sub 6L6 for EL34?

[iknowjohnny]

i believe they are pinned the same right? So can i use 6L6 in my marshall style build is i set the OT tap up or down accordingly and bias them correctly? I realize i might have to change the bias range but i can add a switch to change the set resistor. But i just want to know about the OT taps with a OT that is 3.6k. I would use the 16ohm tap with a 8 ohm speaker, no? anyways, anything to worry about....current? Anything? Also, what is the typical bias current for 6L6?

[Andy Le Blanc]

Lately I've been choosing a bias point based on the PT rather than the tube.
As long as it doesn't exceed the design maximums for the tube type, tube type is irrelevant.

What does matter is loading, and g2 dissipation constraints.

You can noodle with damn near any loading for any tube type.
But you'll what to do the graphical Ip/Ep to find optimal conditions for a given type.

Most every tube type will have a published range, theres a liberty there.

G2 conditions can be satisfied with an extra bleeder and filter cap supplied
with one resistor from the power rail.

You can set nearly any voltage difference between g2 and the plate.

[iknowjohnny]

Ok Andy, you're acting like you don't know what an electronic dufus i am ! seriously, i don't know what G2 is and a good bit of that post went over my head. Not your fault, but in any case let me see if i can main point. Are you saying just worry about the PT's max current rating? because it's a magnetic components that is only rated 150ma. Are 6L6 much higher than el34? Aren't el34's something like 45 ma?

Also it just hit me....i have pins 1 and 8 tied together to ground via a 1 ohm resistor for biasing. is that ok if i try 6l6 or will i have to change that.

[Andy Le Blanc]

My bad... screen grids, g2... To keep things simple I arbitrarily chose 50%
of a PT ma rating
To my surprise all the voltages fell closely in line with published scheme
and was very close to the "70%" bias approach for a pair of 6l6 in typical
fender design, and give a common referenced point to set the voltages around
nearly any tube type, as long as it didn't exceed the dissipation limits.
Seemed a much clearer design approach than quibbling around the best bias point.
tube are in sockets, transformers and circuit components are fixed,
for a reason... you think...

A least it seems like a good practical common reference point, a place to start.

Just as a place to start, it opens up most designs to other tube types
wont reek havoc on the circuit, I've got one project that can take 6v6
right up to KT88, I simply maintain the operating point and mind the screen grids
the voltage difference between g2 and plate, when I change tube type.
Same loading, surprise surprise, nearly the same tone and response, despite tube type.
The bias is maintained not for the tube, but for the circuit conditions.
All the tube types have a different tone of course, but the performance and
presentation of the amp is less affected by type.

If the PT is 150 ma, pick a point, 50% is 70ma... keep it there for each tube type ... do the math and see where the voltage conditions land you
in your practical (real) circuit, mind that the screen grids are negative with respect to the plate, by a similar arbitrarily chosen figure.

I my amp I cant run el84, but I can run 6v6 at nearly 100%, the bias figure goes down as the rating of the tube goes up but the amp plays the same.

[iknowjohnny]

ok, so then i can figure out a good bias point, and screens should be ok with a 1k there as there is now, correct? But what about the OT taps? I mean, i know you said not to worry about that but i'm just talking tone. I'm not sure ii'd want a big mismatch for tonal reasons. i'd really just like to hear what it sounds like with 6L6's as close to a typical 6L6 sound i can get with this amp and OT. So wouldn't i want to use the 16 ohm tap for a 8 ohm speaker to match the OT and tubes optimally?

[Andy Le Blanc]

keep the same loading to start, then change once you establish your operating point.

The 1k series screen grid resistor is fine but you'll want to look at how
your deriving the voltage.

You know what a typical PS filter set up, a couple filter caps series with a
a couple 220k balancing resistors?

give the screens their own and hook that to the main power rail where you usually connect the screens thru a
1k to 5k power resistor to set the voltage for the screens.
Look for a voltage difference between the plate and g2, you want the plate positive
anything 10 to 50 v, to start, you can refine it by ear later.

G2 is from the tubes basing diagram, grid 2.

doing it that way gives you better controll than simply relying on a series resistor alone

[iknowjohnny]

Well, actually i didn't want to change the circuit to 6L6, just make it easily swappable. So i don't want all that. I was hoping i could just get larger bias pots or a switch for the set resistors. (dual bias, hence resistorS) So if thats not going to work i'll probably just pass on the idea. I've done this before with 6550's when i had a 250ma PT so i figured as long as the tubes don't require more current than 34's do i could just bias it and be fine.

[Ian444]

What's the B+ johnny? I assume 2 x EL34 output tubes in this amp? What is the bias voltage range? Again assuming this is a fixed bias amp? From 6L6 datasheets, the recommended load resistance for class AB1 varies from 3K8 to 8K5! Many around 6K6. If your OPT is 3K6 for 2 x EL34, then an 8 ohm speaker on the 4 ohm tap will get you in the 7K2 region (but that is at your own risk). What sort of 6L6 do you have, NOS, Russian, 6L6G or 6L6GC? The last two have different disipation ratings.

[iknowjohnny]

I think the B+ is around 400 but i can't recall for sure. The plates of the 34's read 465 with a bias of around 34-36ma. I have a pair of JJ 6L6 and i just stuck them in real quick to see where the bias range is with the bias pots (again, it's dual bias) turned all the way down and it read 35ma on both sides. So it may even be withing range, but I haven't had the chassis out to check the plate voltage with them but i will. A quick few chords and notes makes me think i may like this. I've been a marshall guy for years but lately as the gray is coming in more and more i like a more fender type sound and feel.

[Ian444]

If they are JJ 6L6GC then it looks like it will be fine. 465V and 35mA is only around 16W dissipation. The 3k6 OPT impedance might work well, but keep an eye out for redplating when you crank it up for a while.

[iknowjohnny]

I pulled the chassis and it's 465 at the plate as before. But using weber's bias calculators it seems they should be closer to 44 Ma so i set it there and it sounds cleaner which i like. No redplating after a few minutes but of course i will have to keep check on it. I'm still in the dar about the best OT tap tho. Are you saying using the 4 ohm may not be good because 7k+ is too high for 6l6?

[Ian444]

Are you saying using the 4 ohm may not be good because 7k+ is too high for 6l6?

It *should* be OK, to connect an 8 ohm speaker to the 4 ohm tap, to double the impedance seen by the primary to 7.2K. But if it blows up, I don't want to be responsible. It is a lot safer going the other way, for example a 4 ohm load on an 8 ohm tap to halve the primary impedance of an OPT. I simply have not tried to double the primary impedance as I have never had to, and don't understand the inner workings of OPT's. I often run small output tubes at much less than supposed optimum load (lower impedance) and never had any issues electrical or sound wise.

[Firestorm]

Try the output impedance both ways and see what you like. Fender usually ran a pair of 6L6s into 4,200 ohms, even though the textbook number is more like 6000. But you might like the 7K2 better. I read an article somewhere about a blind taste test where guitarists played though different impedance settings and were asked to pick the best sound. Supposedly, 9 out of 10 preferred double the stock impedance. Just an anecdote, not a catechism.

[iknowjohnny]

Oh, ok....i didn't realize that. (4.2k) I was under the impression it was more like 6.something. So i'll just leave it there. This is cool because i can make it switchable between 34's and 6l6. But the way it sounds right now i have a feeling i may prefer 6l6 and end up optimizing it with 470R screens and fender correct grid stoppers, maybe a even a fender spec OT. But it will take a while to see if i'm sure. I may play it a week, put the 34's back in and go "oh, yeah, thats what i was missing". But right now it sounds goods even not optimized for 6l6 so i gotta think it could be made better.

[Firestorm]

Don't worry about the 470Rs. 1Ks are better, even in Fenders. (Oh, oh. I can hear the purists stirring).