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Tube Amp - Output Power Fuse Protection

The question is why do some manufactures of tube amps ,not have protection for the output stage of the amplifiers?
I have been auditioning some tube integrated amps that did not have any kind of fuse protection for the output stage of the amplifier.

I was in front of one amp when an EL34 tube flashed and knocked out the left channel. It blew out an output resistor , which has to be replaced to restore the amp. A simple fuse in the plate circuit would have protected the amp and with a new fuse , restored service. Instead , one would have to take the amp to a local repair shop , or have it shipped if not local.

Is this a common practice of manufactures to not have protection? Is it worth purchasing an amp that does not offer protection?

As we all know , it can be fun not to use protection , but sooner , or later it will cost you! One way or the other!
timo62
xti16
1,525 posts
 
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).
grannyring's avatar
grannyring
6,450 posts
 
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.
upscaleaudio's avatar
Ag insider logo xs@2x
upscaleaudio
258 posts
 
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.
timo62 OP
280 posts
 
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.
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