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Getting voltage spikes at idle, with simple triode stages
Getting voltage spikes at idle, with simple triode stages
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Old 19th October 2018, 07:11 PM   #1
Elerion is offline Elerion  Spain
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Default Getting voltage spikes at idle, with simple triode stages

Hello everyone.

I built a simple preamp, using a single ECC88, and I'm getting voltage spikes at idle. Two common cathode stages, capacitively coupled. No bypass capacitors. As simple as it gets. Bias is fine. As designed. Supply voltage is 225 V dc.

I'm getting 20 V peaks at second stage output (anode), every 10 ms, with no signal input.

I see those same spikes (much lower amplitude, around 100 mV) at the input triode's grid. So they are just being amplified by the two stages.

I removed the triode from the socket, and I still get the spikes at the input.

Seem like charging spikes, at exactly 100 Hz. The power supply is just a bridge rectifier, reservoir cap (100 uF), and three RC stages after it. More than enough filtering. No center tap. Filament winding (no center tap either) use two 100 ohm resistors soldered to the ground bus.

Never happened to me before (or at least, never saw it)

Any ideas about where should I look? Thank you!
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Old 19th October 2018, 07:20 PM   #2
Lingwendil is offline Lingwendil  United States
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Getting voltage spikes at idle, with simple triode stages
Disconnect the filament supply, while measuring, and observe the scope. If it stops, you need to dress the wiring differently. Maybe you could also try lifting the heater supply to about 1/6~1/4 of the supply voltage? This reverse biases the internal heater-cathode diode properties, and can be useful in some situations.
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Old 19th October 2018, 07:24 PM   #3
waltube is offline waltube  Italy
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Put also a grid stopper on second stage of 88.

With this two stage you can have around 50 db of gain, if the use is as line preamp it is too high; consider the SRPP, the gain is low and Zout is reasonable low.

Walter

Last edited by waltube; 19th October 2018 at 07:27 PM.
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Old 19th October 2018, 07:36 PM   #4
Elerion is offline Elerion  Spain
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Quote:
Originally Posted by Lingwendil View Post
Disconnect the filament supply, while measuring, and observe the scope.
Just tried. It doesn't stop.

Waltube, thanks for the tips, but that is another story. The spikes are not related to the triodes.

By the way, it won't be a standalone preamp; probably the input stage of a power amplifier, using some GNFB. But that will come next.

By the way, if this info is of interest, this is how voltage varies at stage's supply voltage (yellow trace). It is not ripple. Look at the time base (250 ms). It seems ok to me.
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Last edited by Elerion; 19th October 2018 at 07:41 PM.
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Old 19th October 2018, 08:25 PM   #5
Elvee is offline Elvee  Belgium
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100mV at the input is like an elephant in the room, it's huge, and the cause shouldn't be difficult to find by making some tests and manipulations.

For example, disconnect one wire of the mains input, then the other one.

Check in your house if there is a phase-control supply active: the pulses are spaced 10ms, but they alternate in polarity, meaning the raw 50Hz has something to do with it, not the full-wave rectified supply. (It could even originate from your scope in a way or another)

Etc, etc. : try to be creative
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Old 19th October 2018, 08:32 PM   #6
JoeAlders is offline JoeAlders  Netherlands
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Did you try to supply the necessary supply voltage using a
stabilized power supply in stead of that supply circuit?
If you do that what happens then?
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Old 19th October 2018, 08:33 PM   #7
DF96 is offline DF96  England
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Carefully check grounding in the PSU. These could be charging pulses, although the swap in polarity is puzzling. Maybe some sort of magnetic induction from the secondary wires going to the rectifier bridge? These should be twisted to minimise effective area.
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Old 19th October 2018, 08:54 PM   #8
cerrem is offline cerrem  United States
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If you can put an AM radio up next to the circuit...
Tune the radio to a blank spot where there is no radio station...
Do you hear any hash coming from radio ???
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Old 19th October 2018, 09:20 PM   #9
hpeter is offline hpeter  Slovakia
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i´ve seen this when a "floating filament"
even a charging spikes on one winding can come trough to the another on power transformer
i guess you use cap input filtering....
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Old 19th October 2018, 09:25 PM   #10
DF96 is offline DF96  England
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Yes, could be spikes from the high voltage secondary getting onto the heater secondary.
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