6l6 vs el34

[dscottguitars]

What is the difference between these two?  Is one better than the other?  Why?

I see that the plate voltages of EL34's are higher, but the plate dissipation is lower.  How can this equal more wattage, as I see two of them can produce about 90 to 100 watts output power on the data sheets, in push pull.

Thanks, Daniel

[HotBluePlates]

Is one better than the other?

No. They are just different, but popular, output tubes in the ~20-30w dissipation range.

I say "~20-30w" because the original metal 6L6 was rated at 19w (at its lowest), while later 6L6 variants had ratings at 23, 25w and finally 30w (6L6GC). The EL34 was pretty much always termed a 25w-rated tube.

What is the difference between these two?

The EL34 is a pentode, while the 6L6 is a beam power tube. The difference is pentodes have a suppressor grid (sometimes called G3), while beam power tubes do not. Instead, beam power tubes align the control grid (G1) and screen grid (G2), and use beam forming plates. The net effect is the same as if these tubes had a suppressor grid.

Functionally, beam power tubes do the same thing as pentodes. The exact shape of the characteristics are a little different, so the harmonic content under distortion is different, giving each their own sound.

EL34's have higher transconductance, which generally mean they require less input signal and will likely require less bias voltage.

I see that the plate voltages of EL34's are higher, but the plate dissipation is lower.  How can this equal more wattage ...

As you note, the actual output power to the speaker is similar between the two.

You were looking at maximum ratings, not how the tubes are actually being used. The 800v maximum plate voltage rating of the EL34 is generally pretty meaningless to you and me, because that's not how they're typically used in guitar amps.

Much of the time, the screen voltage is kept pretty close to the plate voltage, because it simplifies the power supply design. When amps take advantage of a higher plate voltage rating, they often have to use a more complex power supply which keeps the screen at a much lower voltage than the plate (see 6550 amps with 600v on the plates and 300v on the screen). But if you go to all that trouble for a watt or two extra, you might as well step up to 6550's or KT88's and get's your money's worth from the power supply.

[dscottguitars]

Thank you, that does help.  But I am wondering why the early Marshalls were changed to the EL34's instead of the KT66's (similar to a 6L6GC?)  I see the early Fenders use 6L6GC's in their bigger amps.  What type of harmonic difference is there, if one can make that distinction? Also, why are the EL34's able to put out more power per pair than 6L6GC's?

I am building a ~100 watt amp and don't know what to use.  Price is a factor as I want to keep it low if possible.

My goal is to go from clean to mean.  I use a pot on the NFB line that does a great job of this, on a couple of 50 watt 6L6GC amps I've made.  But I don't know if EL34's would be better, or 'more popular' as a selling point.

[phsyconoodler]

either will work for what you want.Just think of what sound is in your head and go for it.
I like EL34's for Marshally sounds and 6L6's for that dual rectifier tone but either will do the other's job,they just have a different quality to the tone.
  I personally like EL34's in high gain amps but that's just my own personal preference.
Go listen to a marshal or a fender amp and decide for yourself.No one here is likely to tell you one is better than the other.

[HotBluePlates]

Thank you, that does help.  But I am wondering why the early Marshalls were changed to the EL34's instead of the KT66's (similar to a 6L6GC?)  I see the early Fenders use 6L6GC's in their bigger amps.  What type of harmonic difference is there, if one can make that distinction?

These kinds of changes in the heyday of vacuum tubes were largely done on the basis of production costs rather than tone.

The first Marshall 50w amps were copies of the Febder 5F6-A Bassman, but with British speakers and a subtle change to the feedback loop. Eventually, Marshall switched to the domestically-produced EL34, which was comparable in many ways to the 5881 (except the increased transconductance, as noted earlier). My guess is that Mullard (EL34) offered a more attractive price than Genalex/Marconi-Osram (KT66).

True pentodes, like the EL34, have more 3rd harmonic distortion at some operating points and loads than a beam power tube, like the 6L6 or KT66, operating under the same conditions. In these cases, the beam power tube will often have more 2nd harmonic distortion, which is then cancelled by the push-pull output stage. As a result, the 6L6 will seem cleaner, while the extra distortion of the EL34 may be perceived as a bit more of a brighter edge even before the distortion is obvious.

... Also, why are the EL34's able to put out more power per pair than 6L6GC's? ...

They don't. But data sheet conditions may confuse that, if you're not careful to note everything the sheet is implying. Also, it's worth noting that as the 6L6 was improved (denoted as 6L6GA, 6L6GB, 5881, 6L6WGB, and 6L6GC), in general the data sheet information wasn't updated. You'll see essentially the same curves and conditions listed as in the 1937 RCA 6L6 data sheet.

But in real-world terms, given the inefficiencies of how we operate tubes, there's no meaningful increase in usable output power when you move from a tube rated for 25w plate dissipation and a tube rated for 30w, given the same supply voltages, load and class of operation.

Also, be sure to ignore any data sheet conditions listing "Class AB2" operation, as I can't think of a single guitar amp that uses it. You'll know actual AB2 operation if you see an output tube (like a 6V6) being used as a driver and coupled to the actual output tubes (like 6550's) using a transformer, which often also doubles as a phase inverter. Class AB2 requires driving power to push the output tubes, not just a driving voltage as is the case with class A and class AB amps.

[HotBluePlates]

I am building a ~100 watt amp and don't know what to use.  Price is a factor as I want to keep it low if possible.

Use either. You'll need 4 of either tube to get to 100w, because you probably won't be venturing into class B, insane B+ voltage or complicated rive circuits. The 100w Marshall and Fender Twin circuits have an established track record of reliability. We'd probably do well to work from one of these starting points.

My goal is to go from clean to mean.  I use a pot on the NFB line that does a great job of this, on a couple of 50 watt 6L6GC amps I've made.  But I don't know if EL34's would be better, or 'more popular' as a selling point.

It's a rare player these days that has the venue to crank a 100w amp until the output stage is breaking up. So unfortunately, that seems to peg this as a 100w master-volume amp. Your clean-to-mean will come from the preamp.

London Power (Kevin O'Connor) has a clever idea in the Super Scaler, which allows a small amp (like a Champ or Princeton) to have its power output boosted to 60-100w. I don't have it at hand, but the schematic I've seen uses a pair of 6L6's, a power transformer and two output transformers. Three transformers plus power supply components is not "inexpensive" but is cheaper than building a 100w amp if you already have a small amp that captures the sound you want.

The trick with the Super Scaler is it takes your existing small amp, and is connected to that amp's speaker output in lieu of the speaker. The power output of the small amp is applied through an output transformer, wired "backwards" to the screen grid of the Super Scaler tubes. Those tubes are running class B, and can attain higher-than-typical output powers than you're used to in guitar amps. A second output transformer takes power output from the tube plates and passes it to speaker which can handle the 100w or so.

I haven't built it, so I can't necessarily say it's the best solution. But I sure like the idea, if your customer already gets his ideal tone in a small amp and wants it bigger.

[The_Gaz]

Thank you, that does help.  But I am wondering why the early Marshalls were changed to the EL34's instead of the KT66's (similar to a 6L6GC?)  I see the early Fenders use 6L6GC's in their bigger amps.  What type of harmonic difference is there, if one can make that distinction? Also, why are the EL34's able to put out more power per pair than 6L6GC's?

I am building a ~100 watt amp and don't know what to use.  Price is a factor as I want to keep it low if possible.

My goal is to go from clean to mean.  I use a pot on the NFB line that does a great job of this, on a couple of 50 watt 6L6GC amps I've made.  But I don't know if EL34's would be better, or 'more popular' as a selling point.

It's easy enough to build an amp to accomodate both tube types. To most players, that is the selling point. And if you are really going to be selling these things, I respectively suggest that bit the bullet for an extra quad of tubes and put in the bench/listening time to decide for yourself. With that said, my personal opinion is that they both sound pretty similar when changing them out in the same amp (i.e. a Marshall will still sound like a Marshall with 6L6s.)

[HotBluePlates]

Agreed. When they're clean you can't really tell a 6L6 from an EL34.

It's easy enough to have enough bias pot range to allow the use of both tube types.

[PRR]

You will want iron with that, and iron costs much more than tubes.

For low cost, you use one of the "common" setups: Fender 100W or Marshall 100W.

Output trans Twin reverb
Price each = $80.00
Power trans Twin reverb
Price each = $100.00
============ $180


Marshall 100 watt Output trans
Price each = $90.00
Marshall 100 watt Power trans
Price each = $120.00
============ $210

Small advantage to "Fender". (Mostly because Fenders are more popular.)

Tube choices are basically "6L6", EL34, or 6550/KT88.

You "can" get 100W from a pair of 6550, but it needs uncommon iron. You need _four_ of EL34 or 6L6GC to get above 70 Watts. Four EL34 are around the same price as two 6550; four 6L6 a bit more.

You may use a quad of either 6L6 or EL34. Small re-biasing. (If you bias very cold, no re-bias needed.) They are not the same tube but they are not all that different.

While Fender is usually nominal 4K and Marshall is usually 3.4K, in a loudspeaker amplifier that's not much difference. If you run four 6L6GC into 3.4K, just be sure your G2 voltage is good and high.

> why the early Marshalls were changed to the EL34's

At that time and place, EL34 were much cheaper. It's a cheaper tube and the makers were pushing them across Europe. OTOH 6L6 were imported and KT66 was marketed/priced as an industrial tube.

Here in the US, 6L6GC was readily available and nicely priced.

> why are the EL34's able to put out more power per pair than 6L6GC's?

They don't really. 6L6 suggested conditions were limited so that designers would steer to higher-price tubes in big amps. 7027 is 6L6GC guts on a screwy pinout and shows suggestions to 72 Watts per pair, by "violating" the artificially low "500V/450V limit" published for 6L6GC. GE informally published a report on 6L6GC making ~~70W/pair in ham radio service.

BTW: there _is_ a suggested condition to get 100 Watts from one pair of EL34. It requires a *regulated* 800V supply, and a super high impedance OT. This might make sense in some much larger system such as the driver for a high-power radio transmitter. For a simple amplifier, a second pair EL34 makes much more sense than regulating 800V and custom transformers.

 ----------------------------

> "Class AB2" operation, as I can't think of a single guitar amp that uses it.

Early Ampeg SVT, Fender 300. These are outrageous machines, best not copied.

[dscottguitars]

Thanks again everyone.  I will stick with 6L6GC's because I'm already familiar with them.  I'm going to try a quad for my next project.  I have a few big iron transformers that are in great shape so no worries for a few amps.  I also like the Fender Twin's idea with the ability to pull two tubes for less power and more crunch.  But I don't see anything related to rebiasing so I'm guessing that's not an issue?

Here's a schematic of my plan...  (the bias circuit resistor values are an estimate based on another amp and includes part of a DPDT switch tied to a low power switch in the power supply)

[HotBluePlates]

> "Class AB2" operation, as I can't think of a single guitar amp that uses it.

Early Ampeg SVT, Fender 300. These are outrageous machines, best not copied.

I guess I wasn't too imaginative! 

Yes, the Fender 300 PS shows exactly AB2 operation, with a 6V6 driving an interstage transformer to drive the output 6550's.

[Ed_Chambley]

Why the 12AT7 in the PI?  This looks like a loud and very clean power section.  Very much leaning towards a Twin with a Marshall flair.

You can set it up to switch out the tubes, no need to pull them.  Going from lots of experimenting I have done lately using a 100 watt Plexi, switching to a single pair, I have to take the amp outside to hear any difference in output stage distortion.  I found when using 100 watt OT, there is not a lot of difference in actual SPL.  I ran a thread a while back for feedback on OT's.  I ended up testing a similar output stage with 2 OT, one 100 watt 3.4 for El34 and another 50 watt.  There was a major difference in SPL in doing this.  This would make for a very expensive build.

My input on EL34 as opposed to 6L6.  In the tubing arena it is much harder to get a nice sounding "new" 6L6 (opinion).  The new EH EL34's are getting very close to old Siemens and Mullard quality.  This is a consideration to me if I were planning a line of Branding and Selling.  It is only an opinion, but the term "mean" only comes from cooking EL34 running into closed back cabinets.  There is not a lot of difference prior to saturation.  A Dual Rec Boogie running 6L6 compared to an old handwired EL34 Marshall is very different.

Reading the history of the JTM 45 Marshall, you will find it is true the Blackburn England plant that made EL34 for medical equipment simply gave Jim Marshall a better price on tubes.  Something that has to be considered in selling builds.  They changed and so did the tone.  The late 60's to 1973 was very transitional in rock tone.  I have heard lots of great amps, but there is an element in an EL34 that doesn't exist anywhere else.  Doesn't make them better, just different.  Grab a nice Les Paul and plug into an old 70's Marshall 100 watter, turn the volume to 11.  It will make a sound/tone that I have never heard come from 6L6.  I am not being biased towards EL34's as the majority of my amps run either 6l6 or 5881.  But to me the term MEAN is reserved for Low output humbuckers, 100 watt marshall design EL34 through a couple of 4, 12's running vintage 30's.

[dscottguitars]

I use the AT7 in the PI to reproduce the sound out of the preamp section.  I didn't post it so you may think there isn't one.  My thought on this is I don't want to distort the sound anymore through this tube especially using it as a phase inverter, only get the reproduction.  Sure, at full volume it still adds some preamp distortion, but not too much.  I have my preamp setup to do that, see the schematic following.  The two preamp tubes are wired in parallel to get more current and lower impedance for the next stage, plus I really like the sound I get.  I use a 12AT7 for the preamp and a 12AX7 for distortion.  I have the plate voltages on them somewhat low to get distortion like a pedal does.  It works really well and with a 50 watt amp I built I can get good sounding preamp distortion at a very low volume without having to use a pedal.

For the output tube distortion I use a pot for the NFB.  The Blackface Fender Twin uses an 820/100 ohm divider, about 12% in this circuit.  The JTM 45 uses 27K/5K- but as a presence control with a .1uF cap and I'm not sure how that directly calculates.  My option allows one to cut the amount of NFB to a 57K/470 ohm divider, about 0.8%.  That makes a huge difference in the breakup point of the output tubes as I understand it, (taken from Aiken amps tech pages).  I believe Orange and Trainwreck amps use about 4%.  This is where I get clean to mean...

The other part about the Marshall setup that I know is the speaker polarity.  Which was the main difference between the Bassman and first Marshall.  Celestions used are in forward polar and the amp puts out a reverse polarity adding to the distortion a bit.  And I have my speaker setup for this, a Celestion G12 silver series out of phase or a Jensen Neo in phase.  

Here is the whole schematic...  (the voltages listed are from the 50 watt amp and may be different)

I have a pair of EH 6L6GC's in that 50 watt amp and like them.  I plan on using them for this one too.