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"junker" amp PS woes
"junker" amp PS woes
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Old 7th January 2018, 08:16 PM   #1
bluerooster is offline bluerooster
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Default "junker" amp PS woes

Building a Single ended 6l6 amplifier, from the junque box, and found that my filament transformer is putting out ~7 volts instead of the 6.3 that it should. Upon checking,
I found that mains are 127v RMS instead of 117. So, I guess I can run the filaments on DC and correct that. (probably less chance of "filament hum" anyway) No worries. But, the trouble comes in with the transformer for the rectifier supplying 6+ volts instead of the 5 volts needed. and it needs to be AC, as it also supplies the rectified DC+ for the HV supply. I guess I could install a couple of caps in the feed line, but there would be a significant drop in current.
Trying to get out of buying anything, and just use what I have on hand.

Any ideas?

Thanx
Shorty
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Old 8th January 2018, 03:02 AM   #2
PRR is offline PRR  United States
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Have you checked voltages *under load*? It would not be odd for 5V-6V windings to run a volt high un-loaded.

You can also simply stick a resistor in there. 6V to a 5V 2A rectifier, you need to lose 1V, 1V/2A is a half-Ohm, 1V*2A is 2 Watts. Use one 0.47r 5W or two 0.22r-0.27r 2W parts.

> what I have on hand

Or try several feet of thin/cheap speaker wire. 20 feet of #24 is around a half Ohm.

Or just ignore it. Tubes will last a very long time at the top of their rated heater voltages. Maybe by the time they quit, you will have more junque.
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Old 9th January 2018, 04:24 PM   #3
AndrewT is offline AndrewT  Scotland
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7Vac - 6.3Vac is close to using two diodes wired in inverse parallel.
This would leave a short period at zero crossing when zero current flows.
Would the sudden change from zero current to some current increase EMI?
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Old 9th January 2018, 04:48 PM   #4
kevinkr is offline kevinkr  United States
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"junker" amp PS woes
High mains are not uncommon, but at 127V I would verify that your meter is not lying to you. I have encountered a couple of cheap meters that read several % high.

U.S. nominal line voltage is 120V these days. (In the past it has been 110V, 115V, 117V, and 120V lol)
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Old 12th January 2018, 08:24 AM   #5
AllenB is online now AllenB  Australia
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I think this is an opportunity to use a series impedance for a slightly slower start. It may not be strictly necessary, but I'd gladly do it and be done with it. Personally I'd choose this over the DC option.
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Old 13th January 2018, 03:14 AM   #6
PRR is offline PRR  United States
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Quote:
Originally Posted by kevinkr View Post
High mains are not uncommon, but.... .... ...U.S. nominal line voltage is 120V these days....
My supply is reliably 125V at the street.

One day 2 summers ago it dropped to 124V at the street. It is that steady.

This is monitored on two $5 meters cross-checked with two very good meters (Fluke and Simpson).

At the house it runs lower to lots lower: 123 to 108. My wire is too long/thin. (I can know the street voltage by what I get with all loads off; and estimate with known minimal loads.)

AIR, my last house was steady 128V, lamps would die in a month. I called the company and acted scared, they tapped to 124V, and it was always 121V-124V after that.

Don't forget that 110V was really supposed to be 100V. As DC systems expanded, drop increased, and it became the custom to put 110V in so in busy-hour there would be near 100V left at important customers. This meant that much of the day was up nearer 109V-110V.
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Old 14th January 2018, 12:44 AM   #7
trobbins is offline trobbins  Australia
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Also need to check temperature of transformer has not increased significantly after an hour or so, as that could indicate that primary winding is starting to become stressed. There will always be some nominal increase in magnetising current, but the caution is if that is getting too peaky and if your mains voltage could sometimes go even higher (eg. middle of sunny day due to local PV generation).
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