Testing EL84 tubes

[Coma]

I have an Engl Ironball with EL-84s in the power section. I also have a leftover set of spare power tubes, and I'm curious as to if they would change the amp tone. I don't have the experience nor the tools to adjust the bias, but would it be safe to swap the tubes, fire it up to check quickly and the swap back or do I run a significant risk of damaging the amp?

[Rex_Rocker]

I don't think the risk of ruining the amp is big unless the tubes are bad or really off-spec.

Don't expect them to bias right off the bat, though.

There is a small risk. But at the same time, there is also a risk of the current tubes blowing and taking out a transformer with them if you're really unlucky and something goes catastrophically wrong. Not that it will. It more than like will not. But the risk is always there.

I'd go ahead and pop them, give them a quick try, and then go back to the others.

[beaubrummels]

Wouldn't a 20W 2xEL84 amp be cathode biased? There's little to no risk in changing like for like tubes. Don't know if you'll hear a difference in sound.

[Coma]

Thanks for the input. Did a quick swap - no change in tone, but it seems to have removed some unwanted background whooshing.

It's not self-biasing, at least not according to the user manual. I found the bias pot, but nothing about where to connect the multimeter. Guess I'll take it to a tech.

[misterwhizzy]

Wouldn't a 20W 2xEL84 amp be cathode biased? There's little to no risk in changing like for like tubes. Don't know if you'll hear a difference in sound.

My Jet City was fixed bias, and changing the bias--yes, I know how that sounds--made much more of an audible impact than tube brand. Even more than that, though, changing the PI to a 12AU7 gave it quite a bit more headroom.

[Rex_Rocker]

Yeah, no. I would not think almost any Metal amp would be cathode biased. There are exceptions, of course, like the Krank Rev Jr. But for the most part, cathode biased and Metal amps are kind of antonyms, LOL.

[marcello252]

I found the bias pot, but nothing about where to connect the multimeter. Guess I'll take it to a tech.

Don't do it anyway unless you are 100% sure of what you're doing, voltages there are in the order of 300V or more, I learnt it the hard way, and I'm in electronics for decades

[Coma]

So after a short chat with someone who actually builds hi-fi tube amps, I decided to put the spare (matched) set in and yeah, seem to be working fine so far.

[Ascension]

Wouldn't a 20W 2xEL84 amp be cathode biased? There's little to no risk in changing like for like tubes. Don't know if you'll hear a difference in sound.

Most are but all are not. Would find out if that particular amp was cathode biased to be safe.

[Ascension]

Yeah, no. I would not think almost any Metal amp would be cathode biased. There are exceptions, of course, like the Krank Rev Jr. But for the most part, cathode biased and Metal amps are kind of antonyms, LOL.

With big bottle amps that is true but not EL84 amps for the most part. Every EL84 powered amp I have owned has been cathode biased. That includes several Vintage series Carvins, a few Boogies,Orange Tiny Terror, my Zinky Blue Velvet and several Jet City amps. Almost all my EL84 amps have been gain monsters, so--.

[Rex_Rocker]

With big bottle amps that is true but not EL84 amps for the most part. Every EL84 powered amp I have owned has been cathode biased. That includes several Vintage series Carvins, a few Boogies,Orange Tiny Terror, my Zinky Blue Velvet and several Jet City amps. Almost all my EL84 amps have been gain monsters, so--.

I wouldn't call the Tiny Terror, the Zinky, or the Jet City amps "Metal", personally. They can do it, especially with the right boost. But I don't think that's their primary focus.

All of the current generation of Mesa mini heads are non-adjustable fixed bias. The EVH stuff is adjustable fixed bias. The Ironball is too. The Peavey MH's. 95% of the primarily-Metal-focused EL84's kind of are, dare I say.

FWIW, my EVH LBX was louder and held together better than my Krank Rev Jr. (big bottle 5881-powered but cathode-biased). I bet there was more to that than just being cathode vs. fixed bias, but it also did so compared to my Dark Terror (also cathode-biased). That's not to say the EVH held together in a full-band Metal scenario... but it almost did. At least better than the Krank and the Orange.