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PostPosted: 28 May 2016, 12:51 
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So, I was thinking about putting some 6J6 tubes I got to good use, and I imagined they could make okay mini power tubes for a guitar amp, inspired on Doug Hammond's Firefly and it's descendents.

However, I decided to do the math, just to make sure I wouldn't fry the little guys in the test process.

Problem is, I got a weird result when I decided to calculate the output power from a single 6J6 in push-pull, with a 16k plate-to-plate load running @ 250v B+. I used this datasheet's Average Plate Characteristics graph, and The Valve Wizard's Push-Pull instructions.

I drew rather crude curves on Paint over the graphs from that datasheet, and here it is along with my calculations' results:

Attachment:
6j6-curves-16k-pp.png


I feel very skeptical of this result. Almost 6.7 Watts from a single 6J6 in push-pull??? Is this really correct, or did I calculate something wrong? Is there something missing in the math that I should have accounted for, but is not being mentioned on The Valve Wizard's site?

I'm very confused and I'd love some clearance on this subject. Thanks!


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PostPosted: 28 May 2016, 21:34 
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Hi, The 6J6 is a RF power amplifier and is designed to have some power output. But some things don't seem right in what you have calculated. First each triode has a max dissipation of 1.5 watts. Getting nearly 7 watts from the pair in the same bottle is not likely. For class C operation as an RF amp RCA indicates an output (for both sections combined in push-pull) of 3.5 watts. Class C is totally unsuitable for audio, so an estimate for class AB would be way lower. Likely in the 1-2 watt range. Some common estimates are for AB about 50% of input power (can get a bit more in some amps), 25-33% for class A amps.

Still it would probably be a fun project.

Good listening
Bruce

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PostPosted: 28 May 2016, 22:58 
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Thanks for the input, Bruce!

I am quite aware of the limits of this tube (considering I read the datasheet many times).
It might just be that, considering the tube would be running close to it's maximum plate dissipation at 250v, it's actually running in Class A instead of Class AB? Still, the math gives about 4.5 watts of output in that situation, which still sounds strange from such a small tube, but more plausible than this "magic" 6.7 watts. I'd take 4.5 watts as "peak watts", and being more realistic (but still generous), 2 watts RMS.

I was even considering that the formulas provided by The Valve Wizard would only apply to pentodes/tetrodes, but I have not found any comments in that page about being as such.

Perhaps I'm just being too greedy trying to use a 16k OT for this job, and that messed up the math? Perhaps it's the fact I used 250v as the B+ for these calculations, when the datasheet itself shows usual operation at 150v B+, tho it does say it can handle 300v MAX...

I'm just really weirded out about this. Math is not really strong with me.


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PostPosted: 29 May 2016, 12:17 
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Hi, I used a class A P-P arrangement in my tube headphone amp with 6DJ8s. Except for voltage ratings they are somewhat similar. Running close to max dissipation I could only get abput 500 milliwatts clean and when pushed just under a watt. Similar designs exist that use 12AU7s and they are in the 1 watt range as well. There are a few folks that do simulations on the forum. Perhaps one could sim your design. If you post the schematic it ought to be possible and then you would have a better idea of what it is capable of. I could be wrong, but at 4.5 watts I suspect the tubes will cook in a very short time.

Good listening
Bruce

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PostPosted: 29 May 2016, 16:52 
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I used the 6J6 frequently in RF applications (as a PI in audio). A fresh tube 20% over its ratings ICAS will net you 3.5 watts with tweaked-to-the-nines tuning components. It was driven hard-class-C.

I suggest learning by doing in this case... put away the book, pull out the breadboard, melt a few 6J6's and realize why old geezers like us say the things we do! :D

Have fun! Little anodes turning yellow can be entertaining. Wear goggles, I've cracked some this way :hot:

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PostPosted: 29 May 2016, 18:44 
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Geek wrote:
I used the 6J6 frequently in RF applications (as a PI in audio). A fresh tube 20% over its ratings ICAS will net you 3.5 watts with tweaked-to-the-nines tuning components. It was driven hard-class-C.

I suggest learning by doing in this case... put away the book, pull out the breadboard, melt a few 6J6's and realize why old geezers like us say the things we do! :D

Have fun! Little anodes turning yellow can be entertaining. Wear goggles, I've cracked some this way :hot:


I have already been playing with these tubes in the past. I actually have a full project (that never went beyond the prototype phase) using only this model of tube for the entire amp.

Schematic:
Image
(click for full view)

And how it sounds:
https://a.tumblr.com/tumblr_nla3wi3kUa1r4gqjso1.mp3

The main problem with the 6J6 is that it's prone to being microphonic when you use the two triodes in cascading stages, like I did for the preamp. And the fact it has a rather "high" gain for an output triode means you gotta reduce the signal going into the output stage, otherwise it sounds too "fizzy" (kinda like a small speaker that's about to blow).

It's not actually loud, but it's an okay volume for home use.

I only really made the topic because I found it rather interesting how the calculations could get so deviated from real-life results, and I imagined somebody could explain what is happening with the formula to give such unobtainable values.


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PostPosted: 30 May 2016, 09:34 
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OK. Let's start with the fact that, from the plot above, you are assuming deep class AB2 operation. Without a significant power driver it is unlikely that you would be able to sustain this without significant distortion. At the positive end of the grid swing your grid current is approaching 55mA.

Go back and make sure that you are not driving the power stage into grid conduction. When you make this single change, you'll see output power drop to several hundred milliwatts.Use the plate characteristics with the negative grid swing lines only. This one from the GE (July 1954) data sheet is what you should be using.
Attachment:
Screen Shot 2016-05-30 at 9.32.00 AM.png


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PostPosted: 30 May 2016, 10:46 
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Suncalc wrote:
OK. Let's start with the fact that, from the plot above, you are assuming deep class AB2 operation. Without a significant power driver it is unlikely that you would be able to sustain this without significant distortion. At the positive end of the grid swing your grid current is approaching 55mA.

Go back and make sure that you are not driving the power stage into grid conduction. When you make this single change, you'll see output power drop to several hundred milliwatts.Use the plate characteristics with the negative grid swing lines only. This one from the GE (July 1954) data sheet is what you should be using.
Attachment:
Screen Shot 2016-05-30 at 9.32.00 AM.png


According to those curves you presented, and still following the Valve Wizard method, I got 1.9 watts. That's much more plausible!

But, you mentioned Class AB2. I don't think I've ever heard of that!
How exactly does it work? What would I need to achieve it?


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PostPosted: 30 May 2016, 15:55 
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AB2 is where the output draws some grid current on peaks. You can't get that with R-C coupling. Transformer or some direct-coupled driver, you can.
http://www.duncanamps.com/technical/ampclasses.html

As a side note, one of the best sounding amps I've heard is the McIntosh 245 and it operates class-B. But it takes some OPT and NFB trickery to pull that off.

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PostPosted: 02 Jun 2016, 22:04 
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Geek wrote:
AB2 is where the output draws some grid current on peaks. You can't get that with R-C coupling. Transformer or some direct-coupled driver, you can.
http://www.duncanamps.com/technical/ampclasses.html

As a side note, one of the best sounding amps I've heard is the McIntosh 245 and it operates class-B. But it takes some OPT and NFB trickery to pull that off.


Do you think I could make it a hybrid, and have solid state devices (possibly a couple MOSFETs) as the current driver stage? It's kinda hard to find the appropriate transformers for this kind of application here, and when you do find them, they're more expensive than a whole combo amp.


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