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Author Topic: If your DX-35's 12BY7s are gassy and you don't have replacements?  (Read 17834 times)
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« on: December 02, 2009, 09:24:01 AM »

The 12BY7s were gassy, they would stop oscillating and cause the 6146 to sit at zero bias!! Shocked

I didn't have any 12BY7s around but I scrounged and found a pair of 6HB6s. Their pinouts are almost identical to the 12BY7s.

The filament feed was cut from pin 6. The jumper from pins 4 to 5 was cut. Pin 4 was left as is, the wiring from pin 6 (filament power + small capacitor) was then tied to pin 5. Pin 6 was left alone then.

Then in the oscillator, it was necessary to do a few things:

1) Screen grid feed. Remove the stock 27k resistor that ties the screen to B+. Don't discard the resistor, save it for the next step.

2) Make a voltage divider, 27k to ground, 82k to the B+ where the original 27k screen resistor was attached.

3) At the junction of this voltage divider where 27k meets 82k, tie 100k to the screen grid.

4) Add 100pF in series with the control grid.

5) Put 680 Ohm in series with the Cathode (but leave the stock bypass cap alone).

That was it. The DX-35 can now run with 6HB6s instead of 12BY7s.
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« Reply #1 on: December 02, 2009, 10:22:46 AM »

But are 6BH6's any less expensive or easier to find than 12BY7's?  I didn't have trouble finding some spare 'BY7's for my Gates 160m rig, which uses a pair in the osc/buffer stages.  They weren't very expensive either, about $15 each IIRC.

OTOH, the mod might be tempting if I didn't have any 'BY7's but happened to have a bunch of good 6HB6's on hand.

Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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« Reply #2 on: December 02, 2009, 10:35:55 AM »

I couldn't speak towards the ubiquity of 12BY7s because I don't have any and have never found any.

I lucked out that I happened to pick up a couple of 6HB6s at the Brampton hamfest, just because they looked like nice little power tubes.
AC0OB - A Place where Thermionic Emitters Rule!
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« Reply #3 on: June 13, 2017, 05:10:48 PM »

I have never had any problem with the supply of 12BY7's but this tube is a viable alternative with appropriate modifications.

I do have some questions.

According to my understanding of your circuit as shown in the pdf file (and please correct me if I misinterpreted your description), I have the following comments and questions (note the decoupling of the series circuit has been redrawn by me as I have upgraded DX-35's):

1. The voltage at the junction of the voltage divider would be 80V, so the voltage on the Osc. screen grid would be about 40V since there would be approx. a 200V drop across the 100k for about a 2 mA screen current. This voltage seems too low to generate much RF for the driver-buffer.

2. a 680 ohm cathode resistor would provide a cathode bias of -13.6 volts at 20 mA of  plate current further reducing the gain of the osc. stage. Why insert a cathode resistor here?

As a suggestion for upgrading the DX-35, here is one set of circuit upgrades:


* DX-35 6HB6 OSC Buffer Driver.pdf (43.29 KB - downloaded 591 times.)

Charlie Eppes: Dad would be so happy if we married a doctor.
Don Eppes: Yeah, well, Dad would be happy if I married someone with a pulse.NUMB3RS   Smiley

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« Reply #4 on: June 13, 2017, 08:40:21 PM »

How many 12by7's do you need.?.. I know i have at least 10...

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« Reply #5 on: June 14, 2017, 10:36:51 AM »

FYI - Lesson to be learned: Hang on to your vacuum tubes!!! If you must, throw something else away.

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