EL34 vs 6V6

[daz]

As many of you probably know by now with my incessant posting, i have a amp i built thats similar to a 50 watt jcm 800 2204. Main diff is the trannys and PSU. I can also use a pair of 6V6 with a bias change and setting the PT to 4 ohms with an 8 ohm speaker. When i first was deciding what to build with these trannys i decided on a 4x6V6 amp but changed my mind because i always loved EL34 marshalls and had many of them. Anyways, long story short, i again tried the 6V6's (jj's) after not having used them for a few weeks, and this time they sounded better than the EL34's. The tone is more focused and more springy. Just all around better, but the focus is what i realized the 34's are missing and the main thing i really like about the 6V6's.

Anyways, the question is what do you think the results would be if i went to my original design of a 4x6V6 output section? Do you think it would retain the same great tone? I'd leave it as is but it just doesn't quite have enough clean headroom. I'm pretty sure i know what i'll get as far as headroom, but i'm concerned that i might lose that sound. And i'm not talking about overdriven output tubes, but the sound i get at lower volumes where the outputs are still pretty clean. I just want that spound but a tad louder.

[BackwardsBoB]

I too want that spound but a tad louder.

If you put a current sensing resistor on the cathodes of the 6V6s, you can come up with a cleverly designed circuit to force a multiple of the same current through a pair of 6L6s, EL34s, or even 6550s. Or a quad. Or cascodes of MOSFETs. All you need is sand and feedback.

Of course, a quad of 6V6s is easier, you won't risk being commited by loved ones, and you already have the OT.

Don't expect much more clean headroom from a quad. You can raise your plate voltage to at least 420V, which might help, if you're not there yet. The Gm on 6V6s is really low, so you may need to ask more of your PI. It may be causing the lack of headroom that you are hearing.

[daz]

Can't say i really understood any of that, but i was especially curious about what GM stands for since you say the 6v6 is very low in that regard.

[Leka]

Can't say i really understood any of that, but i was especially curious about what GM stands for since you say the 6v6 is very low in that regard.

http://en.wikipedia.org/wiki/Transconductance

[tubeswell]

Can't say i really understood any of that, but i was especially curious about what GM stands for since you say the 6v6 is very low in that regard.

Ok (watching this out of curiousity) I think I' m beginning to understand - I think this means you can increase the tube current of a 6V6 without it changing the bias voltage that much (or vice versa), compared to if you changed the tube current of another type of tube, because the ratio of tube current to bias voltage in a 6V6 is lower than for (say) 6L6s or EL34s, because a 6V6 is a relatively low power tube. Whereas if you were running a higher power tube, you could increase the tube current more (because you could run a higher plate voltage, without having to adjust the bias point all that much, hence you would get a cleaner signal at higher volume (Is that right?)

[Steve Conner]

Not really. Gm is measured in milliamps per volt. It's the number of milliamps that the plate current will change by when you change the grid voltage by one volt.

So tubes with higher Gm will need less bias voltage to make a given idle current, and less drive voltage from the PI to make a given output power.

More Gm also makes the loop gain of the NFB higher, which will alter the speaker damping and the effect of the presence control.

Sure, 6V6s don't have much of it compared to an EL34 or even an EL84, but they work fine anyway in an amp designed around them, as any 5E3 or Princeton owner would tell you.

[Shea]

I like 6V6s better than EL34s too. Not as mucky sounding, IMO.

I've found that for any given B+ voltage, the range of bias voltages that you need for 6V6s is just about the same as it would be for EL34s. Although they have different Gm figures, they also have different plate dissipation ratings, so it seems to be just a happy coincidence that for a given plate voltage -- say, 420 or whatever -- it works out that the bias voltage you'll need for 6V6s will usually be in the same ballpark as it would be for EL34s. So when you build this amp, you can just copy the bias circuit you used for your EL34 amp.

You might want to try a Weber 100-watt Marshall chassis. It'll have all the holes cut out for your preamp, phase inverter, power tubes, knobs, switches, jacks, light, and IEC inlet. It has 4 input jack holes, so if you're using the 2203 circuit, just don't use the two extra ones. I built a 4 6V6 amp using that chassis. I added a rectifer tube using the the cutout for the filter cap can that's located in the corner next to the power tubes. I had to look around for a tube socket that was wide enough to cover that hole, but I eventually found one.

Shea

[daz]

Actually i'm just going to use what i have and modify it if i did this. The cap can will be removed and strapped to the inside and another octal will go there. The amp is now diode rectified so the 5AR4 socket will be the fourth 6V6 socket. It will be a sloppy mess visually but i really don't care much. Still undecided tho, but if i had all tubes and an extra socket right now i think i'd just go for it. The only thing holding me back is that i always loved EL34's and i can't help but wonder why i'm getting a sound with 6v6's that i think i should be getting with the 34's.

[BackwardsBoB]

You know how V = I x R? Then I = I/R x V. 1/R is called conductance. Now on a tube, we drive one port with a voltage, and the current pops out another port, so they call it transconductance, and label it Gm for short. When you drive the tube, the change in anode current = Gm x the change in grid voltage.

Now a 6V6 has a low Gm for a power tube, at 3.8 mS. This means that for a certain set of operating conditions, a 1V change in grid voltage gives you a 3.8mA change in plate current. This compares to an EL84 at 11mS. The EL84 will need less signal on the grid to fully drive it compared to a 6V6.

But you're starting with a pair of EL34s, which also have a Gm of 11mS. This would tempt you to think you need three times the voltage swing at the grid, which might be a challenge.

But you're replacing each EL34 with two 6V6s. You'll get twice the plate current for a given voltage change compared to one 6V6, so the GM for the 6V6s jumps to 3.6mS x 2 = 7.2mS. You still need more voltage swing at the grid compared to the EL34s, but things are getting much better, and...

I think the set of conditions used to measure the Gm of the EL34 and the 6V6 are different. 6V6s are spec'd with wussy push-pull plate voltages like 285V. Nice, modern 6V6GTAs, etc. have no problem with 420V plate voltage, and this increases the Gm.

For a cathode biased push-pull EL34 pair, running at 375V, I found a source claiming that you need a 42V pp voltage swing at the grids with a 3.4KOhm Raa transformer, producing 35 W, with a -22.5V bias voltage. A 6V6 pair with a 360V plate voltage driving a 7.6KOhm transformer biases at -22.5V, needs a 45V pp voltage swing at the grids, and produces 23W. This is more than a deluxe reverb gets at 420V, but the bias current is 31mA/tube, which is much higher than a stock Deluxe RI, at least until you turn the bias pot.

Since you'll have two of these pairs replacing one pair of EL34s, the transformer reduces to 7.6KOhms/2, or 3.8 KOhms, about the same as the EL34s. Heater current will drop to 1.8A for the quad of 6V6s vs. 3A for the pair of EL34s.

Looks like a straightforward mod. That's cool. I can take a Carvin XV112, drop in a quad of 6V6s, and all I'll have to do is (maybe) drop the supply to the plates and screens by about 30V, and I'm going to want some sag anyway, and I've got a bag of 150 Ohm 20W resistors. That's gonna sound really good. I'll want to hook my speaker up to the next lower impedance output transformer tap, but you won't need to, since you've doubled the number of tubes.