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Old 21st February 2011, 11:36 PM   #1
cricha5 is offline cricha5  United States
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Default small, light dc coupled 6ea7 amp sanity check

While waiting on my edcor transformers, I have started designing my next project. I'm hoping someone more experienced can take a peak at this and make sure that I'm not doing anything too stupid.

The main requirement for this SE direct coupled monoblock tube amp is that it must be small and portable, while still sound good. I therefore used the combo tube 6ea7/6em7 and I am dispensing with a choke in favor of a RC ladder filter like in Morgan Jones' book. When looking at the datasheet for the 6ea7, the recommended operating points are a bit different than with a normal two stage amp. The recommended operating point calls for the driver stage sit at a higher (250V) B+ than the power stage (150V). After looking at power supply designs and grounding schemes that would accommodate this, I floundered upon direct coupled designs, which would solve and simplify the power supply design.

I know these direct coupled designs tend to blow up when things go wrong, and the monkey design fixes it and reduces wasted power, but more components will add size, which I do not want. Also these tubes are relatively cheap and I'll be using properly rated components and fuses, so I'm not too worried about a failure.

Many of the designs I see call for loading the driver anode with a choke. I don't quite understand the need for this and would appreciate an explanation. Since I'm dispensing with a choke in the power supply, one here would not do.

The anode of T1 sits at 250V, idles at 180V, biased at -2V and takes 1.8 ma at idle. The operating point of T2 is at 150V at 50ma, biased at -20V. So, the cathode of T2 needs to sit at 180+20=200V and the anode needs a B+ of 180+150+20=200+150=350V. The cathode resistor needs to drop 200V at 50ma, so 3.2kOhms. Make sense?

Thanks all.
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Old 22nd February 2011, 12:13 AM   #2
wa2ise is offline wa2ise  United States
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Quote:
Originally Posted by cricha5 View Post

I know these direct coupled designs tend to blow up when things go wrong, and the monkey design fixes it and reduces wasted power, but more components will add size, which I do not want.
All you'd really need is an extra coupling cap and bias resistor, which really won't add much if any extra size to the amp. You already have the OPT, power transformer, filter cap and tube, which are the bulky items anyway. And I hate to think of the power you're gonna burn in that cathode resistor. Which means a bulky power resistor probably bigger than the coupling cap and bias resistor anyway. Avoiding that power resistor would allow a smaller power transformer anyway.
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Old 22nd February 2011, 12:21 AM   #3
cricha5 is offline cricha5  United States
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Hmm, good point. I guess I might do the work modding it to a monkey and see whats what.
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Old 22nd February 2011, 02:32 AM   #4
mwiebe is offline mwiebe  United States
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The small triode’s plate resistance according to the data sheet is 30K. Your plate load, R1, is only 39K, which is way too small if you want full frequency response. A choke offers many advantages that you can find searching here.

The high plate resistance of the small triode of the 6EA7 and your small package size requirement may be better served by a constant current source, which can provide high impedance with little loss in voltage, around 30Vs or so.

If that is all the mains voltage you have to work with, you may be hard pressed to monkey with a constant current source, though a high-henry choke works. It’s a nice tube and worth playing around with.
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Old 22nd February 2011, 02:42 AM   #5
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As you noted, the 6EA7 has characteristics very similar to the 6EM7. If someone can tell me precisely where they differ, it would be appreciated. A well tested and proven direct coupled design for the 6EM7 can be found here:
6EM7 DC Coupled Triode Tube Amplifier

I'd suggest you start with that. Don't neglect the high wattage rating of the cathode resistors.

Last edited by Ty_Bower; 22nd February 2011 at 02:44 AM.
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Old 22nd February 2011, 03:08 AM   #6
cricha5 is offline cricha5  United States
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I can't find the graphs for the 6ea7 like I can for the 6em7, so I haven't found out how they differ yet.

I've seen that battery biased design. Somehow I missed it when I was designing mine, but when I did find it I was reassured. Is there a burning reason he decided to use a battery bias?

With my operating points, I'd be burning 10 Watts in the cathode resistor. That kind of dissipation may need a heat sink. I have found that I don't quite fully understand Monkey amps enough yet to go that route. Maybe further down the line. I guess I'll put two or more cathode resistors in parrallel if I want to avoid a heat sink.

mwiebe, I printed out the graph for the 6em7 and drew the load line I wanted and then calculated the plate load resistor. Can you elaborate on the relationship between the tube plate resistance and the plate load resistor? I know that it is generally a tradeoff between linearity and voltage swing. The more plate load you have, the more voltage swing you can get. The less resistance, the more linear.
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Old 22nd February 2011, 03:24 AM   #7
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Originally Posted by cricha5 View Post
Many of the designs I see call for loading the driver anode with a choke. I don't quite understand the need for this and would appreciate an explanation.
It seems to be explained here:
"Usually you would want some margin of voltage to allow the driver to swing more voltage than the output tube would require to drive it to full output, but in this design it is not possible. This led me to believe that a choke was the only appropriate plate load for the driver tube."
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Old 22nd February 2011, 03:33 AM   #8
mwiebe is offline mwiebe  United States
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Radio Designer's Handbook, Morgan Jones, and others go into triode loads. Three times plate resistance is a decent starting point. A constant current source, CCS, is many times higher and can diminish some distortion.

If you just want to build an amp, the Gary Kaufman’s design, linked above, has been built by many and sounds great. If you want to puzzle out your own amp have at it that’s what this is about. Sometimes it’s easier to solder and listen and then figure out what to do to change what you hear.
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Old 22nd February 2011, 04:27 AM   #9
Knarf is offline Knarf  Denmark
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Originally Posted by cricha5 View Post
With my operating points, I'd be burning 10 Watts in the cathode resistor. That kind of dissipation may need a heat sink. I have found that I don't quite fully understand Monkey amps enough yet to go that route. Maybe further down the line. I guess I'll put two or more cathode resistors in parrallel if I want to avoid a heat sink.
There is no need to guess here, you *will* need a heat sink at those power levels.

Attempting to spread out the heat a bit by using two resistors in parallel will not get you anywhere. Ultimately all the heat generated inside/below your chassis has to get out into the free air. So for a given amount of power dissipated as heat, the temperature increase of your chassis will *only* depend on how well it acts as a heat sink. This is regardless of how many components are used to generate the heat. Components heats air below chassis, air heats chassis, chassis act as heat sink, increasing its temperature until equilibrium is reached. Simple.

And ventilation holes will only help in a minor way here, unless you use really big ones. Hot air moves quite slowly through small holes, unless forced by a fan.

So try putting a hot 20W soldering iron inside a scrap chassis about the same size as your planned amp. [1] Use a flame proof surface, please. Close holes with more scrap metal as required, and leave the soldering iron there for a few hours. Then check chassis temperature, and you will have an idea of how hot the chassis will get at 20W dissipation. My suspicion is that it will be hot...

And the components inside a real amp would be way hotter still. They have to increase their temperature up to a point, where they can heat the chassis enough to allow it to work as the global heat sink.

You could also try to calculate how large a heat sink you would need, if, say, you wanted a temperature increase of 20 degrees C (35 F). That is a reasonable figure used if you want long lifetime of components, like electrolytic capacitors. Note again this is the temperature of the *chassis*, not the components and the air inside. They will again be hotter than this, which is why you want to limit yourself here.

20W/20 deg C means a heat sink of thermal resistance of 1 K/W. I happen to have one on my desk with double that value (when corrected for only a 20K temperature rise). It has 11 fat fins, and is 100mm (4") wide, 75mm (3") high and the depth (thickness of base plus height of fins) is 40mm (1.5"). One of these, when placed with fins vertically and in free air, including below (impressive how many forget this...), will dissipate 10W at a temperature increase of 20 degrees C. So you will need two of those. Or have to accept a proportionally higher increase in chassis temperature.

[1) Actually, don't. This is likely to ruin the soldering iron.
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Old 22nd February 2011, 04:47 AM   #10
cricha5 is offline cricha5  United States
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Wow, that is a lot of heat. Thanks for the great description. I guess I'll find a way to fit a heat sink in, or take the time to really learn the monkey design. As a last result, I can use a coupling cap and scrap the direct coupling, but I really wanted to do something different.

Wait a minute. Looking at a couple monkey amps, the cathode resistor still has to drop all that voltage. I thought they were a solution to the heat problem, but I think it only solves the blowing up problem. Do ALL direct coupled circuits have this problem?

Last edited by cricha5; 22nd February 2011 at 04:53 AM.
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