AC Filament Voltage Question ?

[HP8903B]

I have 115V to 6.3VCT filament transformer.
The main voltage at my home is 124V AC.

When the filament transformer is connected to the main 124V AC and 6.3V filament tube, I get 6.8V AC.

How can I adjust the voltage ?

 

[Eli Duttman]

6.3 V. is nominal. 6.8 V. is well within 10% of nominal. Unless you are dealing with a very finicky type, you are OK with things as they are. 😉

 

[HP8903B]

That is good to know.

My amp uses AC filament for 6CJ3 rectifier and 7788 driver.

Both are 6.8V

 

[Ricdelros]

reduce the secondary of about 5 turn..i hope it will exact.

 

[HP8903B]

I didn't make these transformers. I can not adjust the winding of transformer.

 

[HP8903B]

The reason that I was concerned is the 7788 datasheet specifies 6.6V max.

 

[TubeHead Johnny]

For a voltage drop of 0.7V you can use two 6-8 Amp diodes connected in parallel (cathode to anode) in series with the filament circuit. But be warned they will generate a considerable amount of heat.

 

[Eli Duttman]

The reason that I was concerned is the 7788 datasheet specifies 6.6V max.

That sounds finicky to me. However, you still be OK. If you measured 6.8 VAC, with the filament winding unloaded, the voltage will come down, under load.

 

[HP8903B]

6.8V is loaded

 

[Richard Ellis]

Sounds like the P/N junction of a diode drop will clear this up just fine getting you 6.1V..........get a healthy current rating & PIV rating..lots of designers fine tune voltages this way.
PS, At 6,8V.....just about any PIV rating is gonna do.

_________________________________________________________Rick......

 

[Chris Hornbeck]

An alternative of course is just a plain old resistor. It has the advantage of inrush current limiting to a very expensive valve. Might matter, might not, but can't hurt.

All good fortune,
Chris

 

[12E1]

An alternative of course is just a plain old resistor. It has the advantage of inrush current limiting to a very expensive valve. Might matter, might not, but can't hurt.

That's my preference also. You can more easily fine-tune the voltage if you need, and as Chris says, the resistor will limit the inrush current when the filament is cold. (Since a cold filament has much lower resistance, the resistor will drop a much greater share of the voltage until the valve has warmed up.)

 

[tomchr]

For lower drop, use Schottky diodes. Or use a resistor in series with the heater winding. Some of the Vishay/Dale RH-25 series would work.

 

[bill_a]

Uhhh guys, that diode is going to half-wave rectify the AC... Is that really what you want?
Bill

 

[cnpope]

Uhhh guys, that diode is going to half-wave rectify the AC... Is that really what you want?
Bill

Back-to-back (cathode to anode) in parallel, with two diodes.

Chris

 

[jjman]

All 6 of my tube amps run around 6.8vac loaded.

 

[DavesNotHere]

use one of these and select the output to 110VAC :

Line-R 1200VA Automatic Voltage Regulator

 

[Midnightmayhem]

6.8 volts loaded is fine because you are probably losing .1v on each pin connection bringing it down to 6.6 across the filament which is within 10% Good enough for a power amp, for a pre convert to dc. You could drop a resistor in the circuit but i see no need.

 

[HP8903B]

Thank you guys

 

[multi]

I have frequently used both diodes and resistors; even for out put tubes, 811A's 4 amp heaters. Don't find they get very hot certainly not with a driver tube. You can easily work out the size of resistor to use with Ohms law. In Perth west australia the ac voltage is all over the place; supposed to be 240 volts but often rises to 255 volts or more.
Phil