what, technically and subjectively speaking, are the differences?

i also have a question regarding their interchangability... if my amp has been re-biased to take EL34s intead of 6L6s, can I pop 6L6s in safely, or would the bias have to be adjusted again?

shagboy wrote:what, technically and subjectively speaking, are the differences?

i also have a question regarding their interchangability... if my amp has been re-biased to take EL34s intead of 6L6s, can I pop 6L6s in safely, or would the bias have to be adjusted again?

You can pop 6CA7s in there safely, with no rebiasing.
You probably will be able to put 6L6s in there (more likely 6L6GCs) if the amp was originally a 6L6 amp and the transformers weren't swapped out.

You will either need to tell us more about yr amp or do a little research to figure out whether switching tubes will sound shitty or even harm yr amp.

A good starting point would be to print out the two pages below, and check out the differences (Yr amp is probably A/B push-pull, look at those application data). Let us know if this is confusing.

http://tdsl.duncanamps.com/show.php?des=EL34
http://tdsl.duncanamps.com/show.php?des=6L6GC

If this is too much of a bother, just tell us what kind of amp you have.

Subjectively (very subjectively), 6L6s are the sound of Fender amps, Twins especially, and have a "kink" in their frequency response that makes them sound twangy. Or so the lore goes.

EL34s are the tubes Marshall used for their amps, which originally were slightly rejiggered Bassman copies, sort of.

So, in very broad terms, 6L6s are key to the sound of Fender amps, EL34s to Marshalls.

There are, of course, so many other factors involved that the above three paragraphs are kind of meaningless. But that's what goes in those amps, tube-wise, for what it's worth.

Yes, your bias will have to be readjusted. Or rather, you should definitely *check* the bias, and maybe it will be reasonable, and maybe it will need to be adjusted.

In my experience, your bias will need to be adjusted, though maybe only slightly. 6L6GC's have a 30W plate dissipation, where EL34's are only 25W. 6L6GB's have a 23W dissipation, and are closer to EL34's.

The difference is in the construction... both kinds of tube have an anode, both have a cathode, both have a control grid, and both have a screen grid. The difference though is that the 6L6's 5th element is a beam-forming plate (that focuses the eletrons flowing through the tube, kinda steers them) and the EL34 has a suppressor grid (which attracts stray electrons that missed the cathode).

The biggest thing to worry about with regard to 6L6 vs EL34 is that EL34's draw more plate current, so you need to be sure the amp was designed with a beefy enough transformer that it can supply enough current.

In my experience, the EL34 has a sharper more high-endy kinda sound. Also, the EL34 breaks up more smoothly and consistently, whereas the 6L6 goes from clean to dirty over a smaller dynamic range than the EL34 does. Also, the 6L6GC has a 20% higher plate dissipation rating, which translates to "gets a little louder" in the real world.

Check your bias. It might need to be adjusted. I suspect that it will. There's a fine chance you won't *HAVE* to adjust it, in that the amp will operate fine with either tube type. It just won't be operating optimally, for sound or tube life or maybe even for neither.

There is a simple mod...some 2 watt wirewound resistors...that you can put on your power tubes and use a simple voltmeter to adjust your bias. I did it for a decade, and it always worked just fine...

While you're there, run some IN4001 diodes (two groups of three in series) on those power tubes. They will keep poor AC from spiking and eating up your tube life. Best 6 bucks I've ever spent...


Sorry I can't link to some more specific information. There is a book, 'Electronics for Musicians' or something like that, detailing the process. Both of these are cheap, good mods that everyone around my level of (half-assed) knowledge should put to use...maybe someone else knows what I'm talking about?

rayj wrote:There is a simple mod...some 2 watt wirewound resistors...that you can put on your power tubes and use a simple voltmeter to adjust your bias. I did it for a decade, and it always worked just fine...

You don't need 2W wirewounds. They only need to be 1/2W, and not wirewound. The resistor goes from pin 8 to ground, one resistor for each output tube, replacing the wire that connects the cathode directly to ground. The resistor should be 1ohm, or possibly 10ohm. 1ohm is the best, because then it's so simple, no math, whatever voltage you read across the resistor, that's the current, cause V=IR and if R=1 then V=I.

Read the plate voltage, read the voltage (current) across that resistor, take the static dissipation rating of your tube and multiply that wattage 0.5 to find the low end of a "good" bias range, or multiply by 0.7 to find the high end of a "safe" bias range. I usually shoot for about 0.65 but if that doesn't sound good to my ears, then I change it.

I wrote little programs for two different model of TI calculators to do these calculations in a nice quick easy way. The program for the TI92 is quite nice.

If you don't wanna go in and solder your amp to put in these resistors (like if it's a 10 zillion dollar vintage amp, for example) you can just buy a "bias probe" that will work with any amp that has EL34's, 6L6's, 6V6's, 6550's, etc. Any of the normal output tubes that use an octal socket with that same pin configuration that just about all non-EL84 instrument amps use. It's just little socket that plugs into your amp and the tube plugs into this socket, instead of plugging directly into the amp. Wanna know what's inside it? Pretty much nothing... just some wires passing pin 1 to pin 1, pin 2 to pin 2, etc, and then a single 1 ohm resistor like the one you could put into your amp.

If you're not freaked out about losing value of your mint vintage amp, go ahead and put in the 1 ohm resistors. It makes biasing a piece of cake.

I think the guy's reasoning behind the wirewounds were that they are a lot tighter with manufacturer's tolerances, and you want the measured voltage drop to be pretty exact. Not to argue with someone who clearly knows more about it than myself...

rayj wrote:I think the guy's reasoning behind the wirewounds were that they are a lot tighter with manufacturer's tolerances, and you want the measured voltage drop to be pretty exact. Not to argue with someone who clearly knows more about it than myself...

That would make sense. The size might be a little prohibitive in some cases, but in others, probably not. The ones I use are 1% tolerance resistors that were manufactured for some "Mil spec" contract or some such, and then ended up on eBay for whatever reason. They're really nice. And TINY, like, maybe the same size as a piece of rice!!

If you buy enough of them (I think I bought 100 or maybe 200, but even if you buy 20) you can probably go in and pick and choose which ones have the perfect resistance. The ones I got have all been close enough, so...