Tetrode amp moved off work table.

[Naysayer]

I’m still running the 300a tubes as I cannot reach their published max output. Buying two 300y tubes (400w rating) was wishful thinking it seems. Made several mods and improvements but I’m still restricted to a DL although I managed to move amp from the work table to the operating location.

Expanded band testing down through 40 meters. I need band switch work. Specifically, I need to make or find a metal tab that the shorting rotor makes contact with (silver plated would be nice lol). I can’t believe how hard it is to find a band switch! Current old switch came with a copper part someone used as quick fix but it has no spring tension. After a short while it will not make contact with rotor. Looking for replacement parts was a rude experience. High cost and scarcity, it's a damn blight. No one makes shorting band switches anymore? I don’t want a roller but I looked anyway and they do seem plentiful.

Meanwhile it’s best to repair what I have. Ordered new coil stock for the 40-80 series connected section, I’m restricted to 40 and above by my antenna anyway so no great loss there. Had to do many small things, adjustments etc to make it safe for operating position.

I need a Peak reading watt meter despite swearing I'd never pay that much for a PO meter. But, something funny going on with my SSB PO and I need to measure. From my online meter search it seems a Daiwa model is about what I can justify (Spouse lol) financially. I see mixed reviews for the Daiwa models but Birds require far more than just a meter, line sections, slugs, peak kits. It's a whole new way of life.

My other tube amps were triodes (811a & 572b). Each of those Tank sections were the same in that tuning needed lots of Load capacitance and very little Tune Cap. So I built expecting the same but it’s not what I have going on with the Tetrodes. I can peak my PO with the Tune cap at mid-range no matter the Band (10-40) but not for the Load Cap. I cannot go low enough and I'm using only a single section. I’ve never seen that before, it’s always the other way around.

Dinner-time here in NY City so I gotta go help make it. Will return have more to write about moving this amp into operation including a sore back from picking it up. I'll be back soon, maybe revise above material. neil, NYC

 

[nomadradio]

Plate resistance of a tetrode tends to be higher, calling for a bigger coil than a triode might need at the same power level and anode potential.

Seems like, anyway. Might have an influence.

73

 

[Naysayer]

Plate Resistance: Thought I was getting somewhere then, I learn there's a concept at play I know nothing about. A good day to you too, lol. Guess I have more reading to do.

I do have a band switch problem. I'm keeping the original coil dimensions for the Eldico 4cx250 box (another reason to avoid Roller). Perhaps the unshorted coil windings added capacitance, IDK. I was about to try tuning with Load cap disconnected but once I recognized the band switch issue, I disassembled to so I'm back to waiting for parts.

May be next week.

PS
Your recent Pulser Vid about the 811 Ameritron the 600w PO displayed was PEP? I thought it high for 811's but I've never seen a Pulser before neil

 

[Naysayer]

I got lucky and found a relevant article on Tetrode Plate Resistance fast.

Tetrode Construction and Characteristics in electronics

Tetrode Construction and Characteristics- in this article you will learn about Tetrode construction and characteristics in full detail...

www.electroniclinic.com

Sped-Read it just now and it (Revised text Italicized) appears relevant. Thanks for the redirect Nomad. Higher plate voltage may decrease anode current which I found to be holding amp back. It's another dimension of Tetrodes I missed. The tuning has only just begun!

After a Morning Tune session into DL on 10, 15 and 20m is appears I need to revise the Tank circuit. Leads too long for one. Starting to lean toward a roller & moving and replacing Load cap with smaller one. Roller would eliminate my switch issue too. Something is not right in the Tank circuit

 

[DoctorAudio]

The plate output capacitance is also much lower on a tetrode than a triode (of the same plate dissipation ratings) which usually requires more plate/tune capacitance for a given frequency range.

 

[Naysayer]

Plate resistance of a tetrode tends to be higher, calling for a bigger coil than a triode might need at the same power level and anode potential.

Seems like, anyway. Might have an influence.

73

Thank you for the Replies. I'm determined to get at least a reliable amp from this effort but adjustments and revisions go on forever. Trying to follow the 4cx amp diagrams from Tokyo Hy-Power and Pride whenever possible.

High Plate Resistance of Tetrodes
I replaced the single 100R anode suppressor with two, 47R 5W so each tube has it's own suppressor. A small thing, but it all adds up.

I removed the over-sized Load Cap for a more appropriate replacement (pic). It's got 2 sections of 8-42uF. Metal work for mounting has me waiting for some hardware, angle brackets.

I found the post where Nomad noted the DX-300 plate choke was approximately 50uH. The Ameritron choke I was using was 245uH. Another oversized part needing replacement. I have an Ohmite plate choke from a 30L-1 (4x 811a) rated at 44uH at 680mA (pic). My PSU will never clear 400mA. The Ohmite datasheet says the choke is intended for 7-30mHz. Sounds like a fit....I like the small size. Can anyone think of reason to find another plate choke?

I also removed the original 80-40 tank coil section to repair the band switch. I found a better replacement that is the same as the original Eldico 2x 4cx250b Tank Coil (2.5" OD @ 7 TPI).

In a week or so, I'll have it all together again and see how it works. Beautiful weather here in NYC over the last few days.
n2eye,

NYC

 

[nomadradio]

Only reason for a plate choke bigger than a Z-14 would be the 80 meter band. They recommend it for 7 MHz and up for a reason, but I have never tried one below 7 MHz. Works just fine at 10 meters. A check of its self-resonant frequency might be worth the time. A choke coil that becomes accidentally resonant tends to destroy itself. It's purpose is to oppose RF current, and resonance reinforces it. Not healthy for the thin resistance wire inside that choke. Should be hefty enough for two of those tubes, but probably not three or more.

73

 

[Naysayer]

Thank you for your reply. For chuckles I added 2 tiny ferrite beads near the plate choke.

I'll find out if the Ohmite choke can take the voltage soon. It measured 2.5 ohms while the old choke was 7ohms. My antenna system cannot handle 80 so that's a no go area for me. Waiting for the 5w 47R suppressor resistors to finish. Polished the silver parts of tank coil, it gleams now, lol. The new 40/80 coil section isn't plated but it's an improvement and I corrected the shoddy connections that weren't visible before disassembly.

The new Load cap is on it's side to reduce lead length. That is rg8x on the Load cap but the Tune lead (used same wire as the 40/80 coil) is merely covered with insulation as is the plate choke wire (and the soon to explode ferrite beads). I'm hoping for that 600w I expected.

Found wire extenders on Amazon, pictured below. I used one to connect the coil together with solder but it's not visible in picture as I covered it up for more rigidity. These extenders come in all different sizes and they're cheap. Would have used them to extend my aluminum dipole a few weeks ago had I known they exist.

Stay well everyone,

neil, NYC

 

[Naysayer]

Applied power today after rebuilding Tank & band switch. Revisions included new plate choke (series resistance drop of almost 4 ohms), new Load cap and a band switch/pi rebuild/revisions.

The resultant optimal Tune and Load settings for max PO on 28.340 into DL are improved to the extent that the Load setting is now squarely within Variable Cap's range (it was at the very minimum edge previously). The Tune cap's optimal setting is now at the minimum setting possible likely below the minimum value possible. At least it's more normal in that I've always had a struggle to provide the optimal capacitance value from every air variable Tune cap because it's so low. It's certainly usable and peaks the PO nicely and will have to make do, no vacuums for me.

Testing still on bench so PS current & drive level is limited (3-4w) and restricted to 28.340. Conclusion: Some improvement perhaps more to come when I feed more current on different bands.

 

[Naysayer]

Last few days rewired the input section. It was very sloppy, too many parts. I'm surprised it worked at all. What's that saying about a monkey with a typewriter composing Shakespeare?

Deleted a paragraph about input tuning. All progress stopped a week at least. Small feed-throughs on blower box the insulation cracked, short to chassis. Need replacements.

But there is good news: I finally got a proper watt meter. Minty Drake W-4 with the 1971 paperwork to boot. Waiting for K4DPK's Peak Kit.
Next week maybe.
neil, NYC