low voltage, low current 6L6GC sound?

[jaywalker512]

Planning to build my first DIY tube amp in the near future, thinking a Fender 5F1-like amp. I will be using parts I already have available, so obviously some things wont be exactly the same.

I only have 6L6GC tubes here, and with my power supply I'll be able to give it 250v B+ at about 50ma (5K primary impedence on the OT). I know this is very low voltage/current for a 6L6GC, so how do you think it will sound? Like dooky, or no?

[Jazz P Bass]

It will sound clean.

[J M Fahey]

250V is fine, but 50mA is too little, you'll burn the PT.
Idle current alone will be there or beyond.

[jaywalker512]

250V is fine, but 50mA is too little, you'll burn the PT.
Idle current alone will be there or beyond.

Could you expand on that? How would too little current burn the power transformer?

[J M Fahey]

Too low current *capacity*.
If your PT is specified for 50mA it means it can supply *up to* 50mA, within ratings.
More specifically: wire diameter is rated for that.
Whatever "pulls" more than that, will stress/overheat it.
Personally I prefer to have "a little more" capacity, rather than "almost" what's needed.
Original PT is commonly rated 70mA.

[Enzo]

The transformer is only good for 50ma. The circuit will demand more than that, and burn out the transformer.

[Chuck H]

I don't get the "burn the PT" thing either. But the current being too low is at least related to class A operation. You could run at 50mA idle but your drive voltage would need to be reduced to avoid early cutoff distortion. So, as noted, the amp would be clean. Under those conditions attempting to OD the amp probably wouldn't sound very good. And OD is pretty much why we love these little class A amps. For class A you need to bias the tube right between cutoff and saturation. For a 6L6 at 250Vp that requires more than 50mA. Here's an example circuit for a 6L6gc at 250Vp from a tube data sheet:


Class A AmplifierPlate Voltage .............. 250
VGrid No. 2 Voltage ............................ 250
VGrid No. 1 Voltage ............................ -14
VPeak Grid No. 1 Voltage....................... 14
VPlate Resistance (approx) ............ 22.5K Ω
Transconductance ................... 6000 µPlate
Current (Zero Signal) .....72 mAPlate Current
(Maximum Signal) ........................... 79 mA
Grid No. 2 Current (Zero Signal) ......... 5 mA
Grid No. 2 Current (Maximum Signal) 7.3 mA
Load Resistance ..................... 2.5K ΩPower
Output (approx) ............................... 6.5 W
Total Harmonic Distortion ................... 10 %

[jaywalker512]

Oh, I see. I think my wording has been misunderstood. My power transformer can deliver 60ma, and I have 5ma delegated to two preamp gain stages (single 12ax7). The power tube (6L6GC) will be running at 50ma due to the chosen primary impedance of the OT (250v / 5000r = 0.050a), leaving 5ma left over. The idle current should be somewhat less, correct?

[Chuck H]

[Chuck H]

The idea for class A is to get the tube biased in the middle of it's swing capabilities at a given plate voltage. How much that relates to the chosen primary impedance I can't say... But the tube charts can. In the above example, with a 2.5k primary, you need 75mA of current for "correct" class A operation. With your OT at 5k you could run the OT secondary into 1/2 the rated load and reflect 2.5k primary impedance on the power tube. That still leaves your PT shy in capacity. How can we consider voltage as it relates to primary impedance without considering tube dissapation for class A operation? In your equation EVERY tube, regardless of it's dissapation, should be run at 50mA with 250Vp and a 5k primary.

[Enzo]

Don't confuse idle dissipation with audio power. Your 50ma is DC current, steady with no signal. The DC resistance of that primary is much lower than 5k. The 5k impedance involves transferring power to the low impedance speaker load.

[J M Fahey]

The Math is simple: at full (clean) power (well, 10% distortion)= 79mA (plate) + 7.3 mA(screen) + 5mA for preamp tubes=91.3 mA. = 50% above max. rated current.
Ok, maybe it will not *burn* as in "call 911 for the firemen", but it is an abuse and will run quite hot.
As said before, I'd be uncomfortable .
When in doubt, datasheets are a great source of data. (no pun intended).

[jaywalker512]

Don't confuse idle dissipation with audio power. Your 50ma is DC current, steady with no signal. The DC resistance of that primary is much lower than 5k. The 5k impedance involves transferring power to the low impedance speaker load.

I think that's what I'm doing (confusing myself, and consequently you guys). I'm still a little confused about how the power amp components are picked. Is the cathode resistor for an output tube like the 6L6 calculated the same way as for a triode? If so, is my figuring below correct:



Alternatively, would I be better off just wiring it as a triode and using it that way?



Or am I misunderstanding something fundamental :/

[J M Fahey]

No.
To begin with, forget the triode, there's no need for that.
Back to the pentode, you are designing like if you had a *resistor* plate load; while in fact you have a *transformer*, which, among other properties, is an *inductor* and has very low DC resistance compared to its (reflected) impedance.
So, assuming a low DC resistance , you design for 30mA (or whatever value you choose) @ 250V plate voltage.
Or to consider some resistive loss, design for, say, 230V DC. We can tune the design later when you post the actual DC resistance.
Your plate voltage will swing from around 40V (saturation voltage) to its symmetrical value, around 420V.
Delta means "variation of"; I don't have the Greek symbol available
Delta V will be 380V
Delta I will be 60mA (from 0 to twice the idle current).
The optimum load impedance which will make that happen is: Delta_V/Delta_I = 420V/0.06A=7000 ohms.

[exclamationmark]

With a B+ of 250v, with an inductive load (ie, the transformer), the voltage can actually swing to almost double this amount! The quiescient voltage is also always equal to B+. I suggest you read the 'Single Ended' power section on the Valve Wizard website. Takes a while to comprehend, but explains a lot.