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Oscillation in effect loop
Oscillation in effect loop
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Old 24th October 2019, 06:11 PM   #1
razorrick1293 is offline razorrick1293  United Kingdom
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Default Oscillation in effect loop

Hi people,

I not long ago built a 15w el84 guitar amp which for the most part works. my problem seems to be an oscillation in the effect loop recovery stage. the output to the speaker is 20vpp @15kHz.

with the effect loop taken out (volume pot going straight into the PI) there's no oscillation. with a mono jack plugged into the fx loop (recovery stage muted) there's no oscillation. with the mosfet stage bypassed (volume pot -> attenuator -> recovery stage) oscillation comes back. il send a picture of the layout, if anyone can spot some glaring issue i've over looked and schematic of the output stages are attached.

if anyone could help it would be much appreciated
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Old 24th October 2019, 06:16 PM   #2
razorrick1293 is offline razorrick1293  United Kingdom
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This is the layout
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Old 27th October 2019, 08:02 PM   #3
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by razorrick1293 View Post
...with the mosfet stage bypassed (volume pot -> attenuator -> recovery stage) oscillation comes back.
I notice there is neither a "gate stopper", nor a build-out resistor, in the MOSFET buffer stage. MOSFETs have much higher transconductance than valves, so they're even more likely to oscillate.

I suggest inserting, say, a 10k gate stopper (between MOSFET gate and the junction of the two 2.2M resistors), and a 1k - 4.7k build-out resistor between MOSFET source and the 100nF cap.

This may or may not turn out to be the source of the problem you describe, but IMO it should be done anyway.

If it's not the MOSFET, my first suspicion is the gain recovery stage, whose AC operating conditons are affected when you bypass the MOSFET; that changes the impedance it sees at it's grid. It's hard to see why it would oscillate, though, being a quite generic common-cathode inverting gain stage.

I also note that there are no grid stoppers in the LTP phase splitter. I suggest adding them, say 10k - 47k in series with each grid. An LTP circuit has one output that's in phase with its input, and even a little stray coupling between those two can cause oscillation. Leonidas Fender probably left these out to save a couple of pennies with each amp he built, but it makes no engineering sense to leave them out.


-Gnobuddy
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Old 28th October 2019, 07:54 PM   #4
razorrick1293 is offline razorrick1293  United Kingdom
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Thanks for your reply.

I added a gate stopper and build out resistor along with 2 grid stoppers between the 680k resistors and grids. It gave me a lot of hum with no oscillation and when I got rid of the hum the oscillation is still there. Shorting the 33k resistor before the mosfet caused a lot of hum, and plugging a mono jack into the effect loop intermittently causes hum when it's inserted/removed. I've checked my wiring and can see no cause for this.

Ricky
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Old 28th October 2019, 08:30 PM   #5
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by razorrick1293 View Post
I added a gate stopper and build out resistor along with 2 grid stoppers between the 680k resistors and grids. It gave me a lot of hum
Adding a gate stopper and build-out resistor should not cause hum. Something else is wrong. (And you really should put the gate-stopper, grid-stoppers, and build-out resistor back; they are needed.)

I notice your heater wires aren't twisted for a good inch or two near each tube socket. That big open loop of untwisted heater wire will inject lots of hum into nearby components. The attached image shows how to do it (and how not to do it.) Your heater wiring looks like the middle image, which is one of the ways not to do it.

There is a sticky thread on heater wiring that might also be useful: Heater Wiring - the Good the Bad and the Ugly
Quote:
Originally Posted by razorrick1293 View Post
Shorting the 33k resistor before the mosfet caused a lot of hum
Shorting this resistor should make the amp dead quiet. The fact that you hear lots of hum instead is another clue that there are other things wrong; likely either bad grounding layout, or bad heater wiring. It could also be the entire amp is oscillating (at too high a frequency to hear), and the hum you're hearing is an artifact of that.
Quote:
Originally Posted by razorrick1293 View Post
...plugging a mono jack into the effect loop intermittently causes hum when it's inserted/removed.
The word "intermittently" is another clue. I suspect your amp may be oscillating continuously, and bursting into hum at those moments when the oscillation is interrupted (such as by inserting a plug into the effect loop.)

What sort of diagnostic / test equipment do you have? Problems like this are really difficult to track down without an oscilloscope, as there is no way to see what's actually going on.


-Gnobuddy
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Old 28th October 2019, 08:38 PM   #6
razorrick1293 is offline razorrick1293  United Kingdom
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Thanks for the heater tip.

I never took those resistors out. I have a scope. I've probed the entire amp and the oscillation is only in the final stages of the amp. The hum being intermittent leads me to believe it isn't the heaters but that could explain some background hum.
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Old 28th October 2019, 11:18 PM   #7
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by razorrick1293 View Post
I have a scope.
That's good news!
Quote:
Originally Posted by razorrick1293 View Post
...hum...oscillation is only in the final stages of the amp.
You're dealing with (at least) two separate problems: hum, and oscillation.

Oscillation will mask just about every other problem, so it needs to be tackled and cured first. Once the amp is stable, you can then start to address the hum issues.

Okay, then. Let's focus on the oscillation issue.

Is it really only the final stages (output stage?) that oscillates? Try disconnecting the output valves from the phase splitter. Do you still see oscillation on the output valve anodes? (Remember that there's enough DC and AC voltage here to fry your oscilloscope. You'll probably need 100:1 'scope probes rated for a few kilovolts to keep everything safe.)

If the output valves by themselves are oscillating, find and fix the problem. Are the screen grid stoppers mounted right on the valve socket? Are the wires from the anode at a sufficient distance from the control grids? Are there control grid stoppers (your schematic doesn't show any)?

Incidentally, using much bigger than usual grid stoppers on the output valves makes the amp overdrive better, with less blubbering and sputtering and "choking out" (blocking distortion.) Instead of the timid 1k or 1.5k often seen, start with at least ten times that much: 10k - 15k is a reasonable minimum, and 100k - 150k is usually fine (but watch out for the total resistance from grid to ground, which shouldn't exceed the datasheet spec.)

Pentodes have very little Miller effect or input capacitance, so you can use some really big resistors here without affecting the amp's treble response.

Once you're sure the output stage isn't oscillating by itself, it's time to re-connect the phase splitter, and repeat the process. If oscillation re-commences, you now have to debug the phase splitter stage.

Incidentally: the final stages of the amp are where you have the largest AC voltage swings of all. Peak-to-peak signal swing at the output valve anodes can approach twice B+ voltage, so you might very well have 700 - 800 volts peak-to-peak there. Typically this enormous signal will run through several inches of wire to the output transformer primary winding.

Meantime, the input of a typical guitar amplifier will be sensitive enough to drive the amp to full output power with just a few millivolts of input; classic Fender amps often specify 20 mV input signal for full output, and those are low-gain, clean tone only, amps.

So imagine what happens if stray coupling or poor layout or poor grounding allows even a tiny, tiny, fraction of the 700-odd volts at the OT primary to leak back to the input? Your amp becomes an oscillator.

My calculator says that 20 mV is only one part in 35,000 of 700 V. It doesn't take much stray capacitance to cause that tiny amount of output signal to bleed back to the input.

In your photo, I see grey shielded cable (coax cable) running around a bright-blue bit of perfboard, and then terminated within an inch or so of one of the 9-pin valve sockets. Is that coax cable carrying a low-level signal from somewhere near the input of the amp? Is that 9-pin valve carrying a much higher signal level (I'm guessing it's an output valve)? If so, that wire shouldn't be routed anywhere near that close to the valve.

That same coax is also routed within what appears to be a quarter of an inch of some heater wiring. If the coax is carrying a low-level guitar signal, this could cause unnecessary hum.


-Gnobuddy
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Old 29th October 2019, 12:27 AM   #8
cobretti is offline cobretti  United States
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It is sometimes tricky to do a hum free layout. I learned my lesson on my first tube amp. The power transformer was too close to output transformer and I was getting hum through the magnetic coupling. I had to move the output transformer further away from power transf. and add some shielding between those two. Then the hum completely disappeared.
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Old 29th October 2019, 12:39 AM   #9
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by cobretti View Post
...The power transformer was too close to output transformer and I was getting hum through the magnetic coupling...
I briefly owned a Fender Blues Junior, and it had the same problem. It was a rather weird experience to pull the output tubes, and still hear hum in the speaker!


-Gnobuddy
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Old 29th October 2019, 08:13 PM   #10
razorrick1293 is offline razorrick1293  United Kingdom
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Well I pulled the power valves and the oscillation went away. I have a suspicion there's interaction between the wires on the way to the transformer and the mosfet input. However when I turned it on after changing the Penrose grid resistors from 1.5k to 10k (Sorry forgot to draw them) I had a smaller oscillation with guitar signal coming through, 5 seconds later all sound cut and was replaced with loud hum. I think I'm going to get a new chassis and rebuild this amp. With improved layout. Quick question. Can an output transformer be next to a power transformer at 90 degrees? Or should there still be space?
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