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

Ok...well...just a few more comments.

The replacement power supply ckt boards will all provide 2kv or slightly
more on today's line voltages as you said....but, I don't believe that the
higher plate voltage will cause an undue rise in idle current because the
bias voltage is raised (more negative) in a like amount which would offset
or hold down the idle current.

You mentioned you measured across the meter shunt R8.....did you actually
measure it to see if it was correct?
I ask because I had heard somewhere recently that someone had a problem with

R28....this resistor will have the same current flowing through it (if the
amp is keyed) if the amp is at full power or at idle plate current.
I've seen a lot of these resistors overheated etc and you're right....a half
watt resistor here must have been a mistake.....I replace em' with a 2 watt.
No more problems.

I have installed 17 of the YK power supply boards since Jan 2004 when I was
asked by Young Kim to write an evaluation piece which was printed in
Electric Radio Magazine. He's a very clever engineer and that product is top
In fact...the last one I did was 3 weeks ago.
In the process of installing the new board I always go through the unit to
bring it up to date to the latest Service Bulletins which includes replacing
all the resistors we have been discussing.
Every single one of those units worked properly the first time....I don't
mean the YK board.....I mean the idle current was right on.
In fact....the last one I did had the original RCA JAN 811-A mil surplus
tubes that had holes in the plates....but by golly the idle current was
right on and it made 650 watts out....and that was on 120vac. Of course
230vac is better.

So....I still think there is some other problem. 
If you have one or more tubes working a lot harder than the
others...obviously, the harder working tube(s) will crap out long before the
others do.

One last thing.....does your unit have a MOV across C2 and if it
does....what is the rating and part number??

For others that may be responding to Gaylord behind the reflector...please
post to the list so the complete discussion will go to the archives.

Dave Harmon
CCA 97-535
Sperry, Ok.

-----Original Message-----
From: Gaylord Hart [mailto:gahart@xxxxxxxxxxxxx] 
Sent: Saturday, February 18, 2012 7:26 PM
To: Dave Harmon; collins@xxxxxxxxxxxxx
Subject: Re: {Collins} 30L-1, keyed down current @ no excitation.

Hi Dave, et al,

Thanks for the suggestions.  I don't believe there is a problem elsewhere in
my amp that the higher R28 value is covering up.  When I began working on my
30L-1, R28 initially measured 34.4 ohms, and showed signs of heat damage. 
With this resistor, I ran 150 mA key-down idle current.  All of the other
resistors and caps in the bias supply were measured and were right on (R9,
C10, R15, R12, K1).  All supply voltages were well within the expected
ranges.  I also measured every other resistor in the amp, as well, and
replaced 2-3 of these, but none of these were associated with the bias
elements or power supplies.  Grid resistors R21-24 are all CC and are
closely matched in resistance and well within tolerance.  I used an external
SPST throw switch for the antenna relay input since at this stage of testing
I did not need excitation, and I measured 0 volts across this when closed. 
I also had just installed four new 811A's.  I measured the plate current
across shunt resistor R8 with a DVM since the 30L-1 meter doesn't provide
much resolution.

I replaced R28 with a new 39 ohm resistor (and confirmed resistance before
installation).  My idle current dropped to 135 mA, but was still too high in
my opinion.  Some of the higher current can be accounted for by the higher
AC mains voltages we typically see today, and in fact my unloaded plate
supply reads 2 kV, about 10% high.  After measuring and testing essentially
every component in the chain (except as you pointed out, the tubes on an
individual basis) I concluded the other component of the higher current was
either the original Collins design or the performance of the new tubes. 
Since these had all been matched at the factory, I opted at not to test
these further.  I did do a fairly extensive web search on the problem to see
if anyone else had further insights, and what I found was that a significant
number of other 30L-1 owners had experienced the same problem: high idle
current when everything else checks good.  Several others in hear have
commented to the same effect in previous threads.  So I simply experimented
with other R28 values, and settled on 62 ohms, which gives me 108 mA of idle

I subsequently ran extensive tests across all bands with full excitation and
loading, and the performance has been excellent.  Perhaps I have a tube(s)
running a little more aggressively than the original 811A specs, but the amp
works great and I'm satisfied that nothing else is wrong with it.  I'd be
interested in hearing others' experience in this regard.


----- Original Message -----
From: "Dave Harmon" <k6xyz@xxxxxxxxxxxxx>
To: <collins@xxxxxxxxxxxxx>
Sent: Saturday, February 18, 2012 3:32 PM
Subject: Re: {Collins} 30L-1, keyed down current @ no excitation.

> 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.
> 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 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
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