Re: {Collins} 30L-1, keyed down current @ no excitation.



Dave, thanks, this is wonderful.  jm




On Sat, 18 Feb 2012 16:32:59 -0600, "Dave Harmon" <k6xyz@xxxxxxxxxxxxx>
wrote:
> The design values of the bias ckt resistors, relay and L3 will cause the
> tubes when keyed to idle at about 27ma plate current each.
> If you had to change R28 to another value to get the correct plate current
> then there is a problem somewhere.
> Sure...you can make it work like you said but you put a bandaid on the real
> problem.
> 
> I suggest you measure the resistance across the relay winding.
> It should read about 1700~. R12 2k in parallel with K1 10k = 1700~.
> If the relay or especially R12 is low resistance it will still operate but
> change the bias voltage to the grids.
> This would cause R28 to be raised in resistance to get the correct idle
> plate current.
> The required hard ground coming from the exciter through R28 shunts down the
> bias voltage at the junction of R12 and the relay.
> If the ground is not hard ground or the rectified voltage from the bias
> supply through R9 is not correct, you can't get the proper idle current.
> Is R15 10k? Did you measure it?
> Is C10 a new cap or was it mfgd in 61'?
> 
> If that is ok....then I suggest that you test the idle current using one
> tube at a time.
> All tubes should idle at about 27ma each.
> If you have modern metal film resistors R21, 22, 23, 24 replace them with
> regular carbon or the Ohmite OX/OY series from Mouser.
> If you have a combination of mixed type resistors for the grid resistors I
> would replace them all to the Ohmite.
> The 811-A is known to be the weakest component in these amps.
> 
> Also....what exciter are you using?
> Is it providing a good ground on the key line to the ant relay jack on the
> amp or does the exciter provide a voltage to the amp?
> 
> If you are using a non-Collins exciter with a modern relay similar to the
> KWM-2 plug in relay with a plastic contact card....I would inspect that
> relay to see if the card has melted yet. If not yet....it will.
> I destroyed a TR-7 relay this way because I didn't realize how much voltage
> was on that 30L-1 keyline.
> Fortunately, this amp is probably the only amp that is biased this way but
> being aware of the keyline voltage would have saved me some grief.
> 
> 
> Dave Harmon
> CCA 97-535
> K6XYZ[at]sbcglobal[dot]net
> Sperry, Ok.
> 
> 
> -----Original Message-----
> From: Gaylord Hart [mailto:gahart@xxxxxxxxxxxxx] 
> Sent: Saturday, February 18, 2012 3:34 PM
> To: Bill Carns; 'W9HAK'; 'Bent'
> Cc: collins@xxxxxxxxxxxxx
> Subject: Re: {Collins} 30L-1, keyed down current @ no excitation.
> 
> When I re-biased my 30L-1 to get ~110 mA idle plate current, I opted for a
> 62 ohm bias resistor (R28), which gives me 108 mA of idling plate current
> and -4.4 VDC bias voltage.  I've confirmed good operation on all bands with
> output minimum 600W at 600 mA CW plate current.  I installed a 1W 62 ohm
> resistor for R28, assuming I had good margin over the original 1/2W
> resistor.  But after driving the amp at full output, I touched R28 and
> noticed it was hot well beyond the expected idle current rating.  I measured
> the key-down DC bias voltage on
> R28 with 600 mA key-down plate current, and noticed the bias voltage was
> just under -11VDC.  This equates to about 1.8W dissipated across R28 when
> the amp is being fully driven, way above the 1W conservative wattage rating
> I had chosen.
> 
> While this wattage only occurs for R28 during transmitting and is low duty
> cycle by nature, the original R28 (39 ohms, 1/2W) that I replaced had
> suffered heat damage and was actually cracked, though not all the way
> through.
> 
> Has anyone else measured the key-down full-power-out DC bias voltage at R28?
> 
> If so, do you see a shift from about -4.4VDC to -10.8VDC?
> 
> Gaylord
> WB7ODD
> 
> 
> 
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