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Didn't notice this previously when testing, but, when the tube bias is set per the manual, the idling current continuously drifts upwards.? For example, I can set the bias at .9 V (read across the 10 ohm resistor) and it will drift up to 1 V over the next minute or two.? After repeatedly adjusting the bias, it's no longer possible to achieve the 1 volt reading with the bias pot turned full clockwise.? Seems to be that the new tubes I purchased are likely soft?? The plate voltage at the test pins on the power supply is 682 VDC.? The range of adjustment of the bias pot as measured at the wiper is -111 VDC to -154 VDC.? What do you say?

73, Floyd - K8AC

I installed a different set of tubes and after the warm up period recommended, the idle current is stable and I can set the idle current to 1 VDC well within the range of the bias pot.? Guess the other tubes have a problem.

73, Floyd - K8AC

Sounds like they are a bit gassy.

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998

Floyd - K8AC <floydsense@...> wrote

Didn't notice this previously when testing, but, when the tube bias is set per the manual, the idling current continuously drifts upwards. For example, I can set the bias at .9 V (read across the 10 ohm resistor) and it will drift up to 1 V over the next minute or two. After repeatedly adjusting the bias, it's no longer possible to achieve the 1 volt reading with the bias pot turned full clockwise. Seems to be that the new tubes I purchased are likely soft? The plate voltage at the test pins on the power supply is 682 VDC. <snip>

Hi,

Are the two 6DQ5s balanced? You can check by removing one
(NOT just removing a plate cap connector), and retesting the resting
current. Each tube should draw an indicated 0.45V on the 10 Ohm
resistor, when set to the bias voltage you started at.

Also, 0.9V indicates 90mA for the pair. At 680V plate volts,
that means 61 Watts plate dissipation. The 6DQ5s are only rated
at 48W max plate dissipation for the pair, so the pair are already
being overloaded at that resting current, even if perfectly balanced.

If the tubes are unbalanced, one may be hogging most of the plate
current, pushing it way over its maximum rated plate dissipation.
The plate current drifts as one tube is possibly slowly dying.

73,
Ed Knobloch K4PF

On Tue, Apr 30, 2024 at 03:35 PM, k2orsfan wrote:

Are the two 6DQ5s balanced? You can check by removing one
(NOT just removing a plate cap connector), and retesting the resting
current. Each tube should draw an indicated 0.45V on the 10 Ohm
resistor, when set to the bias voltage you started at.

That may be true of your HT-44, but it certainly isn't true of mine.? My 6DQ5s have their filaments in SERIES, so removing one tube removes the filament voltage from the other.? The correct procedure involves removing the screen voltage from one of the tubes, as documented in the Errata Sheet (Form number 094-903794B).

73, Floyd - K8AC

Floyd K8AC wrote
<snip>
My 6DQ5s have their filaments in SERIES, so removing one tube removes the filament voltage from the other. The correct procedure involves removing the screen voltage from one of the tubes, as documented in the Errata Sheet (Form number 094-903794B).
<snip>

Thank you, Floyd. I stand corrected. Never having owned an HT-44,
I extrapolated from my experience with the HT-32B and Collins KWM-2.
The point I was trying to make is that if a plate cap is removed,
but the screen is left connected, the screen will try to act
like the (missing) plate, which could quickly damage the tube.

It might be worthwhile to check the filament voltages
of the two 6DQ5s for imbalance, also. I had a Swan-240
with a 6DQ5 having excessive filament voltage, while several
other tubes were getting an undervoltage, in a series/parallel
12V sharing arrangement. Apparently, the 6DQ5 manufacturer
had shifted to a 'dark heater' design, so its heater resistance
was higher, throwing off Swan's voltage-divider scheme.

73,
Ed K4PF