Need some help with sweep tube amp biasing

[Dmans]

LC,
The two series caps in parallel with the load capacitor are there to lower the capacitance of the variable load capacitor since with them in circuit you effectively have two parallel capacitors of 400pF and 60pF giving a total of 52pF.

How that will change the total value of the circuit(coil) I cannot calculate. A 14 hour work day has sapped my brain.

Your input tune may be helped with a 4-6 turn coil in series with the cathode of the driver tube (if it doesn't already have one). My Hygain Afterburner+ and the Skipper 300 I refurbbed for a friend did not have one. After installing one and pruning a'la the "cut and try" method, it was tuning pretty flat.

Pretty sure nomadradio has the answer to the loading/tuning coil issue. Stay with it! The oldies need to be saved!!!

73
David

 

[loosecannon]

Thanks Dmans,

there is a coil in series with the input to the cathode which is why i was trying to add the trimmer off of it to ground. Its that .75uH coil in the schematic.

it seems like not having that 47 ohm resistor in circuit is messing with the tuning, and im starting to think that could have something to do with the long coax run to the switch.
it runs from one side of the front of the amp to the other, and the ground is established by the switch itself. that means that when it's not grounded at the switch, there is a long open cable run in parallel with the input to the cathode.

im going to try some oddball things like adding the cap after the coil, or figuring out a different way to switch that resistor in.

im also thinking that the notes on the schematic may be because the amp may have been made for 10 or 12 meters and there were "secret" instructions sent with the amp to make it work on 11 like was done with the kenrich and other amps.

i'll be sure to post whatever i try, but it may be a day or two as i think i just need to think and read before digging back in to it.

this thing was such a wreck when i got it, and it has taken so much work just to get it working as well as it does right now that my brain just isn't in to taking things apart again yet LOL.
LC

 

[Dmans]

LC,
I quoted the wrong total capacitance of the load variable capacitor. (Too tired!)

The actual capacitance (if the 2 -120pF capacitors were in circuit) would be 460pF.

The 2-120pF in series = 60pF and in parallel with the 400pF variable would 460pF total.

73
David

 

[Handy Andy]

Just to help...
TUNE - Right Tune-able
LOAD - Left Tune-able​

It was a Slow News day...

 

[loosecannon]

well the fun continues.

yesterday, i tried putting the parallel capacitance on the ground side of the 47 ohm resistor, and with about 680pF of capacitance, i was able to get the input match down to about a 1.3.
good with me.


TUNE and LOAD controls were still "peaking" at around fully UNmeshed positions, so i tried spreading and shorting turns on both coils.

not much change, until today, and the change doesn't seem to be a good one.

so as i left it yesterday, i had two turns shorted on the coil going out to the antenna, after the LOAD control, and the LOAD cap seemed to still be increasing in power when i got to the end stop in the fully MESHED position.

ok i thought, maybe try one turn tomorrow and went to bed.

today, i removed that short, and turned the amp on for an initial test.

nothing, no power out, as if no high voltage was getting to the tubes.

measured AC coming out of the transformer and got 321 vac.
i thought i had measured it previously at 275 but whatever...

let the amp warm up for about 10 minutes and eventually it started making power.
when this happened i started measuring the voltages on the 12 volt winding of the transformer.
(remember i have a relay in the amp now that only produces the HV when the mic is keyed)

the 12 winding was sitting at about 14.5vac. ok whatever...

then keyed the mic and watched it jump up to 23.5vac.

measured the DC after the diode in this line, and sure enough, with mic keyed, the voltage was at about 19 volts DC.

Does this mean the transformer is going bad?
does it get worse when amp starts getting warmer?

why dont the filaments get any brighter when i key the mic seeing as how they would be getting quite a bit more than the 6 or so volts they should have been?

i am stopping for now and hoping someone can process the info i laid out and maybe offer up a couple of tests to do tomorrow.

as of right now, im letting the amp cool off completely and am going to measure the voltages when turned on cold and monitor them as the amp warms up.

thanks for making it through this post!
LC

 

[loosecannon]

OH!

a quick PS-

this is to help with the detective work.

i'll try to be brief.

initially the amp came to me with bent relay contacts, (bent by a person messing with them) and a need for re-capping.

i replaced the relays, re-capped it, replaced a bunch of old over heated (looking) ceramic disc caps, tested for 10-20 minutes of talking on it, and gave it back.

it smoked on the owner the first night he really talked on it, and he probably used it for over an hour at least.

the meter plastic had melted (it sits right in front of the driver tube and is lit with a 12v bulb)

upon initial inspection, i found the top of one of the HV electrolytics blown out.
after disassembling the amp, i found a burnt trace where the 12v enters the bias line.

my question is, could this 12v winding have jumped up on him like i just saw, and the increased voltage on the bias line burnt the trace?

could the increased voltage have caused that bulb to melt the meter?
for reference, i didn't see the bulb get any brighter today when i keyed the mic so not sure on that, maybe the driver tube just got so hot before the HV cap finally blew that it melted the meter.

ok, thanks for following along.
LC

 

[nomadradio]

Odds are that a tube arced over inside and zapped the low-voltage components in the bias circuit with a few hundred Volts or more.

Once they have shorted, the excess current available from the transformer's 12-Volt heater winding will turn a foil trace on a pc board into an auxiliary fuse.

Have seen this sequence play out more than once. Especially if the amplifier was either tuned wrong, driven too hard or keyed too long.

Radiated heat from the nearest final tubes are what melt the meters as a rule.

73

 

[Shockwave]

the 12 winding was sitting at about 14.5vac. ok whatever...

then keyed the mic and watched it jump up to 23.5vac.

measured the DC after the diode in this line, and sure enough, with mic keyed, the voltage was at about 19 volts DC.

I think you'll find the test leads on your digital volt meter have become an excellent antenna when placed inside the cabinet of an energized RF amplifier. When voltage increases on a DVM in the presence of a strong RF field, suspect that RFI is impacting the DVM. Place a .01 cap across the banana jacks on the meter and the problem should go away. If not, add two snap on chokes to the meter leads, at the meter end.

After rectification, filtering, and the voltage dropped across the rectifier diodes, the 14.5 VAC should increase to about 19 volts DC. That's because the DC filter caps charge to the peak of the AC voltage applied to them. That's 1.414 times the AC voltage. If it were really 23.5 VAC when keyed, the DC voltage would be closer to 32 VDC. In this case the DC filter cap in the power supply seems to be removing enough RF to provide a reliable DC reading or, the meter is just much more susceptible to RFI in the AC setting.

 

[loosecannon]

well both of those responses are somewhat relieving, as i think the transformer would be one of the hardest parts to replace.

Nomad, i do believe that the owner of this amp is someone that is under a misconception of how tube amps are supposed to be tuned, as he seemed confused about how i was describing it, and is someone who has been into CB radio for 40 years plus.

if a tube did arc over, would this explain why the amp seems to put out zero RF power for quite a while after being turned on and then eventually produces power?
i am not used to sweep tube amps taking this long to warm up.
the tubes in the amp now are the same ones that were in it when the cap and trace popped.

shockwave, i will try both of your suggestions in order to confirm that is what is going on, and i do hope that is the case.

i love when the problem is that i am mistaken.
LC

 

[loosecannon]

ok, adding the .01uf cap as shockwave suggested did change the vac reading, and although it still changed, the fact that it raised to a bit of a lesser value says to me that i am indeed dealing with RF getting into the meter so we'll let that go for now.

I just put in a set (not matched) of 6LB6 tubes i have lying around for my own amp project and the amp does seem to act more like i was used to seeing last time it was here.

both TUNE and LOAD controls peak very near the fully MESHED positions, and this is with no turns on either coil shorted. once it cools down im going to try spreading both coils a bit to see what changes.

then another test with my personal 6LB6 tubes.

then fully cooled down again, and a test with the owner's 6LF6 tubes and no other changes, in hopes to see what changes.

I wish i had a tube tester here so i could give some more info but alas i don't.

i am now under the strong suspicion that the owner had the amp tuned wrong, talked on it pretty heavily, and may have damaged one of the tubes.

honestly that sucks to have to tell someone they don't know how to operate their own amp, but luckily BBI has a video on tuning old sweep tube amps that seems to be pretty accurate.
maybe he'll believe BBI if he doesn't believe me.
LC

 

[nomadradio]

Umm, okay. Fully meshed position is max capacitance. Capacitors that max out suggest you need more coil, not less. Squeezing the turns closer together is the starting point, until they're close enough to risk touching. Then it's time to add a turn and compensate by spreading the turns back a bit. One whole turn tends to be a lot of correction. The "coarse" tune. Spreading the turns, or squeezing them together is the fine tune.

Oh, and digital meters are famous for getting "bled" by a nearby high-power RF source. You can't trust a reading that changes when the amplifier keys up, most often.

73

 

[loosecannon]

Thanks Nomad.

yes, i know i had previously been talking all about "fully UNmeshed" positions of the tuning caps, and im guessing that using a different set of tubes is the reason this changed?
IDK.

As of right now, the input tune seems to be good after adding 680pF of capacitance on the switch side of that 47 ohm resistor used for HI and LO power.
putting the amp in LO shorts this capacitor in essence, and input tune is only slightly higher in LO position. below 1.5 so im happy with that.

It seems to me that the owner of this amp probably had it mistuned, based on how he told me he'd "always done it", and my advice of how to tune it seemed to go right over his head.

so he got to ragchewing, with the amp mistuned, which burnt the trace that supplies 12vac to the bias line, which im guessing put the amp into overdrive, super heating the tubes, which caused one of the 100uF HV caps to smoke (they were only about two inches away from the final tubes) and the meter near the driver tube to melt.

the amp wasn't visible to him while talking so im guessing that the meter was melting before the cap went and the plume of smoke is what caused him to shut the amp down.

I have been using his tubes for testing up until today, and they seemed to put out decent power, but maybe there is a characteristic of these tubes that im not aware of where they can be damaged from being operated in that manner while still technically not being "bad".

There is a CB shop in town that has a tube tester, so im going to go down there and have his tubes tested, but the guy that runs it probably won't have any insight for me based on what he sees.

right now im tuning the amp with a set of (mismatched) 6LB6's acquired from ebay and they seem to be working so far and im left wondering if this set of tubes peaks the output tank circuit differently than his 6LF6's.

maybe that's why im talking about meshed instead of UNmeshed at this point.

boy if i haven't muddied the waters yet, i don't know what will LOL.
LC

 

[loosecannon]

Success!

again!

just to put a cap on all of this, i found two things that were really throwing me for a loop.

first, the wiring to the ceramic cap of the driver tube had gotten wiggled just loose enough so that tapping on it would cause the power to go down.
replaced and all is well.

the big issue however, can be seen in the pic below:

i was turning the LOAD control back and forth with the amp off, and i noticed a very slight rubbing sound.

so i put a continuity meter across the cap, and about half way through its rotation, BEEP!
continuity.
checked and re-checked, and yep, this cap is shorting out somewhere along its travel.

pulled the part dreading the thought of even trying to bend one of the plates for fear of making it completely fubar, i turned it over, and found that tiny piece of solder that had found its way right down inside of it to where it would never be seen without removing the part.

once that was sorted, i removed a turn from the antenna coil, added a turn to the coil going from the TUNE control to the LOAD control, and BAM.

everything peaks near the middle of its travel.

I think the crappy input tuning plus the solder chunk really had my head spinning and made me run back and forth in circles.

i still have to find a meter for it, and of course it's an odd size. (3" x 2.375" x 1")
and DC 0-1mA meters just don't seem to come in that size anymore.

thought i found one that would work but it was 2 inches deep which would put it right up against the driver tube. the search continues...

i attempted to add a 110vac brushless fan but it creates a bunch of receiver noise so i'll have to sort that out as well, but right now i am very happy regardless.

thanks to everyone who stuck with me through all my questioning and pontificating.
your help kept me on the right track.
LC