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Old 27th February 2014, 01:43 PM   #1
777funk is offline 777funk  United States
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Default Lots of craziness (weird noises and osc) happening in point to point 5W EL84 Amp

I figured with 2 tubes (a 12AX7 and an EL84), it'd be easy enough to just wire it point to point (physically from jack to tube to pot to next tube to OT). I'm getting some crazy oscillations with this thing. I can see it on the scope even after the first gain stage on the 12AX7. It's in a metal enclosure (with the lid off of course when probing). I have all the preamp grounds at one point (at the input jack) and the EL84's ground going to the first cap on the power supply (after the choke).

Curious if the reason for the oscillation is because there are parts and some wires 'floating' in the chassis. I did not use shielded cable but just ran the wires to tube grids right along the chassis floor. If I turn the volume up and down it sounds like an old sci fi film (wobbly frequency sweeps and almost sounds like an old radio being tuned).

I know how to follow a schematic, I know how to shape the tones I want if I need to make changes, I know close to NOTHING about dealing with interference so any advice would be appreciated! Hum isn't a problem by the way. Hum wise, it's pretty quiet.

Last edited by 777funk; 27th February 2014 at 03:05 PM.
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Old 27th February 2014, 02:32 PM   #2
wicked1 is offline wicked1  United States
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can you post a schematic and photos of the amp?
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Old 27th February 2014, 03:03 PM   #3
777funk is offline 777funk  United States
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Sure here's a quick and dirty edit to what I currently have.
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Old 27th February 2014, 03:15 PM   #4
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Its usually ground loops or oscillation on the power lines being fed back.
You need to keep any connections as short as possible.
Keep valves away from transformers.
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Old 27th February 2014, 03:22 PM   #5
777funk is offline 777funk  United States
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I have connections running to and from pots and directly to tube sockets. Layout wise with the chassis, this was originally a crate v5 am and it worked quietly so I know the tranny/tube spacial relationship is ok. This was originally on a 2 layer PCB with one layer being ground. Now it's wired free floating with the parts hard wired to the tube sockets and pots/jacks.
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Old 27th February 2014, 04:06 PM   #6
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Bypass every PSU electrolytic with a film cap. Value not critical, say 220nF. This is usually necessary to prevent oscillation in my guitar amps with multiple 12AX7 gain stages. In HIFI I use at least one big film cap in the PSU, preferably the last before regulation.

Electrolytics don't really shunt radio frequencies.

If this is a HIFI amp not an instrument amp, why use so much gain? Why two volume controls?
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Old 27th February 2014, 04:08 PM   #7
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Power lines were mentioned; I built one HIFI amp that was otherwise stable, but started howling sometimes when switching certain lamps on or off (so it was on the verge of oscillation all along).

Using an AC line filter is a good practice.
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Old 27th February 2014, 04:16 PM   #8
777funk is offline 777funk  United States
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This originally had one volume control but I liked having the two so I could balance how much it distorted (for guitar) between the 12AX7 and the EL84.

I notice however that if it's too loud, it sounds like something's out of bias (fizzy bad distortion) and also that I get much more oscillation.
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Old 27th February 2014, 04:36 PM   #9
kevinkr is offline kevinkr  United States
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I'd add a grid stopper resistor 1K or so right at the grid of the second triode and the EL84 particularly since there is a run of some distance probably to the grid of the output tube. (High transconductance types are very prone to oscillation)

Is the 470 ohm resistor right at the screen grid of your EL84 - if not move it.

Post a picture or two if you can, it should help to address layout concerns.

And finally since this is clearly a guitar amplifier I will move it per forum policy to the Instruments & Amps forum. You'll get lots of good advice both on the topology you've implemented and your specific stability issue.
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Old 27th February 2014, 06:14 PM   #10
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I don't see any obvious major problem with the schematic. Grounding must be all done in one place, not part at the input jack and part over at the power supply. I'd use a separate ground return wire for each section of circuitry. The whole circuit should have only one connection to the chassis. That means the input and output jacks should be insulated from the chassis, unless one of those are the one connection to the chassis for all circuit grounds. The earth wire of the AC line cord (green) should also connect to the chassis, but in a different place (ideally right where it enters the chassis).

Power supply bypass caps of roughly 0.1uF/600V+ are a good idea. The noisy pot could be that there is DC on the pot, which suggests that the cap feeding the pot is leaky or shorted. I'd put at least 1K R's on all grids (right at the tube pin). Going significantly higher in value than that can form a significant low pass filter but can also add hiss noise. Metal film R's have the lowest noise.

When the amp is first turned on, and none of the tubes have started conducting yet, the B+ will be higher than you might realize. Some caps may have had their max voltage spec exceeded.

I'm not sure what the 10 ohm and 0.1uF at the output are for. You never want to run a transformer coupled output without a load (the tranny turns into a spark coil and can blow up the tube and/or itself). The nice thing about the switching output jack is that you can wire it to switch in a 10 ohm 10 watt resistor in place of the speaker, when the speaker is unplugged. This doesn't guarantee safety if a loud signal is present, but is a good idea anyway.

Single ended no feedback circuits are among the best sounding IMO. Personally I'd have added a tone control section after the 2nd triode, and before the master volume control.

Last edited by Bob Richards; 27th February 2014 at 06:21 PM.
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