[Amps] L7 problems

Alek Petkovic vk6apk at bigpond.com
Fri Dec 9 21:36:56 EST 2016 

Did you check the filament feedthroughs like the other Jim suggested?

73, Alek. VK6APK

On 10/12/2016 10:10 AM, Paul Kraemer wrote:
> 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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