I sent this message to Barry, but his site says he stopped dealing... so here I'm with the same message:
a friend of mine bought a WF-55 a good while ago, and tried to build it, he's a naval architect and is not used to soldering and building kits so he failed. I do have some experience with electronics (learned 30 years ago) and did built a few audio amps. This WF-55 landed in my hands to "make it work".
I fixed the wiring errors, re-did the heather cabling better, re-soldered all dry points, and the amp is now working... almost. All tests measurements are within specs. However, I do have a solid hum in the output, without anything at the input. The hum rises with the volume. I verified the grounding, looks ok to me. Below find attached two scope dumps. One at half-volume, the other at full volume.
With a guitar it sounds decent (I'm not a player), but the hum is very disturbing, and in some forum the builders say it's dead-silent. So I think I must have something wrong.
First, output at half-volume, no signal at input:
First, output at full-volume, no signal at input:
the peak-peak hum is about 150mV, which is clearly audible. What annoys me more are the spikes. I verified the rectifier diodes (powered off, in-circuit), they look fine.
Would you have any idea what could cause these spikes ?
The distortion on the full-volume shot is as well strange. I could do more measurements, let me know what you think, and thanks for the advices !
Charles
Edit 8.12.2024 : the schematics is public on this URL. Sorry I should have added it.
a friend of mine bought a WF-55 a good while ago, and tried to build it, he's a naval architect and is not used to soldering and building kits so he failed. I do have some experience with electronics (learned 30 years ago) and did built a few audio amps. This WF-55 landed in my hands to "make it work".
I fixed the wiring errors, re-did the heather cabling better, re-soldered all dry points, and the amp is now working... almost. All tests measurements are within specs. However, I do have a solid hum in the output, without anything at the input. The hum rises with the volume. I verified the grounding, looks ok to me. Below find attached two scope dumps. One at half-volume, the other at full volume.
With a guitar it sounds decent (I'm not a player), but the hum is very disturbing, and in some forum the builders say it's dead-silent. So I think I must have something wrong.
First, output at half-volume, no signal at input:
First, output at full-volume, no signal at input:
the peak-peak hum is about 150mV, which is clearly audible. What annoys me more are the spikes. I verified the rectifier diodes (powered off, in-circuit), they look fine.
Would you have any idea what could cause these spikes ?
The distortion on the full-volume shot is as well strange. I could do more measurements, let me know what you think, and thanks for the advices !
Charles
Edit 8.12.2024 : the schematics is public on this URL. Sorry I should have added it.
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It's a Champ clone with a solid-state bridge rectifier: https://www.ampmaker.com/shop/wf-55/
Check that one the bridge diodes has not failed.
Check that one the bridge diodes has not failed.
Zoom in on one of the spikes, and see if it’s actually a burst of RF oscillation. A solid state rectifier with a transformer that has a high leakage reactance is a recipe for doing this. If it is, the transformer/rectifier requires a snubber.
Check that the grounding switch of J1 is actually grounding the input when no instrument is plugged in. Otherwise, I would look at supply point C for noise.
A snubber can’t hurt, even if ringing is not the issue. Ringing usually has more cycles to it, at higher frequency (narrower). This has a discontinuity in the “fundamental”, which suggests a mechanism with hysteresis. Because of level dependent behaviors with snap-back that I have seen, I’d suggest looking for h-k issues in the tubes. At least try to rule it out.
I'm applying your suggestions, one after the other:
- J1 grounding : 0.1Ω when disconnected (shorted by the connector mechanic), 1MΩ when open (R1), looks ok.
- replaced all 4 Uf4007 diodes.
- so far no effect, same noise on the scope.
- next step is testing noise on C. I need to pull out my differential probe, first time I will use it
- actually easy to use.... no noise on C, about 260V DC and if I can trust the 500x sonde, about 400mV of noise, no spike, no change when rising the volume.
- now to check if I have spare tubes of the correct / similar types
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I digged my stock and made a 100Ω / 100nF snubber, applied to the transformer HV output, the spikes are gone. so the main issue is solved, thanks for your suggestions! I still have about 180mV hum, which when playing is not disturbing, but still a bit annoying. What are common /acceptable values for this kind of amp ?
Attachments
While it does not seem significant, 400 mV noise at C could easily result in 180 mV at the output. Would need to run a sym to verify. I would look to reduce it via adding capacitance to the supply especially at C. This could be temporary as a diagnostic. The noise at C would not be dependant on the volume setting as you observed.
I tested adding 33 uF in parallel with the last PSU capacitor, C5. No change, but I was mislead by my differential probe, I think the ripple is much lower, barely measurable. I then shortened diverse wires, brought the grid resistor R2 closed to the pin 2 of the input tube, no change. I then measured the signal at the top of the volume pot VR1, and there I already have this weird signal, about 30mV peak-to-peak.
considering the two following stages, no wonder my output at full volume is very noisy. Now the question is why does this tube (12AX7) creates such a noise. Of course my lab is noisy with all sorts of switching PSUs, LEDs, etc, but that seems quite a mess. I tried another tube, same business.
Any hints ?
considering the two following stages, no wonder my output at full volume is very noisy. Now the question is why does this tube (12AX7) creates such a noise. Of course my lab is noisy with all sorts of switching PSUs, LEDs, etc, but that seems quite a mess. I tried another tube, same business.
Any hints ?
It's frustrating, considering what chatgpt says about such an amp...
Typical Acceptable Noise Levels
- High-quality SE guitar amps: Around 1-2 mV RMS of noise at the output. This corresponds to a low hiss or hum that is generally imperceptible during performance.
- Budget or vintage-style amps: May have up to 5 mV RMS, which could be more noticeable but is often considered acceptable for guitar amplifiers due to their "character."
I might suggest that R2 is a bit large in value. That makes it easier to pick up filament supply noise. Also check out the filament wire routing. Should be twisted all the way as close to the tube socket as possible. Also avoid filament wiring proximity to the grid or R2.
Not sure if it's still relevant , but the scope pics from post #1 look like typical heater buzz.
As the amp uses heater elevation, that's a bit surprizing.
Nevertheless you could try a cathode bypass cap of 25µ or larger at the input triode.
Some tubes are more prone to heater buzz than others.
But also check the heater supply wiring.
As the amp uses heater elevation, that's a bit surprizing.
Nevertheless you could try a cathode bypass cap of 25µ or larger at the input triode.
Some tubes are more prone to heater buzz than others.
But also check the heater supply wiring.
I tried a cathode bypass cap, no impact. I will rewire the heater supply. I'm in the middle of a move of my lab so I might get silent for 1-2 weeks. Nevertheless I'll go on with this issue, and meanwhile thanks to all contributors and ideas !