What am I doing wrong... my amplifier - won't!

[WB3JOK]

OK, some measurements and things I have learned:

First, the huge 14 MHz spur was an artifact because I did not have a direct connection to the spectrum analyzer, but a short length of wire antenna. Now that I've run the analyzer input through a large resistor directly to the dummy load connection, it looks clean. The 2nd harmonic only comes up above the noise floor when whistling or otherwise overdriving the exciter/amp 

Idling: Ep 2.8 kv, Ip 70 ma
Driven at 7.2 Mhz: Ep 2.6 kv, Ip 130 ma dipped, (160 ma off-resonance), Is 2 ma
On the grids: 120 v pp drive, bias -70v (i.e. swing from -20 to -140 Eg)

The output power does tune through a broad peak while adjusting the loading cap.
Plate dip is "normal", not too sharp, not super-shallow.

The power output is much better with the plate supply all the way up, 3.6 kv and 100 ma idling.

The input toroid is doing its job correctly - the output voltage (to the 200 ohm swamper and the grids) is exactly twice the voltage at its input. However, that signal is attenuated because... I'd forgotten that at some point I'd installed a t-pad on the input (20 ohms in each tee arm, 50 to ground) which is a 7.5 db attenuator. I had been planning to drive it from my Heath SSB rigs in that other mode 

I have found that there's plenty of power gain even with the 7.5 db attenuator and I can drive it hard (up to 200 ma plate current) on either 75 or 40 meters. Spectrum analyzer looks clean, too. But although the tank will resonate on two different inductor taps, the "wrong" one really kills the output power. It's considerably more critical than I thought. Guess I do need to study pi-net design more carefully!

It also seems that I have picked the wrong operating point? This amp originally was intended for use (and still may be) on AM, driven from my PW ARC-5. Somehow I had picked 450v for screen, possibly because my plate transformer gives 3600v when directly connected to the ac line...

[WQ9E]

Charles,

It sounds like you are on the right track now.

Going with  3.6 Kv. on the plates is well within 4-125 specs but you end up with a very high plate load resistance which changes your pi net calculations. 

How much power do you plan to extract from the amp? 

[WB3JOK]

Well... as much STRAP as I can, and still stay under 500 watts plate dissipation! 

Although my Thordarson plate transformer says 900 VA on the nameplate, so I can't run 1+ KW input continuously if I want it to last. Can't afford Peter Dahl iron 

[WA1GFZ]

since the rig runs better at higher voltages I suspect the pi network is set up for a high plate Z. You are at a low q point if the dip is shallow. The tubes will love the extra voltage. You will get more power if you reduce the inductance a bit.
130 ma resting it getting up there for dissipation at 3600V. I ran my old 4-1000A GG at 110 ma. You might want to blow some air on the tubes and pressurize the chassis to keep the seals cool.   

[WB3JOK]

Actually set to 100 ma at 3600v for now, but still 90w per tube.

The sockets are mounted above the chassis on 3/4" standoffs so I can't pressurize the chassis with a blower. A big muffin fan is blowing across them, (hopefully) keeping the seals as well as the envelopes within ratings!

I designed this amp about 6 years ago and can't find the schematic (if I ever drew one), so the details are a bit hazy, but since I could make the 3600 volt plate supply with junkbox parts (good for 250 ma continuous), that determined the plate load resistance

It IS 3600/(2*0.250) = 7200 ohms, right? 

[WA1GFZ]

I'm looking at an Eimac Data sheet (for 4 tubes)
Class C 3000, 600, -150, 1500 watts out
AB1 2500, 600, -96, 660 Wo
AB2  2500, 350, -43, 800 Wo
3500V you could get a little more if you keep it cool.
These AB1, AB2 specs are for audio service so RF you might get get a bit more at low duty cycle of SSB.

Get an old copy of the ARRL SSB handbook. It had a grid driven linear using 4-125s, 4-250s or 4-400s
Build a modulator and kick butt with it on AM.

[WB3JOK]

Building a modulator is one too many projects and I don't have the parts lying around anyhow... How about I just use this one as a linear on AM or SSB? 

After all, my original reason for building this amp was to amplify the 10 watt (carrier) PW'er up to 300 watts or so. A power gain of 16 dB should make me audible at least some of the time... 

I just need to find the right operating point and I should be able to make smoke...

[WQ9E]

Most of the spec sheets list AB1 and AB2 for two tube push pull.  The single tube AB1 RF data I found specifies a maximum of 200 watts output per tube.  This is with 3,000 plate, 510 screen, -95 bias, 30 Ip idle, 105 Ip max, and 6 mils screen current.  The 200 watts output is peak and although it does not specify I believe this is not including output circuit loss so your realistic RF output will be a little less. 

Running AM linear is pretty rough service for the tubes so although the output figures lead you to believe you could run up to 200 watts carrier (net of output tank losses) you would be beating up the tubes pretty badly.  To run at this power I would want to use proper air system sockets with chimneys and a properly sized blower.  Around 15- watts carrier/600 peak would be a more reasonable figure for AM linear and at this level you could probably get away with a pair of fans-one to blow through the sockets and another to blow air past the envelope and plate seal.

Tube parameters for that level are 2000 volts, 615 screen, -105 bias, 40 Ip idle, 135 Ip peak, 14 ma screen.  But if it were me I would use the 3,000 volt parameters and reduce drive to the proper level for a reasonable output.  Then if you decide to also use it with your SB-102 or similar you could run it as a pretty high power SSB rig.  It would also be easy to change the bias if you ever wanted to run it as a conservative 1 KW CW/RTTY amp.

[WA1GFZ]

How about screen modulation? That should be easy to do. AM linear would limit you to 150 to 200 watts carrier and make a lot of heat. Heck you could modulate the 4-65 voltage regulator.

[KM1H]

To put it in perspective you have the equivalent of a single 3-500Z. That means about a 200W carrier at most with plenty of air.

Thats a lot of work for only 13dB of gain. A rework with a pair of cheap 4-400's would give almost another 3dB with the same drive if your PS is up to it.

Screen modulation makes a lot more sense as Frank suggested at minimal added cost or complexity.

Carl
KM1H

 

[WB3JOK]

True, but the reason I decided to use parallel 4-125A's is one of the great ham homebrew traditions - they are dirt cheap! Especially compared to 3-500's or 4-400's.

Anyway it seems to be working better since I increased to full 3.6 kv on the plates and figured out better inductor tap settings. Today's a rainy day and I have a bad cold, so while stuck inside I built a soft-start for the plate transformer.

Now if someone would come back to a CQ (40m... daytime...) I can test it out   

[WB3JOK]

It works, on SSB anyway 

No ugly spurs unless badly overdriven. No parasitics on the spectrum analyzer either (despite not sinking the sockets below the chassis for more grid-plate isolation). 200 ohms swamping resistance appears to be keeping things under control.

Just had a nice long chat with a couple of guys in IL and OH... the large muffin fan is quite audible in my D-104, which they originally thought was 60 or 120 Hz ripple. 56 uf of filter capacitance is plenty 

Aside from that, it does provide the expected 2 S-units of improvement 
Now maybe I'll try it out with the screen-mod ARC-5!

[KM1H]

4-400's aint expensive, about $20 NIB at most hamfests around here. I was given a NIB pair in 1964 USAF boxes last month....the guy liberated them when he was stationed on Guam back then   Might have to build a more contemporary amp one of these days and quit messing with WW2 and earlier glass.

However if anybody has a nice 6C21 to go with the NIB one here I may return to the dark side I like my tubes in pairs at least.

Carl

[WB3JOK]

that's a lot cheaper than I had been seeing them... of course I haven't looked lately. No hamfests that fit my schedule or within a reasonable distance here... can't make it to Dayton this year 

For $20 NIB I'd definitely buy a pair 

Just searched ebag and the 4-125's seem to be gold plated these days!

[KM1H]

I think I paid $20-25 for a pair of NIB 125's at Nearfest a few years ago. Put them in my 2M FM amp for contests; the old ones from the amps FAA days were real tired and I didnt loaf them either for 25 years.

[WB3JOK]

I'd miswired something in the relay circuitry so I had to flip the standby/operate switch instead of keying from the SB-102... so today I went to fix it.  While doing so I broke one lead of the bias-cutoff relay off flush with the epoxy. Don't have a spare 24 VDC relay either, naturally.  At least I got to use it once before I broke it!
There's a major kwermtest started this weekend anyway, so I won't be operating very much.

After searching my previous posts on this subject I realized I started building this amp in 2004... where on earth did the last six years go?? 

[Opcom]

After searching my previous posts on this subject I realized I started building this amp in 2004... where on earth did the last six years go?? 

I ask myself that a lot.

[KA2QFX]

I unsure about something. Was this amp working and crapped out, or is this a HB in initial testing? 

Assuming the latter...
At what frequency are you measuring your plate inductors? Could they have significant shunt C between the windings at RF? 

[WB3JOK]

Yes, homebrew in initial testing.
My Tek 130 measures at 125-140 KHz.
Not sure how to calculate stray capacitance on a single-layer toroid coil... what would you consider "significant"?

[KA2QFX]

I would try to measure the inductance at the frequency which they will be used. Maybe with a signal generator and known approximate value R in series. Even a rice box at low power might suffice. You could measure the drop across the R and XL and determine the L.  Try it at different frequencies, if there's a considerable change (>10%) in calculated L over an octave I'd say you might want to consider a different inductor design.
unds too like
I sounds like your just not achieving any reasonable Q at resonance at RF frequency.