Discussion in 'Amps and Cabs' started by rollyfoster, Aug 30, 2008.
i was told EL84 amps don't need to be biased as long as the tubes are matched. true???
Not true. Depends on the amp, not the tube type.
Adam is right. Most EL84 amps are cathode biased (meaning you won't need to 'set' the bias manually, it's done as part of the operation of the circuit to dynamically 'adjust' it while you play), leading to the misconception that EL84s don't need it.
Any tube in a cathode biased amp doesn't need bias adjustment or setting, and any tube (including EL84s) in a Fixed or Fixed/Adjustable bias circuit DOES need to be set to a preset (or idle) level.
And cathode biased amps should be checked to insure good functioning of parts and good tubes.
Let me ask you guys this: So, if an amp is indeed cathode biased and has, let's say for example, EL84's, can one simply swap in 6V6's, EL34's or KT66's? Just curious as I'm no amp tech, but I hear about certain amps that are supposedly able to do this because they're cathode biased. Thanks in advance for the education!
No you can't do that, because EL84's have a miniature 9 pin base, the other tubes you mentioned all have a larger 8 pin base. Besides that, they all have different operating parameters, so it's not so easy to just swap and go.
i had no idea EL84s are so damn little.
any info about the budda SD series II as far as replacing tubes? it doesn't seem like there's much variety in the world of EL84, either.
There is a ton of variety in the EL84 world and the best part is even for an amp with 4 of them, you can afford to try out a few sets.
hmm.....maybe i'm looking in the wrong places....
i only have room for 2 of them and so far have only found GT and JJ as sources.
Well for current production theres only 3 places they are made today. Russia-(Sovtek, Mullard, Electro Harmonix-all are New Sensor Corp.)
Theres Ei's(dare I call ythem NOS now? lol!) is available cheap. NOS RFT's a bit more. After that NOS get more pricey.
So as far as new stuff goes New Sensor has a few,JJ makes one and Tads's (China) is another.
Groove tube just tests and rebrands the above mentioned tubes.
I might have missed one or 2 EL84 manufacturers butthats most of them.
To get an idea whatas available heres a couple links.
Thanks for this information! OK, I see and this makes sense. EL84's having 9 pins and the others having 8.
That being said, if we take EL84's out of the equation, can 6V6, KT66, 6L6, and EL34's be swapped (or are there different operating parameters between them)?
"Besides that, they all have different operating parameters, so it's not so easy to just swap and go."
You missed that part of the answer.
In a non-cathode biased amp you should re-bias even if you install another power tube of the same type (i.e. 6L6->6L6) and also if you replace the rectifier tube. You may also need to re-bias after a few months of using the same tubes, they can often 'drift' as they break in. In other words: if in doubt, re-bias.
Again, you should check the bias in every amp, every time you change tubes. While it is true that most cathode biased amps are forgiving for throwing tubes into them, the safe thing to do is to at least check to make sure the bias is correct, and the tubes are both working correctly.
There are ways to make amps that will allow swapping of 6V6's, 6L6's, and EL34 type tubes, but it is always done as a comprimise where they will all make sound, but none will preform to their optimum output and tone.
Thanks guys. I have a multi-meter, but don't know how to check/bias the voltage. Are there simple instructions on how this is done or can someone provide this info (assuming it's not too indepth for a forum thread posting)?
I'm assuming the black lead goes to ground (on the chassis) and the red will contact somewhere (tube socket or close by?) and the multi-meter is set to some parameter range (DC millivolts?). Thanks again for this info!
Its easy enough with the right information. Its also dangerous as your dealing with lethal voltages. A bias probe is your safest bet. You can always approximate the plate voltage . Then you dont need to do any poking around inside the amp except maybe to turn a trim pot. What type of amp is it. Bob
Thanks Bob. I'll do a search for a bias probe, as it sounds like something I should probably own. I have a Marshall 1974x clone (GDS kit) that runs two EL84's. I suppose a bias probe would provide me the info I need when I start to swap out for other EL84's
There's an Alessandro amp I'm considering getting that (according to the website) can readily accept a pair of KT66, EL34, and 6L6's. A current owner chimed in on another thread and told me he's running two 6V6's right now with no issues. Just wondering how an amp can do that and adjust to all these different tubes without any adjustments under the hood?