Filament voltages

[RPMac]

I'm using JJ 300Bs and measured across the filaments (pos-out on the board to the top of the resister pair)

Right 4.57V
Left  4.62V

Then measured across the .62 resister

Right .788
Left  .779

Plate

Right  101
Left    100

122VAC main in

Should these filament measurements be closer to 5V?

 

[Paul Joppa]

The voltage is a bit low because the JJ does not meet the WE 300B specification of 1.2 amps filament draw - the spec sheet says 1.3 amps, and the drop on the 0.62 ohm resistor agrees (1.27 amps). This is the nature of the filament bias used in the BeePre.

I do not expect this to be an actual problem - most tubes have enough excess emission to handle +/-10% filament voltage variation.

The theoretical risk is that, as emission falls near the end of the tube's life, the reduced space charge is less able to repel massive high-velocity ions that lead to cathode poisoning. However, the velocity of any such ions is greatly reduced by the low plate voltage in the BeePre - so I doubt it is a problem in real life. It's even possible that the reduced filament temperature will extend the life.

 

[RPMac]

TJ 3ooB/n

Filament
Right 4.67V @ 1.25A
Left  4.77V @ 1.23A

Plate
Right  131.5V
Left    114.5V

Is the right plate voltage a concern?

6A3

Filament
Right 6.04V @ .953A
Left  6.16V @ .926A

Plate

Right  100.4V
Left    102.4V

Is this good for the 6A3?

PJ, thanks for the detailed explainations. Hopefully I'm starting to learn.

 

[Paul Birkeland]

Man, those 6A3's are dead nuts on!

Does that high plate voltage follow one of your TJ's?  How much time are you allowing them before measuring the plate voltage?

With Doc B's pair of rather ancient TJ's, they come on and function immediately, but they take 3-5 minutes to drop down to proper operating voltage (which for them is ~110V).

-PB

 

[Chris65]

Does this confirm the 6A3's are a straight drop-in? Would like to have a set of back-up tubes & NOS appeals (plus the lower prices!).

 

[RPMac]

I'm listening to the BeePre with 6A3s and the 3b7 Stereomour you built...all DHT, all Bottlehead...can't get much better.

I measured the TJs twice...first time as quick as I could get measurements. The second time was probably 3-5 minutes. I put the tubes in the same sides, but I suspect the tube because the measurements were the same and the other tubes I've measured don't indicate a problem with the amp. My concern is if the high plate voltage will damage the amp. I also tried some 183 tubes (45s with 5V filaments) which checked good for the filaments, but the plate voltage was in the 130V range. The 183s have a higher plate resistance. I would like to give them a longer listen if they won't hurt the BeePre.

Chris, the 6A3s were plug and play.

 

[Paul Joppa]

The plate voltage will vary more than usual due to the fixed bias voltage. As long as it's less than 450 volts, it won't harm the tube! No seriously, as long as it's less than 130v there will be plenty of compliance for preamp output.

The 183 is spec'd for 25mA maximum (183B 30mA), so you should probably avoid them in a circuit that draws 60mA. The filament will die very quickly.