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 Post subject: Odd Tube Voltage in Drake 2B
PostPosted: Apr Sun 13, 2008 8:29 pm 
Member

Joined: Apr Sat 29, 2006 4:48 pm
Posts: 111
Location: Bel Air, Maryland
Recently I acquired a Drake 2B receiver and have had a tremendous time with it. This really is one powerful receiver for SSB. I did some minor restoration, changed out the old paper caps with newer polypropylene versions, peaked up the alignment, etc. As far as I can measure with my signal generator (frequency synthesized), its sensitivity on the 80-40-20m bands is spot on and the audio is great. As a final step I did a final voltage check. All of the tube voltages check out OK & are within reason for a receiver of this vintage except for pin 1 (oscillator grid) of V3 (the 6BE6 2nd mixer & VFO). Instead of –0.5V, I measure +2.99V. Now, according to my knowledge & I’m no expert, oscillator grids of converter tubes should have a negative, not positive, voltage on them. Yet, this receiver performs just fine. I can’t figure it out. I checked out the all the resistances in the osc circuit which includes the T3 transformer & all checks out OK. I hate to take out the T3 transformer to check out the components inside it since the receiver is performing so well but this positive grid voltage is bothering me. I’m wondering if anyone else has noticed this issue with the 2B? I don’t know if it matters or not but I do notice that the only ceramic socket in the receiver is that for this V3 tube (heat concerns?).


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 Post subject:
PostPosted: Apr Sun 13, 2008 10:03 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2356
Location: Mt. Airy, Maryland
Measure with respect to the cathode.

Also, by connecting the probe, you might be killing the oscillator which would cause incorrect voltage readings.

The ceramic socket is for low-loss, not necessarily heat concerns.


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 Post subject:
PostPosted: Apr Sun 13, 2008 11:50 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3640
Location: Monterey California USA
I was wondering what paper capacitors you changed since I don't recall there being any, other than perhaps one. It's been quite a few years since I worked on one but I seem to recall papers were not used...


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 Post subject:
PostPosted: Apr Mon 14, 2008 12:01 am 
Member

Joined: Apr Sat 29, 2006 4:48 pm
Posts: 111
Location: Bel Air, Maryland
Sorry, "tubular" caps is probably a better word choice.

Voltage measured at cathode (pin 2) of 6BE6 is +2.79V (Manual says it should be +2.7V).


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 Post subject:
PostPosted: Apr Mon 14, 2008 12:17 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20548
Location: Sandpoint, IDAHO 83864
I had the same question as Geoff did. I think the only capacitor in the set other than the electrolytics that is not ceramic is the AGC filter/ time constant capacitor, which is a 1uF job.

Glad to see another 2B being put back into service. It is my all time favorite for cruising the pre-WARC ham bands. A couple of crystals would cure that problem also.

I had mine on 40 meters one day several years ago and I worked John Chenoweth on CW. I forget his W8 call at the moment. When I told him my receiver was a Drake 2B, I could almost hear him fall off his chair. I knew who he was, but he didn't know me, so I thought I would rub it in a bit. After all, he was the engineer at Drake who designed it! We had a splendid CW high speed QSO after that and it only ended when the band dropped out of under us.
Curt

_________________
Curt, N7AH
(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: Apr Mon 14, 2008 1:48 am 
Member

Joined: Apr Sat 29, 2006 4:48 pm
Posts: 111
Location: Bel Air, Maryland
Just solved the problem. W3JN was right, the probe from my Fluke 87 was affecting the oscillation & producing an erroneous voltage. I wrapped a 100K resistor around the probe tip and pin 1 of the 6BE6 now measures -1.01V. Got that info from surfing Google Groups. I assumed that the 10-meg input of the Fluke 87 would be OK but not so. All other voltages measure OK also. Thanks gents & BTW, this really is one fantastic little receiver. I'm amazed at the sensitivity & the quality of the audio. I'm listening to hams as far away as Jamaica, Argentina & England with just a long wire in the attic.

Great forum here!


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