6g5

[mike hadley]

The Philharmonic has two eye tubes, the expander has a 6e5, and the tuning uses a 6g5,

as the 6g5's are getting harder to find and there is an abundance of 6e5 tubes, both from Russia and Japan,

this makes a great is a temptation to fit one of these to the 6g5 socket,

yes the tube has the same pin out, yes the tube will work after a fashion, but the tube cutoff voltages and curves are different,

please see the chart below,

the charts show that the 6e5 has a cutoff of -8v and the 6g5 is -22volts, this problem can be fixed by adding a 1.8meg resistor to the grid input of the 6g5 to chassis, this will bring the cutoff voltage to within the 6e5 range, final trimming can be achieved with the control connected to the cathode,

the extra resistor is marked in green see the schematic below.

This does provide a solution to swapping the tube out, the only difference is the 6g5 grid curve is almost logarithmic where the 6e5 is closer to linear, there is noticeable difference in performance as the 6g5 has a more snappy action when operating, a small price to pay as the 6e5 tubes are in plentiful supply at the moment.

If you intend to fit the Russian 6e5s tubes the base will have to be adapted as these tubes are octal and not ux6 as fitted to the Philharmonic.

Mike

[David C. Poland]

I vote for using the correct 6G5/6U5. They are the same tube and interchangeable.

[Scott Seickel]

If the 6E5 works decent with the added resistor, why not.  It is easily reversible.   I would never go so far as to change to an octal socket for the Russian version.  

I have been successful finding 6G5/6U5's on E-bay that are NOS in the $40 range.  You just have to stalk the auctions. 

On the other hand, the 6J5G's are impossible for me to get NOS.

[mike hadley]

I find that the 6e5 with the added resistor works really well as I have two nos Japanese tubes fitted to my Philharmonic, I bought a number off of e-bay, they came to a little over $13 each
The tube is set to close on a strong local station, there is plenty of action on the shortwave bands,
I did my initial tests with a tired 6g5 tube as a benchmark, there is really not a lot of difference in action between the two tubes,
the extra resistor under the chassis is labelled with "6e5 mod", for future reference,
as Scott has noted this mod can be easily removed,
The Russian tubes would have to be fitted with an octal to ux6 adapter socket rather than hacking the loom, make the new part fit the chassis and not the other way around.
Mike

[David C. Poland]

That 1 meg resistor is usually hidden in the socket base. Tight fit.

Running another wire from pin 2 to ground be a challenge and a bit unsightly even if wrapped around the eye tube cable. Go for a NOS 6G5/6U5.  Try radio club meets, too. 

[Scott Seickel]

Did you fit a small 1/4 watt resistor in the socket  easily Mike? 

And on a different note, how is your power transformer doing on 50 Hz?  Does it get fairly warm? I believe the late transformers were painted black to help dissipate heat better, but I am not positive about that being the reason why.   

[mike hadley]

Well this is embarrassing, not only was the original schematic wrong, I put the resistor in the wrong place.

the extra resistor 1.8MΩ is connected between pin three, the grid of the eye tube and to chassis, making a voltage divider, the mod is easily placed under the radio chassis, on the small tag strip between the sensitivity diode and the first AF tube, the connection between the 3MΩ resistor and the wire leading to the eye tube grid is there.

I have pinched a little part of Scott Seickel's diagram, (thanks Scott), from another posting on this site, and revised this posting.

Scott, to answer your question, after checking through the IF alignment, I had the set running for over four hours, the transformer was running around 100f so I don't think that there is too much to worry about.

Mike