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"Lacewood" Amp V2.0
https://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5736
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Author:  mwhouston [ 08 Jan 2020, 17:23 ]
Post subject:  Re: "Lacewood" Amp V2.0

john55 wrote:
My wife turned up with a 1947 Marconiphone radio the other day. She thought I might want to try to get it working.
I don't have much interest or knowledge in radio but decided to give it a look over. I managed to salvage the little mono amp that I can play with later, but as for the receiver..... I didn't have a chance (I don't know about the USA but here in Spain I don't think anyone broadcasts on SW / MW / LW anyway).
Then I looked at the case, dull brown bakelite but not a scatch or crack on it anywhere. A bit of soapy water and dasboard polish and look what I've got.
This is going to house the 6V6 and tone stack that I've been asking questions about in the last two or three posts on this thread. It might be a bit of a squash but I've got height as well as width and depth. Any thoughts?

Looks good.

Author:  john55 [ 13 Jan 2020, 12:22 ]
Post subject:  Re: "Lacewood" Amp V2.0

This chassis is a bit tight to fit everything, but that's all I've got if I want it to fit in the Marconiphone case.
I've kept the two 12AU7's and the 6SN7 as far away from the mains transformer as possible. Are they far enough away or am I asking for problems?
I would prefer to keep the 32uF+32uF capacitor on the top plate if possible but could move it underneath if the heat from the 5U4GB and the 6V6 on either side of it would cause problems. Asking for advice here as I have read that capacitors don't like heat.

Author:  john55 [ 24 Mar 2020, 06:34 ]
Post subject:  Re: "Lacewood" Amp V2.0

If nothing else, a four week lockdown concentrates the mind. I’ve been trying to get this finished for months.

Author:  Suncalc [ 24 Mar 2020, 17:44 ]
Post subject:  Re: "Lacewood" Amp V2.0

john55 wrote:
If nothing else, a four week lockdown concentrates the mind. I’ve been trying to get this finished for months.
John;

It looks superb! As always you workmanship is top notch. The integration with the case is really nice.

I'm here in the USA outside of Seattle, WA and we are just starting a 14 day lockdown to contain virus spread. I'm still working but saving a couple of hours a day in commute time. I really should put all that extra time to good purpose. ;)

Author:  john55 [ 30 Mar 2020, 11:39 ]
Post subject:  Re: "Lacewood" Amp V2.0

I noticed something strange when I turned off my latest amp (pictures above) today. The ammeters needles bounce instead of dropping smoothly. Could someone take a look at this short video and perhaps tell me what's going on. https://theleathershop.biz/page212.html

Author:  mike567 [ 30 Mar 2020, 14:34 ]
Post subject:  Re: "Lacewood" Amp V2.0

Possible "pop" sound when you turn off the power. Will cause power spike with gradual bleed off of the capacitors

Author:  Suncalc [ 30 Mar 2020, 16:14 ]
Post subject:  Re: "Lacewood" Amp V2.0

john55 wrote:
I noticed something strange when I turned off my latest amp (pictures above) today. The ammeters needles bounce instead of dropping smoothly. Could someone take a look at this short video and perhaps tell me what's going on. https://theleathershop.biz/page212.html
Your power switch should be behind the power transformer and PS filtering. As such, the currents should just slowly fall as the B+ fall while current is pulled off the PS filter. Does this happen on both channels or only one? If both, do the needles move in synchrony?

The power switch transition on the primary side of the power transformer should not cause this behavior. Do you have an As-Built schematic? We need to see details of exactly how the power supply was wired. Honestly, for the time being I am at a loss.

Author:  john55 [ 31 Mar 2020, 05:04 ]
Post subject:  Re: "Lacewood" Amp V2.0

I have wired the power supply as per the schematic on the previous page (dated 13th April 2019) with the exception of the 350 ohm 5 watt dropping resistor which I've left out (I always have slightly less voltage here in Spain). I have to do this with all builds.
My voltage readings are: 280v to output transformers, 250v to 6SN7 and 275v to tone stack pre amp. All readings taken with amp fired up and under load.
No "pop" when switching off.
It happens on both channels and the needles move in synchrony.

Author:  Suncalc [ 31 Mar 2020, 07:27 ]
Post subject:  Re: "Lacewood" Amp V2.0

Where exactly is the power supply bleed resistor connected and what is the value of the resistor.

Author:  john55 [ 31 Mar 2020, 10:34 ]
Post subject:  Re: "Lacewood" Amp V2.0

Matt, My understanding of a bleed resistor is a resistor in parallel with the cap or caps so I would expect to see it across the power supply outputs. I assumed the resistors in the schematic (20k and 2k7) somehow do the same job.

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