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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 31 Mar 2020, 12:36 
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Upon further contemplation, I am suspecting that you're getting some current reversal in the power supply filter as the various capacitors draw down and change the voltage profile across the 5H choke. I would put a ≈250kΩ 1/2W resistor across the 100µf capacitor following the choke. See if this modification tames the transient behavior.

john55 wrote:
I assumed the resistors in the schematic (20k and 2k7) somehow do the same job.
These won't provide a bleed function because of the DC blocking behavior of the filter capacitors. Bleed resistors need to be parallel the the capacitors in order to provide a current path to ground.

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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 01 Apr 2020, 04:28 
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Suncalc wrote:
I would put a ≈250kΩ 1/2W resistor across the 100µf capacitor following the choke. See if this modification tames the transient behavior.


Unfortunately, doing that has made no difference. Non at all.
I've made a twisted triple cable to the ammeters, from the chassis, two wires from the 392 ohm resistors and a common signal ground. The bypass caps still go directly to the signal ground. The twisted triple is over 2 feet long, could this have an influence on the ammeters?


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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 01 Apr 2020, 10:22 
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Would you advise trying bleed resistors on all of the power supply capacitors?


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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 01 Apr 2020, 10:25 
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john55 wrote:
Would you advise trying bleed resistors on all of the power supply capacitors?
No, that's really not required.

I am wondering from where the current reversal are coming. I'm not sure I understand exactly how you've wired in the meters. Can you post a simple sketch?

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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 01 Apr 2020, 10:31 
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There's one post above the last one Matt. Explains how I've wired the ammeters


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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 01 Apr 2020, 10:51 
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The three wires to the ammeters are a twisted triple over 2 feet long.


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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 01 Apr 2020, 12:36 
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Could be back EMF off the output transformer.

If it is then its because the DC B+ rail is dropping to fast.
But its just a guess. It would cause a POP in the speaker at power off.
There are other things it could be but do a few checks first.

Put a multi-meter on the B+ rail and watch to see if its a sudden off condition.
The only other thing I can think of at the moment is the screen supply dropping.
ie the tube shutting down.

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M. Gregg

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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 01 Apr 2020, 17:52 
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I know if I’m using a reg on the heaters if I don't put a shunting diode across the reg for when the power is turned off it will take out the reg. Nearly every time.

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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 01 Apr 2020, 18:25 
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M. Gregg wrote:
Could be back EMF off the output transformer.
I am inclined to agree with this assessment.

I don't have an octal socket extender but I may take some transient measurements off one of my UL amplifiers. I don't think that this is anything about which you should be worried. But it has made the Engineer in me curious.

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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 02 Apr 2020, 02:33 
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Have you tried using one gnd wire for each tube/valve return from the meters.
(4 wires instead of 3)?


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