Rick Denney wrote:
> Ed writes...
>
>> On the B- side of the HV is a diode D16 according to the schematic
>> in the metering circuit. Its a 1N4007, make sure it isn't open.
>
> I will do that.
You need to measure this for forward/reverse resistance, not continuity.
This diode acts as a fuse to protect the metering circuit. With no drive
applied, you should see no grid current.
Also check the 2.5mH choke, L3 on the output of the tank circuit.
>
> I did test it for DC continuity, and I suppose for there to be
> anything else wrong there would need to be some signs of heat damage,
> which I don't. If you think I should, I'll test RF through it using my
> scope.
This also needs to be checked for resistance. IIRC it probably should be
in the 50-200 ohm range.
>> Other than that its time to pull the tubes and check them for shorts.
>
> I hadn't considered that, though I did inspect them for what I could
> see through the glass. I will check those, too.
Sorry,you need to check for grid to plate and grid to filament shorts,
with the tubes out of the circuit. Also check the plate caps, I've seen
too many 811's with loose,broken anode caps.
> This evening, I injected RF using my MFJ-259 as a source, and traced
> it through the circuits with my scope. RF input makes it reliably to
> the cathode inputs on the tubes, and RF output makes it reliably from
> the anode connections to the output connector. The tubes were
> connected and in place, so I'm wondering if they were shorted,
> wouldn't my RF get shunted to ground somehow? The two sides of the
> cathode aren't shorted or the filaments wouldn't light, and if the
> bias input side of the anode was shorted, I would have seen big
> sparks, it seems to me. Maybe it's just wishful thinking--I don't want
> to have to replace those three tubes again!
>
> This has been a real learning experience for me--I'm sorta glad my
> ebay amp has had (the expected) problems--but now I'm ready to just
> fix it. I will check the things you suggested. Thanks.
>
> Rick, KR9D
If I understand it correctly there were/are 2 versions of this amp.
Would you take the time and look here to see if this schematic agrees
with yours ?
http://www.ameritron.com/man/pdf/AL-811.pdf
The reason I need to know this is on the schematic it shows a string of
diodes in the filament ct that they are using to obtain operating bias.
These 5 diodes produce about 4-5 volts of bias, if one of them is open
you will see results as you are seeing.
Another thing that bothers me is the ALC circuit. again an old and
proven design. However it picks off some of the RF output, rectifies it
and sends it back to the input circuit. If it were me I would take this
out of the RF path, especially since TT does not provide ALC.
Keep in mind that TT stopped producing the Centaur, not because it
didn't sell, but they could not find acceptable tubes.
I'm not going to comment on the parasitic discussion other than to say
that most catastrophic events in an amp are caused by component failure
and/or operator error.
**** KEEP IN MIND HV CAN AND WILL KILL YOU **** BE CAREFUL ****
Ed W3NR
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