Bandmaster 6l6's glowing

I repaired a Bandmaster amp head for a friend.

It works ok, is pretty quiet at idle, power seems good, but the 6l6 plates glow orange after a minute or so.

I have not had an amp with 6l6's for many years.

The 6l6's are right below the power and standby switches and they get really HOT !!!

Is that normal before I take it apart again to check on the 6l6 voltages ??

I repaired a Bandmaster amp head for a friend...



One statement, seems to make sense...


It works ok, is pretty quiet at idle, power seems good, but the 6l6 plates glow orange after a minute or so.

Is that normal before I take it apart again to check on the 6l6 voltages ??



If you repair amps, you should NOT be asking if red-plating 6L6's is "normal."

1. What did you "repair?"
2. Take it to a real repairman before trying to measure voltages.

If its normal tube glow, cool, BUT you should know enough to know what "normal glow" looks like at this point. Be carefull, there are some nasty voltages in an amp... take care of yourself and do not get zapped.


.

I have worked on electronic equipment,radio,radar and TVs and computers for many years even military equipment that had 6l6's or 5881's in them but they were not driven hard in that application and believe it or not, I have never had an amp with 6l6's in it although I have oned many amps and own two presently that do not have 6l6's.

Anyway I did not mean gas glow, I should have been more clear about that.

I talked to the owner of the amp and he said it was like that before I fixed the power supply problem, but I am going to take it apart again and check the tube voltages because I don't think it is right even though it sounds fine.

Those tubes are on the way to dying an early death.

First thing is to make sure that it's biased correctly. I use this table all the time. In fact, I have it posted next to my bench.

http://members.shaw.ca/house-of-jim/Html/bias_tables.html

If it is and the problem persists, then start tracking it down.

As mentioned, most likely, the idle current through the tubes is too high causing there to be too much DC power dissipated at the plates. Having too much DC power there will cause the plates to glow orange as the structure cannot cunduct/radiate the heat away rapidly enough.

You'll most likely need to adjust the control grid bias voltage to relieve this condition. As mentioned, the tubes will burn up fairly quickly otherwise.

Find a way to measure the Plate Current through the output tubes. If you do not have a bias tool, then you can (among other methods) measure the DC resistance of each half of the Output Transformer primary windings. They should be somewhere around 200 ohms.

Next, fire up the amp and measure the voltage drop across each primary winding. Then do a E/R = I, and get the Plate current.

Use the chart listed above to get you in the ballpark for Plate Current specs.

Typically, you want the DC Power at the Plates to be 70% of what the tube is rated for, in Class AB1 operation. Use the Plate Voltage * Plate Current for this number.

Thanks for your responses..

Pvee

Finally got around to fixing it today.

The bias was way off. Someone had worked on the bias power supply before and it was put together wrong plus the electrolytic cap was open also.
.