Date: Thu, 8 Dec 2016 20:52:20 -0600
From: "Paul Kraemer" <elespe@lisco.com>
To: <amps@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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