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Old 12th June 2012, 04:41 AM   #21
taj is offline taj
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A couple things catch my attention...

There's no decoupling network(s) after the power supply. With that DC-DC converter you're using, I haven't got a clue whether it's still necessary, but it's certainly unusual to see. Could there be a high frequency AC component from that module getting through to the B+ or ground causing instability?

Also, the coupling caps are substantially larger than is typical for a guitar amp. The typical value is .022 (22nF) or thereabouts, which would block a lot more of the low freq. spectrum, possibly curing the motorboating. You generally don't worry too much about about LF fidelity in a guitar amp, though it is a taste thing.

Lastly, the operating point you're using for the input triode is unusual. Not bad though, I've just never seen those values used before (which doesn't mean too much really). I'm curious if you copied them from a known amp design? or how did you decide to use those numbers? Just curious, that's all.

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Old 12th June 2012, 05:42 AM   #22
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Something oscillates non-stable: either load disrupts oscillations in your SMPS, or your amp oscillates due to some parasitic feedback (through ground wire, or coupling between input and output).
Do you have oscilloscope?
If I disappear suddenly, that means I finally created a time machine and pushed wrong button that brought me to Stalin's Russia. In any experiment any result is the result. Even if it is negative.
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Old 12th June 2012, 07:20 AM   #23
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The cathode bias resistor on the input stage (470 Ohms) needs to be 1K to 1K5.
The output Tranny is unsuitable, 10K primary, you want about 3K5 for Triode Mode EL84

Triode mode EL84 is the way to go as you don't have much current capability from the SMPS.

As stated above the volume pot on the input should be 500K log not 10K and you need to have that grid leak resistor 470K from the 5K6 / 0.1uF junction to 0V. I would also reduce that cap to 22nF but that can wait till its going.

The EL84 cathode bias resistor (470 Ohms) seems a bit high. try 270 Ohms once it is up and running.

HINT: Class A Triode mode EL84 is normally run at 250V with a 270 Ohm cathode bias resistor into a 3K5 primary impedance Output Tranny. For that set up EL84 idle current is 34 to 36 mA.
At 200V you would normally want a bit lower primary impedance in the output tranny (say 3K) and a slightly lower EL84 cathode bias resistor to run more current (in this case that (reduced bias resistor) might not be a good idea due to the limited current capability of the SMPS.

Add a 47uF/350V capacitor directly across the output of the SMPS. I am guessing that the SMPS is shutting down and starting up over and over , which is the "motor boating" you are hearing. This cap should stop that happening. That is Wavebourn's point in the post above.

If using an 8 Ohm speaker then connect it to the 4 Ohm tap so that it reflects an anode load (to the EL84) of 5K instead of 10K and 5K will be close enough to the ideal 3K to 3K5.

That should get it going!!! - Best of luck with it.


Last edited by gingertube; 12th June 2012 at 07:23 AM.
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Old 13th June 2012, 04:18 PM   #24
taj is offline taj
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Originally Posted by gingertube View Post
The output Tranny is unsuitable, 10K primary, you want about 3K5 for Triode Mode EL84
It looks to me like 3K5 would work pretty well for pentode mode too, when using Vg2=200.

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