50A1/1L6

 


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Evaluation of a solid state (SS) 50A1

with the 1L6 

Line meter - Fluke 8022B

DC meter - Extech 22-816

Radio - Restored Transoceanic chassis 6A40  (see here for the chassis).

For Schematic click this link

 

Original/used 1L6 and SS 50A1

Line input (Vac) Measured DC across 700 (680) ohm R3 filament series resistor.  (Vdc) Calculated current

(ma)

1L6 filament voltage across  Pins 1 and 7

(Vdc)

Local Oscillator active on 18.2MHz
90.0 34.09 50.1 1.01 No
95.1 34.09 50.1 1.04 No
100.0 34.09 50.1 1.05 No
105.0 34.09 50.1 1.08 No
110.0 34.06 50.1 1.09 No
115.1 34.06 50.1 1.10 No
120.0 34.06 50.1 1.12 No

Table 1

 

New 1L6 and SS 50A1

Line input (Vac) 1L6 filaments voltage

(Vdc)

Local Oscillator active on 18.2MHz
88.2 (below spec) 1.11 No
90 1.12 Yes
100 1.18 Yes
110 1.21 Yes
115 1.22 Yes
117 1.23 Yes

Table 2

 

Sylvania Tube manual cut sheet.

scan0001.jpg (869688 bytes)  The Original 50A1 operated above 50ma from 59 to 54 ma for different voltages.

 

Current through the 1L6

 

There is a 100 ohm resistor parallel to the filament Pins 1 and 7 that divides the filament current.  Using ohm's law I=E/R the current through the 100 ohm resistor is 1.09v/100ohm = 10.9 ma.  That leaves only 39.1 ma for the tube filament.  Also the tube filament is rated at 1.4 volts.  A bit more current from the regulator would help to boost the 1L6 filament voltage.  This should help the tube oscillate at the higher frequencies around 18 MHz.  

The objective is to get 1.3 to 1.4 volts on the used 1L6 filament at the top end of the line voltage input.  Since the SS 50A1  has fantastic regulation, as reported in Table 1,  a bit more current allowed into the filament string should improve performance.  

As it turns out, below, a fresh NOS 1L6 and the solid state 50A1 perform just fine.  No drop out of the oscillator on the high short wave band at 18MHz.  However, I would like 

   

Additional data

These results are for the fresh new 1L6 and a fresh new 50A1 (D5TF30) ballast tube.

 

Line input (Vac) Measured DC across 700 (680) ohm R3 filament series resistor.  (Vdc) Calculated current

(ma)

Measured 1L6 filament voltage across  Pins 1 and 7

D5TF30

(Vdc)

Local Oscillator active on 18.2MHz
71 (below spec) -- -- -- No
90 33.7 .050 1.11 yes
95 34.1 .050 1.18 yes
100 34.6 .051 1.29 yes
105 35.0 .051 1.29 yes
110 35.4 .052 1.36 yes
115 35.8 .053 1.41 yes
117 35.9 .053 1.44 yes
120 36.3 .053 1.49 yes
125 36.8 .054 1.57 yes

Table 3

 

 

Solid State 50A1 and fresh New 1L6

Line input (Vac) Measured DC across 700 (680) ohm R3 filament series resistor.  (Vdc) Calculated current

(ma)

Measured 1L6 filament voltage across  Pins 1 and 7   SS50A1

(Vdc)

Local Oscillator active on 18.2MHz
90 34.08 .050 1.16 yes
95 34.08 .050 1.185 yes
100 34.08 .050 1.20 yes
105 34.08 .050 1.22 yes
110 34.08 .050 1.24 yes
115 34.08 .050 1.25 yes
117 34.08 .050 1.25 yes
120 34.07 .050 1.27 yes
125 34.07 .050 1.29 yes

Table 4

 

Conclusions:

The used 1L6 needs a bit more filament voltage, closer to the tube specification of 1.4 volts, to function on the high shortwave band of 18 MHz. The solid state 50A1 holds the current constant at a lower filament voltage than the used tube desires to oscillate on the high SW band.   It oscillates with increased filament voltage (observed but not documented).

The new D5TF30 (50A1) ballast tube has a current range from 50 ma to 54 ma.  

It appears that a new 1L6 works with reduced filament voltage on all bands and all frequencies with the new D5TF30 ballast tube and the solid state 50A1 ballast tube.

The solid state ballast tube works great with the fresh new 1L6 and has rock solid current regulation.

 

 

I have been in contact with the seller of these solid state replacements.  Ed is a courteous gentleman and has provided the following correspondence to be shared:

 

Hello Paul,

There is a chip resistor that set the current to about 50mA and a 1/4 watt axial resistor that is a trim that is selected in test to set the current between 50mA and 51 mA. On most devices this resistor is 1K. Current increase is about 1.2 / R Amps. Thus, if you reduce this resistor from 1K to 510 Ohm you will increase current by about 1.2 mA, 330 ohms will increase it to about 2.4 mA, 220 Ohms will increase it about 4.3 mA, etc.

I hope this helps,
Ed

Seller: eea101( 570Feedback score is 500 to 999)
Feedback: 100% Positive

 

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