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 Post subject: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Fri 14, 2018 5:01 pm 
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Background, I am servicing a Zenith 11-S-474 and was measuring tube socket DC voltages.
https://www.radiomuseum.org/r/zenith_11 ... h1103.html

At the G2 pin of the 1232 RF amp I expected to see nominal 110 volts DC, but instead measured 230 volts DC? After much head scratching and replacement of a couple resistors I still measure 230 volts on G2. Whats going on here?

Now a little later I tune the radio off-station and note the G2 voltage drops to 107 volts DC. Tuned back on-station, the voltage goes back to 230 Volts DC. Not an AGC issue since AGC is a negative low voltage.

Can someone with a better understanding of tube theory than me please explain why I am seeing the voltage swing on the G2 pin of the RF Amp tube.

Attachment:
Zenith 11-S-474.jpg
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 Post subject: Re: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Fri 14, 2018 5:20 pm 
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R6 is 47K, so the screen voltage will swing a lot with tube current.

Strong signal, AVC increases, tube current decreases, plate and screen voltages both increase.

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 Post subject: Re: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Fri 14, 2018 5:43 pm 
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Interesting.

I guess I was under the incorrect impression that screen grid voltages were more or less fixed and screen grid current flow was insignificant.

This voltage swing would result in about 2 mA of screen grid current above what ever the idle current is.

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 Post subject: Re: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Fri 14, 2018 5:55 pm 
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Another way to look at it: the AVC swing on the grid changes the plate current and the screen current. The change in current then results in a change in voltage. The screen has the larger resistor, so the voltage changes more..

Does a change in current cause the screen voltage to change?
OR
Does a change in screen voltage cause the current to change?

Answer: Yes

In this case, the Tung-Sol data sheet (at Frank's) shows both plate and screen current as a function of grid voltage. I don't see that very often....

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 Post subject: Re: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Fri 14, 2018 6:01 pm 
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As Mark has pointed out, that is perfectly normal behavior for that particular tube in that RF stage in that set. Other tube types in an RF amp in a different radio may or may not have such wide swings on G2. When G2 voltage reads high, the tube is conducting less due to the AVC action on G1.

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 Post subject: Re: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Fri 14, 2018 6:51 pm 
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Thank you, Mark and Dennis, for your informative replys.
Always good to learn at least one new thing every day. :D

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 Post subject: Re: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Fri 14, 2018 7:27 pm 
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Also worth noting, the local-distant switch which is located on the Wavemagnet antenna assembly in this radio changes the resistance in the RF stage cathode circuit, and will cause a pretty dramatic change in the voltages on that tube depending on which position it's in.

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 Post subject: Re: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Fri 14, 2018 7:46 pm 
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Mr. Detrola wrote:
Also worth noting, the local-distant switch which is located on the Wavemagnet antenna assembly in this radio changes the resistance in the RF stage cathode circuit, and will cause a pretty dramatic change in the voltages on that tube depending on which position it's in.


Yes, I observed the effect of the L/D switch position while I was monitoring socket voltages.

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 Post subject: Re: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Sat 15, 2018 7:22 am 
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Did you use the kind of meter specified on the service data?

RFRM


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 Post subject: Re: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Sat 15, 2018 3:04 pm 
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Retired Radio Man wrote:
Did you use the kind of meter specified on the service data?

RFRM


A loaded question, similar to what I was looking at... Love it.

Possibly not the same page, but I would like to see the same measured with an analogue meter if it wasn't. Why you ask? Aside from modern digital meters often being unable to handle tube radio volts. We have RF and some have issues with RF & get into a real pickle. They read it process it, & it becomes part of the result: With some it actually causes an internal "flash over". Analogue meters on DC range will not see RF.

Marc


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 Post subject: Re: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Sat 15, 2018 6:45 pm 
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Retired Radio Man wrote:
Did you use the kind of meter specified on the service data?

RFRM


The available service literature specified a 1000 ohm per volt meter.

I used both a 20 K-ohm/volt meter, and a 10 Meg-ohm per volt meter, each with similar results as mentioned in my first post.
Simpson 260, Fluke 73

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 Post subject: Re: DC Voltage Measurement On G2 Of RF Amp
PostPosted: Sep Sat 15, 2018 11:41 pm 
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If you post twice on the same radio, with no reference to the previous, that is how information gets lost and tends to be duplicitous, which is why it is frowned on.

Marc


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