[Roger A. Modjeski]

This is my first poll, I may need some help.

After many years of helping people get their tube amps back in order I find that often just the tubes are at fault. Tube testers, even the simple emission testers are suitable for finding dead, shorted or weak tubes but wont tell you if the tubes are really happy in their sockets. In situ testing (Latin for "in place"). I often worry that people are listening to amps where one output tube is loafing or the driver tube is so far off that it causing distortion and premature clipping. I know this happens because I have amps come to me in that condition. Recently I serviced an RM-9 with a quad of Svetlana EL-34s from Tube Depot. He was blowing fuses and one tube was running red hot. He had the bias properly adjusted but the one tube was handling 90% of the total bias. Of course it ran hot.

So what can I do. Not everyone buys matched tubes from me (though I've told you again and again how others match tubes. You could do better with a yardstick!)

Given this the questions are: how many people have a meter, how many people with a meter use the test points  twice a year? How many people use the test points to determine if something is wrong with a tube?

[DustyC]

Every time I've had a output tube blow or act up I've always replaced them as a set. Sooner rather than later another one in that set will go so why not replace them all.

[Tubo]

Of course I have a meter. Of course I do not use it to check bias twice a year! This hobby is too much work! What you propose is brilliant. Do it!!

[Clio09]

Well I do like the fact that I can check the bias on my RM-10 MkII and do so monthly. It never drifts too far, maybe 1% - 3%, so obviously there is something to Roger's tube matching process. In fact, since I bought the RM-10 MkII, after trying a set of Telefunken EL-84's and some Mullard 12AX7's, I went back to the stock tubes supplied by Roger and have no intent on using anything else in the amp from this point forward. NOS tubes are nice and it's fun to tube roll, but in the end I learned an expensive lesson. Other than Roger, the only person I would buy tubes from is Andy at Vintage Tube Services.

Now Roger, how about putting some of those Gold Lion EL-84 tubes on sale so I can buy myself a quad for Christmas.

[Roger A. Modjeski]

Of course I have a meter. Of course I do not use it to check bias twice a year! This hobby is too much work! What you propose is brilliant. Do it!!

Thanks for your candid and humorous answer. I have a meter attached to my body so its good to know anothers opinion. There are some folks who get their meter out daily and wear out the bias pots.

[Roger A. Modjeski]

Every time I've had a output tube blow or act up I've always replaced them as a set. Sooner rather than later another one in that set will go so why not replace them all.

I agree if you have an amp that is hard on tubes like some ARC, CJ and Rogue amps. Though, if you have one of mine this is not the case. Although I sell tubes, I hate to waste them. If an output tube goes in it's first couple thousand  hours and the remaining ones still have good gettering still why replace them. Their survival thus far is an indication of their goodness.

If you bought the tubes from me and saved the boxes I can match in another for you. A friend here has one of my oldest RM-9's with Siemens EL-34s. They are the longest lasting tube out there. He has run the amplifier many hours every day. About 5 years ago he was blowing fuses. We replaced one output tube. He has had no trouble since. His tubes are over 22 years old. They must be getting up to 20,000 hours now.

For those of you who have a large collection of tubes I do offer testing and re-matching on my tube tester for $100 up to 12 of one type. In the process if you are a few short or if I can't make good pairs, I can often add in a tube from a large collection I have of good used tubes. If you have more tubes or several different types contact me for a quote. In times like these your used tubes may be a great benefit.

[Roger A. Modjeski]

Well I do like the fact that I can check the bias on my RM-10 MkII and do so monthly. It never drifts too far, maybe 1% - 3%, so obviously there is something to Roger's tube matching process. In fact, since I bought the RM-10 MkII, after trying a set of Telefunken EL-84's and some Mullard 12AX7's, I went back to the stock tubes supplied by Roger and have no intent on using anything else in the amp from this point forward. NOS tubes are nice and it's fun to tube roll, but in the end I learned an expensive lesson. Other than Roger, the only person I would buy tubes from is Andy at Vintage Tube Services.

Now Roger, how about putting some of those Gold Lion EL-84 tubes on sale so I can buy myself a quad for Christmas.

A new batch just showed up for my evaluation. Let's hope these are good. I've had to send back several batches due to performance problems and drift, lots of drift. As you know I reject lots of tubes and lately lots of batches.

[rbwalt]

HI roger. roger i love the tube test points on my RM9SE.It makes it so easy to check bias and balance. love how i can check the condition of the driver tubes via their voltage. if they fall out range just replace them. it is a no brainer to check these things via a multi meter. i check my balance about once a month or so.Please  keep this feature on your amps.

Yes there might be some that will find a excuse in doing this but those are the ones that do not know how to check their tire pressures or their oil in the cars or are just to lazy to do so.

You can only do so much hand holding.

rob walters.

[BobRex]

Yes, I have a meter.  But no, I rarely check the bias.  Why not?  Well let's break this into 2 sections:

1) Low level tubes (phono, pre-amps, amplifier input stages...) - For the most part manufacturers do not make test positions accessible for these, the best you can do is probe the socket pins on the circuit board - and that's not always possible.  Even if probing is possible, these tubes are typically fixed bias and would require changing resistors in the circuit.  Kinda blows the warranty all to hell......

2) Output tubes - Again, assuming they can be field biased, you may be dealing with some hefty voltages.  Not something I'd suggest a newbie try without some help. (I'm thinking specifically of some SET amps here, 300Bs use what - 450 volts on the plate??, the Almarro 318B amp uses even higher plate voltages...)  My current SET amp is a fixed bias design, so once again, I really can't do much without changing the circuit.

[Ericus Rex]

I bought this:

http://www.amp-head.com/product_info.php?cPath=21&products_id=70

...which makes testing bias safe and a breeze.  Sure you still need to buy a meter but there's no opening up the chassis and no probing around 450 Volt+ connectors.  This one has the added feature of being able to test for plate voltage.  Well worth the dough and IT'S CURRENTLY ON SALE!!!

[Roger A. Modjeski]

Thanks for your comments so far and we need more votes. In discussing this with some dealers I find the continuous monitoring via an LED indicator per tubes is the best bet.

As to the danger of checking bias there is no such danger in my amplifiers. There is never more than 1 volt on any of my test points. Test points are always conveniently located but I still find people who don't use them. These bias test points also let you know the condition of your output tubes, though not always individually. That is another reason I am tending toward individual indicators.

[jamesgarvin]

I keep the meter next to my RM-200 II amp, and probably check once a month or so. I have one tube that consistently tests around 50mv when playing music, while the others are around 38-40mv. I have put in some other tubes from RAM labs, and changed the fuse to that socket, but it biases high. I do not hear any deleterious effects, and the tube has run high for more than a year and is still going strong, so I am not too concerned.

I do wonder, though, if it would be better to be able to bias each tube individually rather than two together.

[davidrs]

Roger,

I do have a meter. I bias the output tubes, once a quarter, and when newly inserted.

Wish there were easier solutions to check the signal tubes.

And I did vote.

- David.