Lots of craziness (weird noises and osc) happening in point to point 5W EL84 Amp - Page 2 - diyAudio
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Old 27th February 2014, 06:18 PM   #11
M Gregg is offline M Gregg  England
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A few things I would try,

Disconnect C23 from ground just to try it.

Put 10K resistors in series on the input (grid) to 2nd tube and power tube right on the valve socket.

The heater resistor CT what value are the resistors? (2X 220 Ohm would do it)
what's the PSU circuit like?

If the above has no effect move all the grounds to the negative of the first cap on the PSU.
Make sure the input ground is isolated from the chassis and only grounded at the same point.

M. Gregg

Last edited by M Gregg; 27th February 2014 at 06:37 PM.
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Old 27th February 2014, 07:44 PM   #12
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One thing to keep in mind with any amp circuit is that you want to keep any high impedance nodes, and/or input circuits, as far away from output or power supply wiring as is practical, to minimize the probability of the circuit oscillating. Also, keep all leads as short as is practical. Especially with high impedance circuitry (grid circuits in this case), parts should be as close to tube pins as is practical. Stray capacitance and inductance is not a good thing.

Also, put a passive Rf filter at the input (maybe a 10kohm R in series, and a 330p ceramic or mica cap to gnd., which will give you a -3dB point at 24kHZ if the source Z (guitar pickup) is about 10kohms - which it is on a strat and I think most guitars).

Also a 0.01uF 3kV ceramic cap across the AC input, or better yet from each of the hot and cold AC connections to the earth gnd which connects to the chassis.

A guitar makes a nice antenna for Rf, and any audio amplifier is highly likely to act as an AM detector for those frequencies, which will result in audio band noise and distortion (not the good kind), especially if you're in a place that uses light dimmers, or any switch mode electronics.

Last edited by Bob Richards; 27th February 2014 at 07:50 PM.
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Old 27th February 2014, 10:14 PM   #13
Enzo is offline Enzo  United States
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Does turning the volume down turn the oscillation down or otherwise affect it? Does pulling the preamp tube kill the oscillation? Poke around with a wooden chopstick while it is doing this, does moving anything affect the volume or tone or frequency of the oscillation?
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Old 28th February 2014, 12:20 PM   #14
teemuk is offline teemuk  Finland
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I'm not sure what the 10 ohm and 0.1uF at the output are for.
Zobel network. The capacitance counteracts loudspeaker's increasing inductance at higher frequencies.
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Old 28th February 2014, 05:09 PM   #15
777funk is offline 777funk  United States
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Loads of good information here! Noise and hum have always been kind of a mystery to me.
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Old 1st March 2014, 07:32 PM   #16
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The Zobel won't hurt anything, but is generally only needed when the amp is using negatory feedback. If negative feedback gets shifted in time (phase) by the reactance of the load (speaker), then it may cause the amp circuit to oscillate spuriously or worse.

For hum problems, all gnd connections need to come back to a "star center" gnd post (bolt is my rec). The exceptions are the AC cord green "earth" wire (tie it to the chassis in a different place), and the wiring between the power tranny secondary ground connection (often the center tap) and the large filter caps of the power supply. You want to connect that secondary gnd connection to the filter caps separately, and then run a wire from there to the "star center" location for the entire rest of the circuit. The idea is to keep those high current surges away from the rest of the gnd. connections. The circuit ground must only tie to the chassis in one place, so any I/O connectors should be floating.

The early guitar amps didn't do this right; there were ground loops from non-floating 1/4 phone jacks, and no Rf filtering anywhere. They hummed, squeaked, sputtered and didn't need to.

For noise issues, use all metal film R's in the early stages and anywhere the R value will be over a few kohms. Those who think metal films sound metalic are fools. The hiss that carbon films or comps make have a psychological effect that makes some people think they sound more or less metalic. The only other noise of any real significance is from Rf being demodulated (AM detected), or from the tubes themselves - some tubes are significantly noisier than others, even of the same tube number.
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Old 2nd March 2014, 03:56 PM   #17
mike567 is offline mike567  United States
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Your Schematic says your heaters are artificially center tapped to ground thru 1 ohm resistors? Seems to put a lot of load on your heater windings. usual value is 100 ohms
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Old 2nd March 2014, 06:20 PM   #18
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There is a spec for heater to cathode max voltage, which many designs ignored. It's usually around 100 volts. It's important that the heaters are somehow referenced to ground, so they can be reliably referenced to the cathode. In the case of a follower topology, the cathode will be sitting up around 1/3 to 1/2 of B+, which can cause this spec to be exceeded.

I make a power supply filter cap bleeder resistor that is split giving me a roughly 90VDC tap, which I tie to one side of the heater supply, with an AC shorting cap off that tap to gnd. If you're running the filaments on AC, use at least 18AWG single strand wires, and twist them fairly tightly so the electromagnetic field they emit largely cancels itself out. Keep that twisted pair as far as you can from any high impedance wiring.
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