tube paranoia

[oldguy]

Your Hot Rod Deluxe is a fixed bias amp. I'm not sure if it adjustable or not.

But since your amp is fixed bias, you should have the bias adjusted every time you change power tubes.

That seems confusing somehow.......

[kiteman]

That seems confusing somehow.......

I think he means resistor wise. You can change resistors to change the bias.

[tunghaichuan***]

I think he means resistor wise. You can change resistors to change the bias.

Kiteman is correct. In a fixed bias circuit there can either be a resistor which sets the negative bias voltage to a predetermined level or there can be a pot which can be adjusted within a certain range. These days, the resistor needs to be changed or the pot needs to be adjusted every time the tubes are changed out. Back in the good old days when tubes were manufactured in the West, it was not as critical. RCA/GE/Sylvania/Tung-sol/et al. made tubes to very tight tolerances and very strict quality control. Nowadays, not so much.

Also, as tubes age, their bias voltage requirements change as well.

In a cathode bias circuit, there is also a resistor, from the power tube(s) cathode(s) to ground. This circuit is self adjusting, so it is not as critical to reset the bias every time tubes are changed out. However, I usually like to check it anyway just to make sure the tube isn't dissipating too much power.

Sorry for being unclear.

[tunghaichuan***]

That seems confusing somehow.......

Sorry Oldguy, I meant user adjustable. Fixed in my post.

[oldguy]

Thanks for explaining, Tung, that makes it easy to understand!

[redgibson]

I think I need to sit down with a "Tube Amps Explained" book or something. This is all very good information I just don't quite understand half of it. I like the way they sound / react! Now I have to figure out how they work. Thank you everyone. BTW, the wedding went great without a hitch. Packed the dance floor and got a cheering rowdy crowd. It was a great confidence booster :-)