[TMC] This darn amp

Wed Oct 20 21:25:43 EDT 2021

Thanks Larry!

I appreciate the input and will squirrel it away. However, my amp does not
seem to have the symptom you described so I do not suspect the plate
blocking cap.

Kind regards,

Chris

On Wed, Oct 20, 2021 at 5:56 PM Larry Godek via TMC <tmc at mailman.qth.net>
wrote:

> I found, after a few years of operating my PAL-500 amp that the tuning
> is not a cut and dried thing.  Takes some playing with to learn the
> right way to do it with good results.  Also i had a plate blocking cap
> go out on me which kept the amp from doing anything.  Never found a
> correct replacement but one of the old mica or whatever they were caps,
> the old style molded ones with a 5 KV rating held up nicely, and still
> doing the job.  Had plate voltage and minimal plate current but couldn't
> dip the final.  The replacement plate blocking cap fixed the problem.
>
> Larry W0OGH
>
>
> On 10/20/2021 2:02 PM, Gary Schafer wrote:
> > Ok, what band are you doing the test on?
> >
> > An open screen bypass cap would probably cause the amp to go into
> oscillation.
> >
> > If the screen current goes negative then the screen voltage will rise
> provided there is a high resistance in the screen supply.
> > The screen meter being at 1/3 scale is because they have a resistor in
> the meter circuit that makes the meter draw some current, to where they
> indicate zero current, so that the meter can show positive or negative
> screen current.
> >
> > A quick indication of good tubes is to drive them rather hard and turn
> the plate tuning off resonance to see how high the plate current goes.
> > When the amp is properly loaded you will not get much plate current dip
> but you should see a peak in screen current. If you are getting a large
> plate current dip then it is not loaded properly
> >
> > 73
> > Gary  K4FMX
> >
> >
> >    _____
> >
> > From: Chris Bolkan [mailto:bolkyboats at gmail.com]
> > Sent: Wednesday, October 20, 2021 3:04 PM
> > To: Gary Schafer
> > Cc: tmc collector's group
> > Subject: Re: [TMC] This darn amp
> >
> >
> > Hi Gary!
> >
> > Thanks for the quick reply. Answers in Italics. I still haven't been out
> to the garage today so don't know any more than the last reply but.......
> >
> > if you had a shorted screen bypass capacitor then you wouldn't have
> proper screen voltage when measuring at the tube socket.
> > Agree. The cap is NOT shorted. It is open or appears so. It only
> measures 4pf, not 1000pf. I believe the built-in cap in one tube socket is
> defective (open). Will check the other socket shortly.
> >
> > If there is a high resistance in the screen voltage circuit the screen
> voltage will fall as the screens draw current.
> > Good to know! I did not know if the screen voltage would rise or fall. I
> assumed it could do either since screen current could go either positive or
> negative on the meter. I am going to check to see if there is high DC
> resistance in the screen supply circuit as you suggested earlier. At this
> point I know the AC impedance of the screens to ground is high because of
> the failed bypass cap built into the tube socket. Do you know the effect of
> a high AC impedance on the screen grid, i.e. failed bypass cap? I do not.
> >
> > It is sounding like you don't have enough drive to the amp. There are
> two input jacks, one is 100 mill watts and the other is 1 watt. If you put
> too much drive power in you will burn out the load resistor(s) at the input.
> > I seem to get adequate drive, if by that you mean adequate drive voltage
> displayed on the "drive" meter setting. Drive can EASILY be set to pull so
> much plate current that the HV breaker trips off. Problem is the amp is
> only putting out about 70 watts when it is drawing 300ma of plate current.
> >
> > A few, 5 to 10 ma of NEGATIVE screen current is normal when the amp is
> fully tuned up.
> > No matter what I do adjusting the amplifier I do not see any change in
> screen current in the "screen current" meter position. This is very
> frustrating as I have to the best of my ability determined that the meter
> is actually measuring screen current. It's kind of a funky circuit that
> measures screen current where a resistor divider has zero screen current
> displayed at about 1/3 scale on the meter. it all seems to be working
> correctly, but I never see a change in screen current on the meter. Hope
> that description was not too confusing.
> >
> > Check your driver meter readings as the manual shows.
> > Drive is monitored per manual. I have noticed this....when the amp is
> outputting power, the drive reading will be say 20-30 on the drive meter.
> When I unkey the amp (bias the finals off or kill the HV) the drive as
> reading as indicated on the meter rises significantly
> >
> >
> > Try the SSB and CW positions of the bias switch.
> > There is no SSB/CW switch. The amp I have is a PAL350A.
> >
> >
> > Thanks again for the questions and suggestions. This is frustrating but
> I am learning a lot, and in the end that is what it is all about. I
> appreciate everyone helping me through this.
> >
> > best regards,
> >
> > Chris
> > KI7NXY
> >
> >
> > On Wed, Oct 20, 2021 at 12:29 PM Gary Schafer <
> garyschafer at largeriver.net> wrote:
> >
> >
> >
> > If you had a shorted screen bypass capacitor then you wouldn't have
> proper screen voltage when measuring at the tube socket.
> >
> > If there is a high resistance in the screen voltage circuit the screen
> voltage will fall as the screens draw current.
> >
> > It is sounding like you don't have enough drive to the amp. There are
> two input jacks, one is 100 mill watts and the other is 1 watt. If you put
> too much drive power in you will burn out the load resistor(s) at the input.
> >
> > A few, 5 to 10 ma of NEGATIVE screen current is normal when the amp is
> fully tuned up.
> >
> > Check your driver meter readings as the manual shows.
> > Try the SSB and CW positions of the bias switch.
> >
> > 73
> > Gary  K4FMX
> >
> >
> >    _____
> >
> > From: Chris Bolkan [mailto:bolkyboats at gmail.com]
> > Sent: Wednesday, October 20, 2021 10:13 AM
> > To: garyschafer at largeriver.net
> > Cc: tmc collector's group
> > Subject: Re: [TMC] This darn amp
> >
> >
> > Hi Gary!
> >
> > Great questions. I'll do my best to answer.
> >
> > 1) Check that your screen voltage at the socket is holding steady as you
> drive it. Could be a high resistance in the supply circuit.
> > I have not checked this. I did find a power supply wiring error which I
> corrected and that brought the screen voltage up to spec. I then measured
> proper screen voltage at the socket. I have not however verified that the
> screen supply is stiff at the socket. I will apply some resistive loading
> and verify that the voltage remains the same.
> > I have begun checking the capacitor on the screen pin in the sockets. I
> have only started this effort and found right before bed last night that
> one of the sockets (haven't checked the other yet) has only 4pf of
> capacitance. The pin is not leaky to ground. It's like the cap in the
> socket just opened up. I do not know what the effect of this bypass cap
> being open can be. If it is degenerative feedback this might be my problem
> but I have to check the other socket first and learn what the effect of a
> high impedance screen to ground is.
> >
> >
> > 2) What is your plate idle and screen idle current?
> > Plate idle current is set to 90ma on V203 and then to 160ma with V204.
> Screen current reads 0
> >
> > 3) Also check the band switch, may be the TANK is way off frequency.
> > The tank tunes nicely dipping plate current with dune and peaking output
> with load
> >
> > 4) Do you get any dip in plate current when you turn the plate tune cap?
> > Yessiree a nice dip
> >
> > 5) Shorted output padder cap?
> > What is a padder cap?
> >
> > 6) Do you get a peak in output power when tuning the plate tune cap?
> >
> > Yes, I think so but I can't positively remember. I have been looking at
> the plate current when dipping and the scope in parallel with the dummy
> load when loading for peak. I dip the plate current with the tune and load
> for peak output and dip the plate and load for peak etc while bringing up
> the drive. I can't actually say if the output was peaking at the dip. I
> should know but I don't. Sorry.
> >
> >
> > Obviously I need to sort out the tube socket screen bypass cap thing,
> and I can easily check for screen supply stiffness at the screen pin on the
> socket. Can you tell me the effect of high impedance on the screens?  There
> are inductors in series with the screens so without the bypass capacitance
> the screen(s) would be high impedance.
> >
> > Thank you for the great questions Gary. I appreciate everyone's help.
> >
> > Chris
> > KI7NXY
> >
> > On Tue, Oct 19, 2021 at 11:04 AM Gary Schafer <
> garyschafer at largeriver.net> wrote:
> >
> >
> > Check that your screen voltage at the socket is holding steady as you
> drive it. Could be a high resistance in the supply circuit.
> >
> > What is your plate idle and screen idle current?
> >
> > Also check the band switch, may be the TANK is way off frequency.
> > Do you get any dip in plate current when you turn the plate tune cap?
> >
> > Shorted output padder cap?
> > Do you get a peak in output power when tuning the plate tune cap?
> >
> > 73
> > Gary. K4FMX
> >
> >
> > Sent from my iPhone
> >
> >> On Oct 19, 2021, at 10:31 AM, Chris Bolkan <bolkyboats at gmail.com>
> wrote:
> >>
> >> Hi TMC lovers!
> >>
> >>
> >> Got the power supply corrected. Now puts out a nice solid 360V to the
> final
> >> screens and all other voltages are correct.
> >>
> >> The good: All voltages correct, finals now bias nicely just as I think
> they
> >> should and it tunes up nicely.
> >>
> >> The bad: At 240ma plate current, single tone with a drive of 20, it puts
> >> out 120V p-p into a 50 ohm load. That calculates out to 35 watts RMS
> and is
> >> almost exactly what the inline wattmeter shows. Even when I pump it up
> to
> >> over 300ma it only puts out just 50-70 watts. I don't like doing that
> for
> >> long as occasionally the breaker trips because I have it set
> conservatively
> >> until I figure out what is wrong.
> >>
> >> So correcting the screen voltage and making it all "right" still did not
> >> change the output power. That's what it was before even with what I
> >> consider a profound change.
> >>
> >> This is also puzzling: There is no change in screen current (on the
> meter)
> >> no matter what I am doing tuning this thing up. The meter reads very
> close
> >> to 0 (about a third of the way up the scale as it should, but nothing I
> do
> >> changes screen current. I verified the proper screen voltage on the
> final
> >> tube socket pins so I know the sockets are not shorted. Since the
> screens
> >> get their voltage off the measurement divider, if the current is
> changing
> >> it should register.
> >>
> >> I still have not checked the capacitance on the screen pins of the
> sockets
> >> yet. If those were somehow low would that have a degenerative feedback
> >> effect? Even so it seems highly unlikely that both sockets have failed
> in
> >> exactly the same way. This is so confusing.
> >>
> >> I've been posting a bit lately about my PAL 350. I've not seen a single
> >> comment so I guess I don't know if the posts are even going out. Well,
> >> here's the latest. Any ideas would be greatly appreciated!
> >>
> >> Chris
> >> ______________________________________________________________
> >> TMC mailing list
> >> Home: http://mailman.qth.net/mailman/listinfo/tmc
> >> Help: http://mailman.qth.net/mmfaq.htm
> >> Post: mailto:TMC at mailman.qth.net
> >>
> >> This list hosted by: http://www.qsl.net
> >> Please help support this email list: http://www.qsl.net/donate.html
> >
> >
> > ______________________________________________________________
> > TMC mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/tmc
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:TMC at mailman.qth.net
> >
> > This list hosted by: http://www.qsl.net
> > Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> TMC mailing list
> Home: http://mailman.qth.net/mailman/listinfo/tmc
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:TMC at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html