Rough 6L6 wattage output with given B+

[Dagan8]

Hello

Hopefully this is a fairly easy question.

I am planning on using a 275-0-275 230mA transformer in a Fender Blackface Reverb inspired build. This will be amp used mostly for cleans and I was aiming for around 35W of power output. What kind of power output can I expect using this transformer with SS rectifier, Bassman OT, Choke, 2 X 6L6, fixed bias. I realize that I could easily use 6V6 for around 20-25W but I would like to use 6L6/5881 if possible.

Cheers.

[J M Fahey]

You'll be closer to 30W, but since I guess you already have the transformer (or it is within your budget), "just do it" .
The amp will be fine.
Measure it after you build it, if you wish.
As of 25W with a couple 6V6, not that easy.
Consider the 6L6 as a *very good* 6V6 and you'll be fine.

[Dagan8]

30W is more than fine. Thanks.

[Jazz P Bass]

You really have not defined your output transformer or the load.

[Dagan8]

You really have not defined your output transformer or the load.

Hammond 1760K
4K Primary
89.43ohm DCR across the primary

[Jazz P Bass]

Thanks.
What is the load on the secondary? (speaker dc resistance)
Am I wrong or is that not where you measure the output wattage?

[Dagan8]

What is the load on the secondary? (speaker dc resistance)

Does it really impact the answer to my original question? I thought that as long as I correctly match the impedance with the correct output tap the outputs tubes will "see" the OT's input impedance? A 30W amp plugged into a 1 X 12 cab and the same 30W amp plugged into 10 4X12 cabs is still a 30W amp.

I'm still learning so I could be wrong.

[Steve Conner]

Well, let's run the numbers.

SS rectifier, so the B+ is 275 * 1.4 = 385V.

If we assume a tube drop of 50V when fully on, then the peak power is (385-50)^2/(4000/4) = 112W
and the average ("RMS") power is one half of that: 56W.

I'm being optimistic: 50V is a low tube drop for a 6L6 passing 335mA with only 385V screen voltage available. It would be over 100V, which gives us 40W RMS.

And I didn't account for sag in the power supply either, which will compound the problem by sagging the screen voltage. The actual output will certainly be nearer 30W than 56.

Indeed, when an 8 ohm load is connected to an 8 ohm tap, or a 16 ohm load to a 16 ohm tap, then the 4k primary will present a 4k ohm load. I divided by 4 in the above example, because I wanted the impedance between the CT and one end.

[Dagan8]

Thank you Steve

28W makes sense when I think about some of the boutique AB763 based clones on the market that claim interchangeable power tubes. Most seem to claim 22W with 6V6 and 30W with 6L6. I guess as long as I correctly adjust bias and compensate for the load impedance I can interchange 6V6 and 6L6 to see which I like. I can judge at that point whether the extra headroom of the 6L6 is needed.

Thanks for your help everyone.

Cheers!

[Dagan8]

6L6 passing 335mA

Does this mean that my PT may not have the required power for 6L6 since it is only rated for 230mA? If that's the case I will stick with the 6V6's and call it a day.

[big_teee]

Check this out!
PT Current Draw
Calculate Bias
Good Luck,
B_T

[J M Fahey]

Don't worry.
Mind you, it's just usable, but if it were capable of more, better, not for increased power but for cooler operation
Your 6L6 will *never* pass 335mA when saturated.
4000 ohms plate load is extremely optimistic, and only works because speakers usually have *more* than their nominal impedance over most of the range.
Since I'm a boring Engineer instead an illuminated Guru , I found 5500 ohms to be an optimal impedance for run of the mill 6L6 which can be purchased anywhere.
Find OEM STR Sylvania 6L6 a nice reminder of a glorious past, but don't design based on them.
And have no use for 6V6 since they cost more and deliver less than a 6L6, go figure.
Bad in my book.
Oh well.
Can get 40 *real* RMS watts into any matched load with 430V +B, screens same voltage through 1K resistors, 5500 ohms, any day of the year, any reasonable tube.
50/60W? = never seen. (unclipped that is)

[big_teee]

Don't worry.
Mind you, it's just usable, but if it were capable of more, better, not for increased power but for cooler operation
Your 6L6 will *never* pass 335mA when saturated.
4000 ohms plate load is extremely optimistic, and only works because speakers usually have *more* than their nominal impedance over most of the range.
Since I'm a boring Engineer instead an illuminated Guru , I found 5500 ohms to be an optimal impedance for run of the mill 6L6 which can be purchased anywhere.
Find OEM STR Sylvania 6L6 a nice reminder of a glorious past, but don't design based on them.
And have no use for 6V6 since they cost more and deliver less than a 6L6, go figure.
Bad in my book.
Oh well.
Can get 40 *real* RMS watts into any matched load with 430V +B, screens same voltage through 1K resistors, 5500 ohms, any day of the year, any reasonable tube.
50/60W? = never seen. (unclipped that is)

Is there any sound difference using 1K screen Resistors vs. 470 Resistors?
T

[pdf64]

Just checking over a DrZ EZG50, a take on a BF 6L6 reverb amp.
Single channel, GZ34 rectifier, VB+ ~ 455 static, 425 max clean, ~19Vrms@8 ohm (~45 watts).
The output tubes are GT, they have seen maybe 2000 hours of gig usage.
The OT primary works out at ~5k5; it's core stack is about 25% thicker than a regular 50/60 watt OT.
470 ohm 3W resistors on the screen grids.
The schematic is ~95% AB763.
Pete.

[J M Fahey]

DrZ is confirming what I found, that with modern cheap run of the mill 6L6, 5500 ohms is a conservative, "easy" load, which delivers at least 40 clean RMS watts.
They got a little more with a better , oversized OT? Good for them.
Besides, it's easy to check, just vary load impedance in, say, 20% steps from half to double and check which is the best.