Need help: Aa driver not passing AC signal

[banpuku]

Hi. My Aa driver tube is not passing AC signal to the coupling cap. I believe it might be caused by the LED bias which is measuring positive voltage (+). Here are the measurements:

Vac at Grid: 50mV
Vdc across filaments: 3.8Vdc, 0.5A
Ua = 160Vdc
Bias = 2Vdc (note, this measures (+) Vdc, not (-)).
Vac at Plate: 0mV

I have tried 2 different NOS Aa tubes. This does not resolve the issue.
I am curious about the bias. Using an LED. The + leg (long leg) is attached to the + filament pin. The - leg (short leg) is connected to ground. Yet, the bias measures positive (+). I thought this should have measured negative (-).

Anyways, no Vac coming from the Aa tube. Schematic and Aa datasheet attached.

Thoughts?

SOLVED: See #2 below.

 

[gerrittube]

You short the AC (audio) with your 30 uF capacitor. Try without this capacitor!
And connect your 70 K resitor to B+ instead of ground. You have no plate voltage, so no amplification.

Regards, Gerrit

 

[banpuku]

I love DIYAUDIO.COM. Always a helping hand available.

Gerrittube: Yes, this was the issue. Thank you!!! It works now.

Modified schematic attached.

 

[56085]

The grid resistor of the EML50 being 5K, the AC load of the Aa is only 4667 Ohm. I think it's better to use a higher value for that grid resistor, like 220K (the datasheet states 250K as the maximum value).

 

[gerrittube]

Glad it’s working now. And I agree with PFL200, a higher value grid resistor makes sense too.

Regards, Gerrit

 

[euro21]

If DC voltages are real ones in #3 post, the tube anode current is about 4mA, which is really false data.

At 160V, -2V the estimated current is 1 or 1.1mA (see #1 post curves), so anode resistor must be:
(450V-160V)/1mA= 290k.
(450V-160V)/1.1mA= 264k.

BTW are you read Ale's web site?
Aa tube – Bartola(R) Valves

 

[rayma]

I agree with PFL200, a higher value grid resistor makes sense too.

Yes, it should be 220k or 250k. The amp then will have much more output and better bass.