Re: G3SEK TRIODE BOARD on eBay
This listing sold on Thu, 9 Jan at 4:18 PM GMT ...
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Must be on the European Ebay? Does not show up on mine.
These are now as rare as hen's teeth, so I thought that I'd flag it to the list in case anybody is interested.
I have no connection with the seller.
See eBay Item No. 116441758244
73s Tracey G5VU
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Re: G3SEK TRIODE BOARD on eBay
Must be on the European Ebay? Does not show up on mine.
These are now as rare as hen's teeth, so I thought that I'd flag it to the list in case anybody is interested.
I have no connection with the seller.
See eBay Item No. 116441758244
73s Tracey G5VU
|
G3SEK TRIODE BOARD on eBay
These are now as rare as hen's teeth, so I thought that I'd flag it to the list in case anybody is interested.
I have no connection with the seller.
See eBay Item No. 116441758244
73s Tracey G5VU
|
Re: GG Amplifiers Bias Cut Off
The old man who sold it to me is reliable. The PA has hardly worked but it has not been turned on for about ten years. ? I will at least regenerate, a little, the filter capacitors, one by one, with an additional voltage so that they reform quietly; that, I know how to do. ? I will try a start-up under reduced voltage by leaving the two tubes in parallel. I will make the cathode and grid modifications afterwards, if the tubes seem ok. ? Thank you for your precious help. De la part de Alek Petkovic via groups.io
贰苍惫辞测é?: jeudi 9 janvier 2025 11:21
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Re: GG Amplifiers Bias Cut Off
I'm not very familiar with the L7 Francois but if I guess you
could do two tubes at once.
Even if you just turn on the amplifier and key it, at full plate
volts, if you think the tubes are OK. Run it like that without any
drive and maybe you will get enough anode current for you to see
the anodes glow red after some time. If you get 100mA per tube,
that is 250W. The anodes should glow red after a while.
If you know the tubes are not good, you should go with the W8JI
method of lowering the voltage, if possible.
On 9/01/2025 2:04 pm, Fran?ois via
groups.io wrote:
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See
Thanks
for this very interesting post. It should definitely be
adapted to a 3-500ZG (graphite anode) because the photograph
shows a 3-500Z with metal anode
?
So
900 V 250 mA (225 W) and the author states:
Watch the color of
tube anodes. Anodes should be a very faintly detectable
red. Do not run the tubes too hot.
?
In
the case of the L-7 which uses two tubes in parallel, what
would be the best solution to achieve cooking? (remove one
tube?)
De
la part de
Alek Petkovic via groups.io
贰苍惫辞测é?: jeudi 9 janvier 2025 00:12
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Re: GG Amplifiers Bias Cut Off
See Thanks for this very interesting post. It should definitely be adapted to a 3-500ZG (graphite anode) because the photograph shows a 3-500Z with metal anode ? So 900 V 250 mA (225 W) and the author states: Watch the color of tube anodes. Anodes should be a very faintly detectable red. Do not run the tubes too hot. ? In the case of the L-7 which uses two tubes in parallel, what would be the best solution to achieve cooking? (remove one tube?) De la part de Alek Petkovic via groups.io
贰苍惫辞测é?: jeudi 9 janvier 2025 00:12
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Re: GG Amplifiers Bias Cut Off
Yes. Very true but I' had it where the tubes will flashover at
1900/2000V, so by changing the HV output wires on amps like the
AL-80B and TL-922, you can easily get to around 1000V to start the
cooking process as per Tom's article. If the tubes flash at that
voltage, they become desk ornaments.
Alek.
On 9/01/2025 7:20 am, Jim VE7RF via
groups.io wrote:
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He can also just use the low 1900 vdc CW position, apply a
bit of drive, and tune the amp a bit off resonance.? ?That will
get the tubes good and red.?
A good Eimac 3-500Z? will? hi-pot? 16 kv? (between anode and
grid)..... and 5 kv? (between grid and cathode).?
Chinese? 3-500Z's? ?will hi pot 9-15 kv? from anode to
grid.....and only 1 kv between grid and cathode.?
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Re: GG Amplifiers Bias Cut Off
He can also just use the low 1900 vdc CW position, apply a bit of drive, and tune the amp a bit off resonance.? ?That will get the tubes good and red.?
A good Eimac 3-500Z? will? hi-pot? 16 kv? (between anode and grid)..... and 5 kv? (between grid and cathode).?
Chinese? 3-500Z's? ?will hi pot 9-15 kv? from anode to grid.....and only 1 kv between grid and cathode.?
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Re: GG Amplifiers Bias Cut Off
Francois...... Eimac and the other makers of the 3-500Z? used this chemical, called? 'Zirconium'? ?on the anodes.? ?The Zirconium coated anode IS the getter.? It is heat activated when the tube gets red.?
IF you want to operate the L7 at only 500-600 watts pep output....... use the LOWER? 1900 no load? B+ (CW)? position.? IE: 500-600 ma.....and aprx 1770 vdc loaded.? Then you will get better efficiency.
With 7.2 vdc of bias, and the lower 1900 vdc position used, the tubes will idle at just 40ma (normal).? ?IMD is superb? with 600 watts pep out.? I have measured the IMD on my spectrum analyzer.
Yes, you can use a Zener diode...BUT? remember that the zener is in the cathode..... and cathode current is the sum of the DC plate current and DC grid current.? ?IE: 800 ma of dc plate current and 300 ma of DC grid current = 1100? ma of cathode current.? ?1.1 amps? x 7.2 vdc (zener vdc) =? 7.92 watts of zener dissipation.? ?Zener's require a heatsink.? A 10 watt zener will run warm to hot.? ? You could use? 2 x 10 watt zeners in series, like a 4 vdc zener and a 3 vdc zener.?
The reason we use 10 x 1N5408 diodes in series for bias is..... they are rated for 3 amps CCS? and have a 200 amp surge rating.? ? The 10 x diodes can be mounted either horizontally or vertically...on some small perf board.? Non critical, since the vdc is so low.? 6A10 / PM600 diodes are rated for 6 amps CCS...and have a 400 amp surge rating.? ?Diodes are cheap to buy,? and do not require a heatsink.?
And yes, I use 2 x 100k? @ 3 watt MOF resistors in parallel (=50k)? ?for the cut off bias.? That's called....'redundancy'.? IF one resistor went open, you still have the 2nd resistor in the circuit.?
Jim? VE7RF
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Re: GG Amplifiers Bias Cut Off
Hi Francois.
Running the tubes on filament only does not help at all. You must
apply drive to activate the gettering process.
See
That is a good method. Run the amplifier at low anode volts
(around 1kV) and apply drive, with plate and load not tuned and
and get the anodes red.
If you have a 1kV Megger, you can check the tubes initially. It
will show if the tubes need attention. There should be no
breakdown between filament and grid on good tubes. If there is,
then you can do the above process. I have rescued a few tubes in
that way. It works very well.
73, Alek VK6APK
On 8/01/2025 4:27 pm, Fran?ois via
groups.io wrote:
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> This happens in Australia as well where we have 400W
PEP limit. At that level, the anodes never get red, so the
tubes fail very quickly.
> You will need to exercise your amplifier at full
power regularly to ensure healthy tubes.
Thank
you for this very important information. The PA has not been
used for 10 years. So there is no point in heating the
filaments for only 1 or 2 days to hope to degas.
?
I
need to heat the anodes. I did not know this particularity.
De
la part de
Alek Petkovic via groups.io
贰苍惫辞测é?: mercredi 8 janvier 2025 08:35
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Re: GG Amplifiers Bias Cut Off
> This happens in Australia as well where we have 400W PEP limit. At that level, the anodes never get red, so the tubes fail very quickly. > You will need to exercise your amplifier at full power regularly to ensure healthy tubes. Thank you for this very important information. The PA has not been used for 10 years. So there is no point in heating the filaments for only 1 or 2 days to hope to degas. ? I need to heat the anodes. I did not know this particularity. De la part de Alek Petkovic via groups.io
贰苍惫辞测é?: mercredi 8 janvier 2025 08:35
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Re: GG Amplifiers Bias Cut Off
If you run 500W PEP only, your tubes will quickly become gassy.
They have no separate getters. The gettering is done by the anodes
and so they must become red during operation to keep them healthy.
This happens in Australia as well where we have 400W PEP limit.
At that level, the anodes never get red, so the tubes fail very
quickly.
You will need to exercise your amplifier at full power regularly
to ensure healthy tubes.
73, Alek VK6APK
On 8/01/2025 2:16 pm, Fran?ois via
groups.io wrote:
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Thank
you for your patience
?
As
for the resistors at the terminals of the filter capacitors,
I understood that they allowed to distribute the voltage at
the terminal of each capacitor fairly. It is like for the
rectifier diodes with respect to the reverse voltage. This
point is understood.
?
Indeed
the quiescent current of the 3-500Z tubes is probably
sufficient without requiring the bleeders. You confirm to me
that the no-load voltage (in Rx) will not exceed 2650 V, or
330 V per capacitor.
?
Ok
for the 50 k? resistor in the grounding of the cathodes. It
is the equivalent of what was called automatic polarization
and it is indeed stabilizing. It is probably preferable to
constitute it by two resistors in parallel in case one cuts
out. You suggest that in Tx, the cathode circuit (the CT of
the xfmr wire as you say) is made as you indicate to me:
?
Note,
Drake did not use any bias at all on TX....and idle
current was very high ( 210-230 ma). Wire 10 x 1N5408
diodes in series....... in series with the CT of the xfmr
wire. The 10 x diodes will provide for 7.2 vdc of bias.
This will reduce the idle current down to 100 ma.
?
·????????
Do
you think that we could use a zener to replace the string of
diodes?
·????????
Have
you noticed the appearance of linearity distortion by
reducing this quiescent current?
?
Be
aware that in France, our HF power is legally limited to 500
W PEP. The two 3-500Z will therefore generally operate at a
slow trot
--
F1AMM
Fran?ois
De
la part de
Jim VE7RF via groups.io
贰苍惫辞测é?: mardi 7 janvier 2025 22:35
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Re: GG Amplifiers Bias Cut Off
Thank you for your patience ? As for the resistors at the terminals of the filter capacitors, I understood that they allowed to distribute the voltage at the terminal of each capacitor fairly. It is like for the rectifier diodes with respect to the reverse voltage. This point is understood. ? Indeed the quiescent current of the 3-500Z tubes is probably sufficient without requiring the bleeders. You confirm to me that the no-load voltage (in Rx) will not exceed 2650 V, or 330 V per capacitor. ? Ok for the 50 k? resistor in the grounding of the cathodes. It is the equivalent of what was called automatic polarization and it is indeed stabilizing. It is probably preferable to constitute it by two resistors in parallel in case one cuts out. You suggest that in Tx, the cathode circuit (the CT of the xfmr wire as you say) is made as you indicate to me: ? Note, Drake did not use any bias at all on TX....and idle current was very high ( 210-230 ma). Wire 10 x 1N5408 diodes in series....... in series with the CT of the xfmr wire. The 10 x diodes will provide for 7.2 vdc of bias. This will reduce the idle current down to 100 ma. ? ·???????? Do you think that we could use a zener to replace the string of diodes? ·???????? Have you noticed the appearance of linearity distortion by reducing this quiescent current? ? Be aware that in France, our HF power is legally limited to 500 W PEP. The two 3-500Z will therefore generally operate at a slow trot -- F1AMM Fran?ois De la part de Jim VE7RF via groups.io
贰苍惫辞测é?: mardi 7 janvier 2025 22:35
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Re: GG Amplifiers Bias Cut Off
Francois,? you do NOT require the? 2 x? 50K bleeder? resistors.? ?
?
Each of the 8 x 200 uf filter caps already has a 100K resistor across it.? ?2650 vdc / 800k = 3.3 ma?
?
The 8 x 100k resistors? serves? ?2 x functions.? (A)? equalizes the voltage drop across each 200uf? electrolytic.? ?(B) provides for some bleeder current...... so when HV shut off, the 2650 vdc? will drop down to 0 Vdc after a few mins.?
When I removed the 2 x 50K @ 50 watt wire wounds (and also the 5K @ 7watt resistor)? ?from the L4PS/ L7PS? HV supply,? the B+ did NOT increase ......remains at 2650 vdc? on RX.??
You won't find bleeder? resistor's being used on any other doubler power supply, none of them.? They all use just a resistor across? each HV electrolytic filter cap.?
Bill Orr's handbook has it wrong.? He shows a single bleeder resistor? ?between? B+ and B-.? ?That will only function as a bleeder resistor.? It will NOT equalize the voltage drops across each HV capacitor.?
And no,? don't insert resistors? ?R1? or RS.? That will make the voltage regulation much worse.? ?That same error is? in some of the older ARRL? hand books.?
In my opinion,? ?we could care less what the B+? is on RX.? ?The real? NO load B+ is what you have when drawing 100 ma idle current.? ?The amplifier has to draw idle current 1st...... before you can apply drive.?
Note, the drake L4PS? and L7PS? HV supplies are identical.? ?I have 4 of the L4PS supplies....and all 4 of then are modified the same way. (I have 4 x drake L4B linear amps).?
The 50k resistor in the CT of the fil xfmr.? It's value is not critical.? ?Anything from 10k? to 100k? will work to cut off the tubes on RX.? ? The dissipation if a 100K resistor? is used? is only .005 watt.? ?Dissipation if 50k ohms is used is? only .01 watt.? ? Dissipation if 10k ohms is used = .050 watt.? ? A 3 watt? MOF resistor is more than ample.? ?NO need for a 25 or 50 watt wire wound as seen in older handbooks.?
Does this make sense now ??
Jim? VE7RF
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Re: GG Amplifiers Bias Cut Off
Hello ? Thanks for the dictionary; it was essential. ? Fortunately I made a diagram so that you could tell me that it was not the right solution. ? I now have to translate your text to make a diagram of the device. It is indeed very difficult to understand the text. ? Concerning the removal of the two 50 k? resistors. One of my references is the ARRL Hand Book 1965. Attached is an extract relating to voltage doublers (as on the L-7). Oddly enough, it is a voltage doubler (a Latour in the L-7) that provides the most constant as a function of the flow rate, provided that a bleeder resistor is used. ? The removal of the bleeder means that when the HT current is low, the peak voltage is reached. What do you think? ? Ok for the direct grounding of the 3-400Z grids. ? In the same Hand-Bokk there is the schematic of a "Compact 3-400Z grounded-grid amplifier" but with only one 3-400Z in class C. It uses a 50 kΩ resistor in series with the cathode to block the 3-400Z during reception. This resistor is short-circuited in emission. Il read : ? The 50.000-ohm resistor in the center tape of the filament transformer biases the tube to cut-off during "stand-by" periode and eliminates the "diode noise" caused by static plate current. -- F1AMM Fran?ois De la part de Jim VE7RF via groups.io
贰苍惫辞测é?: mardi 7 janvier 2025 04:27
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Re: GG Amplifiers Bias Cut Off
Dentron Clipperton L and Yaesu FL-2100B have no grid current
metering, so B- is chassis ground.
I just did the hard grounding of grids on my FL-2100B. cathode
goes through bias diodes to ground with cutoff resistor switched
with an extra high speed 12V relay. Also changed the amp to +ve
12V keying and removed the ALC stuff. It works really well.
73, Alek VK6APK.
On 7/01/2025 11:27 am, Jim VE7RF via
groups.io wrote:
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On Sun, Jan 5, 2025 at 07:02 AM, John Sparkes wrote:
Hi fellas
Sounds like you guys learned in your hobby what
I used to teach my junior electrical engineers. Of course,
they all worried about the hot wire. I told them in order of
importance, it’s earth, then neutral, with the hot wire way
down the list. The green (E) and black one (B-) will kill
you or land you in prison (as the designer). Same applies to
amps !
John
You can either float the supply, OR grnd the B+.... OR grnd the
B-.? ? If you grnd the B+ to the chassis.... you now have a -5kv
supply...which will also kill u.? ?This assumes the safety
diodes are either not installed or have blown open ( normally
wired between B-? and chassis).??
You never grnd the B- to the chassis....... it's always floated
a bit.? ? Done that way so you can measure plate and grid
current.? ?In the old days, a 10 ohm resistor was wired between
B- and chassis.? Works BUT, if the anode arced to the grid in a
GG amp...or B+ arced to the chassis, the path for fault current
is up through the resistor, then back to the B- of the filter
caps...completing the loop...and of course the 10 ohm resistor
grenades.?
These days? RVS connected diodes are? used between B- and
chassis....then the B- is always within +/- .7 vdc from chassis
potential.??
?
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Re: GG Amplifiers Bias Cut Off
On Sun, Jan 5, 2025 at 07:02 AM, John Sparkes wrote:
Hi fellas
Sounds like you guys learned in your hobby what I used to teach my junior electrical engineers. Of course, they all worried about the hot wire. I told them in order of importance, it’s earth, then neutral, with the hot wire way down the list. The green (E) and black one (B-) will kill you or land you in prison (as the designer). Same applies to amps !
John
You can either float the supply, OR grnd the B+.... OR grnd the B-.? ? If you grnd the B+ to the chassis.... you now have a -5kv supply...which will also kill u.? ?This assumes the safety diodes are either not installed or have blown open ( normally wired between B-? and chassis).?? You never grnd the B- to the chassis....... it's always floated a bit.? ? Done that way so you can measure plate and grid current.? ?In the old days, a 10 ohm resistor was wired between B- and chassis.? Works BUT, if the anode arced to the grid in a GG amp...or B+ arced to the chassis, the path for fault current is up through the resistor, then back to the B- of the filter caps...completing the loop...and of course the 10 ohm resistor grenades.? These days? RVS connected diodes are? used between B- and chassis....then the B- is always within +/- .7 vdc from chassis potential.??
?
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Re: GG Amplifiers Bias Cut Off
On Sun, Jan 5, 2025 at 08:42 AM, Fran?ois wrote:
I have drawn a simplified diagram of the L-7 power supply. I have drawn in red an additional bridge to establish to prevent the cathode from being isolated.
?
?
In emission, the ammeter is shunted by the 5000 ohm resistor; this will have no impact on the measurement.
?
Do you confirm this modification?
You still have it wrong.? ?The 2 x 50K? @ 50 watt? wire wound resistors in the L7PS are removed.? ?The 5K @ 7 watt resistor below the chassis? of the L7PS is also removed.?
(Originally, Drake used the +120 vdc? Voltage drop across the 5 k resistor to cut the tubes off on RX. ) This modification alone will reduce the heat in the L7PS? (66 watt CCS load removed).? 66 watts continuous.? The top lid on the L7PS will now run cool.? Each of the 8 x 200uf @ 450 vdc electrolytics in the L7PS already have a 100K @ 2 watt Carbon comp resistor across them.? ?Those are called equalization resistors? (EQ resistors)...used to equalize the V drop across each electrolytic.??
?
These days, to cut the tubes off on RX, a 50k to 100k @ 2-3 watt? MOF resistor is wired into the Center tap? of the fil xfmr.? ?(The CT of the? fil xfmr has the yellow stripe).? ?Re wire the center pole of the 3PDT? TR relay...so the new resistor is in the circuit on RX.....and shorted out on TX.? ?The V drop across the new resistor? will cut the tubes off on RX.? ?On TX,? with resistor shorted out, the tubes will draw normal idle current.
Note, Drake did not use any bias at all on TX....and idle? current was very high ( 210-230 ma).? ? Wire 10 x 1N5408 diodes in series....... in series with the CT of the fil xfmr.? ?The 10 x diodes will provide for 7.2 vdc of bias.? ?This will reduce the idle current down to 100 ma.?
The 10 x diodes? are wired in series with the new? 50k - 100k? bias cut off resistor.? ?It's an easy modification.?
For safety, wire reverse connected diodes (1N5408)? between? B-? of? HV filter caps.....and chassis.? ?This will protect the plate and grid current meters.?
Does this all make sense ??
Jim? VE7RF
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Re: GG Amplifiers Bias Cut Off
On Sun, Jan 5, 2025 at 06:12 AM, Fran?ois wrote:
Hello
?
Your participation is very interesting because I am in the process of putting a Drake L-7 back into service. I understand very well the electrical problem that you mention and the original diagram also worried me (open cathode circuit).
?
?
I am French and it is already difficult to get a correct translation. Could you explain the abbreviations that you use and which do not seem universal to me:
?
·???????? HV
·???????? B+
·???????? B-
·???????? MOF
·???????? CT
·???????? xfmr
·???????? millen(s)
?
Could you send us a diagram
?
On this L-7 do you know why the grids are not directly grounded (RFC 1)
?
?
Thank you
HV =? high voltage? B+ = high voltage DC output..... ( like the +1900 / +2650 vdc on the L4B/L7).? B-? ?=? the B- end of the filter caps.? MOF =? Metal Oxide Film? resistor.? These have replaced carbon composition resistors..as used for the 100 K @ 3 watt? MOF across each of the 8 x electrolytic? caps used in the L7PS? HV supply.?? CT=? ?center tap.? ?IE: the center tap of the filament transformer. xfmr =? transformer? ?Example,? fil xfmr and plate xfmr. millen? =? the? red high voltage? connector used on the back of the L7 / L4B? amplifier.? (They also come in BLACK). Back in 1963, Drake came out with the original L4.? ?It RF grounded the grids with 200 pf capacitors.? ?Capacitors won't pass DC grid current, so the RFC1 choke was also used..... to pass DC grid current.? The concept was..... the? 7.4 pf? grid to cathode Capacitance of the tube formed? 1/2? of a voltage divider.... with the 3 x 200 pf grid caps? forming the other 1/2? of the voltage divider.? The concept / idea was it would provide for some NFB? ( negative feedback)? ?and improve IMD (inter modulation distortion products...splatter reduction).? ?It doesn't improve IMD at all, and makes the amplifier slightly more unstable.? ?It also reduces the gain of the amp. With the grid caps and grid choke removed.....and EACH grid bonded to the chassis? with copper strap (10 mm wide copper strap), aprx 20-25 watts? LESS drive required.? Does this make sense now ?? Jim? VE7RF
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Re: GG Amplifiers Bias Cut Off
Along the same lines as I mentioned the other day, QRO Technology made a blunder on the HF 2500 amplifier. ?I found this problem when a local ham called me and told me that his HF 2500 would restart the initial warm up timer sequence every time he switched the fan speed switch. ?I reiterated what he said and he confirmed I didn’t hear incorrectly. ?i?said to him that is next to impossible and he said it does it every single time without fail. ?I kinda thought it was nuts but I said bring it over and I’ll look at it. ?So he came by and I hooked it up on the test bench. ?Turned on the amp with the fan speed switch in low speed. ?Amp started up and went through the warmup cycle. ?I then switched the fan speed to high and damn if the warm up cycle started up again. ?Well I saw it happened and I was a believer. ?I told him he would have to leave the amp while I tried to find out why that was happening. ?I started out ?by trying to wrap my head around the problem by thinking to myself.how the two circuits might be intertwined electrically. ?No matter how I?tried to do it there simply was no relationship between the two circuits. ?Now I’ve seen the timer circuits in these amps go crazy with leaky tantulum caps and a bad transistor and on some that used old carbon resistors. ?So I just went in and replaced all three. ?Afterwards it did the same thing. ?That’s when I pulled the schematic and mulled over it. ?I particularly concentrated on the fan speed circuit. ?Drawing on my many years experience as an electrician I know full well the difference on switching resistive loads and inductive loads. ?Subconsciously we probably all know when we switch on a gang of old fluorescent lights. ?Remember that familiar click ?In the switch box? ?Well when I saw the way they wired the speed switching of the fan I instantly knew what the problem was. ?So I went and rewired the fan speed switching circuit, end of problem.
On Sunday, January 5, 2025, 1:02 AM, John Sparkes via groups.io <vk6jx1@...> wrote: Hi fellas Sounds like you guys learned in your hobby what I used to teach my junior electrical engineers. Of course, they all worried about the hot wire. I told them in order of importance, it’s earth, then neutral, with the hot wire way down the list. The green (E) and black one (B-) will kill you or land you in prison (as the designer). Same applies to amps ! John On 5 Jan 2025, at 2:03?am, Jim VE7RF via groups.io <jim.thom@...> wrote:
? Lou can chime in on this one.? On? the..... "big 3"? ? ameritron amps....... they don't use a resistor at all.? On RX it's wide open !? ?I got into an argument with W8JI about this years ago.? ?IMO, it's a stupid idea...... and 'saves' all of a few pennies at best.? ?While it ( a wide open across the relay contacts)? might? just work on the ameritron amps...it sure as hell doesn't work on that single and also dual GS35B? 6m amp I designed for a buddy in W7 land years ago.? ?I contend that with the B-? in the cathode open on RX........? the cathode will? try and assume partial to full B+.? ?The handbooks say the same.?
On those GS35B amps, the builder used red millens for the B+...and black millens for the B-.? ?One day, while putting it back together...( HV supply in a separate box) the black millen was not making contact.? ?With fil lit, then the B+ on next, all hell breaks loose...with an open B-.? ?The cathode indeed assumes B+ potential.? ?Those amps had aprx 4500 vdc no load on em.
I had the? exact same thing happen on my hb 4-1000 amps.? ? HV was in it's own box...... and separate? connections for the B+ ( millen)......and similar for the? B-.? ? One day, in super haste, zero patience....and only being 20 yrs old back then,? I wired it all back up....and proceeded to get in on the contest that night.? ? Like an idiot,? I completely forgot to hook up the B-? at the amp end.? ?All hell broke loose... with loud snapping and crackling going on.? ?Hooked up the B- and al was well.?
Next time I had it apart, the .01uf @ 1 kv disc caps used on the cold end of the fil choke were fried... with huge black smudge marks down the side of em.? ?With no B- hooked up, the cathode assumed all of the? B+....and arced across the bypass caps.? ? ?I was lucky it didn't arc across the coupling caps... ( between cathode and tuned inputs)..and start rfying the caps on the tuned input.?
These days, I take all that B-? wiring seriously.? ?The B-? from the supply goes to the negative side of the plate current meter.? Positive terminal of plate current meter is bonded to the positive terminal of grid current meter.? At that junction point,? it goes off to the bias cutoff resistor ( 2 x 100k @ 3 watt MOF's in parallel....for redundancy)...then through a mess of series bias diodes, then to the? CT of the fil xfmr.? ?
IF the plate current meter ever went open,? you have just opened up the B-.....and again, all hell breaks loose.? ?So in goes safety diodes across the plate current meter ( in addition to safety diodes wired directly between B- of? HV supply....and the HV supply chassis...and? also between? B-? and chassis? in the RF deck. ).?
On a similar note, drake used this stupid scheme? for cutoff bias? in the L4B / L7 amps.? ?They applied +120 vdc (+90 vdc when in? the lower 1900? voltage CW position) to the? CT of the fil xfmr...on RX.? On TX the center contact removed the +120/+90 vdc.... and instead? the CT? went off to the metering etc.?
Problem is...when relay contacts? in mid air? ( rich measures called that....'no mans land')? the CT has gone open.? ? ?And again, the cathode try's for a split second, to assume? full B+ potential.? ?Their scheme works.... but after years of use, the center contacts of the oem 3PDT TR relay were fried...and I mean burnt so bad.... it would not pull normal idle current.? With amp keyed, the CT was open.? ( The 100w input +? kw output contacts? were fine).?
?
?The easy fix of course was to toss the +120 vdc bias junk..... and install a 100k / 3 watt resistor in the CT of the fil xfmr.... an re-wire the contacts of the center pole of tr relay so resistor is inline all the time...and shunted on TX.?
I also added 10 x 1n5408's for bias....and also grnded the grids etc, etc.?
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