Ferguson 391T Radiogram

[Murphyv310]

Hi.
I have one of these in for repair at the moment. It's been lying for 25 years. So far it's recapped and a couple of valves replaced, radio performance is excellent as is the Audio quality. The turntable was seized and is in bits. Photos to follow

[Murphyv310]

       

Hi.
Here is the radiogram in question. Since it's re-cap I have checked some of the high value resistors around the PCF80 AF amp and phase splitter and have found the values above 1.5m to be way off, the 3.3m was up to nearly 10m. after replacing them the performance was...... just the same
All we are waiting for is a new cartridge that is stereo LP compatible (mono) and the job will be complete

[Geordie McBoyne]

Hi Trevor,
Found the circuit diagram of the 391RG in the 1955/56 Radio and Television servicing book.
Pentode section of the PCF80 functions as the audio amplifier and the triode section is directly coupled from the pentode and serves as the phase splitter. Tone control is in the negative feedback loop.
I reckon this a very good amplifier.
Neat table radiogram.

Geordie McBoyne.

[ppppenguin]

Why the PCF80 TV tuner valve in a radiogram? Supplying 9V for the heater would have been an extra expense. Maybe Ferguson had loads of them around from TV production and it was more economic to make a 9V heater supply than stock the ECF80 or other small signal triode-pentode in small quantities.

[pwdrive]

Better heater/cathode insulation?

Lawrence.

[ppppenguin]

The cathode of the triode section is at about 60V because it's being used as a phase splitter. The cathode of the pentode is close to 0V. In its original TV application it would have been very close to the bottom of the heater chain. Just about any valve will take 60V between H and K.

Looking at the circuit (pp100-101 of R&TVS 1955/6) it looks like Ferguson provided an overwind on the heater winding to power the PCF80 heater. All the other valves are 6.3V.

[Murphyv310]

The pentode of the PCF80 runs in starvation mode, stage gain is high like the 3-3 so there is quite high negative feedback. The only drawback being AC coupling to the output valves. Not only is this radiogram loud but it's quality is very good. Can't understand why if they insisted on a TV valve they didn't use the ECF80.

[Geordie McBoyne]

Circuit diagram of the Ferguson 391RG push-push audio amplifier.  DC stabilising of the pentode amplifier is performed in a similar manner as the 3-3 amplifier.  Note the separate unbypassed bias resistors for the EL84s. I think if I was to construct this amplifier I'd add a DC balance control.

Geordie McBoyne.

[Murphyv310]

A balance pot is a good idea. I have to say the amp is good and I would say it has be around seven real watts output, bass response is excellent even with the original Acos crystal cartridge.

[Synchrodyne]

DC stabilising of the pentode amplifier is performed in a similar manner as the 3-3 amplifier.

A difference though is that in the Ferguson case, the pentode screen grid bypass capacitor is taken back to the (unbypassed) pentode cathode, not to ground.  But as the screen grid feed is taken from the unbypassed cathode of the triode, it would also be modulated with AF as well as carrying the desired positive DC voltage.  This AF would be antiphase to that on the pentode cathode, as seen on the other side of the screen bypass capacitor, so presumably the arrangement allows the desired DC feedback to the pentode screen grid without also providing a material AF feedback path.  In the Mullard 3-3- case such gymnastics were unnecessary, as the EF86 screen feed came from the bypassed EL84 cathode.

 
The use of the PCF80 rather than the ECF80 is hard to rationalize, given that the ECF80 was a common and abundant valve type, and had a 100-volt heater-to-cathode rating.  Possibly there was an element of chronology, in that in the 1950s Philips was inclined to introduce P-type TV valves ahead of their E-counterparts.  But the ECF80 was certainly used in some of the first crop of UK FM receivers, so it would have been available to designers during 1954 at least.
 
 
Cheers,
 
Steve