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[K1LEM]

 

Having had over 13 years, two New AL572 and each time having attacks of gas destroying the soild state bias circuit in the AL 572... I come to this conclusion.

Its not worth the trouble. I run VOX anyway and no CW with an amp. So the tubes resting currents are hardly contributing to what is a very HOT RF Box.
The issue is not so much if at all, with amp design, its the Chinese tubes and there poor quality. Using 572B is a low demand application with less than 2kv plate voltage hardly tickles them and there is less stress. But, the 572 Amp is a big boy and does use the four 572B to the end of their CCS design limits.

The suggestion I have is , if Ameritron has no engineer who will address this, then it is a far better thing, to bypass the entire circuit. I as a tech  am sick of replacing this circuit when the 572B has a flatulence issue.
So has some dedicated lover of this amp, and it is a "honey", decided to do a bypass and if so how did you do it?

By the way I took four new chinese 572B from RF parts tuesday and installing same:
1. At power up 1300 watts. Then nothing. Examination showed a 572B blushing always, a sure sign of a grid to filament short.
oops.
2. Later things get even more dicey. I am on 40 and suddenly a shot rings out.. Blam.. boooom.. no fuses blow, but the 50 Ohm fuseable resistor to ground on one tube bites the dust. Power drops to about 970 watts. Expected. Three Tubes active.
So much for the robust power supply, huh?

So I have out of four new tubes received yesterday, two left. One may work, because it is not showing a constant Grid to Fil short. However, seriously its not worth it.

I am told the Russain and more expensive tubes are better, but quite frankly Christine, at this point I could give a damn.
 

[W8JI]

First of all, you are not current on mods to the amp. Before I did anything I would bring the amp up to current state with this change. The 51 ohm resistors and capacitors should be replaced with direct connections to the PC board.

http://www.w8ji.com/arc_protection_al572.htm

If you still want to remove the bias it is pretty simple. The only problem is you will have to add a large Zener diode (or a long forward biased string of rectfiers). Pull the 3055 bias transistor and the collector pad becomes the connection to the tube cathodes and the emitter pad becomes the lead to the metering circuit. I'd run 8 or 10 volts bias.

As far as I know, there are no Russian 572B tubes. If you find a plant manufacturing 572B tubes in Russia, please let me know. All the 572's I know of come from China.

When I prototyped the AL572 it was with Svetlana tubes. When we got production tubes in, virtually all were bad. After a variety of questions and varied responses I was not even sure those tubes were made in Russia in the first place. Of course this was many years ago when the amp first was released. If that has changed, let me know.

[QRP4U2]

I would recommend replacing the 2N3055 PT and wiring an NTE 5185A 9.1 Volt 10 Watt zener or equivalent with the anode to point I and the cathode to point H.

Phil

[W9PMZ]

I installed the circuit that W8JI referenced and have had no arc's since.  It's not all that hard to install, just takes a little patience.

If you want some Svetlana tubes there is a guy on eBay that sells them.  When I had my last arc I replaced the Chinese tubes with the Svetlana ones.

Out of curiosity, when the arc occured in your amp did your amp now experience the negative grid current thump at turn on?  I can't remember if this is normal operation from when I first purchased the unit or if there is still something wrong that I have not corrected.

73,

Carl - W9PMZ