Can I use a 6L6 in a 5F1 kit instead of a 6V6?

[leevc5]

I want to use a 6L6 in my 5F1 Build.  Is there anything I need to do?

[pompeiisneaks]

At a minimum you need a different output transformer, as the 6L6 has a much higher output impedance than the 6V6, but you may also need to bias the tubes differently (I don't know off the top of my head).  You could look at a similar circuit that uses 6L6 like a Fender Bassman 5F6A that uses the 6L6 and see if that transformer and setup may work, but you should setup a schematic on it and share for the gurus to tell you if it won't be a good setup.   

Edit:  The more I read up on it, they're pretty close and it may work, but the OT still needs to handle the higher current, and you may need to adjust the impedance of the speaker on the opposite side. 

I.e the 6L6 seems to run at 4500 ohms at 300V, and current anywhere 55 mA but the 6V6 at 315V is 8500 ohms and 34 mA so they're a bit different, so there is an impedance mismatch and difference in current.  If the OT can handle them, you would then just need to adjust the bias etc.   I'd peek at a single ended 6L6 schematic and see how it looks comparatively to the 5F1

~Phil

[PRR]

Just pop it in. Keep a hand on the power transformer, it will be running warmer.

Since you are not adapting the 6V6 bias or load to make the most of a 6L6's ability, and the iron would not support more, it won't be "bigger". You are sending a 1-ton truck to do a half-ton job. The half-ton truck should be cheaper, and a little less wear on your roadway. But we all drive 5-passenger sedans to haul a 6-pack of beverage.

[HotBluePlates]

At a minimum you need a ...

Edit:  ... the OT still needs ...

... Since you are not adapting the 6V6 bias or load to make the most of a 6L6's ability, and the iron would not support more, it won't be "bigger". ...

Translation:
As long as the power transformer heater winding will allow it, you can just plug a 6L6 into a Champ and be done.

But power supply voltage, available current capacity from the power transformer and the impedance of the output transformer are the things which determine how much output power you will get. When those things are in place, using the bigger tube will lead to more output power.

But the higher output transformer primary impedance of a standard 6V6 circuit won't allow the 6L6 to suck more current from the power transformer without running out of available supply voltage swing. Many amp makers keep the same supply voltage when switching to bigger tubes, then lower the output transformer primary impedance to allow more current-swing with the bigger tubes (and more current capability in the PT), which then results in increased power output.

[Paul1453]

So, this might also work to swap in 6L6s into Doug's 6V6 Plexi?

I know the Plexi is not Cathode biased, so as long as I biased the 6L6s for output the OT can handle it might be OK?
What I mean is to calculate a bias voltage that gives the 6L6s 18W or less output.
My PT has more than enough heater and HV current available.   
From your expert responses, what I think I understood was that with a fixed bias Plexi,
if I biased the 6L6s for full output I could possibly smoke the OT with that much power.
With a cathode biased Champ, that is not really possible? 

[pompeiisneaks]

Sounds right.  Basically confirm the delta current draw for the different tubes and ensure that the current setup can handle it (I would guess both PT and OT, i.e. heater current capacity and such) as well as that the OT won't be drawing too much current (Which you said it can handle).  Both cases should work, with the re-biasing condition you mention for fixed bias.  The cathode may need a change too just because the operational parameters have changed, but I'm not 100% sure on that.  You can easily check what cathode bias requirements are in the 6L6 datasheet and check.  Then if needed adjust the cathode resistor.

~Phil

[PRR]

Fix bias push-pull works different.

You could idle-bias at "zero" and still get large power output.

The B+ and the load limit power output.

[Paul1453]

OK.

So it's fine to slap a 6L6 in for the 6V6 in a Champ.

But don't try it with a fixed bias PP setup.

I appreciate you setting me straight on that PRR!   

[HotBluePlates]

PRR posted while I typed.

So, this might also work to swap in 6L6s into Doug's 6V6 Plexi?

Your PT has a huge amount of 6.3vac current capacity. So yes, no problems just popping the 6L6's in.

Why do I keep harping on the 6.3v? A 6V6 draws 0.45A of heater current (0.9A total for 2x tubes); the 6L6 draws 0.9A of heater current (1.8A for 2x tubes). You'll want to verify your PT's 6.3v winding has an extra ampere of current capacity when swapping.

I know the Plexi is not Cathode biased, so as long as I biased the 6L6s for output the OT can handle it might be OK?
What I mean is to calculate a bias voltage that gives the 6L6s 18W or less output.
My PT has more than enough heater and HV current available.   

You can't "bias the 6L6 to get 18w of output power"... You can only bias to set the idle dissipation (heat in the tube plate).

You can apply more negative volts if the tube is redplating during actual signal conditions, which amounts to cooler idle bias.

-  Output power will be a function of output transformer primary impedance, supply voltage and power transformer current capacity. Say you have 285v of supply voltage, and the 6V6 can pull its plate as low as 50v on signal peaks (corresponding to the area of the knee at the 0v gridline on the plate curves). Then peak plate voltage swing can only be 285v-50v = 235v.
-  If your OT is 8kΩ of primary impedance, then if the tubes stay class A (never run to plate current cut off), the load to one side is 4kΩ; if class AB, the load at the peak will be 2kΩ.
-  Given the 4kΩ load to one side in class A, 235v peak equates to a peak current draw of 235v/4kΩ = ~59mA.
-  Peak power output will be 235v peak * 0.059A peak = ~14w peak; or 14w peak/2 = 7w RMS
-  If the output stage actually runs to class AB (would have to verify with plate curves at the correct G2 voltage), then 235v/2kΩ = ~118mA, 235v * 0.118A = 27.7w peak = ~14w RMS

If you plug 6L6's in the same circuit, and even if the PT can supply well over 300mA, you will still only get 14w RMS of output when using the same 8kΩ plate-to-plate OT. That's because when the bigger tubes attempt to draw more a.c., the current is pulled through the OT primary impedance to that tube, so there is an a.c. voltage swing happening to swing plate voltage towards ground, and the tube runs into its saturation voltage (the voltage where the knee of the 0v gridline occurs). Below that plate voltage, current drops rapidly.

... if I biased the 6L6s for full output I could possibly smoke the OT with that much power.

The power rating of the OT is indirectly associated with current-handling capability. It directly relates to the size of the core, and how much power can be passed from primary to secondary, at some specified bass frequency. You can pass more power through most OT's than they're rated to handle, but bass response will be shaved off (and possibly treble response). OT's on vintage Fender amps, especially the smaller tweed amps, are tiny compared to hi-fi OT's rated to handle the claimed power of those Fender amps.

Really tiny OT's (or those with very high primary impedances) typically have thinner-gauge primary windings. That's where you have a risk of burning the OT. You generally won't burn a 8kΩ OT by plugging in 6L6's, because the current drawn won't really go up much (as explained above), and because the wire itself is thick enough to handle the current even if it did go up.

Usually, the OT gets burned because of some other fault in the amp, or due to a build error which pulls excessive current through the primary.

[HotBluePlates]

The repeated bottom-line here is the power output capability is decided first by the OT primary impedance and its interaction with the power supply. Those two set a limit on maximum power output no matter what/how-many tubes you use. The tube is then the means to allow a controlling voltage (output tube grid drive) to control (tube plate) current through the OT primary. The output tube selected will have its own voltage/current constraints, which are investigated/verified after you've selected a PT/OT combination to deliver the desired power to the speaker.

Example tube constraints are maximum electrode voltages, saturation voltage at the intended screen voltage ("knee"; limits peak plate voltage swing), peak plate current for the desired screen voltage ("knee" again; sets maximum current the tube can pass at the specified G2 voltage), and plate dissipation (superimposed on grid curves, with loadline drawn; does the tube exceed plate dissipation for a significant amount of time anywhere on the loadline?).

The root of the misunderstanding is you learn tube electronics with already-designed guitar amps. The designer of those amps set a particular plate load, and used a particular supply voltage, power transformer, output tube set, etc. Transformer makers don't wind an infinite array of primary impedances or transformation ratios, so there's a limited set of "common parts." Amp manufacturers don't reinvent wheels: if 450vdc and 4kΩ primary Z with two 6L6's got manufacturer a 40w of claimed output power, then manufacturer B can probably easily acquire a transformer set enabling 450vdc and 4kΩ to get 40w from their amp. And then guitarist-techs come in trying to change speaker loads and find out about "impedance matching"... But they're really just "matching" the original-design primary impedance to get a consistent output power level from the amp...

[HotBluePlates]

Fix bias push-pull works different.

You could idle-bias at "zero" and still get large power output.

So it's fine to slap a 6L6 in for the 6V6 in a Champ.

But don't try it with a fixed bias PP setup.

That's not what PRR said.

Class B theoretically is where the output stage is biased right-at the point it cuts off ("... idle-bias at 'zero' ..."). Each side of the output stage amplifies 1/2 of the incoming waveform; the OT stitches those halves together to create the complete waveform.

Class A theoretical max efficiency is 50%, while class B ax efficiency is theoretically closer to 71%. "... and still get large power output."

Class AB is a middle ground, where each side cuts off for some amount of the total signal cycle. More power than Class A, but less power than Class B.

Fix bias push-pull works different.

PRR said this because you can't reliably bias a tube to Class B (plate current cut-off) by cathode bias. You can bias somewhat into Class AB with cathode bias, but the closer you get to Class B (still in Class AB), the bias method will have to be switched to fixed-bias.

So it's fine to slap a 6L6 in for the 6V6 in a Champ.

Ever heard of a THD Univalve? (If not, Google...)

SE amp, you can plug in a 6V6, 6L6, EL34, KT66, 6550 or KT88. Output power never really changes, but impression of cleanliness/tonality when using some tubes will change. That's because the amp is cathode-bias and has a set primary impedance & speaker load.

[Paul1453]

Thank you HBP!   

This is part of the reason I wanted to try a fixed bias amp this time.

I have a whole lot more to learn, just to become competent.   

So, the bias is just like the idle setting on a engine.
If you put in a 400HP engine in, it doesn't matter what you set the idle at it can still make 400HP.

I really wasn't planning on sticking 6L6s in my Plexi when I get it finished.
I just wanted to check my understanding of these principles, which is obviously lacking.   

On the other hand, I do plan on putting a 6L6 in my reel to reel Champ conversion after I verify my heater capacity.   

[HotBluePlates]

So, the bias is just like the idle setting on a engine.
If you put in a 400HP engine in, it doesn't matter what you set the idle at it can still make 400HP.

Sort of. I've learned the hard way over the years that the idle bias is really decided when the engineer designed the amp.

But Ken Fischer pointed out that setting bias to a specified voltage doesn't tell the tech whether the individual tube used (whose performance will vary from the ideal "bogey" valve) is running stone-cold or at plate-melt. So he recommended biasing by measuring idle current.

Then those who popularized Ken's approach didn't emphasize that there's no one "right" idle current for any/all amps, and that you have to know something about the designer's intent.

But learning amp design is beyond the scope of the average hobbyist/wannabe-tech, so recommendations of a specified idle current (but for all(?) amps) of dubious provenance emerged and have been repeated for years.

Pete Millett's site has many scans of the old tube books. The reading will be dry if you're searching for "Guitar Amp TONE!!" and will assume you know/learned the very basics of electricity (though some books there may have that as well; certainly, NEETS does). Some books will be math-heavy, others not so much. But you'll learn how tubes work in a (non-guitar) circuit, and can then apply that to guitar amps.

[Paul1453]

I hope I'm not Hijacking this thread.

The Army trained us with basic tube operation fundamentals.
Nothing about design criteria or circuit optimization.
I was trained to repair equipment with the exact parts and layout as designed.
Any substitution or optimization was strictly forbidden.

That is why I'm kind of struggling with the design concepts.

I can troubleshoot and repair a as designed circuit rather quickly.
Figuring out how to redesign a circuit to work with the parts I have, is a monumental task for me at this time.   

So for the engine analogy.
The PT is kind of like the carburetor and the OT kind of like the exhaust.
If you put a 400HP engine in that wants a 4 barrel Holly double pumper carb and a dual tuned port exhaust,
but you feed it with a 2 barrel carb and a single output restricted exhaust, you're not going to get 400HP out of it.