6DE7 moduator plate voltage

[wa2tak]

I'm restoring a DX60B.
Got good power out.

Have no audio...now.

The schematics for the 60 and 60B show plate pin 1 having 800 on the 60..and 720 on the 60B.
Wow!

The tube data shows MUCH LOWER voltages...275...150.

What's up?

[DMOD]

I'm restoring a DX60B.
Got good power out.

Have no audio...now.

The schematics for the 60 and 60B show plate pin 1 having 800 on the 60..and 720 on the 60B.
Wow!

The tube data shows MUCH LOWER voltages...275...150.

What's up?

1) If it has not been modified (always check the wiring against the schematic) , check the HV B+ power supply voltage first as it should be > 600V at the top of R20 (a center tapped 30k).

2) Check the Function Switch (FS1) wiring according to the schematic.

The 2nd stage of the 6DE7 is a Cathode Follower which means the cathode pin 9 'follows' the voltage on the grid (Pin 2) without any phase changes in the signal. Pin 2 should be greater than 22V, usually 27V or higher.


In AM mode (no audio) your 6146 Screen voltage Pin 3 should be greater than 65V.

Check the voltages on the speech amp 12AX7A plates (pins 6 and 1) as well and let us know what they measure.

If any of the filter capacitor electrolytic's are original, replace them and check the values of R20, R18, R19, R25, R27, and R30.

Electrolytic C32 has been known to short or become leaky starving the 12AX7A of plate voltage.

Phil - AC0OB

[DMOD]

I'm restoring a DX60B.

The tube data shows MUCH LOWER voltages...275...150.

What's up?

On the DX60B the component numbers changed wrt the DX60 and the HVB+ supply has a different configuration.

If any of the filter capacitor electrolytic's (C39-C42) are original, replace them and check the values of R34, R35, R36, R37, and R25.

Electrolytic C33 has been known to short or become leaky starving the 12AX7A of plate voltage.

A bit of Theory: The overall modulation Method is called, "screen grid modulation." The first and second stage of the 6DE7 form a DC coupled Cathode Follower. A Cathode Follower (Stage 2) has a high input impedance and a low output impedance. The stage gain is about 0.85.

http://www.valvewizard.co.uk/dccf.html

In the Cathode Follower of the DX-60's, the plate voltage can be much higher than tube specs because a relatively low current passes through it. The max plate voltage it can sustain for short periods is 1500 volts. The tube dissipation is 7.0 Watts max. but the current through it is about 5 mA so the tube plate only dissipates about 3.5 Watts on the average at 650 Volts.

The second stage's cathode Pin 9 usually rests at about 105V when the Final's screen is about 65V. The 47k R27 and the 0.1uF cap C36 is a time constant circuit which forms the Controlled Carrier (CC) circuit. With audio applied, this circuit generates an average carrier output that varies with the audio level, instead of remaining constant as in conventional plate modulated systems.

A CC transmitter can be designed to modulate at approximately > 100% while automatically adjusting the average carrier level to accommodate the audio input signal. When the audio level increases the “average” carrier level increases as well. This action prevents over modulation. The commercial CC version of SGM was designed to improve operating efficiency since the amount of peak power available is double that of a conventional AM transmitter system.

In practice, unmodified ARS CC transmitters rarely achieve a modulation percentage of greater than 85%.

Phil - AC0OB

[wa2tak]

Very interesting!

Tnx! for the lesson!

So much for 'checking' tube voltages...esp when the tube data shows otherwise!
Big learning experience.

Now got audio....luv the smell of solder in the morning.

Rig is now up & running FB!...
...and it's looks like new.

[DMOD]

Very interesting!

Tnx! for the lesson!

So much for 'checking' tube voltages...esp when the tube data shows otherwise!
Big learning experience.

Now got audio....luv the smell of solder in the morning.

Rig is now up & running FB!...
...and it's looks like new.

What did you find that was preventing AM modulation?

If you are interested in improvements to achieve greater than 85% modulation, then I can post some circuits here.

Phil

[wa2tak]

No AM due to internal shorts in the 6DE7.
Simply replace it...got audio.
How good is it...don't know yet.