6L6GC Load Resistance

[triplejjr]

I'm modding an EVJ and I have output transformer with a 5000 ohm primary and 4, 8, and 16 ohm secondary. Can I run a 6L6GC without any problems? The tube specs show maximum load resistance of 4500 olms at plate voltage at 300.
Thanks for any help.

[triplejjr]

EVJ is an Epiphone Valve Junior. It is far away from what it orginal was in it's previous life. The reason I asked the question is I would not like to have to spend the money on a new output transformer, if I didn't have too, because I already did that when I had an EL84 output tube.

[jazbo8]

I challenge anyone that can hear the difference between 4.5K and 5K OPT (assuming all else being equal), just make sure it can handle the power.

Jaz

[phsyconoodler]

A cathode biased single ended amp can only draw so much current anyway so the OT will likely be fine.It's the power transformer that you need to worry about.The filament current is double that of an EL84 so it loads down the PT more.
  You can figure loads a little better by understanding that a normal push-pull amp with a pair of 6L6's likes to see a 4k load from the OT and a single ended amp with the same tube would be happy at 8k.But that's not set in stone as they will likely sound and work fine with less load.
  2-EL84's like anywhere from 6.6k to 8k and a pair of 6L6's like around 4k and are happy as hell at 6.6k too.
And remember you can alter that load relationship by increasing or decreasing the speaker impedance on the same output tap.

[darryl]

The filament current is double that of an EL84 so it loads down the PT more.

The 6L6 heater current is only slightly more than the EL84. The 6L6 is 900mA, the EL84 is 760mA. This relatively small difference should not cause any problem.

[HotBluePlates]

The tube specs show maximum load resistance of 4500 olms at plate voltage at 300.

Not a "maximum". Just a condition.

It's like a recipe: if you use this plate voltage, this screen voltage, this bias voltage, this load impedance and apply this input signal, you get that power output and amount of distortion.

I'm guessing you saw the middle of the three conditions under "Class A Pentode" as found on this G.E. 6L6GC data sheet. Note the screen voltage is 200v, so it probably won't apply to your case much anyway (cause you'll likely have the screen voltage closer to plate voltage, for simplicity).

Note also that particular condition must have been taken from the original (19w) 6L6 data. It notes idle plate current of 48mA, which with a plate voltage of 300v is an idle dissipation of only 14.4w.

Will you always use true 6L6GC's, or might you use Russian types labeled as 6L6's (or 5881's) but only rated for ~19w plate dissipation?

Let's assume a middle-value of 23w idle dissipation (safe for true old 6L6GB/5881's). Let's also assume you'll land on 300v after whatever extra B+ current is drawn over the original Valve Jr conditions.

The a good load impedance is found from
23w/300v = 0.076A idle current
300v/0.076A = 3947Ω

That's an odd value, but dang close to the commonly-cited 4kΩ SE load. You have a 5kΩ OT in hand. If the PT can deliver the current (76mA idle * 2 = 152mA peak current), you'll probably land on a little less output power and a little more distortion than the ideal case of 4kΩ.

At this point, if you were really trying to determine the ideal load for this SE amp, you'd draw loadlines on plate curves for load both higher and lower than 4kΩ, but all running through the same idle point of 300v @ 76mA, and calculate power output as well as distortion using some arcane methods. But your new tube probably doesn't exactly match the curves, a watt either way doesn't materially change your ability to use the amp, and you have volume controls to decrease/increase distortion.

So it sounds like you're all set, as long as the PT can deliver. Reading some other postings over the years from Valve Jr modifiers, it seems it probably can.

So what cathode resistor to use? Let's guess 5mA is a safe guess for screen current (which seems plausible from the condition shown on the data sheet with plate and screen voltage the same). Noting the trend of bias voltage for the 3 SE conditions given, it seems -19v to -22v of bias might be likely for 6L6's at 300v plate and screen. So with ~81mA of cathode current and, say 21v of bias, we get 21v/0.081A = 259Ω. 250Ω is the closest standard value, and is typical of 6L6 amps.

Look up the specs for a 6l6 tube, with a plate voltage of  300, the tube chart I have indicates the tube has a resistance of  about 4500 ohms.

Check out the 5th line of each of those SE conditions. Plate resistance of the 6L6 is actually very much greater than the load impedance, and is typical of all pentode/beam power tubes.

[PRR]

> If the PT can deliver the current

That's the question. If still stock Jr PT, and already cheating-up the heater power a hair, how much more abuse can the PT stand.

Twice the tube (23W vs 12W) on the same PT.... either we cook the PT or we under-utilize the big bottle.

A conservative choice would be to run the big-bottle at the same voltage and current (and load) as the original mini-bottle. What is that, 330V 36mA? 9K nominal, 7K or 10K will be fine.