2E26 Transmitter

[Carl WA1KPD]

That screen voltage is too low at 70 volts.  Is the plate dip rather shallow?  Or does it peg off resonance?

Nice deep dip and no peg

...The transmitter looks like it's loaded lightly or not loaded at all......So maybe the output coupling Capacitor is at fault /defect,or the output coupling circuit is not right value/not the right load/antenna ......have You tried it with a dummy load?

Agree on the loading. My testing is in a 50 ohm dummy load with a Bird & 50 watt slug

The output circuit looks more like link coupled output stuff, where the cap tunes out reactance, is not used to load the final, the link position does that.
Its not pie net, so it wont tune like a pie net....What does that output coil and link look like?

Going to tear that apart this afternoon

...Can you switch the coils or is it built for one band?

You build it for one band

Yes, 70v is low; specs give 200v max. Something is going on there. 3.5 or 7 MC crystals? Could this be a 20M tx? Doubling in the oscillator and straight through or doubling in the PA would get it there.

Coils are for 75, as is xtal. Have checked them with a GDO and they dip fine on 3885 +/-

How many paper caps are in there?  You might find a few resistors well out of tolerance or even the wrong value. Ive picked up several old HB projects that looked darn near professional but the builder never got them working due to wiring or component mistakes.Carl

All caps are disc and check out OK. All resistors are well within +/-10%. It looks used Carl so I think it once worked, but who knows....

Also, as Gito pointed out: if this rig was designed for mobile operation, in the 1950's  then it is possible that in the expected application, the resistance of the antenna (after adjusting the output capacitor) would be

Since SO239s were used in the ARRL circuit I am ASS-U-MING it was designed for 50 ohms....

I am going to focus on the output side of the circuit, downstream of the PA coil for now

Thanks for all the help as someone else said I think this is going to be a "duh" moment when I find it

[KC4VWU]

Do you have the construction article on it? Maybe it is in one of the period handbooks. I may even have the particular QST issue.

Phil

[AB2EZ]

Carl

Looking at the QST article that you attached to your last E-mail:

For 80 meters, the article lists L2 as having 80 turns, and L3 as having 3 turns.

If I understand the link-coupled design correctly (I haven't studied it)... that means that the impedance step up ratio is (80/3) x (80/3) = 711.

That is much to large for a 50 ohm load. I.e. 711 x 50 = 35,556... but it is just about right for a 5 ohm load (80 mobile whip antenna with a loading coil).

Therefore, using a toroidal  step up transformer should help a great deal. 1 turn on the primary (facing the rig) and 3 turns on the secondary (facing the antenna) will convert a 50 ohm load to 50/9 = 5.6 ohms

Stu

[KC4VWU]

Found it! It's in the 1961 handbook. It explains that 80M operation would work O.K. into a loaded mobile whip by tuning the output link line. Tank Q on that band = 6. The PA is operated straight through on all bands and the multiplication is accomplished in the oscillator only.

Phil

[W1UJR]

I realize that I am late to the game, but wonder..

Tuning on a harmonic, and not the fundamental freq.?
Easy to do with the old buzzard rigs, often can't tell unless you measure the output freq directly.

[Carl WA1KPD]

 will convert a 50 ohm load to 50/9 = 5.6 ohms

Stu

Hey Stu,
I just tried it with a 4 ohm resistor for a dummy load and it seemed to act pretty much the same. Pulled the output assembly apart and the link tested out about right on inductance and no shorting to the coil.

I am thinking of drilling and blasting the output network and putting in the one from a April 54 QST (attached) which is designed for 50 ohms out. I would just use the 80 meter portions, no switch. looks like 123-350pf +/- plate cap and up to 1,000 pf of loading-I'll try and calculate it a bit closer

I realize that I am late to the game, but wonder..

Tuning on a harmonic, and not the fundamental freq.?
Easy to do with the old buzzard rigs, often can't tell unless you measure the output freq directly.

Checked it with the freq counter Bruce both on osc power only and PA. right on the money with VFO or xtal

[W1UJR]

Ok, how about lashing up a output network on the bench just for testing purposes, just to make sure that the rest of the rig/power supply is ok?
A simple res circuit going right into your dummy load.
No need to mod rig, just JS clip it in place, if all works well, then you know where the fault is.
Should take 10-15 minutes to whip something up with a wiz like you at the helm.

Plate blocking cap bad/open?
Jump out for testing?

[AB2EZ]

Carl

Was the 4 ohm resistor you tried a non-inductive type?

At 4 ohms, just a little inductance might mess things up in a way that might not be compensated by the output capacitor.

Stu

[KC4VWU]

WU2D did a good write up on the ARC-5's in ER a few years ago and he gives advice on matching networks for them. You're facing a similar matching challenge with that tx. Experimenting is fun!

Phil

[AB2EZ]

As an alternative to reducing the effective antenna impedance... try increasing the number of turns on L3 from 3 turns to 9 turns.

You could test this by just using some insulated hook-up wire to add turns to L3 or to substitute for L3

Stu

[KM1H]

A 8' whip on 80M will be a lot less than 12.5 Ohms.  There is no rocket science here, use a 20 to 40W light bulb as a load, the link tuning will have sufficient range as this was the same circuit as used for ages prior. The typical mobile setup then was a 50 Ohm output from the rig to a roller or tapped coil and variable cap in the trunk to match the antenna. Thats what I used with a TBS-50 in the late 50's and it worked fine.

Id get out a GDO and dip the driver and final to be sure where your at and then switch to diode mode and see what frequency the RF is. BTW, what is the 2E26 grid voltage and current? Current should be 3-4 ma according to the article which is a bit high but it should be measured.

Carl

[AB2EZ]

Carl

The output circuit design concept, used in this rig, has been used successfully in "ages prior"; but one still needs to implement it properly.

In the QST article, the table shows L3 having 3 turns for any band between 20 and 80 meters.... while L2 goes from 20 turns to 80 turns (as the band goes from 20 meters to 80 meters).

With a 50 ohm load, the turns ratio is simply too high on 80 meters (maybe the author never actually tried running this rig on that band).



Stu

[KM1H]

When working with link coupling there is more involved than just the turns ratio Stu.

The primary is usually designed with a Q of 10 to 12. The secondary is typicaly a Q of 2-3 to give some QSY ability.

In the rig under discussion the coupling is very tight and C4 is there to adjust to a feedline/load which is usually reactive in a mobile setup. In this particular rig C-4 is too small and should be around 450pf to give some leeway. Adding turns and keeping the 140pf cap will also accomplish the same but substantially change the Q of both the primary and secondary. With such tight coupling I would keep the coil as small as necessary to transfer power to the load.

BTW Carl, is the link wound in the same direction as the primary?

Carl...the other one

[Carl WA1KPD]

Lots more interesting ideas here.

Family time took over today and between some work and family obligations it will be about 3 weeks before I can crack into it again.

I appreciate all the ideas. I think I will do as Bruce suggested and lash up a PI net on the bench and see how that works.

Carl, link turns are symmetrical and ocs/PA dip OK on the GDO. Freq counter shows a 80 meter signal.

Stu. I may also just try adding some more links to see what happens. This ain't rocket science so it should be able to get going. Yes it was an inductive resistor so maybe it was enough to screw up the test.

I keep going back to being able to pick off 10 or so watts with a .001 cap attached to the PA. so I think it is a mismatch expecting a whip antenna of some sort and not a 50 Ohm dummy load.

I'd rather have it work into 50 ohms so I amy have to make some changes.

Also noted that the author W1TRF is still listed as active on QRZ (1200+ lookups) so maybe I can contact him.

Thanks all for the help. I've learned a lot today and will keep you posted.

73

Carl

WA1KPD

[WB6NVH]

Assuming that it was intended to match a mobile whip which was probably in the neighborhood of 4-10 Ohms, I am thinking that such a load was not a pure resistive one either.  Might try putting something like 50-100 pf in series with the output to see what happens.  The screen voltage is way too low, either because the screen is drawing too much current or some issue with the dropping resistor.

[KX5JT]

This is certainly a very well timed thread for me as I have a 2E26 aircraft band transmitter (now in parts) that I was wanting to eventually rebuild into a nice little plate modulated exciter for an amp on the low bands.  Nothing to add other than I'm watching this thread very closely, like a sponge hungry for knowledge. 

John KX5JT

[WB6NVH]

I looked at the article in the '61 Handbook and it shows that the author used two 28 Volt # 1820 lamp bulbs in series as the dummy load.  I wonder how close that would be to 10-50 Ohms resistive.   He also states that the transmitter is intended to be connected to a "loaded" whip and that the PA tank circuit on 80 Meters has a Q of 6.  That's not very good in my opinion and you are relying on stuff downstream to do the harmonic suppression.

A few pages on in the handbook shows feedpoint impedance of the typical loaded whip of the era at 16-22 Ohms on 80 Meters depending upon location of the loading coil, so I think it's safe to say that this transmitter would want something in that neighborhood to match.  But I think you should still get way more power out at 50 Ohms than you are.  Back to investigating why the screen voltage is so low.

[w1vtp]

<snip>...
  Back to investigating why the screen voltage is so low.

Yup!

[Gito]

Hi

After a reread my first reply,I  mistakenly wrote that the screen VOLTAGE soars up when there's the drive is normal,but the transmitter is loaded lightly,or not loaded at all,Actually What I'm trying to write is the CURRENT of the screen soars up, that makes the screen voltage drops ,because the screen voltage get's its  Voltage from a dropping resistor from the B+ (plate supply).

When using dropping resistor from B+ for the screen supply,We must have the right Grid drive, the right loading of the transmitter.
These 3 conditions has a correlation,
Low grid drive/no grid drive ..... the screen has low current,the voltage will soar up to B+/plate supply.(the transmitter at resonance)
high grid drive......the screen current gets high ,the screen voltage will drop  .

Normal drive ... the transmitter is loaded lightly/plate current is low (at resonance) ,the screen current will get high ,so the screen voltage gets low.

A correction of what I wrote before

Gito

[AB2EZ]

Gito
et al.

I agree with you (Gito).

I have always found the expressions "lightly loaded" and "heavily loaded" to be sources of confusion. I keep away from those expressions.

When people use the expression "lightly loaded" ... in the context of a pi network... they mean that the loading capacitor is set to a relatively small value of capacitance. This causes (through equations that are simple, but non-intuitive) the r.f. load (at resonance) on the tube to be relatively low. As a result, when the tube draws plate current, the fundamental frequency component of the plate current current flows through a relatively low impedance.

The consequence of that is that the plate voltage on the tube doesn't drop very much (from nominal B+) on positive swings of the plate current.

The consequences of that are: lots of plate current flowing (not much of a "dip" when tuning through resonance), lots of plate voltage on the tube during the entire r.f. cycle (well above the screen voltage)... and no reason for the electrons to stop at the screen. Therefore, relatively low screen current.

When people use the expression "heavily loaded"... in the context of a pi-network, they mean the the loading capacitor has has been set to a relatively high value of capacitance. This causes (through equations that are simple, but non-intuitive) the r.f. load (at resonance) on the tube to be relatively high. As a result, when the tube draws plate current, the fundamental frequency component of the plate current flows through a relatively high impedance.

The consequence of that is that the plate voltage on the tube drops to well below the screen voltage (from nominal B+) on positive swings of the plate current.

The consequences of that are: a big "dip" in plate current flowing, when tuning through resonance, a plate voltage on the tube below the screen voltage for a significant portion of the r.f. cycle... and during those times when the plate voltage is below the screen voltage, a good reason for the electrons to stop at the screen. Therefore, relatively high screen current. [The electrons will pass through the screen only if they see a field pointing toward the plate. When the plate voltage drops below the screen voltage, the electrons have no reason to continue on to the plate]

In the situation at hand, we do not have a pi network (please refer to my posting of today regarding coupled inductor output circuits]... but the end result is the same as what is referred to a "heavy loading" in the context of pi-networks.

Too high an r.f. load impedance on the plate of the tube (at the fundamental frequency) causes the plate voltage to drop significantly below the screen voltage for a significant fraction of each cycle. The electrons have no reason to continue past the screen to the plate during those portions of the r.f. cycle. Thus they stop at the screen. The screen current goes up significantly, and (because of the screen dropping resistor) the screen voltage drops.

Stu

[WU2D]

Oh The Humanity!

Don't worry Carl - the whole forum is on this little transmitters case now.

My 6AG7 6L6 rig is essentially that circuit and I don't even have a loading cap on the link. It puts out 12W on 40M and almost 15W on 80M into 50 Ohms. This is with 280V on the plate of the old 6L6.

The 40M coil has a 3T link on the cold side and the 80M coil has a tightly coupled 3 T link about in the middle of the plug in coil.

Mike WU2D

[Carl WA1KPD]

Mike,

That is one purty little rig. I heard you on 75 yesterday, nice sig. inot south central CT.

Interesting that the best coupling you got on 75 was in the center. When I was playing with the .001 cap as a direct connection that is about where I got the highest output.

There certainly has been a lot of great input (pun intended) and I look forward to getting my hands back into the little beast.

I was not an electronics student (other then getting my license 40 years ago) so I stumble around these things, learning as I go. The group here has sure spent a lot of effort and provided a variety of ideas.

73
Carl

[Gito]

Hi Stu

What I meant by loading  is like this.

suppose We have a transmitter with an 807 final.
The operating data says uses 600 VDC on plate ,275 VDC on screen.load plate current 100ma.screen current 6.5 ma using a 85 kohm as dropping resistor from the B+(plate power supply).

Loading heavily/full loaded means that We can load the transmitter to its full rating ,like  600VDC (on plate) and 100 ma at resonance(dip),transferring all the power to the load (60 watt input)

Lightly loaded means instead to load it to 100 ma, we load it only to 40 ma at resonance (dip) so the power output is smaller.Since this "two" transmitter has the same drive and circuit.

in heavily/full loaded transmitter ,the screen voltage is normal ,since it's  current is normal.

In lightly loaded transmitter the screen voltage drops since the screen current get higher than normal.( the screen voltage gets it with dropping resistor from B+/600v , V=I X R).

It's like this the plate current get higher...the screen current got lower.
the plate current get lower.....the screen current got higher
with the same drive and plate voltage.

So in this case(2E26  transmitter)  has a low screen voltage/the screen current get high .it is because it cannot  be loaded to its full capacity at resonance.
and It is Because it has not the right load ,design for this transmitter.

Gito

[AB2EZ]

Gito

I understand what you are saying. We are in total agreement as to what is actually going on....

However, your latest post (which probably gives the correct interpretation of the expressions "lightly loaded" and "heavily loaded") illustrates why I (speaking only for myself) try to avoid using those expressions.

In the definition of "lightly loaded" which you provided (and which is probably the correct definition)... the transmitter is "lightly loaded" when the r.f. load impedance (at resonance, and with respect to the fundamental frequency) on the tube is too high.

What makes this even more confusing is that... with a pi-network ... if the transmitter is lightly loaded, then the way to reduce  the r.f. the load impedance on the tube (to make it more heavily loaded) is to increase the effective resistance of the external load (e.g.the antenna). This is accomplished by reducing the capacitance of the loading capacitor.

 I believe that these expressions have misled and/or confused a large percentage of ham radio operators... to the point where they have trouble diagnosing why a particular transmitter is not working as it should.

Again... you and I are in total agreement as to what appears to be the cause of the low output power and the corresponding low screen voltage (at resonance) of this transmitter.

Best regards
Stu

[KM1H]

It sounds interesting except that you have the terms backwards Stu.

Why the loading terminology is confusing to many is that it is an inverse relationship. In a pi network as mentioned Light loading means more capacitance and Heavy loading means less. In a linear amplifier the proper adjustment is to tune for maximum power and increase the loading (less capacitance) until the power drops 3 to 5% which is the sweet spot for best IMD performance. In a Class C AM amp this adjustment is also critical in obtaining minimum distortion; drive level and bias also are important factors.


A Q of 6 is of no concern on 80M as mentioned in the article as a 8' single band loaded whip makes for a good lowpass filter.

As far as the radiation resistance of a 8' (12 degrees) whip on 80M it is between .4 and 1.6 Ohms depending upon location of the loading coil. Measured readings higher than that are a result of ground loss resistance which will be substantial on 80M and vary by installation.

Carl
KM1H