Tube Amp - Output Power Fuse Protection

ARC doesn't use fuses. Cary and CJ and most others do. The thought would be that a fuse in the circuit path isn't good sonically. There is current going through the fuse up to the fuses limit or rating. I've never been able to compare the two. I have mixed feelings.

But regardless I think it's not reasonable to expect a customer to repair an amp by replacing a resistor if a tube fails, and that's in essence what we are talking about.

PrimaLuna has a trick way of doing it. The Adaptive AutoBias circuit senses when a tube fails and opens a very high quality Fujitsu relay in the circuit path protecting the amp from the failure. Once the amp is turned off and the tube is replaced you simply turn the amp back on.

It addresses the issue of repairs and fuses as well as sonics.

Kevin , your comment below is exactly what I meant by my post. Thanks for your response.

" But regardless I think it's not reasonable to expect a customer to repair an amp by replacing a resistor if a tube fails, and that's in essence what we are talking about."

It just seems like an unnecessary risk and hassle to have to take with a tube amp with no protection.

I was curious what the general consensus was on this matter. If one really likes an amp without protection , do you purchase it and hope you never have a power tube fail that would take out your amp?

I ask this because I have only owned Conrad Johnson amps for the last 16 years , which have the output of the amp protected by fuses. I have been recently
auditioning amps from different manufactures and was really surprised that some choose not to protect the output stage of their amps.

It comes down to sound quality vs. risk of repair. If one does not like the "risk" involved, then buy an amp with a fuse at the output. We do have choices.

I and others bypass fuses and even place copper lugs in the fuse holder. I have not had one issue with this practice, but I am also able to fix any possible issue so for me the "risk" is not big.

I think it is reasonable to leave the fuse out as do others. So it is very subjective and linked to our risk & reward makeup with gear.

What you have to add into the equation is how hard does that manufacturer run the tubes? They won't tell you, and most customers don't understand anyway. Here's a simple tip:

You need to know plate voltage and how the tube is biased to determine dissipation and how hard the tube is run. They won't tell you. So simply look at the watts per channel, and divide it by the number of tubes. 80 watts per channel out of two KT88's. That's 40 watts per KT88. The maximum dissipation of a KT88 is 42 watts. So that's toastier than you might want.

It's desirable to run tubes at 65-75% of their maximum dissipation IMHO and the opinions of many others.

Maximum dissipation of a EL34 is 25W, KT88 42W, KT120 60W. So you do the math. I will go on record saying power has zero to do with sound quality. Or bass quality. Manufacturers run tubes harder to make their amps stand out showing more power. Newbies will often buy based on these larger numbers not knowing they create more headaches later.

This is just a way to help you figure this out. It's certainly not an exact method. It applies to ultralinear amps (the vast majority of amps made) with manual bias or a true autobias (which is very very rarely seen). It does not work with fake autobias which is "cathode bias". But I'm not a fan of cathode bias anyway as 30% of the power goes up in heat. Which is why many cathode bias amps run in Class A operation. Hot hot hot.

Great points Upscale. I agree 100% with running tubes at 65%!