[Amps] L7 problems

Fri Dec 9 21:10:42 EST 2016

Jim
Good to hear from you.  I was hoping you would have comments.

I had already made the change on the keying going to a 100K 5w and series 
string of 1N5408 in ct for bias.  The relay shorts the resistor.  Just like 
my ES4L4B kit includes with the soft start and soft key.  I also don't like 
putting the 120v on the ct and running zero bias on keyed.

I had also done the diodes HV- to gnd and the one across the grid meter.

Of course I made sure along the way the changes didn't make any difference 
to the problem

Interlock was checked first off and has a stiff bit of pvc cable jacket in 
there to keep it defeated while I work on it.

Feel like only thing left in the equation is something is wrong with that 
transformer.  I have some lv high amp industrial transformers that with a 
little adjust from a variac I can run at least one tube on the bench and 
I'll disconnect primary and secondary of the Drake transformer.  Don't need 
the relay power just to test so that should say yay or nay and I can send 
the transformer to Heyboer for rewind if it shows that IS the culprit.  I'll 
let everyone know how it works out.

Any notes or history would be appreciated

Paul K0UYA

-----Original Message----- 
From: Jim Thomson
Sent: Friday, December 09, 2016 12:05 AM
To: amps at contesting.com
Subject: [Amps] L7 problems

Date: Thu, 8 Dec 2016 20:52:20 -0600
From: "Paul Kraemer" <elespe at lisco.com>
To: <amps at contesting.com>
Subject: [Amps] L7 problems

Group
I have an L7 that was sent to me for repair and I am getting nowhere with 
it.

The problem is the grid meter has a small negative reading in standby even 
with NO tubes installed.
If the antenna relay is keyed the reading goes to zero.  The amp does work 
if you ignore that pesky off scale negative reading in standby.  The amp 
does same thing on three different supplies so it is NOT the power supply. 
I have an L7 of my own that is fine on the supplies so it is the L7 at 
fault.

Here is an A/B comparison of the bad amp (customers) and a good one (mine)

The ONLY things I have been able to observe between bad and good is:
(1) Bad the center tap of the filament winding is off center by about
0.2vac.  In other words, it should be the center and instead measures like
2.6vac one side to center and 2.8vac other side to center whereas good will
measure EXACTLY the same either side to center
(2) Bad there is a measureable 25mv (dc plus 60hz)to ground on the negative 
connection
of the HV supply.  Since that voltage is across a 1 ohm resistor it makes
the amount of negative grid current.  If the amp is keyed the voltage goes
nearly to zero as it should be all the time.  Good amp is ZERO all the time.
Same supply, same location, just swap the linear.

I hope someone can give some insight to this.

Thanks

Paul K0YTA

##  I had the exact identical problem on a hb  4-1000 GG amp years ago.  Neg 
grid current on RX..... then rises to zero when u hit the PTT  (no drive 
applied).
With drive applied, it operated normally.  Lemme dig out my old notes as see 
if I can find it.  But it only happens when I cranked the B+  way up to 5600 
no load..on RX.
I also own 4 x drake L4Bs...with mating 4 x L4PS power supplies. L4PS is 
identical to the L7PS.   I have not had the neg grid current issue on the 
drake amps.
I did have one of them with a fil to cathode short..that resulted in idle 
current on RX.  One bad tube, thats it.

##  IF the CT is not dead center on the fil xfmr sec, you will hear 60 hz 
hum over the air.  A buddy re wound a fil xfmr for a hb 2 x 3-500Z amp. 
The loaded voltage was a
bit high, so he took turns off of one end only. Voltage was back down to 5.0 
vac..... but the CT  was no longer dead center, but offset....since he 
removed turns from ONE end
only.   You could hear the slight bit of hum over the air.... but in his 
case, it was a lot more off, vs  ur .2 vac offset.

##  I had another buddy with the neg grid current issue on RX...on a drake 
amp, dont recall if it was an L4B or L7.  Again I will attempt to dig up 
notes.
Check the B+ interlock circuit.  On the L4B...with top lid removed, it just 
doesnt short the B+ to the chassis.  It also shorts the B- to the 
chassis..and does that
1st, a split second b4 the B+ gets shorted to chassis.  You can see that on 
the spring loaded plunger rod assy.  Rls it by hand in slow motion... and 
you will there is
2 x sets of contacts.... with the lower set grnding the B- to chassis...then 
followed by grnd the B+ to chassis....last.

##  On my l4bs.... I tossed the pair of 50 watt bleeder resistors in the HV 
supply.  The no load B+ didnt budge...2650 on RX.   2500 under load of 800 
ma.
Ok, oem deal was to use a 7k 5 watt  resistor in series with the former 
bleeders.  The V drop across the 7k was + 130 vdc..used to cut the tubes 
off.  On CW position,
its  +90 vdc to cut the tubes off.  Its a dumb fubar method imo, since the 
CT goes open when TR 3PDT relay is in mid air.    With big bleeders removed, 
you no longer have any
90/130 vdc for cut off bias.   No problem just use a 100k at 3 watt mof..or 
a pair in parallel for redundancy... in the ct lead.   Then re wire the 
center tr relay contacts so they
just short out the 100 k..when on TX.

##  RVS  connected 1N5408 / 6A10s  wired between B-  and chassis.   Then the 
meters are protected.  B- cant wander more than  +/- .7 vdc.

stay tuned.

Jim   VE7RF

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