questions concerning a 6l6 in a champ, can i uese the same OT?

[jimmybjj]

... I've seen several post about people putting a 6l6 in a champ and just changing out the bias resistor and cap. I am still pretty ignorant to electronics and amps but doesn't the 6l6 have an impedance of 4000 ohm's and he 6v6 about 8000. would a stock be ok with long term use of a 6l6? I have a bandmaster OT that has a 4000 ohm primary, can I install this in the champ for use of the 6l6, and have the added benefits of larger iron?

[sluckey]

doesn't the 6l6 have an impedance of 4000 ohm's and he 6v6 about 8000.

yes

would a stock be ok with long term use of a 6l6?

Probably. Ideally, you would use 1/2 the speaker load too.

I have a bandmaster OT that has a 4000 ohm primary, can I install this in the champ for use of the 6l6, and have the added benefits of larger iron?

No. The BM OT is a push/pull OT. The Champ needs a single ended OT.
 

[jimmybjj]

doesn't the 6l6 have an impedance of 4000 ohm's and he 6v6 about 8000.

yes

would a stock be ok with long term use of a 6l6?

Probably. Ideally, you would use 1/2 the speaker load too.

I have a bandmaster OT that has a 4000 ohm primary, can I install this in the champ for use of the 6l6, and have the added benefits of larger iron?

No. The BM OT is a push/pull OT. The Champ needs a single ended OT.
 

thanks for the quick response. I knew the answer to the bandmaster ot question I just needed you to help me remember, thanks :-)

[bigdaddy]

Using a 6L6GC in a champ will not deliver the goods people expect. In fact the amp need a major overhaul for it to work correctly. The JJ 6V6 can handle the higher voltages you have now and it sounds very close to the 6L6GC with higher voltages on it and more like a traditional 6V6 with lower voltages. I know I have tried it......

By using the 6L6GC you have made most 8" speakers unusable if you drive the amp, they will blow. Unless you get a higher powered speaker like the models from Weber or Jensen reissues. Or a 10" and that won't work well in such a small cabinet. Also that little output transformer is not going to like being stressed so much.

The best way to make the champ work is to leave it alone and just make sure all you caps are good, all your resistors are within range and you have new fresh tubes. Also that it's biased correctly with the higher AC voltages a 470ohm 3 watt carbon resistors won't cut it. You're pushing the 6V6 too hard and it's best tone is within the 12-14 watt range. I would go higher and use a 5 watt ceramic minimum. There are other ways to make this little guy crank. I lay it all out on a previous post.......4 or 5 little mods makes this amp come alive. But it really needs to be brought up to spec and there is no real advantage to using a 6L6GC in my opinion.

[jimmybjj]

thanks for all of your advise, it is well taken. I still want to try it anyway. I am still very new to all of this and want to do it primarily for the experence. this is a new build,all of the components are new. the pt is rated at 100ma but the ot is closer to factory spec primary is 5000 with 4 and 8 ohm taps. I am using a 8 ohm 12" 25 watt speaker. which tap should I use with 6l6, and if you have time, why is that rap better?

I tried looking for the thread that you were referring to but had no luck, can you provid a link please.

thanks, Jim

[PRR]

> doesn't the 6l6 have an impedance of 4000 ohm's and he 6v6 about 8000

No. 6L6 internal impedance is 30K, 6V6 internal impedance is 60K.

We ALWAYS load a pentode with a much lower impedance.

What you actually want to do is use a load similar to the Volts and Amps which you work the tube at.

A typical SE 6V6 stage may work at 300V and 40mA, 0.040A. Then 300V/0.04A gives 7,500 ohms. Indeed we load such 6V6 stages with 7K or 8K. We can instead run 250V 48mA, which suggests a 5K load.

We can run a 6L6 with these exact same conditions. And get essentially the same output power. And between 6V6 and 6L6, not a big difference in tone (6V6 was 6L6 technology in a 6F6 size package). This means paying for a 19W-30W bottle to do a 12W job. This may be wise for reliability. Especially since many 6V6 amps push their bottle past 6V6 ratings. But it isn't any "Wow!" difference.

OR we can use the fact that a modern 6L6 can dissipate twice the power of a 6V6. Since supply voltages tend to stay nearly the same, we may choose to run twice the current. From 300V 40mA to 300V 80mA. You would think this needs a new PT, but Fender used the same PT for both the one-6V6 Champ and the two-6V6 DeLuxe, and it will carry 80mA. Now the ideal load is more like 300V/0.08A or 3,750 ohms, commonly taken as 4K.

[alerich]

What you actually want to do is use a load similar to the Volts and Amps which you work the tube at.

Thanks for that explanation, PRR. I've never seen that relationship between the output tubes and the OT explained before. Good stuff.

[tubeswell]

Hi PRR,

So the formula for working out 'proper' load resistance is:

Zout = Va/(Pa/Va)

which, as far as I can make out, if you reduced it to Ohms law (E = I x R) terms, would be in this case R = E/(E.I/E) or R = E/I

where;

Va = plate voltage
Pa = maximum power rating of the tube

so for a 6V6GT running at 415

415/(12/415) = 14k4

so how come we run BF 6V6s (i.e. in a BF Deluxe Reverb) at 6k6 415 V?

[HotBluePlates]

so how come we run BF 6V6s (i.e. in a BF Deluxe Reverb) at 6k6 415 V?

Or more than 415v.

You can design on a cocktail napkin, or you can design very accurately, but rarely both at the same time. Class A operation is one of those times where you might be able to do both.

Also, if you really understand what's going on, you can cheat by making simplifying assumptions, and know how reality will differ from your simple result.

So first off, I think he's thinking class A. 300v * 0.04A = 12w.

I also think he's using another little cheat to make things easy: assume the plate swings all the way to ground (0v) and the tube swings from 40mA to 0mA. Truth is, a pentode will get closer than a triode, but the plate will almost never swing to truly 0v. For 6V6, look at the 0v grid line at a plate voltage below the "knee" of the curve, and it looks like you really won't swing the plate below 40v. Also, it's quite hard to cut tube current off completely.

The formula gives a.c. impedance of the output transformer; meaning the voltages and currents we are talking about are not the dc plate voltage and idle current. But if we have a tube being run in class A, it idles at/near 100% and ideally swings above and below idle by an equal amount. So instead of figuring the resulting ac voltage swing at the plate and the current swing through the tube and load, take a shortcut and call those swings equal to d.c. conditions.

You also imagine the pentode behaves like a resistor, which it doesn't. But it's a close enough place to start to figure if the idea has merit.

so for a 6V6GT running at 415

415/(12/415) = 14k4

so how come we run BF 6V6s (i.e. in a BF Deluxe Reverb) at 6k6 415 V?

I figured your numbers came from a silverface Champ, but I'll answer to the Deluxe: it's class AB, a higher supply voltage is used, a smaller primary load impedance is used, with more bias voltage to keep the tube from melting at idle or during the signal swing. PRR's approach will work, but you now have to know that plate current will swing to a value 2 times or more what it is at idle, and the ac plate voltage swing is bigger. And we can't just plug in the value of the max dissipation for the tube. In other words, we're moving away from "cocktail napkin design".

But let's say you were thinking about that silverface Champ, with 395v on the plate. First, remember there is also a cathode resistor underneath, with maybe 20v across it, so your plate-to-cathode voltage is more like 375, and maybe 45v of that has to exist across the tube at maximum current so you have 330v left. Also remember by this time, Fender knew they could push the 6V6 as a 14w tube (there's plenty of boundary-stretching that occured to 6V6's), so we're idling around 20v/470 ohms = ~42mA.

So if we have a tube that has 375v across it, but will swing a total of 330v, and we assume that it will swing the idle current to 0, then
330/.042 = ~7860 ohms, or about the 8k that would be typical in a Champ. Happily, 330v * 0.042A = 13.89w, so dissipation checks out.

This is all being a little loose and free with the numbers, but it is a way to see whether a proposed operating point passes a first stage sanity check.

[PRR]

> Deluxe Reverb

Push-Pull amp.

The original question seems to have been about a Champ with just one big bottle.

Two is different from one.

[PRR]

> you really won't swing the plate below 40v. Also, it's quite hard to cut tube current off completely.

The two effects are sorta complementary. You can't swing to zero volts, you can't swing to zero current, opposite ends of the load-line. Power is less than "perfect", but if both losses are "small", the optimum slope is still close-enuff.


> if we have a tube that has 375v across it, but will swing a total of 330v.... Happily, 330v * 0.042A = 13.89w

The dissipation comes from the 375V. Your hypothetical example has 15.75 Watts dissipation.

[HotBluePlates]

The dissipation comes from the 375V. Your hypothetical example has 15.75 Watts dissipation.

Dang.

And I can't find a cocktail napkin here either, so I guess I'm really up a creek! 

I promise I'll get it straight sooner or later!

[tubeswell]

> Deluxe Reverb

Push-Pull amp.

...

Two is different from one.

So should that formula for PP be: Zout = [Va/(Pa/Va)]/2?

Or maybe something like: Zout = [Va/(Pa/Va)] x .707? (???)

[jojokeo]

So should that formula for PP be: Zout = [Va/(Pa/Va)]/2?
Or maybe something like: Zout = [Va/(Pa/Va)] x .707? (???)


My guesstimate would be for the 2nd as it takes into account the biasing coefficient of the two tubes in class AB1 operation which seemingly would be more warranted for an output load resistance-determined starting point. But then what if there were four output tubes? 

[PRR]

> So should that formula for PP be.....

It depends.

My answer is for Class A.

You "can" run a push-pull amp Class A. Cathode-bias push-pull amps come very close; but I'd hesitate to design with a simple approximation.