6L6GC vs EL34 vs KT77 Tubes... Educate me please

M

MetalThrasher

Well-known member
I never paid much attention to the various tubes out there. Most of my amps have EL34's which I like. My KSR PA50 has 6L6GC tubes in it. I'm thinking about trying a set of EL34's or KT77's. Does changing the types of tubes in a power amp make a big difference?
 
It does, 6l6 will have more headroom vs el34 and have a grittier distortion typically.

KT77 should be somewhere in the middle between 6l6 and el34 and are known for a bigger low end.

6CA7 should also work with that amp since they're a drop in replacement for el34 but i would still confirm with KSR beforehand to be safe.

Imo you'll probably notice a difference between all of these. Might not be drastic and completely change your tone, just different characteristics.
 
Thanks! This makes sense. I feel the 6l6 lacks a lower end that I like. So the KT77 is in the middle of the both? I'm looking for more low end. What should I get?
 
MetalThrasher said:
Thanks! This makes sense. I feel the 6l6 lacks a lower end that I like. So the KT77 is in the middle of the both? I'm looking for more low end. What should I get?
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I would get the Gold Lion KT77.

That is what I am considering buying for my Fortin. Idk theres a lot of good tubes but doesn't hurt to test them all.

The Shunguang black with gold are considered the best but theyre hard to find and pricey so the Gold Lion is usually the most popular.
 
MetalThrasher said:
What about a grittier distortion? I think that's what I'm aiming for.
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Having to have tried/used/owned amps with all 3 types of tubes, to my ears I'd say EL34s due to their lesser headroom or earlier point of breakup compared to KT77s and 6L6s.

Cheers
 
It depends on the amp. I much preferred 6L6s in my Mesa Marks and obv like 6L6s in old Fenders and EL34s in old Marshalls. They all have their own voice and match up better with different amps.
 
Might as well throw E34L in the mix if good low end response and aggressiveness is something your after. I think the bulk of the tone is determined by circuit design more so than tubes IMHO, but there are undeniable subtle differences between tubes. JJ34L in a Wizard MCII are rough, aggressive, gritty vs. =C= which are creamier in the breakup/smoother. Still pretty subtle to my ears, but some folks have ah ha moments when swapping tubes.
 
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MetalThrasher said:
What about a grittier distortion? I think that's what I'm aiming for.
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Here's a small article by Cliff Chase of Fractal Audio, whose job is to analyze and understand tube amps as much as possible because he has to emulate them accurately enough so that people buy his modeler:

Cliff Chase of Fractal Audio said:
You'll often read that 6L6's sound "full" whereas EL34's have more midrange and other colloquial descriptions of the tone of a power tube. These myths are perpetuated by forum dwellers, uninformed tube "experts" and even amp manufacturers as marketing tools.

Well, the fact is that power tubes do NOT sound different. They do not have any intrinsic tone.

"But I can hear the difference when I change to a different type of power tube. How can that be?"

A power tube has a very flat frequency response and they all clip roughly the same. If you put a resistive dummy load on a tube power amp (assuming it doesn't have any intentional frequency shaping) it will measure very flat. However a speaker is not a resistive load. A speaker is a highly reactive load. As I've mentioned in the other threads in this forum section a speaker has an impedance that is sort of scooped at the midrange frequencies.

It is the impedance of the speaker that affects the tone of the amp and different types of power tubes react differently with that impedance. As I've mentioned before a power tube is nearly a current source. The operative word here is "nearly". No power tube has an infinite plate impedance and that's why power tubes sound different. A current source has infinite output impedance, an actual power tube has a finite output impedance.

The output impedance of a power tube (or any active device for that matter) is defined as delta V / delta I which is the change in voltage vs. the change in current.

Let's take a 6L6 for example. Let's assume that the tube has a quiescent operating point of 300V and let's assume we swing +/- 100V around that point. If we look at the plate graphs for a 6L6 at a bias of -10V we see that the plate current at 200V is 95 mA and at 400V it's 105 mA (roughly). Using our formula for impedance we get 200/0.01 = 20 Kohms.

Now let's take an EL34. At 200V the current is 130 mA and at 400V the current is 150 mA. The plate impedance is therefore 10 Kohms which is half that of the 6L6.

This lower output impedance "de-Q's", or flattens, the speaker impedance. Essentially the EL34 has a higher damping factor than a 6L6. This higher damping factor reduces the mid-scoop due to the speaker impedance. This makes the tone have more midrange.

There's a little more to it as the output transformer plays a role as well and 6L6 power amps typically have a slightly higher impedance ratio. There's also different operating voltages and bias points but I'm trying to keep this simple.
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So to sum it up, mostly, any differences in tone between tubes will manifest as being slightly more or slightly less mid scooped due to whatever differences in impedance and therefore damping there will be between the tube types. You're not going to get vastly different, or more / less distortion or anything cast like that.
 
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MetalThrasher said:
That's exactly what I wanted to hear! Thank you. Should I try a KT77 or just get an EL34.
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If you want more low end, KT77’s have more than EL34’s. 77’s are a great tube, they have lows similar to 6L6’s, but they’re tighter, and have ripping mids like an EL34. JJ makes a great KT77, and Eurotubes is a great seller of them. They match them really well also. As MHM mentioned, E34L’s are killer tubes, and I prefer them over EL34’s. They run hotter, so they’re much more aggressive. They’re like a really pissed, harder hitting EL34, with punchier lows and snarling mids. They’ll definitely sound grittier due to the aggression and snarl in the mids, and cutting highs.
So basically, a KT77 sounds like a mix of an EL34 and 6L6, and an E34L is a super pissed EL34 from Hell.
 
I mean, if you want to be a complete tech junky maybe Cliff is right but ehhh.

Maybe it's how that amp or how those speakers react to the tubes instead of the tubes themselves but bottom line you'll hear a difference with a tube change like that.

I forgot about the E34Ls though, I had a quad in my old Pitbull and that amp was mean af.
 
maddnotez said:
I mean, if you want to be a complete tech junky maybe Cliff is right but ehhh.

Maybe it's how that amp or how those speakers react to the tubes instead of the tubes themselves but bottom line you'll hear a difference with a tube change like that.

I forgot about the E34Ls though, I had a quad in my old Pitbull and that amp was mean af.
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Agreed. Whatever the reason is, there’s a difference in the sound as a result of the tube change...
 
You have to try some options in your amp - it's that simple. Makes clips before and after, as your ears will lie to you, especially after spending money. Do not rely on forum people telling you 6L6's are scooped, and EL34's are more midrange crunchy. It's true sometimes, in some circumstances. Other times it's the complete opposite - tubes do not have a frequency response!

Good luck on your tone search.
 
EL-34's & KT 77 are "power pentodes".
6L6 are "beam tetrodes"
Not all amps can use both types.
Its usually one or the other.
And if you use the wrong tube type anyway can cause catastrophic results.
YMMV.
 
Not withstanding screen resistor values for the different tube types, it should be noted that putting el34 / 6ca7 / Kt77 tubes in an amp designed for 6l6's is usually a bad idea due to the heater current draw of the el34 variants.
6l6 tubes draw .9 amps, while el34s draw 1.5 amps per tube. In a 100watt 6l6 amp, you would be increasing the current draw on the heater tap of the pt by 2.5 amps.
Tread carefully here....
 
mhenson42 said:
I have a huge stash of KT88s, 6L6s, el34s/e34Ls, 6550s and KT77s and all sorts of pre-amp tubes. None of them will change the tone/feel of an amp as much as simply swapping the speakers and/or cab.
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Agreed, the differences are generally pretty subtle. I've found adjusting pickup height a bigger influencer, unless you're replacing a faulty tube of course.
 
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