Ok so I get this radio working really well. I am running it on the test bench when it starts to break out into a medium - pitched audio howl, which seems to somewhat respond to a change in audio control. In order to attempt to squelch it I put my fingers on the grid caps of all the tubes. When I put my fingers on the detector tube grid cap ( with some pressure ) the howling does stop. It does NOT appear to be an RF-related howl but maybe a bias or bypass cap related howl? I changed the 0.01 mfd bypass to ground on the audio output and the series grid cap to no avail. This is a medium pitched growl - almost sawtooth like. Bias related? Oscillating detector tube? Unknown but disappointing after all that work.
Lou
Try a little capacity from the audio line to ground. Add a .001 mf from grid #1 of the audio output to ground.
Since the radio was working well placement of wires alone may be enough to stop oscillation.
Be sure tube shields make good contact with the chassis. Any bypass caps disconnected?
Norm
:Guys:
:
:Ok so I get this radio working really well. I am running it on the test bench when it starts to break out into a medium - pitched audio howl, which seems to somewhat respond to a change in audio control. In order to attempt to squelch it I put my fingers on the grid caps of all the tubes. When I put my fingers on the detector tube grid cap ( with some pressure ) the howling does stop. It does NOT appear to be an RF-related howl but maybe a bias or bypass cap related howl? I changed the 0.01 mfd bypass to ground on the audio output and the series grid cap to no avail. This is a medium pitched growl - almost sawtooth like. Bias related? Oscillating detector tube? Unknown but disappointing after all that work.
:
:Lou
:
One thing..
When I connect my Fluke multimeter to the grid of the 45 output tube, I get -45 volts or so instead of the -3 volts advertised as correct for this tube. This is even with the volume pot turned way down ( no signal ). The radio plays fine - any reason for this? Is the multimeter being fooled?
Thanks Again All
Lou
:Lou
:
: Try a little capacity from the audio line to ground. Add a .001 mf from grid #1 of the audio output to ground.
:
: Since the radio was working well placement of wires alone may be enough to stop oscillation.
:
: Be sure tube shields make good contact with the chassis. Any bypass caps disconnected?
:
:Norm
:
::Guys:
::
::Ok so I get this radio working really well. I am running it on the test bench when it starts to break out into a medium - pitched audio howl, which seems to somewhat respond to a change in audio control. In order to attempt to squelch it I put my fingers on the grid caps of all the tubes. When I put my fingers on the detector tube grid cap ( with some pressure ) the howling does stop. It does NOT appear to be an RF-related howl but maybe a bias or bypass cap related howl? I changed the 0.01 mfd bypass to ground on the audio output and the series grid cap to no avail. This is a medium pitched growl - almost sawtooth like. Bias related? Oscillating detector tube? Unknown but disappointing after all that work.
::
::Lou
::
:
:
Minus 45 volts is reasonable when you measure between grid and filament of a 45 tube. A 45 requires high negative bias.
Minus 3 volts may be an error or measured from a different place?
Norm
:Ok - after a bit of searching and some detective work I saw that the 24A tube used in the detector slot had a blue internal glow!! That explains the strange oscillations, etc. A new (NOS) tube cured the problem.
:
:One thing..
:
:When I connect my Fluke multimeter to the grid of the 45 output tube, I get -45 volts or so instead of the -3 volts advertised as correct for this tube. This is even with the volume pot turned way down ( no signal ). The radio plays fine - any reason for this? Is the multimeter being fooled?
:
:Thanks Again All
:Lou
:
:
::Lou
::
:: Try a little capacity from the audio line to ground. Add a .001 mf from grid #1 of the audio output to ground.
::
:: Since the radio was working well placement of wires alone may be enough to stop oscillation.
::
:: Be sure tube shields make good contact with the chassis. Any bypass caps disconnected?
::
::Norm
::
:::Guys:
:::
:::Ok so I get this radio working really well. I am running it on the test bench when it starts to break out into a medium - pitched audio howl, which seems to somewhat respond to a change in audio control. In order to attempt to squelch it I put my fingers on the grid caps of all the tubes. When I put my fingers on the detector tube grid cap ( with some pressure ) the howling does stop. It does NOT appear to be an RF-related howl but maybe a bias or bypass cap related howl? I changed the 0.01 mfd bypass to ground on the audio output and the series grid cap to no avail. This is a medium pitched growl - almost sawtooth like. Bias related? Oscillating detector tube? Unknown but disappointing after all that work.
:::
:::Lou
:::
::
::
:
:
Lou
:Lou
:
: Minus 45 volts is reasonable when you measure between grid and filament of a 45 tube. A 45 requires high negative bias.
:
: Minus 3 volts may be an error or measured from a different place?
:
:Norm
:
::Ok - after a bit of searching and some detective work I saw that the 24A tube used in the detector slot had a blue internal glow!! That explains the strange oscillations, etc. A new (NOS) tube cured the problem.
::
::One thing..
::
::When I connect my Fluke multimeter to the grid of the 45 output tube, I get -45 volts or so instead of the -3 volts advertised as correct for this tube. This is even with the volume pot turned way down ( no signal ). The radio plays fine - any reason for this? Is the multimeter being fooled?
::
::Thanks Again All
::Lou
::
::
:::Lou
:::
::: Try a little capacity from the audio line to ground. Add a .001 mf from grid #1 of the audio output to ground.
:::
::: Since the radio was working well placement of wires alone may be enough to stop oscillation.
:::
::: Be sure tube shields make good contact with the chassis. Any bypass caps disconnected?
:::
:::Norm
:::
::::Guys:
::::
::::Ok so I get this radio working really well. I am running it on the test bench when it starts to break out into a medium - pitched audio howl, which seems to somewhat respond to a change in audio control. In order to attempt to squelch it I put my fingers on the grid caps of all the tubes. When I put my fingers on the detector tube grid cap ( with some pressure ) the howling does stop. It does NOT appear to be an RF-related howl but maybe a bias or bypass cap related howl? I changed the 0.01 mfd bypass to ground on the audio output and the series grid cap to no avail. This is a medium pitched growl - almost sawtooth like. Bias related? Oscillating detector tube? Unknown but disappointing after all that work.
::::
::::Lou
::::
:::
:::
::
::
:
:
Lou
:I might be thinking of the 24-A. Guess I will check again.
:
:Lou
:
:
::Lou
::
:: Minus 45 volts is reasonable when you measure between grid and filament of a 45 tube. A 45 requires high negative bias.
::
:: Minus 3 volts may be an error or measured from a different place?
::
::Norm
::
:::Ok - after a bit of searching and some detective work I saw that the 24A tube used in the detector slot had a blue internal glow!! That explains the strange oscillations, etc. A new (NOS) tube cured the problem.
:::
:::One thing..
:::
:::When I connect my Fluke multimeter to the grid of the 45 output tube, I get -45 volts or so instead of the -3 volts advertised as correct for this tube. This is even with the volume pot turned way down ( no signal ). The radio plays fine - any reason for this? Is the multimeter being fooled?
:::
:::Thanks Again All
:::Lou
:::
:::
::::Lou
::::
:::: Try a little capacity from the audio line to ground. Add a .001 mf from grid #1 of the audio output to ground.
::::
:::: Since the radio was working well placement of wires alone may be enough to stop oscillation.
::::
:::: Be sure tube shields make good contact with the chassis. Any bypass caps disconnected?
::::
::::Norm
::::
:::::Guys:
:::::
:::::Ok so I get this radio working really well. I am running it on the test bench when it starts to break out into a medium - pitched audio howl, which seems to somewhat respond to a change in audio control. In order to attempt to squelch it I put my fingers on the grid caps of all the tubes. When I put my fingers on the detector tube grid cap ( with some pressure ) the howling does stop. It does NOT appear to be an RF-related howl but maybe a bias or bypass cap related howl? I changed the 0.01 mfd bypass to ground on the audio output and the series grid cap to no avail. This is a medium pitched growl - almost sawtooth like. Bias related? Oscillating detector tube? Unknown but disappointing after all that work.
:::::
:::::Lou
:::::
::::
::::
:::
:::
::
::
:
: