[Central-Electronics] Fwd: 20A 6AG7 screen grid resistors

[k6jek]

"Use only RCA 6AG7s."  That's what the manual says along with a line about surplus 6AG7's being prone to oscillate.  I figured the resistors had something to do with that.

Do you suppose the different ESR is why CE used micas in places where we're used to seeing ceramics?  I figured they just got a deal.  

Push has come to shove with the ones in this transmitter, at least the big bruisers. They're shot so they have to go.  I'm putting in modern micas which probably have different characteristics than the older ones did even when they were new.  I'll cross my fingers.

How many gray hairs did you get puzzling out that mixer decoupling business?  I'll not mess with that unless I'm sure it's kafluey.

Jon

On Mar 21, 2011, at 2:44 AM, Jim Candela wrote:

> Jon,
> 
>   I'm wondering about those resistors. I do recall reading somewhere that 
> certain 6AG7's types were preferred, and we know those tubes work pretty hard in 
> the 20A. This is especially true with 125 vac line voltage, where the 
> B+ Hoovers around 400v when the rig is in standby.
> 
>   Just thinking out loud here, maybe the 6AG7's can get destroyed when driving 
> them into a high impedance antenna. Here the plate will swing pretty low making 
> the screen grid become the plate for a portion of every cycle. A lightly loaded 
> tube when driven hard, will have high screen current. So 100 ma G2 current 
> through 180 ohms, will dump 1.8 watts into a 1/2 watt resistor. Could these 
> resistors be 'fuses' like the 560 ohm B+ resistors are?
> 
>   Or taking from what Colin said, about these resistors being an 'isolation' 
> resistor, maybe these resistors help stabilize the 6AG7's when loaded a certain 
> way into a complex load impedance.
> 
>  As far as the bypass capacitors go, there are a few that are critical. Both 
> are at the Mixer plate circuit. One is in series with the mixer tuning condenser 
> (must be Mica), and the other is the B+ decoupling capacitor at the mixer coil. 
> Here we see Mica or Ceramic disk type. The resonant 'Q' of the mixer tank is 
> very high, so CE put a resistor across the coil. The rigs that bypass the B+ 
> with a mica capacitor use a lower value resistance than the rigs that use a 
> ceramic bypass. The ESR is higher with the ceramic, so the stage gain is lower. 
> They get the gain higher with a higher resistance across the tank coil.
> 
> I found this out once the hard way. Where has all the gain went? :-)
> 
> Jim
> WD5JKO 
> 
> 
> 
> 
> ----- Original Message ----
> From: k6jek <k6jek at comcast.net>
> To: central-electronics at mailman.qth.net
> Sent: Sun, March 20, 2011 5:33:34 PM
> Subject: [Central-Electronics] Fwd:  20A 6AG7 screen grid resistors
> 
> It's a world gone mad: paper caps good;  mica caps bad.
> 
> As Colin points out the screens draw almost nothing so 180 ohms won't drop the 
> voltage more than a smidgen. This is another strike against the mica bypass caps 
> since something was drawing enough current to fry the resistors.  So I checked a 
> few more or those big micas.  They are all very leaky except, interestingly, the 
> ones used to bypass the AC mains which I replaced anyway with X1/Y2 rated caps.
> 
> Meanwhile several of the Sangamo paper caps have tested good!  That was the last 
> thing I expected after my earlier experiences with them.
> 
> Thanks again.
> 
> Jon
> 
> On Mar 20, 2011, at 1:52 PM, Colin Lamb wrote:
> 
>> Hi Jon:
>> 
>> At 180 ohms, it is not a dropping resistor.  Calculate the voltage drop of the 
>> very small screen current through 180 ohms.  It will probably be no more than a 
>> couple of volts.  180 ohms would be an isolation resistor.
>> 
>> Also, unless the 6AG7 is operating class A (it probably is), then a dropping 
>> resistor would not be appropriate as it would cause the screen voltge to change 
>> with power level, increasing distortion.  Screen voltage in a linear amp (unless 
>> class A) should be regulated.
>> 
>> 73,  Colin  K7FM
>> 
> 
> 
> 
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