Thanks Einar. All most interesting and useful. I used this tube initially
according to the Mullard Handbook conditions for SSB service but this gave
poor efficiency and low output. Recently I moved to class B operation based
on the Mullard data for audio use. I now have the data for the QBL5-3500
which helps, plus the info about its use in the '2DAF circuit.
Although I never actually met him, I remember as a youngster listening to
Dick Thornley G2DAF testing his amplifiers on 80m and discussing principles
with other operators back in the early 1960s. There was controversy at the
time because his circuit was similar to those invented by others. This
resulted in some "heated" correspondence in the radio magazines of the time.
I have never actually tried the '2DAF circuit with any tube. My experiences
have been mainly grounded grid triodes, or triode connected tetrodes,
cathode fed. This arrangement failed with the QY5-3000 many years ago, and
the experiment was ended when a parasitic or internal tube flashover (not
sure which) caused a catastrophic failure. This resulted in the magnetic
force generated completely unwinding the anode choke without fusing the
wire. The surge also caused the mains 240 volt wiring to the outdoor shack
to burn out and the accoustic bang deafened me for some time. There was over
100 uf of reservoir cap in the h/b psu and the 2 big HV transformers used
were good for 2amps continuous between them. It taught me not to use nails
as fuses in the 240 volt ac line!!
It was some years before I felt like another try with the QY5-3000. These
more recent experiments gave initially poor results. Emission appeared low
with several bc tx pulls. Comparing notes with another G stn indicated he
also had low emission problems with that tube type. Then I acquired a brand
new QY5-3000 in original packing, and the emission appears ok though I only
have 4kv on the plate right now
The problem I see is the need to run a substantial amount of grid current,
to get anything like a respectable output. This may be the reason why people
are using the 2DAF circuit to avoid the need for a complex g1 regulated bias
supply. I am not too keen on the '2DAF design since a spectrum analyser is
essential for proper setting up. Not having such an expensive piece of test
gear makes me prefer sticking with more conventional circuits and with
electrode voltages similar to those given in the book.
Using the latest amp with this tube, tests have been undertaken with a dummy
load except for one session on 80m. All was fine until the HV disappeared
after a bang. Nothing serious, but the problem became repetitive even on
dummy load. After searching with a 10 Kv insulation tester, it was found to
be a hairline carbon track formed on a ceramic standoff in the plate
compartment pulling the HV DC plus HV RF to ground but the arc, only struck
on an RF voltage peak was then sustained by the DC even when no RF was
present. No probems have been encountered since that was sorted.
Best wishes
Howard Aspinall G3RXH
----- Original Message -----
From: Einar Persson>
>> Hi Howard
> That was a very common tube in Philips radio/TV transmitters in Sweden so
> they were quite common here also.
> Philips name is QBL5-3500. The tube is also known as 6076.
>
> Here is some info: http://www.tube.be/6076.html
>
> A few Swedish hams are using that tube type in a G2DAF circuit for the HF
> bands. On air, as well as spectrum analyzer tests says it's quite clean.
> I have some drawings I can scan and send to you if you are interested.
> My own experience with the tube is so far in a 2m amp.
> 73 // Einar // SM3MXR
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