ckyratm wrote:Thank you guys for the help! So I'm making a guess that the 2 sets of tubes should be fairly close enough that the swap should be ok. In some other research I found this: "Orange Rockerverb 50 is about 400pv and uses 6V6's (14 watts dissipation) which we bias between 24 to 28mA." from this site:
https://www.eurotubes.com/eurotubes-Gen ... rshall.htm" onclick="window.open(this.href);return false;
Don't know how accurate that is though.
SO NOW, I'm having a new problem. I turned on my amp with the old tubes still installed (I wanted to record it to get a before and after tone comparison) I noticed that my dirty channel volume is not responding. A little bit of sound does seem to come through, but it's very faint. The gain seems to be the only knob on that channel that is working. Clean channel is fine.
I tried swapping the power tubes and the problem persists. I just used my amp at a show a couple days ago and it worked great. Now not so much. I am leaving on a 1 week tour this Friday so this is obviously the worst possible time for this to happen. Hopefully I can figure it out. Heads up, I may repost this in a new thread to cover a bit more ground. Please let me know if anyone has any info on this. Thank you all!
Since your new problem is limited to one channel (the dirty one) that has nothing to do with the power amp section. (Otherwise
both preamp channels would be affected!)
Suggestion would be to do a little preamp valve swapping first to find the culprit.
As far as biasing goes, you need to be absolutely clear that this is a dangerous procedure. The voltages under the chassis of valve amps are easily high enough to kill you. (And can remain so even long after powering many types of amps down!) Working inside them isn't to be taken lightly and you need to know the right precautions to take (eg NEVER touch the chassis with one hand when working inside with the other, always keeping one hand well away from the amp!) Didn't see any mention of that on the video.
Also many (myself included) would disagree with the 'bias to 70%' suggestion. For many amps 70% (of maximum plate dissipation at idling) is an absolute maximum. as soon as the amp is running hard this can take it above 100% and lead to VERY short valve life. So 70% is more a limit than a target, and I don't agree with the statement that most amps will start into crossover dissipation at 65% - often it is much lower, between 50 - 60%! Using a bias probe between the socket and valve includes the screen current giving a bit of extra leeway. (The calculations aren't correct, but the direction of the error is safe leading to lower actual current settings!
)
For big fixed biased amps I prefer them biased at the cooler end (though obviously above the crossover distortion threshold). This gives tighter punchier sounds and actually gives more clean headroom. Amps biased hotter lose clean headroom and the sounds get generally warmer, mushier, less distinct. Some people may prefer that even at the cost of big reductions in valve life. That obviously includes people selling you valves, but doesn't include me!
[quote="John Phillips (
http://www.thegearpage.net/board/showpo ... stcount=12" onclick="window.open(this.href);return false;)]Given that like many other EL34 amps that put out more power than the tube set is really capable of while staying below the maximum dissipation rating, you need to bias it cooler than the 70% of that often recommended. I would go no higher than 60% (15W) and preferably slightly lower still. 15W at 480V is 31mA, so 30mA is what I'd suggest, as Mike did.
If you're measuring using a plug-in bias meter between the tube and socket, your reading will include the screen current though. This will be about 2 to 5mA for EL34s typically, so you could probably go as high as a reading of 35mA safely.
If you can test the amp at full power into a dummy load if you have access to one (an attenuator set to minimum volume will do). Watch what happens to the power tubes - if the screens (inside the plates) start to glow very bright orange, you're at risk of a tube failure. Some extra glow is normal, but it shouldn't be a lot brighter or more intense in color than the filament glow. You may even get redplating if it's biased hot enough, in which case you're definitely running it too high.
[/quote]
If you don't
fully understand any of this I'd ask yourself if you should really be biasing the amp yourself. Personally... though I'm well up on the theory I prefer to leave it to my tech who can also do a thorough health check when inside the amp for valve changes. Valve amps aren't zero maintenance appliances so this pays big dividends in reliability. (And in both amp reliability and personal safety!!!)
Andy.
