Gary has, I think, hit the tube on the plate cap.
I went looking for a set of curves for the 6L6 in my pile of old books
and data but no find yet.
Just from "rule of thumb" for audio tube amps you should get a couple
hundred volts audio swing (not related to BIRD WATTS) at each plate to
ground. 50 v is only a couple of watts audio. Soft tubes or/and
shorted xfmr.
Try disconnecting the RF section from the power supply and put a 5K
power resistor across the output winding of the mod xfmr. Now you can
test the audio chain and figure out where it is failing. This removes
the possibility of RF back into the audio chain and power supply
variations from the load of the 6146. Actually, just pull the 6146 AND
hook the 5k resistor across the mod iron output leads.
Now, Redemption! I earlier said to reverse the OUTPUT leads on the mod
iron. Reversing the plate leads on the 6L6s would definitely make the
ampalator into an oscillafier. It isn't often that I'm really wrong.
Just ask any of my ex-wives.
73, Larry K2LT
Gary Schafer wrote:
It's beginning to sound like a partially shorted mod transformer. With 20
volts signal on the grids of the mod tubes I would think that you should see
at least 200 volts on the plates??
Could be bad 6L6's.
The reason that you see a slightly distorted audio pattern on the audio
amplifiers when in tx and not in standby could be the result of the negative
feedback loop trying to correct distortion from the mod transformer.
Can you easily get high modulator plate current with audio applied? Will the
mod plate meter peg or nearly so if you hit it hard? If it will that is an
indication that the tubes are good and the transformer may be shorted.
73
Gary K4FMX
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:amradio-
[EMAIL PROTECTED] On Behalf Of Jack Schmidling
Sent: Friday, November 17, 2006 9:45 AM
To: Discussion of AM Radio in the Amateur Service
Subject: Re: [AMRadio] Ranger Audio... the numbers
Gary Schafer wrote:
Check at the center tap of the mod driver transformer with your scope
and
see if you see anything there. Could be a bad or missing bypass cap
there.
It's brand new but I didn't have a 15mf so I used a 47mf.. ditto for C59A.
Check the jumpers on the plug in the back to be sure that the proper
taps on
the mod transformer are selected.
All ok.
Check the clamp tube circuit. Could be holding the screen slightly low
on
the 6146 and clipping the modulation peaks to the screen.
Screen voltage is 150 and the nom is 190.
However, R35, the Modulator Screen Voltage Adjustment resistor seems to
go to the clamp circuit and it is not at all clear what is going on
here. It seems to me that diddling with it changes the screen voltage
on the 6146.
So does diddling with the modulator current effect the 6146 screen
voltage or do I have a screen voltage problem?
Check modulator screen voltage.
It is the nominal 250V... but again, why is it nominal if that resistor
is there to diddle with the current?
js
--
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