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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.

..Todd
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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?
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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.

Cheers,
Ian

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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Quote:
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.

..Todd
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Old 22nd July 2014, 01:48 PM   #25
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Default Reflected output transformer impedance

GingerTube your instruction about reflected impedance is very wrong. The reflection is relative to the output tap so, if using 8R speaker on 8R tap and you now wish to halve your output transformer primary impedance, you must use the 16R tap. This way the transformer 'sees' half it's expected load so the reflection gives half the primary impedance. Putting an 8R speaker on a 4R tap Robles the primary which is a sure way to blow the valve and/of the transformer. Using a reduced primary impedance is much safer for a valve than a much higher than expected impedance.
This often stated incorrectly with dire consequences for the uneducated reader.
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Old 23rd July 2014, 05:08 AM   #26
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And here is my 2c. Get a proper power transformer. That SMPS stuff is great for micropowered amps but will not supply continuos power to an El84 type amp, no matter what the manufacturers say on their spec sheet.

Experimentation is great, but I recommend going the tried and true route of using known good components, specially if this is your first build(assumption). Once you have a working unit, then experiment with new components, at least you know that if it doesn't work, you can always go back.

Good luck with your build"
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Old 23rd July 2014, 05:10 AM   #27
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Btw, I agree with Loopytoots.
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Old 24th July 2014, 12:54 AM   #28
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I would have guessed the EL84 would have red plated with no grid resistor.

On one of my builds I missed out the grid stopper resistor and that red plated pretty quickly.
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