Metropoulos Forum

I recently blew the first Power tube (Valve Art KT66) on my Metro Super-Plex. Using the Apex ID number I ordered 4 matching New tubes. They sent me four with different currents than the originals ... The original tubes had a current of 64 and the new ones have 30. The new tubes sound fine. Break up way sooner and get way hotter. The heaters are glowing red after just mins of being powered on... I feel I need to adjust the plate voltage down... bias the amp... ? Please help...

Thank you,
Aaron Winton

Heater filaments will begin to glow within seconds of powering up the amp. Some tubes may glow slightly brighter or darker than others and this is normal. If you start seeing the grey plate structure inside of the tubes turning orange or red, you have a serious problem because the tubes are redplating. Never play the amp if you start seeing any signs of redplating. You can play the amp in a dimly lit room and watch the valves if you really want to be sure. The only glow should come from the filament in the very center of the tube.

Another thing to mention is that one sellers matching system does not match another's so never trust the numbers written on the tube. Once your amp is biased to a certain sellers tube, you must use their tubes with the same rating or you run the risk of being over biased or under biased.

Thank you. I have tried to find out where George got the tubes for this amp. I followed what info was on the tube. All i know is they are Valve Art and bought from some amp supply store. Several of these stores use a system from a company called Apex. www.apexmatching.com How do you check the bias and how do you bias a Super-plex? Thanks.

How hot should GT66 get? The bases are not glowing but the tubes are getting to hot to touch in a matter of mins. Is this normal?

Call George,

I do not know what system George has in these for checking/changing the bias.

You must do this properly.

The Superplex is his baby and I am pretty sure he will want/be able to help you get these KT66 into a safe range so that you do not fry them or your amp.

Like neikeel mentioned, contact George. He will definitely get you the right answer. But to answer your question, power tubes should get pretty darn hot even when biased properly. They will be too hot to touch for more than a second when operating properly. The term redplating means that a red or orange spot starts to appear on the plate of the tube. This indicates that the tube is pulling way more current than it should and it will ruin the tube within a few minutes if severe enough.

I believe George uses ValveQueen for power tubes