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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 28th April 2016, 01:56 AM   #11
Guarnera is offline Guarnera  United States
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I replaced R3 with a jumper. Now I get 6.93 volts. I have some 3 watt, .75 ohm, 1.1 ohm, and 1.5 ohm resisters coming from Mouser. Probably be here tomorrow. I'll see if any of these work.
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Old 28th April 2016, 11:57 PM   #12
Guarnera is offline Guarnera  United States
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My resistors came today. R3 was 2.2 ohms. I put a 1.1 ohm resistor there and now my filament voltage for the 5842's is 6.36 volts. Great!
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Old 29th April 2016, 08:35 AM   #13
Mooly is offline Mooly  United Kingdom
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Filement voltage Tolerence?
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Originally Posted by Guarnera View Post
I put a 1.1 ohm resistor there and now my filament voltage for the 5842's is 6.36 volts. Great!
=

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Originally Posted by Mooly View Post
....or just parallel your 2.2 with another 2.2 and see what that does.
Pleased to hear you have it sorted that's great.
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Old 29th April 2016, 04:51 PM   #14
Guarnera is offline Guarnera  United States
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Thank you Mooly. You have been a big help, and I am grateful. Now I need to sort out why my B+ voltage drifts? But that's on another post.
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Old 29th April 2016, 06:14 PM   #15
Mooly is offline Mooly  United Kingdom
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Filement voltage Tolerence?
You're very welcome

B+ drifting (and I don't even 'do' valves normally lol, why am I here ). Depends how much its drifting. Possible suspects could be mains voltage fluctuation. That can be more than you think on a supply (the mains supply that is) of marginal capacity. Many years ago now and we had an issue with excess volt drop. Loading the supply to near its maximum (240vac nominal, 100amp) pulled the voltage down to around 217 volts.

Imagine that change reflected in an HT rail.

Long term drift could be bias points changing and various stages pulling more or less current.
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Old 24th May 2016, 01:39 AM   #16
Guarnera is offline Guarnera  United States
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Mooly,
You are right. The B+ drift has been traced to drifting AC line voltage. The amp should be running a B+ voltage of 365 volts, with 120 volts AC line power. My AC line starts out as high as 124 volts in the morning and has dipped to as low as 120.5 volts in the evening. This 4 volts extra in the mornings gets me starting with a B+ of 380 volts. Most of the time I'm at about 375 to 377 volts. One night it got down to 120.5 volts AC, and my B+ was 365 volts.
I'm ordering a 120 volt regulated UPS, to give me s steady 120 volts AC.
The other strange thing that has happened is that my 300B filament voltage started out at around 4.75 volts. I figured I had some voltage drop along the umbilical cable, since I built the power supply in a separate chassis.Then some time into breaking in the tubes it upped to 4.88 volts. Yesterday at about 250 hrs on the amp I noticed the 300B filament voltage was 5.04 volts. Today its 5.09 volts. I'm thinking maybe its the cheap Chinese DMM I have on the filament voltage. I think I'll change it to a good Fluke meter.
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Old 24th May 2016, 02:05 AM   #17
PDL is offline PDL  United States
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I wouldn't worry about a 3% change in voltage. Adding an UPS will likely inject more noise than anything else. Back when tubes were the norm, tolerances of plus/minus twenty percent were common.

What do you expect to gain from adding an UPS?
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Old 24th May 2016, 04:31 AM   #18
Guarnera is offline Guarnera  United States
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What do you expect to gain from adding an UPS?


A steady regulated 120 volts AC.
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Old 24th May 2016, 07:37 AM   #19
Mooly is offline Mooly  United Kingdom
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Filement voltage Tolerence?
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Originally Posted by Guarnera View Post
The other strange thing that has happened is that my 300B filament voltage started out at around 4.75 volts. I figured I had some voltage drop along the umbilical cable, since I built the power supply in a separate chassis.Then some time into breaking in the tubes it upped to 4.88 volts. Yesterday at about 250 hrs on the amp I noticed the 300B filament voltage was 5.04 volts. Today its 5.09 volts. I'm thinking maybe its the cheap Chinese DMM I have on the filament voltage. I think I'll change it to a good Fluke meter.
Is it possible that the filaments permanently increase in resistance a little as they age in use ? Because of the finite source impedance of the filament supply, any change in resistance would reflect in a different current draw, and that would show as a rise or fall in the overall filament voltage.
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Old 24th May 2016, 09:29 AM   #20
Guarnera is offline Guarnera  United States
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It was, as I expected. The cheap Chinese DMM changed its reading. I checked it with a good Fluke meter and it is still 4.8 volts.
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