Danger for OT?

[Bieworm]

Hey guys, I just built a 36 watt marshall type circuit. So it's a dedicated 36W PT and OT running the show. For power stage there are 2x 6L6GT tung sols .. cathode biased.
After measuring the voltages I have 344V on the plates, 310V on the screens and 20.0V on the cathodes. This calculated in the tube bias calculator gives me around 20.5W dissipation per tube. Does this mean I'm pushing 41W to a 36W OT? That sounds like a bad idea... or am I missing something here? It's about 68% biased, on a 30W tube that is.. should I raise the cathode resistor and bring those 6L6's to the 18W target?

[Latole]

41 to 36 watts on the power tubes = same on PT

Why don't you use 6V6s ?
 

[Bieworm]

41 to 36 watts on the power tubes = same on PT

Why don't you use 6V6s ?

The 6L6 is the whole idea...most pit el34 in these.. so why not 6L6

[sluckey]

What is the resistance of your cathode resistor?

[pdf64]

Idle anode dissipation is unrelated to audio power output.
In the same way that idle rpm and a vehicle’s top speed are unrelated.
The only thing that’s directly affected by idle dissipation is the valve’s operational life.

[Bieworm]

What is the resistance of your cathode resistor?

150R

[Bieworm]

Idle anode dissipation is unrelated to audio power output.
In the same way that idle rpm and a vehicle’s top speed are unrelated.
The only thing that’s directly affected by idle dissipation is the valve’s operational life.

What I'd like to know is if it's hard on my transformers. Should I raise the cathode resistance? I only have 5W and 10W resistors in my stash. I gues that 180R 5W is going to get a little too hot?

[thetragichero]

for some an underspeced ot is a tonal "goal"
if it sounds good and the tube is not running under unsafe conditions then all is well in my book

[pdf64]

What I'd like to know is if it's hard on my transformers. Should I raise the cathode resistance? I only have 5W and 10W resistors in my stash. I gues that 180R 5W is going to get a little too hot?

Why do you think it might be hard on either the PT or OT?

[Bieworm]

Well I just ruined a fine tung sol 6L6 GC STR by hitting it abrubtly to the bottom while pulling it out. I wanted to try the stupid 5881WXT's. Well I will have to use the now...
They dissipate 19.9 Watts at 150R . No redplating or anything...

[Latole]

Idle anode dissipation is unrelated to audio power output.
In the same way that idle rpm and a vehicle’s top speed are unrelated.
The only thing that’s directly affected by idle dissipation is the valve’s operational life.

You are right
See this Hammond push pull transformer specs. It work for two 6L6 at 30 watts

1645     30 watts audio  Primary Impedance : 5,000 ct   Push-Pull (2 Tubes)   6L6GC, 6V6, 807, 5881, EL34


https://www.hammfg.com/fr/electronics/transformers/classic/1608-1650?referer=790

[Bieworm]

What I'd like to know is if it's hard on my transformers. Should I raise the cathode resistance? I only have 5W and 10W resistors in my stash. I gues that 180R 5W is going to get a little too hot?

Why do you think it might be hard on either the PT or OT?

I'm not educated on that material. It makes le wonder if the tube is going to force the trannys to deliver 40 watts, when they aren't designed to... Just wondering if I'm doing anything wrong..

[Bieworm]

I have the tube town 36W OT.  It's 4k primary

[pdf64]

From the data you’ve supplied, the PT, OT and output valves look to be cruising along well under their rated limits.
eg 4 x EL84 would draw more current, both at idle and with signal, and hence, in conjunction with the increased cathode cathode voltage of 6L6 types, the EL84 would put out more power.

[HotBluePlates]

... 36W PT and OT ... 2x 6L6 ...
... 344V on the plates, 310V on the screens and 20.0V on the cathodes. ...

I have the tube town 36W OT.  It's 4k primary

Check out the 6L6GC data sheet.  Follow along with the top graph on Page 6 (peak plate current when drive signal makes the control-grid-voltage equal to cathode-voltage; i.e., "0v" from G1 to K).

     - You have 344v plate - 20v = ~325v plate-to-cathode.  Put a dot on the X-axis at 325v and 0mA.

     - A pair of 30w (6L6GC idling at 100%) can only make 30w audio in Class A (60w total heat, 50% efficient).  You're idling cooler, getting more than 30w (or will you?), so this must be Class AB (remains to be seen below).

     - When the Class AB output section is driven to peak output, one side is cut off.  Primary Impedance of the remaining side is (plate-to-plate Impedance)/4 ==> 4kΩ/4 = 1kΩ.

     - 325v / 1kΩ = 325mA ===> Put a dot at 0v, 325mA on the Y-axis.  Connect the dots (Red line below) and you have your loadline.

     - Look at your screen voltage: 310v screen - 20v cathode = 290v screen-to-cathode (what the tube actually "sees").  Round it up to "300v" because there happens to be an "Ec₂=300v" curve on our graph.

     - Draw a line from the X-axis up along the loadline to the 300v curve for screen voltage (Blue line overlaid on the Red line below).

     - Where this line touches the screen-voltage curve indicates the "peak plate current" and "minimum plate voltage" when we drive the output section to its maximum clean power output.  Conveniently, this happens at 225mA Peak Current, and 100v plate-to-cathode.

     - Find Peak Plate Voltage Swing: 325v plate-to-cathode - 100v = 225v Peak Voltage

     - Find Peak Plate Current Swing: easy, it's 225mA Peak Current

     - Calculate Peak Power Output: 225v Peak x 0.225A Peak = 50.6 Watts Peak

     - Calculate RMS Power Output: Peak Power / 2 = 50.6w / 2 = 25.3 watts

We could overlay a dissipation curve for the 6L6s, but it seems clear they wouldn't likely run too hot.  Your amp could really have another 100v added to the power supply to get a higher power output.  We could also re-run the calculations assuming Class A (plate-to-plate Impedance / 2, loadline goes through the idle bias point) to get a better estimate of power output, but I don't think it would change things too much.

As you hopefully see, what matter for Power Output is how much supply voltage is available to become voltage-swing, how high plate current can get (due to screen voltage & OT impedance) to become current-swing, and what results from that.  Your idle bias has more to do with how easy the output section is to drive and whether the tubes tend to overheat, than the actual power-output the stage will deliver.

[PRR]

There's NO way ALL the plate heat can appear as audio output. (Even if it did, it is not about to melt a guitar-amp's OT.)

In a self-biased power stage, the Audio Sine output is, at most, half the idle dissipation.

41W idle heat is maybe 20, 23 Watts audio output; but more likely 16W-19W pretty-clean.

It takes *time* to melt a transformer. One hot lick, even a hot song, won't do it.

[Bieworm]

Ok I am assured...
Gonna start building the combo shell for the new born today...
Here's the chassis from control plate view.
And here's a sound clip... who says 1-tube reverb doesn't sound good and lush?
https://www.dropbox.com/s/qg5bekvobi8tytr/Tremolo%20TMB%20reverb%2036W%20first%20take.m4a?dl=0

[2L man]

On idle there comes very little power loss to OPT. Measure voltage between B+ and anodes and multiply using current and add together and there is idle power.

You can also measure both  secondary resistance and then you can calculate individual tube biases after you measure voltages over secondarys.