Single 4CX250B base amp for the masses!

Hello Nomad.
Still having issues with the bias of the Tube. When the bias is turned up the relay latches, tube gets hot and the Parasitic burns up.
The Amp has the original Low Voltage board. I have been studying your new Low Voltage board schematic that you have posted on eBay. I have adapted just the NEW Bias circuit to the old board.
Any and all feedback would be appreciated.

Really makes it sound as if turning up the plate current with the bias pot is kicking the tube into oscillating.

That would explain the toasted parasitic.

Next question would be why. The parasitic choke on the tube's anode connection is a big deal. Doesn't have to be the hairpin-style the factory used, just needs to have enough inductance to prevent it from oscillating. I found that just substituting larger resistors on the 'hairpin' widened the spacing and reduced the inductance enough to cause trouble. Got in the habit of inserting the resistors' wire leads from the outside of the hairpin, leaving the too-large resistor alongside, rather than inside the hairpin. This allowed using the original spacing across the inside of the hairpin.

The tiny parasitic choke wound on a half-Watt resistor on the tube's grid terminal is equally important.

Ground connections always come to mind when an amplifier oscillates. You did the 'suicide intervention' with the output-side coax cable, didn't you?

The factory used push-on pin contacts for the coax between the Load control and the output relay. They always come loose.

If those pins have not been replaced by a soldered connection, now is a good time.

More than once I have seen the rivets come loose that secure the input and output coax connectors to the rear panel. That can cause odd behavior until the rivets are drilled out and replaced with a machine screw and suitable tooth washers.

The relay can create odd symptoms, but usually gives itself away by arcing on the contact points, or making an area of the clear plastic cover dark near the arc.

The layout in the Pride has a good record of being stable, so long as everything is nailed down and grounded properly.

Cool upgrade to the factory Low-voltage board. More or less what we were doing back in the day. The foil traces get pretty fragile on that one. Biggest reason we started making our own was foil traces falling off the factory board.

Here's hoping the stability problem has a simple cause.

73

I would like directions/instructions on how to make the 9 to 1 input torid

I know I shot some pics showing how to configure the one that RF Parts sells.

Like, ten or more years ago.

Never figured out the exact type of the ferrite core they use in the Pride for this. Just easier to buy them from RF Parts.

But you still have to wire the three windings in series, use the first winding for input and the third one for output.

I'll dig around and see if I can find those.

73

1 Will those little blue chinese caps hold up if used to ground the cathode tube sockets, or do I need silver micas?
2 Have located a 500v CT transformer, 40 ma, good enough for 1 tube?
3 How do you determine what value doorknob cap to use?
4 What valvue for plate choke? One from an SB200 ok?
5 I seem to remember using a 50 wt, stud mount zener..

DannyAbear said:

1 Will those little blue chinese caps hold up if used to ground the cathode tube sockets, or do I need silver micas?
2 Have located a 500v CT transformer, 40 ma, good enough for 1 tube?
3 How do you determine what value doorknob cap to use?
4 What valvue for plate choke? One from an SB200 ok?
5 I seem to remember using a 50 wt, stud mount zener..

Click to expand...

1) Chinesium might hold but, digikey
2)you're looking for 1800 to 2k , That's more like 800v x 2.42 cap input
3)enough
4)close
5) or a stack of diodes
6) I'm a hack , Nomad will be along eventually

I have had good results from the blue chinese ceramic disc caps. Silver mica is overkill to use as bypass caps.

That transformer should do fine.

The doorknob cap between the plate choke and the Plate Tune control should be at least 500pf. This represents a reactance of about ten ohms at 27 MHz, plenty low enough. A higher capacitance value works, but won't affect performance at all.

The plate choke we use is about 50 uH, same as the Pride. Also the same plate choke used in the SB220 as it turns out. Haven't experimented with other values. Haven't needed to.

A 50-Watt zener to regulate the bias is overkill. The Pride used a single 5-Watt zener that was extreme UNDERkill. We use five 5-Watt zeners in series. A single 10-Watt is probably big enough. We also put a 'reverse-polarity' diode from the cold side of the grid choke to ground. Any pulse of positive voltage that comes out of the tube if there is an arc inside goes to ground through the diode and protects the bias control. The 6-Amp rectifier we use for this seems stout enough for this.

The schematic diagram of the low-voltage board we sell for the Pride is one of the pics in the Fleabay listing. The transformer it uses is about 500 Volts center tapped.

73

500 Volts center tapped...

Told you he'd be along shortly
Oh yeah , I'm a hack

Naturally you'll also need a transformer for the anode supply. The 500-Volt center tapped part will supply only control-grid and screen-grid bias.

The Pride's design pushes the tube's upper plate-voltage limits. Puts 1750 Volts AC into a bridge circuit. Gets you 2450 Volts DC to the tube's plate.

To use a full-wave voltage doubler would call for a transformer with half that output voltage, around 875 Volts AC.

Best deal I have seen for a high voltage transformer is a toroid from Antek.

Their type AS-4T430 is rated at 400 VA, but at 50 Hz. Add 20 percent to that when running from 60 Hz makes it nearly a 500-Watt rating. Plenty enough for one 4CX250. It has two 430-Volt secondaries. Wire them in series to work with the Pride's full-wave bridge type anode supply.

https://www.antekinc.com/as-4t430-400va-430v-transformer/

At 57 bucks plus shipping you won't beat it anywhere else.

And there's a bonus. Has two 6.3-Volt 4-Amp secondaries.

I ended up with a basket-case Pride years ago. Both transformers were blown, blower was gone, Load control replaced with some tiny thing. By the time we had it working, it got nicknamed the "Havana Taxi". Had way more substitute parts than original parts in it.



The Antek HV transformer would never have fit unless the blower was changed to the "biscuit" blower seen in the pic.

73

Thanks for the info; I am building/converting a Heathkit SB200 to run a 4cx250 or 350, haven;t bought tube yet, I remember trying a 350fj in my pride 25-30 years ago, with very little improvement; I have all of the major components, just built a Harbarch HV board for it.. The hardest part is making the air chamber to cool the tube but got that figured our just got to install it. Medical problems have me kinda slowed down right now, but hope to get it running in the next couple of weeks.. Thanks again

Stay away from the 4CX350 tubes. The control grid in that series of tubes is much more fragile. It's rated for zero Watts. The grid in the -250 tubes is rated at two Watts. Doesn't sound like a lot, but the '350 is designed for circuits that hold the drive voltage below the grid-current level. The '250 will tend to show better peak power in spite of the 350's higher heat rating of the anode. Harder to blow out from driving too hard.

No easy way to protect a 4CX350 from that.

73

Anyone tried the Russian 4cx-400? Is it a worthwhile upgrade from a 250b?

Leaning more to the Swetlana 4cx400, I have enough power supply for it; would the input circuit be the same as for a 250b and would something change?

The original russky name for this tube was GS36b. Svetlana renamed it the "4CX400B" as a marketing tactic in the '90s.

The differences look minor. The control grid is rated for less power than the 250B, so you may need to be back off the drive power a bit.

It draws about 15 percent more filament current, but probably won't overload a transformer meant for the 250B.

I bought a couple of these, but never got around to trying them. You'll need a larger chimney, since the anode ring is larger.

More power rating also means more air. All other things being equal, you should need twice the airflow needed by the 250B, more or less.

Without that, the higher power rating is meaningless. If you only have 250 Watts worth of airflow, it won't be enough to push this tube any harder than that.

Gotta try them and see for myself.

73

Hello All: Air flow is very important, knowing this from my latest experience...….

Using a Commander 1250 Amp on 20 meters I noticed the amp was running very HOT, why is this all of a sudden when the amp has worked great for years. Looking over the amp The Teflon chimney looked like it got super hot. Wow why? Found unsoldered joints in the output circuit from the factory, and a clogged fan.

Solder joints fixed, fan super cleaned and all shinny and pretty. Put a fancy temp gauge on the amps output air temp monitor to monitor the output air temp. Noticed the air temp raises to 135 F when used. Thats too much in my book. Added a muffin fan just above the tube on the outside of the amp, to suck air out as it was being pushed out from the internal fan.
Now the temp doesn't raise to about ballpark 100 F now. New internal Bodacious Fan ordered.

Eimac's book on "The Care and feeding of Power Grid Tubes" says that extra cooling air pushed thru the tube is a good thing and will not hurt anything. And may allow the ratings to be slightly exceeded. So more cooling air the better. Yeah what nomadradio said.

Jay in the Great Mojave Desert