The AM Forum

My homebrewed 4x4-125A amp has been sitting for a couple of years. Will check the big electrolytic filter (a 58 uf surplus defibrillator cap) for re-forming, and dust off the tube envelopes as well. I can hook up a variac to the plate transformer primary to bring it up slow. Max is 4.2 kvdc unkeyed, 3.8 kv idling.

I seem to recall a recommendation to run the 4-125A filaments (only) for some period of time, before bringing up the HV?  Thanks. ???

I believe the 4-125A uses a zirconium getter applied to the anode which only becomes active, as a getter, at high temperatures. I don't know if filament power alone will get you there - maybe. It couldn't hurt to try, as long as you don't exceed the seal temperature ratings. When you do apply HV, use a series resistor to limit current in case of a flash-over. 4.2KV is 140% of the data sheet rating of 3KV so you're prudent to be cautious.


Don

It's not quite that bad - I remembered wrong. The defibrillator cap is rated at 4.2 kv. The actual voltages are something like 3.5/3.2 kv. But you're right, it's a bit over the Eimac datasheet rating.

I've used this amp before and not had any problems with flashover (but have a "glitch resistor" in the circuit anyway - 4-125A's are cheap but not that cheap).  ;)

Ran the filaments for about 20 mins, then set it to TUNE and hit the switch for the plate transformer.  :-X

Meter swung up to 3.2 kv and the plates gradually turned a dull orange (bias set to 100 ma idling current for the four tubes, or 80 watts dissipation for each 4-125A). Let it sit that way for another 15 mins. with no flashover or other ugliness  8)

Now to get it back on the air with my Command Sets (80 and 40m rx & tx with homebrew screen modulator)!  ;D

I had a set of new to me 500Z tubes that arc'ed over in my sb220 on ssb.

Put in CW mode, no arc.

Soon as I applied 50 watts of drive, arc city again.

Ran them in CW, increasing the drive until just a TRACE of blue would appear.  The plates where gently blushed.

As soon as the blue went away, would increase the drive again, to the point of just blue.

Took me a while to get to full output.

Then switched to ssb and dropped drive.  Keyed the amp, did the same process.

That was a year ago.  Tubes still running great.

Thought I'd pass it along just in case you have a problem when drive is applied. Start low, watch for blue (indicative of gas), and slowly take care of the problem


--Shane
KD6VXI

In preparation for the Bouvet DXpedition last month, I looked at my 30S-1 linear with lightning damaged HV On and Step-start relays .   I decided that I didn't have enough time to do a proper  restore.    Back in the corner of the hamshack was an old SB-220 that was last used in the late 80s ( 88 ? ) .      I didn't have a 240VAC variac, so I temporarily wired the 240vac plug to put 120vac across the 240 leads.     Obviously, the center tapped 120vac fan might not run.       I turned the power on, and saw that the filaments had a weak glow.     I left the half voltage on for maybe twenty minutes and then rewired the power cord back for normal 240VAC operation.   Filaments were glowing , just as I remembered them 31 years ago.    I closed the PTT relay, and after maybe 10 minutes, I opened the PTT and changed the HV to SSB position.   In SSB position, I again closed the PTT  ( no rf drive ) and watched the 3-500 tubes start to show a dull grey-orange color. 

On 20 meters, I tuned the SB-220 into a dummy load slowly .... 250 watts, 400 watts, 800 watts and finally 1100 watts.     No arcs / no sparks.     Since then, I have run filaments several more hours when cleaning up another project.       Tough tubes those 3-500Zs.

Don W4DNR