tubes and tube color

[o)))]

hello,two questions.i was just wondering about tube color. i have a 73 model t and the tubes glow orange and a blue/purple color.also,I've noticed that the blue/purple color will pulsate while playing,is this okay or bad?second question.I'm using electro harmonix 6550 tubes right now and I'm thinking about trying kt88's.would the 6550's be better than the kt88's or vice versa for this amp? thanks o)))

[Robbie Nuke]

The pulsating blue color is an indicator of a gas/vacuum fault. Happens from time to time, usually with older power tubes. I would replace them whenever possible, nothing dangerous... Your replacement tubes should not have this symptom. Keep the old ones if they are original USA brand for momentos!

[Soundmasterg]

The pulsating blue color CAN be a problem, but often is not. The orange CAN be a problem if the plates of the tubes are what is orange. The plates are the big, flat metal pieces in the tube. If they are orange or red then the tube is not biased correctly, or there is some other problem. I'd check the bias on the amp to make sure it is correct for those tubes and the voltages in the amp. Check out Kevin O'Connor's FAQ at London power regarding this.

http://www.londonpower.com/faq.htm

"Q: What does it mean when my power tubes glow blue? Should I toss them?

A: No, you don't have to toss them just yet. However, a blue glow indicates the presence of some gas in the tube, which is not a problem in most situations. If you have noticed an increase in the amount of blue, then you might want to get a new set of tubes sooner than later. Too much gas in the tube can lead to bias failures, blown fuses, and possibly blown output transformers.

WARNING!
If your tubes are glowing bright orange - that is, the grey parts inside the tube are glowing orange - TURN THE POWER OFF IMMEDIATELY! A bias failure is in progress. Usually, this is the fault of the tube itself, but can also be caused by an open grid resistor, bad socket connection, or poor solder connection. Check the bias voltage right on the tube socket with no tube plugged in. If the bias is okay, plug in a new tube. "

[JoeArthur]

 +1.

Normally the orange glow from deep inside the tube is the heater and this is normal.

All power tubes will have a slight bluish glow that will pulsate with the signal, simply because it is impossible to achieve a perfect vacuum inside the tube.

Normally, when the bias goes and the tubes glow - it will be a bright red that lights up everything, and not the nice soft orange glow from the heater.

[o)))]

thanks alot.i just purchased jj/tesla kt88's as well as jj/tesla 12ax7's.the store i bought them from says the bias rating is on the kt88 box.could i bias this amp myself?  thanks

[Soundmasterg]

Could i bias this amp myself?

Thats a loaded question.....it depends on your knowledge level, and what equipment you have. The bias level that they are saying is on the tube box doesn't really make sense. They may have put the number the tubes match up to on the outside of the box, but that isn't the same as the bias.

To bias the amp, you need a meter that you can use to measure the plate voltage. Any DMM that can measure up to 1000v DC should be fine. You also need some way to measure the cathode current. Many places sell bias probes that you can use for this purpose. Basically you stick the bias probe in the tube socket. Then you put the new tube into the bias probe socket, and the bias probe will measure and display the cathode current when the amp is turned on and put into operate mode. Then you have to plug these numbers into a simple formula, and compare the result to published specs for the tube in question to see if it is in the right range or not. If not, then you need a straight slot, completely insulated screwdriver (a plastic one would be great) to adjust the bias adjust pot. Lastly, and most importantly, you need the knowledge to do this safely without killing yourself, which is very possible when the amp in question has at least 560v on the plates of the tubes.

Best advice would be to take the amp to someone who can do this for you, unless you want to spend the money to get the equipment necessary, and the time to learn how to do this safely and accurately. It isn't that hard, but it is critical to follow the proper protocols so that you don't hurt yourself and that you don't damage your amp or the tubes.

If you decide you really want to do this yourself, then I can follow up with a post telling you how to use the formula and how to use the equipment.

Greg

[o)))]

thanks soundmasterg i will take the amp to a tech,it would be the best thing to do.Thanks again