[R-390] Thrifty Tubes

[Perry Sandeen]

Yo Bubba Dudes!,
Bob kb8tq wrote:  summarized approximately, use some common sense and don't expect miracles.
His comments are good advice.
One could try to use a 6GM6 12,000 uMmhos in place of a 6BA6 4,300 uMhos in let's say the IF of a SP 600 which does doesn't have shielding in the band switch or between IF cans one could have bucket load of grief.
The tubes I mentioned, 6AB4, 6BY6, and 6JH6 all have relativity modest gain improvements over the OEM 6C4, 6BE6, and 6BA6 and are listed as substitutes in the tube manual.
When working for Hallicrafters from 67 to 69 I learned a lot about tubes from the *old timers*.Any tube manufacturer would house label tubes for you (for a price).
And it gets even more interesting.  At that time there were three large tube manufactures.  RCA, GE and Sylvania.  Since volume is the key to low cost they would have a I'll do this type and you do that arrangement. So the 6BA6 could have been made just one manufacture and then the unbranded tubes sent to the others.
Adding to the mix were manufactures like Zenith.  Now supposing the Zenith engineers wanted to use a 6BZ6 but the patent owner wanted more than Zenith wanted to pay.
For a moment let's assume it was GE or RCA.  Zenith would go to Sylvania and say we need a 6BZ6 spec't tube and we'll buy a million for a certain price.Sylvania engineers make a tube with the almost the exact 6BZ6 spec's and then call it a 6JH6.  Zenith and Sylvania are both happy.
That's why you'll find many tubes in a tube manual that seem to be almost exactly alike.
In 1959 I was dating a young lady whose father was an EE at the plant that made Silvertone TV's for Sears.  He told me that they had fierce arguments over which brand of resistors to use based on a cost difference of 1/10 of a cent per unit.  That's how cut throat the business was.
In the late 60's or so Phillips invented two nine pin frame grid pentode tubes. The 6EH7 was semi-remote cutoff with a gain of 12,500 uMhos. The 6EJ7 was a sharp cut-off pentode with a gain of 15,000 uMhos.  Both were low noise as well.
Everybody was using them in consumer products as well as the US military.  One use that i was told about from someone whom I made a purchase was in TV distribution amplifiers for motels.
John R Leary used the 6EJ7 as the 2nd mixer in his SP 600 mods.  QST has several articles about using the 6EH7 in ham gear.

As I was interested in using them in my B/A receivers I purchased a large lot of *good* used tubes.  They were house branded as Mullard, Sylvania, GE, RCA, Sylvania, Zenith et. all.

In his seminal *Competition-Grade R390A* Ray Osterwald, now editor of ER magazine, replaced the 6C4 triode mixer (3,100 uMhos) with 6HA5 triode (4,500 uMhos) as well as the 6JH6 for the IF tubes.
He made a spectacular conversion and had the numbers to prove the mods.
So high gain tubes have been used successfully in many B/A receivers but as Bob noted, it needs to be done carefully.
For those interested in more information on frame grid tubes see the articles in the Y2KR3. Ray Osterwalds mods are documented in the almost ultimate R390A ski on the R390FAQ site.
Regards,
Perrier