agree Andy. I'll continue to put them in. The question still remains whether I'll make them accessible to clients. In the interests of safety I suppose I will. It's interesting that if you measure your 1 ohm resistors in parallel you get an automatic averaging for one tube bias. Having two pays off though because the first two tubes I pulled out of my Russian bulk pack turned out to be 12ma different at idle!! And this on a 7189! And they sounded fine though somewhat pinched. The problem is that folks become slavish to this 70% business instead of listening to the amp and setting bias by ear.

Do your amps have accessible bias pots or do you expect folks to get inside?

[FUCHSAUDIO]

I agree Andy. I'll continue to put them in. The question still remains whether I'll make them accessible to clients. In the interests of safety I suppose I will. It's interesting that if you measure your 1 ohm resistors in parallel you get an automatic averaging for one tube bias. Having two pays off though because the first two tubes I pulled out of my Russian bulk pack turned out to be 12ma different at idle!! And this on a 7189! And they sounded fine though somewhat pinched. The problem is that folks become slavish to this 70% business instead of listening to the amp and setting bias by ear.

Do your amps have accessible bias pots or do you expect folks to get inside?

We had a "pot per tube" on all amps except the 30-w, which is cathode biased. I may (in a future chassis incarnation) remote the bias pots and test points, but prefer to encourage users to simply have someone who knows wtf they are doing, bias their amps...lol.

[Structo]

I installed dual bias pots on my D'Lite 44 (2 x 6L6GC) amp.
I really like it and like the ability to bias each tube individually.

[drz400]

Just because two tubes are idling at the same current doesnt make them matched, in other words you can have them matched at idle but the Transconductance could be different when signal is flowing. I do agree that the 1ohm 1% 3W is the way to go though and is a quick check on matching WHEN They are already matched for transconductance.

[Andy Le Blanc]

people do get in to ruts very easily ....... I been takeing cues from some of
the older amps I've had a chance to work on.... tending to favor self bias....
very close to the published max dissipations..... my ear seems to like it....
but the correct bias dont do a thing if other aspects are off.. if the pairs
transconductance is way out joint the bias wont hide other distortion issues
and performance wont be the best... the model for push pull is two identical
tubes.... Ive had a practical experience of trying to balance an amp with four bias controls.....
it was a chore...... two might be a good practical limit..
keeping maintanence practical for the owner\user I think would be the best
I liked the way fender has three tip jack with a bias and balance...
but it deosn't give you B+..... for a reason...... drunk stoned and trying to
cram pokey metal bits into the the back of your amp in the dark sounds like
a bad boo boo waiting to happen

[Lonely Raven]

Not trying to derail the thread, and hopefully not too pedestrian of a question.

If my Heathkit TT-11 tube tester measures TC, it is a reliable way for me to match tubes? Or is the voltage typically too low to really give the tubes a fair matching?

I ask because I'm thinking about buying a bulk box of Russian power tubes and I'd like to match them myself to save money.

[Ron Worley]

Eric-

Unfortunately, no... you need to measure mu (and there ends my understanding), which is a measure of the health of the tube under load.

It's this measurement that is used to match the tubes. It's plate current or something like that.....

And now the experts can chime in with a real answer!!

Ron

[Lonely Raven]

I figured more data was required to properly "match" tubes.

I'd love to build a tube tester...basically an amp, to match and burn in bulk tubes, with built in meters and everything. I've heard of people doing this, but currently is beyond my skills.

[Ron Worley]

Me too.... Anybody know a decent set of plans / schematic??

Ron