Drake T4XB/R4B Info?

Steve Harrison ko0u at OS.COM
Wed Dec 9 00:00:06 EST 1998

At 04:17 PM 12/8/98 -0500, Stan Laevens wrote:

>a) The RF amplifier tube in the R4B has been switched from a 6BZ6 to a
6HS6.The manual, schematic and chassis label all state that a 6BZ6 should
be there. When I returned the 6BZ6 to its proper place the receiver did not
perfrom as well. Was this a common change that some R4B owners liked to
make? In this particular unit the 6HS6 provides much better sensitivity on
the higher bands but at the expense of distortion.

Replacing the first RF amplifier tube with a 6HS6 has long been de rigeour
for many owners of receivers using tubes of the basic 6AU6 class; the 6AU6
and 6HS6 have the same base pinout, with the suppressor grid on pin 2 and
the cathode on pin 7. The 6BZ6, on the other hand, swaps pins 2 and 7. Very
often, the suppressor grid is grounded directly while the cathode has a
bias resistor to ground (sometimes, the suppressor grid is connected to the
cathode on the socket). Thus, the 6HS6 might draw higher cathode current
and thus run physically hotter than the original 6BZ6 since the cathode
would be grounded with the resulting loss of cathode bias. Because the
suppressor grid current is almost zero at all times, the presence of the
originally-small cathode bias resistor in the suppressor lead probably has
negligible effect.

But the real difference between the two tubes is that the 6BZ6, being a
semi-remote-cutoff pentode, changes gain relatively slowly with a change in
the grid bias (that is, with application of AGC) while the sharp-cutoff
6HS6 changes gain much more quickly. In fact, while the 6BZ6 was designed
to handle both large and small signals with aplomb, the 6HS6 was originally
designed for use in limiting IF stages in FM/TV receivers. Thus, the
application of AGC to the 6HS6 can cause it to begin cutting off much more
quickly than the 6BZ6, which can also result in severe distortion (that is,
it tends to either run wide-open or cutoff). This occurs because the 6HS6
cuts off sooner than the 6BZ6. As a result, you may find the 6HS6 to cause
severe AGC "pumping" on a busy and strong band compared to the 6BZ6. This
situation can be made worse by the fact that the 6HS6 requires only a
couple volts of grid bias for full cutoff compared to -15 or -20 volts to
fully-cutoff the 6BZ6.

Another reason why the 6HS6 may seem much hotter (both physically and
sensitivity-wise) is because it does not require as much screen voltage as
the 6BZ6: 75 volts vs. 125 volts. It may be, however, that running the
screen voltage so much higher than the design intention might help
alleviate the very-low-grid-bias-for-full-cutoff situation.

How to fix the problem? Eliminate the AGC on the 6HS6 altogether and hope
it's strong enough to handle anything on the band. Also, move the original
6BZ6 cathode resistor from pin 2 to the 6HS6's cathode on pin 7; it should
not be necessary to change the cathode resistor value, although the screen
voltage should be reduced to around 75 volts.

A better and more-palatable solution might be to try the 6EW6, which not
only has the same base pinout as the 6BZ6 but also has higher
transconductance. However, it might be necessary to drop the AGC voltage
range as the tube curves show it reaching cutoff with only a couple volts,
the same as the 6HS6, while the 6BZ6 requires several tens of volts of grid
bias for full cutoff (try a resistive voltage divider). The 6EW6 also is
rated for the same screen voltage as the 6BZ6 (125 volts), and was
originally designed for 45 MHz TV IF use; it should do very well in
receivers covering 10 meters. The RCA transfer curves are just a little
steeper with application of grid bias than the 6BZ6 although the 6EW6 is
called a sharp-cutoff pentode like the 6HS6.

As a minor sideline, the filament current of the 6BZ6 is rated at 0.3 amps;
the 6EW6 at 0.35 amps; and the 6HS6 at 0.4 amps. Your receiver's power
transformer has probably been running just a tad warmer than originally
intended with the 6HS6 replacing the original 6BZ6.

Isn't talking about tubes fun? 73, Steve Ko0U/1

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