Is 3.5V too high for artificial heater taps?

[davesi]

Just measured my voltages on a pj to Spitfire conversion. The pj OT had no taps so I installed bridge rectifier and artificail taps on heaters. Overall voltages about 25vdc lower but heaters are measuring 3.5vac on all. I know this is getting close to the limit but will I harm my tubes?

Any advice appreciated

Dave

[davesi]

Uh that was pj PT

[Phil_S]

I think it is possible you are not measuring correctly. Set the meter to ACV. Place one probe on one leg of the heater supply and the other probe on the other leg. Get the reading "across" the winding, not one leg to ground. I'll bet you see 7V or there about.

[davesi]

That is correct.. 7VAC across or 3.5VAC to ground. Is this too high? Or should I consider dropping this a little?

[Phil_S]

I'm no expert, but the rule of thumb that I see around is that 10% under or over is OK. 7.0 is a bit more than 10% over 6.3, which is 6.93. I assume this is with the tubes in sockets. If you don't have tubes in sockets, the reading is bogus. Socket the tubes and measure again.

This happens with old transformers that have 115V primary ratings when plugged into today's 120+V mains supply lines or sometimes it just happens. You can try a dropping resistor (a 5 watter?), or you can try a small light bulb like a #47 to see if you can add a bit more of a load to the filament string.

If you use the amp a lot, the higher voltage will burn out the filaments a little faster, but I have no idea how much faster. If this were my amp, I might fiddle with it a little to see if I could get it more in line, but I wouldn't spend a lot of time on it. Your amp should run OK at 7V. In the bigger picture, this is fairly minor.

[Mikka]

What was the mains voltage when you did your measurements ?

Maybe could you wire tranny primary to a 115V or 120V from 110V or something like that. It should be better than a dropping resistor.

[Firestorm]

Put a couple of high-current silicon diodes "back to back" on one side of the heater supply and they'll drop the voltage nicely. See Merlin B's explanation:
http://www.freewebs.com/valvewizard/heater.html

[davesi]

Mains voltage at the time was 122vac. It is constantly over 120 every time I've measured. Measurements were with all tubes in, on each feed. I will look into dropping diodes. Do these need to be zeners, or will standard 1000v diodes be ok? Thanks for the link. I am pretty new at this and appreciate your help.

Dave

[Mikka]

Mains voltage at the time was 122vac. It is constantly over 120 every time I've measured.

That explain why your heater voltage is so high !

122 / 110 = 1.11

6.3 x 1.11 = 7V



So if you have the possibility to wire the primary of your tranny for 120V mains voltage, it would fixe it.

The Merlin's methode can work well but this is a plaster on a wooden leg.