[AMRadio] Ranger Audio... the numbers

Gary Schafer garyschafer at comcast.net
Fri Nov 17 20:05:33 EST 2006

Plate to plate impedance on the 6L6's is going to be in the neighborhood of
5000 to 6000 ohms. 400 volts plate to plate voltage swing would be around 26
watts of audio.

Plate voltage swing to ground would be 1/2 the above or around 200 volts to
ground for 1 tube.

Since there is about 26 volts of bias on the grids the peak grid voltage at
greater than 26 volts will cause grid current to start to flow and the tubes
will be slightly into AB2 operation, which I think they do get into.
I believe you measured around 20 volts on the grid to ground so it is in the
ball park. But the plate voltage swing seems awfully low.

It is difficult to detect a shorted modulation transformer sometimes. As
Larry says it is hard to detect shorted turns by measuring with an ohmmeter.
I once had a mod transformer that looked ok at low signal levels and
resistance measurements looked fine. But when a small amount of audio power
was applied it would break down and short.

Look at the grids again when applying audio and see if the peak voltage
reaches the bias voltage. Also put the scope on the center tap of the driver
transformer and DC couple it and see if you see any movement when applying
audio. That will be an indication of grid current being drawn.

Then if you still have only 50 volts on the plates of the mod tubes--
something is amiss.

Gary  K4FMX

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Jack Schmidling
> Sent: Friday, November 17, 2006 5:50 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] Ranger Audio... the numbers
> Larry Taft wrote:
> > 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.
> Could you explain the math on that?  How do you arrive at watts knowing
> only volts?
> As a data point, 50v into a 50 ohm load is 50Watts
> js
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