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Hallicrafters SX28 - LF Amplifier & Power Supply

This page is dedicated to describe and understand the SX-28's LF-amplifier circuit.

Having a look at the original diagram from Hallicrafters, it was not so clear to me what kind of circuit it is. So I have re-drawn it in a different and more understandable way.

This is the original Hallicrafters diagram   and this my redraw of it:   

The two triodes within V12 are configured as voltage amplifiers providing two phase opposite input signals to the push-pull power tubes.

Let’s examine the first stage working conditions. The following table compares the conditions as required by the 6SC7 specs and the value required by the Hallicrafters manual. By the diagram we can see as the voltage supplied to the triode circuit is limited and stabilized to 200Volts by R38 and C44.

Parameter

6SC7 specs

Required by SX-28 manual

My rig

Plate current (mA)

2 (max)

0.7 (read text)

Not measured

Cathode bias (Volts)

-2

-2

-1.7

Plate voltage (Volts)

250 (max)

130 

127

Control grid (Volts)

0

0

0.01

Plate load line:

to define a line I need two points;
as reported by the SX-28A manual (TM 11-874 @ page 34), the specified working conditions are: plate voltage of 130V and bias (V12pin6) –2Volts. Looking at the 6SC7 characteristics this condition is not possible with a 100K load resistance as @-2 volts in bias we should have a 150 Volts plate voltage. Working graphically it seems that 1,5Volts is a reasonable bias value to have 130Volts as plate voltage; under this condition the plate current results 0,7mA, so the first point is: (130Volts,0,7mA). B+ for the tube is @200Volts, giving us the second point of the line (200Volts, 0mA) so here is the correct working conditions. Measures in my rig confirmed the values graphically derivable from the diagram (plate voltage ranging from 95 and 165 volts) - but only at very low frequencies. As soon as the frequency increased over the 50Hz value, the voltage range dropped significantly. It remains to be verified if the dropping is compatible with the bandwidth diagram given in TM 11-874 manual….

 

The push-pull working condition is AB1. The working condition chosen by the designers is the one highlighted  on this 6V6GT specifications. Those values exactly match the SX-28 specifications, the output choke specs as well as the voltages required by the SX-28 maintenance manual.

Here are the figures:

Parameter

Required

Measured my rig

Plate current

35 mA

32 mA

Cathode bias

15 Volts

14.5 Volts

Plate voltage (Vs Cathode, not GND!)

250 Volts

250 Volts

Screen grid

250 Volts

264Volts

Control grid

0 Volts

0.014 Volts

 

XXX          construction ahead          XXX      12/3/2001 

load dynamic line:

to define a line I need two points;
the specified no signal working conditions are: plate voltage of 280V (a little less in SX-28As..) and bias @ –15Volts. Reading the plate characteristics we get a plate current of 35mA. The first point is: (280Volts,35mA).
With a dynamic load impedance of 10K plate-to-plate each tube sees a 5Kohm load resistance. With a 35mA current the voltage drop at the load resistor [EG1] is 35mA*5Kohms=175 Volts, meaning that the B+ has to be (250+175)=375 Volts to compensate the dropping.; this gives us the second point of the line (375 Volts, 0 mA) ; by connecting the two points, it is possible to trace the dynamic load resistance line for this power amplifier. Such a diagram should have been on the designer shelves for a while, back in the forties…

With regard to the output transformer, we can say that its primary must have an DC resistance of 500 ohms and 10000 ohms as impedance plate-to-plate. Its windings ratio should be given by the formula: n=SQRT(Rl/Ro) where n is the ratio, Rl is the required load impedance as per the tube specs and Ro is the output (speaker) impedance. This means that the original choke should have n1=SQRT(10000/500)=4.5  and n2=SQRT(10000/500)= 1.4  for the 500 and 5000 ohms outputs. If you want (or need) to substitute it with a 8 ohm output, the formula becomes: n=SQRT(10000/8)= 35 (it can be challenging to build such a choke in the same space as the original one… many people report Hammond has made such a transformer for their guitar amplifiers…).

XXX          END of construction          XXXX

Here is an excerpt from the user manual (as you can find @ bama) describing the LF-amplifier and the power supply;



Waiting for the net to clarify:

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 [EG1] For this computation it is allowed to consider the dynamic load as a simple resistor…