Tube Amp - Output Power Fuse Protection

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.

Kevin, in your example I quoted above the likelihood is that the tube is dissipating much less than the rating! In this case it might only be dissipating 20 watts at full power.

The dissipation is important- it gives you some idea of how much power you can dissipate in the tube. But if the tube is producing 40 watts of output power, there is a larger amount of power that is needed to cause it to do that. Typically it might be about 60 watts or so.

A better approximation of how much power is being dissipated in each tube is to look at the total power draw of the amp at full power. Subtract about 7% for the power transformers themselves. Then subtract about 10 watts for the driver circuit. Then subtract the output power. The remaining power is divided by the number of power tubes and will be pretty close to the dissipation of each tube at full power. Some variance will exist on account of the filaments and class of operation.

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

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.

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.

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.

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.

Simple. Fuses sound like crap! Forget them and do not use them anywhere
if you want your gear to sound great! Ha!

A circuit breaker on/off switch is the way to go and no fuse at output. I
bypass them in digital gear with never an issue.

They really do sound awful in all honesty.

First you would want to get the most out of the circuit. By adding a fuse in line with the resistor the fuse would have to blow before the you damage or burn the resistor. That would limit the output power to less than the design without the fuse. Tough call. I personally would take a little less power to save a repair. But OTOH I know how to solder.

That said I do have an integrated that protects against frying the resistor. For whatever it's worth it's an Octave. Back when I had a V70SE I did lose some tubes with flash over and dead shorts. These tubes were also bought used. Never know what you are going to get. I later upgraded to the V110 and not had any failures for over a year (but a good chance I weeded out the bad tubes).