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Old 2nd January 2011, 07:06 PM   #1
Arnulf is offline Arnulf  Europe
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Default Guitar amplifier PSU hum woes

Schematic of PSU in question is attached below. Current draw per tap is indicated, as are measured AC ripples. Mains frequency is 50 Hz, I've measured the bridge diodes and they appear to be fine.

Do these AC ripple values make sense to you ? They seem to be awfully steep, considering the capacitances involved, it is almost as if some capacitors were duds

The problem I'm facing is hum which is apparently picked up in the input stage, guitar cable and so on. I cannot resort to GNFB just yet because I don't know what kind of insertion loss to expect from tone controls so I'm running things open-loop right now.
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Old 2nd January 2011, 07:21 PM   #2
M Gregg is offline M Gregg  United Kingdom
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Arnulf,

Normally the hum will reduce when the guitar is plugged in due to the low DC resistance of the guitar pick up. It is not uncommon for PSU caps to go dry or blow out in guitar amps.

Is it DIY built or what make is it.

Sometimes the reverb will become disconnected and this will cause pick up.

The other common fault is a bias problem or Leakey coupling cap.
What is each tap connected to?

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M. Gregg
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Old 2nd January 2011, 08:05 PM   #3
Zibi is offline Zibi  Poland
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100R after bridge?
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Old 2nd January 2011, 08:52 PM   #4
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Arnulf, Can I suggest you short the grid input of the first stage to 0V (to eliminate anything before hand from influence) and check hum. Then obtain a known good electrolytic of suitable voltage and capacitance ratings (eg. 100uF), and solder directly across first filter cap (with correct polarity and safety concerns), and recheck hum and measured AC voltage levels. I presume your meter is rated for measuring ACV only with high DCV.

Ciao, Tim
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Old 2nd January 2011, 10:02 PM   #5
Arnulf is offline Arnulf  Europe
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M Gregg: guitar is plugged in all the while. I wouldn't dare test the Oscillatingmachine without it

Zibi: yes, forming a LP filter with the following capacitor, and (in a way) "emulating" the "sag" of a vacuum rectifier. At least that was the plan ...

trobbins: amplifier topology is as follows: two cascaded common cathode stages, followed by a floating paraphase (= common cathode stage + inverting stage with full feedback), driving a PP pair of output pentodes. Volume potentiometer is inserted between 2nd common catode stage and the input of the phase inverter (= its common cathode stage).

With volume potentiometer turned all the way down there is esentially no hum at the speaker (whatever is left is merely a result of output tube disparity and isn't obtrusive).

With grid of 2nd common cathode stage (this is second stage after the input) shorted there is no hum at the output. Rotating the volume potentiometer will alter the speaker noise somewhat (it will be at its worst at some 50% of log pot's scale), but it doesn't sound like hum, more like some sort of oscillation. I can live with this, until I find out what *this* is and squash it. It's annoying but only if you hold your ear directly to the speaker so it doesn't bother me.

With grid of 1st common cathode stage (this is the input of the amplifier !) shorted any changing of the setting of the volume pot will change the level of hum as well, indicating that hum is coming from the 1st stage, or, in worst case, from second stage itself (potentiometer represents AC load for second stage). This is understandable, afterall the calculated gain of first stage is ~20, of second stage is ~20, of phase inverter (its common cathode stage) is ~40 and the gain of the output stage is just short of canceling out OPT transformation ratio (some 15-20x versus 25x).

The very first capacitor down the PSU chain is a known good one (pretty much, there's no certain thing in life than one's demise ). The rest are of unknown origin, it's reasonable to assume they are ~10 years old and of dubious quality.



Now what I'd like to know is whether the specified AC noise figures, measured at each lowpass R-C intersection, make any sense to you guys. If this first capacitor (a known good one) tames the ripple down to under 10 volts AC (= less than 5% of rectification ripple, DC ends up at 210-220V at its positive terminal), shouldn't all the follow-up capacitors reduce the ripple far more ? The ripple should have been negligible by the third capacitor, let alone fourth, which caters to both 1st and 2nd stage, yet I'm seeing almost 2V at the top side of 1st and 2nd stage anode resistors
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Last edited by Arnulf; 2nd January 2011 at 10:05 PM.
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Old 2nd January 2011, 10:30 PM   #6
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It sounds like your ACV meter is not reading correctly. As you summise, the ACV should be down in the mV level with that amount of RC filtering - possibly the first capacitor may be in the V range.

Given your hum interrogation, I'd suggest looking at the heater supply you are using for the first 2 input stages. Do you have a humdinger pot? Are you using DC elevated AC heaters, or are you using a DC heater supply?

Ciao, Tim
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Old 3rd January 2011, 07:06 AM   #7
Arnulf is offline Arnulf  Europe
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All heaters are AC. Output tubes have one side of their heaters referenced to the ground (this has reduced noise considerably). Referencing them to 90V didn't make any difference versus connection to the ground. Input tube (dual triode, encompassing both 1st and 2nd stage) has its heater floating, referencing it to any potential (GND or 90V) didn't make any difference.

I did try DC supply for the first two stages ("wall wart" with DC output + 1000 uF) but it didn't make any audible difference whatsoever.

I don't have any other reliable high voltage capacitors handy, I'll have to buy some and replace existing ones to make sure filtering is allright. I can't think of any other reason for the ungodly large AC swings at the indicated points apart from meter possibly being defective, as you pointed out. Stores should be open, I'm not sure whether I'm going to make it to one today though.
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Old 3rd January 2011, 08:03 AM   #8
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It sounds like you didn't use a humdinger pot, but just connected one side of the heater to ground, or a DC voltage. I suggest it is well worthwhile trying a 100 ohm pot across the heater supply, and taking the wiper to ground, or more preferably to a well filtered +V DC (> about 10V, and < 100V). There is usually a sweet spot on the wiper setting where hum is nulled, and it is typically a lower level of residual hum than just grounding a heater end or mid-point, or using a fixed humdinger resistor.
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Old 3rd January 2011, 08:33 AM   #9
Zibi is offline Zibi  Poland
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Old 3rd January 2011, 08:50 AM   #10
M Gregg is offline M Gregg  United Kingdom
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The ripple you have does not look correct it should be based on the load "current drawn", to have "loaded the capacitor" to give values as high as you show! You say the cap is good something is not right.

What is the volt drop across the 100 Ohm ? The reason I ask is becase if one of the capacitors down the line is going short it will increase the volt drop. Our old friend >>>(Kirchhoffs current Law).

---------
Does it have a spring delay line unit "reverb"
If it does check the connections.


Regards
M. Gregg
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