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Question about RLa-a for *LW6 in triode connection?

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If I was to try the LW6 in P-P triode I would try 275 to 300 volts. That's the G2 spec plus a little bit.

I'm trying to burn through the wife's honey-do list of outdoor tasks while it isn't raining. That has allowed me only one indoor day this week and I spent that replacing the motherboard in my other PC that unexpectedly checked out in the middle of a video editing session last week. Unfortunately her list is growing faster than I can cross stuff off the list......but it's got to rain sometime.

Tomorrow it's off to her father's house for Fathers day. I get to let the grandkids throw water balloons at me until they run out of ammo.

I have a driver board that can drive the grids out af near any tube made, a box full of LW6's and a selection of suitable OPT's. Should be easy to try something like 300 volts in triode into 1250 ohms.

they appear to be more suitable for Class aB operation in order to keep the idling power as low as possible.

Horizontal (line output) sweep tubes were never designed for class A operation. It has been stated that the most linear part of the curves lie in the red zone. Vertical (frame output) sweep tubes are DESIGNED for class A operation, but the biggest ones are only good for 15 watts or so. I'm sure that an LW6 can work in class A triode, but how much power will it make, and under what conditions remain unknown. Perhaps a point where the first 10 watts of so can come in class A with more power available in AB.

There are several different varieties of LW6's. The internals are probably all similar but there are several variations of heat radiating fins attached to the plates, as well as none at all. There are also two different bottle diameters.

All the Sylvania made tubes have no supplemental fins and the smallest bottle, even still it takes 60+ watts to get a faint redness in a dark room. It was a well used Sylvania that I torture tested to 600+ watts, not a GE like I thought. The tube is still in the test socket.

There are several different GE fin structures and two bottle diameters. The fattest tubes with the large wrap around fins need 80 to 90 watts to provoke glow.....but those are the ones I had trouble with. I was initially running them at around 40 watts, and went down as I lowered the B+.
 
Ok. so I ran some sims with Push-Pull Calculator and I'm thinking triode connected with a B+ of 220V and Ia of about 100ma and RLa-a of 1k25. It should make 35W, 15W class a limit.
Apparently this doesn't exceed any specs.

Thoughts?

Here's what I'm thinking, minus the power supply, of course.
 

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PPcalc is screwing up. It's telling me that I can get 50.9W power output with 0W plate dissipation. Then I hit "find Iq" again, and now it's a 2.4W amp with 347W Pd. I wish he'd rewrite it.


I restarted it and now I'm getting plausible results: B+ 200V, Ia 100ma, Raa=1k25. It gives 32W into the load, 5W class a limit. and Ik limit= 24.8k% what ever that means...
 
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Thanks, George! I've already ordered this based on the 12V model. If this doesn't work I'll try it out!
AC-DC 100-240V to 36V 5A 180W 50/60HZ Power Supply Switching Board Module AF | eBay

Upon more tests it seems I can use 300V B+ with 120ma and be well within specs. 75W, 8W class a limit into Ra-a 1k3. Cathode peak is only 560ma and the tube is spec'd for 1400ma peak (using the 6LF6 sheet as a basis).

It also means I can add a switch to change the Ra-a to 5k2 (secondaries in series or parallel) and have 25W output, 24W class a limit.
EDIT: If i trust the results of PPcalc.

OK. So I've built an amp. It's triode connected, 300V B+, 120ma fixed bias, and RLa=~1k3.
I haven't made any non aural measurements yet, but it sounds great.
Push pull calculator says it'll deliver 75W at 0.3% 3rd. That's before gNFB.

I'll probably start a new thread for it, but here's a glimpse.
 

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Horizontal (line output) sweep tubes were never designed for class A operation. It has been stated that the most linear part of the curves lie in the red zone. Vertical (frame output) sweep tubes are DESIGNED for class A operation, but the biggest ones are only good for 15 watts or so.

The "or so" part is true. Here's what I get for the 6LR8 (I have a bunch of the 21V version). 30W with a PD of 13.87W -- less than 7.0W/tube for a tube rated for PD= 14W is running them pretty conservative. You could do more power with higher voltages, deeper into Class AB1, and larger plate loads. At the expense of more distortion, of course.

The 6LR8 was one of those specialty tubes that included a small signal triode and a power pentode in the same glass to make vertical time base oscillator/amplifiers. Still convenient as you have your grid drivers right there. Looks rated for big screen, color TV use.

I wouldn't use *LW6s as pseudotriodes. Delicate screens that can't take much voltage goes against that. Besides, as pents, you can get north of 100W with a PP pair. If these are anything like the other TV HD finals, they should sound real good as audio pentode finals, requiring just a bit of gNFB to take the "edge" off.
 

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The screen is rated at 280V, it's getting 320V through a 220R resistor. The plate is still the main target. Also the screen can tolerate 40ma at 320V for a few seconds (the plate cap fell off when I was testing it upside down). I'm thinking in tetrode, they would make 150W or so. I wouldn't run them at 400V with lower current, but 320V, 120ma is well within spec, even if the max screen voltage is exceeded slightly.
 
Here's the current schematic.

It takes about 15 minutes to stabilize at it's bias points. This is by far the best demonstration I've made to date in favour of toroidal power transformers as OPTs. These transformers still sound fine (at low volume, while the amp is heating up) with a 40ma DC mismatch.

The model I'm using is rated at 250VA.

The calculated output is 75W. The class A limit is about 8W. The tubes idle at about 40W Pd.

To me this is nothing because these tubes ran for years basically full out as horizontal outputs in televisions. I've kept the voltage low (for a tube with a peak of 7kV anyway) to better serve the screen grid, and to maximize the power from the OPT. High current and low voltage is preferable with an off the shelf PT as OPT in my experience. And as George (Tubelab) has said, they tend to die at higher voltages, and his suggestion was to keep it under 350V which I've done.

Any comments?
 

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The only comment I have is good job!

My thoughts are for the bias mismatch saturating or magnetizing the output transformer.

You said the bias stabilize about 15 minutes , how much variation? 2. is it tube related or simply the circuit heating to stable températures?

In my amps the bias is offset for at most 1.5 mA for the first 5 minutes, then it gradualy sets to the desired bias within +/- 0.25 mA often and 0.5 sometimes depending on temperature of the room.

I am currently building such an amplifier with a toriodal with 2mA tolerance, So I though about using an auto bias board... not very familar with it and its compatibility with class AB , I want to bias to be like 50% higher when tubes goes into class AB, goal is around 60 watts 1% with KT-77
 
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