What is the correct load for an 845 triode? - diyAudio
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Old 23rd February 2013, 10:32 PM   #1
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Default What is the correct load for an 845 triode?

I have a pair of output transformers on a stereo 845 SET amp and I question whether they are wound correctly. I measured their turns ratio and found the 8R taps to be 26:1. The 4R are 36.6:1. For those without calculators within reach, that works out to about 5K4. That doesn't seem like the correct choice to me.

The attached file is plate characteristic curves drawn with an 80ma OP at 1000v which represents the amps design. The BLUE line represents the transformer loaded as speced. The RED line is a 10.9K load; the transformer's 4R taps connected to an 8R load (or the 8R taps connected to a 16R load).

If I have this thing drawn correctly, it seems to me that this amp would best be operated with the doubled loads to follow the red curve. This conclusion would mean that the manufacturer screwed up the transfo design on a production amp. An equally probable conclusion would be that I have a misunderstanding of the of the plate characteristics.

Please have a look and tell me what you think.
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File Type: jpg 845 plate characteristics.jpg (199.8 KB, 280 views)
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Old 23rd February 2013, 11:24 PM   #2
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http://tubedata.milbert.com/sheets/111/8/845.pdf

According to the above 9K is recommended, so your calculations seem right on. The Manufacturer probably used 5Kwhatever as a cost saving measure.
The 845 will work well with the Manufacturer's transformer, but with a 10K load the distortion will be less, damping facter higher so it will probably sound better. Triodes arent that critical with load presented to them, but there will be an optimum operating point, which 10K seems to be more so than 5K.
Daniel

Of course, connecting a 16 ohm speaker to the 8 ohm tap of your transformer will effectively double the primary impedence, but then the primary inductance of the transformer will be too low for corresponding impedence of the primary, so low frequency response of the transformer will suffer.

Last edited by danFrank; 23rd February 2013 at 11:29 PM.
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Old 23rd February 2013, 11:26 PM   #3
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I agree with Dan here, the 10k seems more optimized load here.
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Old 24th February 2013, 09:42 AM   #4
45 is offline 45  Italy
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Depends on the application. For example, at about 750V plate voltage and class A1 operation 5-6K is a very good choice. You get about 12W with low THD.
10K is better for high voltage. However a 10K OPT for such application is a lot more difficult to make in comparison to 5K. If the transformer is a standard type (and even more if cheap) a 5K OPT is typically a better performer than a 10K OPT of the same quality
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Old 24th February 2013, 09:56 AM   #5
45 is offline 45  Italy
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Anyway you should get about 24W at 5% THD from the load line at 1000V 80mA which is not bad at all! Driving the 845 into class A2, even for few volts, is quite challenging....

Last edited by 45; 24th February 2013 at 09:58 AM.
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Old 24th February 2013, 02:24 PM   #6
Merlinb is offline Merlinb  United Kingdom
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The 'traditional' load for a power triode is 2*ra, plus a little bit to reduce distortion perhaps, so 5.4k is a perfectly reasonable choice. It doesn't matter that the load line goes over the dissipation curve for part of its length, since the average power is roughly constant, and is determined by the bias point. Notice that with the 5.4k load the bias point is almost dead centre, which looks deliberate to me. You might prefer a larger load to reduce distortion more, at the expense of some output power, but they certainly didn't screw up the design.

Last edited by Merlinb; 24th February 2013 at 02:29 PM.
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Old 24th February 2013, 04:04 PM   #7
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Thanks everyone.

Merlinb, I'm very glad to hear you weigh in for the design. I posted hoping to hear that the tranfos are just fine and that my understanding is to blame. But, I don't completely follow you. Can you please elaborate on these two questions?

I didn't know that 2*ra is a traditional load for a triode. I can see that ra is about 2K and so it follows that 5.4K could be a very reasonable choice. So, I went back to the Amperex data sheet and noted that they show values for load resistance of 3K4 @ 750v, 9K @ 1000v and 16k @ 1250v. But ra is virtually unchanged at these three OPs. What IS changing is the max power dissipation curve. I think I read in Morgan Jones (or somewhere) that the load line (for a small signal triode) should be tangent to the dissipation curve which is more in line with the specs I just sited. Those two criteria seem quite at odds with one another. (What is it they say about a little understanding?)

I get that the load line can exceed the dissipation curve for half of its cycle, but I don't really see that the 5.4k line is dead center any more than is the 10.9k. I suppose I'm asking, dead center of what?

Thanks for your help.

Last edited by Captn Dave; 24th February 2013 at 04:23 PM. Reason: added: (for a small signal triode)
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Old 24th February 2013, 04:20 PM   #8
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Quote:
Originally Posted by Captn Dave View Post
........Please have a look and tell me what you think....
My method to determine which output give best performance would be to measure the frequency response, max. output power and THD, also at the low bass frequencies.
Playing with the load line is just theory and can lead to an incorrect result.
And of course listening is important.
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Old 24th February 2013, 04:27 PM   #9
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Originally Posted by artosalo View Post
Playing with the load line is just theory and can lead to an incorrect result.
It's difficult to design the driver stage without selecting a load line. The 10.9K line is much more difficult to drive than the 5.4K line. And perhaps that is the reason for the 5.4K windings.
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Old 24th February 2013, 04:47 PM   #10
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This is one view to the design procedure, but now the case is a bit different.
You have an existing amplifier with existing output transformer and two possible ways to connect it.
Which one is better (objectively) is simply determined with the measurements.

Usually driver stages should be designed by looking at the loadline but in addition with plenty of headroom.
The driver stage can also be analyzed with suitable measurements.
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