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Filement voltage Tolerence?

TSE 300B amp. I built it in two chassis, separating the amp and the power supply. They are connected with a 36" cable. I may have some voltage drop over cable length. On 300B filaments I have 4.85 volts. But on the 5842's I only have 5.62 volts. The amp seems to be working, and sounding just fine. Any ill effects from low filament voltage? Thank you all.
 
When you say you separated the amp and the power supply, what exactly does that mean?

Putting the transformers on a separate chassis shouldn't cause any trouble ( other than additional cost and inconvenience ) . Putting the HV and bias rectifiers, filters, etc., on a separate chassis probably wouldn't cause you any trouble ( other than additional cost and inconvenience ) because the dc currents are low and you can adjust for the small variance with the amplifiers adjustments.

Putting the DC filament rectifiers, filters, regulators, etc., on a separate chassis is asking for trouble because of the high DC currents involved. You can't really adjust the DC filament regulator up, because, IIRC, it has a max output voltage of 5 volts - so no headroom. That was one of the issues I ran up against on the TSE 801 diversion.

You could consider moving the 6.3 volts AC to the main board, and do all your rectification, regulating, etc., on it, although I think 4.85 volts should be fine for your 300B. The 5842's are a little low - there is a dropping resistor on the board you can change, I think, that could take care of this. My 5842's run at 6.0 volts and they seem fine at that voltage.

Win W5JAG
 
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See page 2; http://www.tubebooks.org/tubedata/we300a_b.pdf
The filament should be within 10% of ratings otherwise damage may occur. Too cold = electron stripping, too hot = burnt out filament.
The 5842s should have 6v3 but 5v67 is as low as I would go. 4v5 is the minimum for your 300a or b, so that is OK.

Western Electric didn't specify a tolerance that I could see on that datasheet.

When I was looking into this for my TSE 801 project, and I'll concede my "research" was admittedly casual, the tolerance rule of thumb, when not actually specified by a manufacturer, was plus or minus five percent, not plus or minus ten percent.

So, for a 300B 4.75 to 5.25 volts. He's okay there. For the 5842's ( they may have an actual spec - I haven't looked ) 5.985 volts - 6.615 volts. He's not Ok there, by that metric.

Win W5JAG
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Its trial and error really. If you already have 2.2 ohms in series then that suggests you probably have around 0.6 volts dropped across it at the moment. So it seems you are down to pretty much shorting it out to bring the voltage up.

Try a 1 ohm to start (or just parallel your 2.2 with another 2.2) and see what that does.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
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.
 
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.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
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.