[Amps] SB-220 rebuild advice

[Christopher J Galbraith]

Hi Tom,

Thanks very much for the suggestions.  I really appreciate it.  I have some
follow-up questions on some areas I don't understand.  Thanks for any
additional help.

1. Follow-up: What exactly does the glitch resistor accomplish?  Is its
purpose to drop the plate voltage during very short duration transients of
large current?  If so, does dropping the plate voltage somehow stop the
'glitch'?  Also, what is the nature of these glitches?  An anode-to-grid
short via plasma caused by a gassy tube?

A 50-Ohm resistor will only drop a few volts under normal operating
conditions.  To drop the plate voltage by just 500V there would have to be
10A of plate current.  With 10A of plate current, this resistor would be
dissipating I^2*R=100*50=5000W (hopefully for a *very* short amount of time,
until it becomes a fuse and does open up!)

2. I will ground the grids with copper straps to chassis.  The RFC and
capacitor parallel-tuned circuit resonating in the 100s of kHz makes me
nervous!  Also, a parallel-tuned circuit above resonance looks like
capacitive, and the only time I've seen active devices (FETs and BJTs) with
their gates/drains grounded through capacitors was in oscillators!

3. Follow-up: What sort of fault condition are we dealing with here?  Are we
concerned that if the Zener fails or the center-tap of the filament
transformer opens up that we will get a negative voltage on the cathode (I'm
not sure why this would occur), causing plate current to run away?

Would this clamping be accomplished in an SB-220 by placing a series of 3x
1N5408 (3.6V clamping voltage, thanks Paul) from the cathode (filament) to
ground?

4. OK, will do (the soft-key board will do this).

Other comments: Thanks for the procedure to de-gas the 3-500Z, a great idea.
Also, thanks for the parasitic resistor info and the advice on the other
components.  I found a 6 kV Fluke probe on eBay for $5, woo-hoo.

VY 73, Chris KA8WFC


---- (Part of original message below) ----


There are only a few things in the SB-220 to do:

1.) It needs a properly rated HV glitch resitor, and that 
doesn't mean sticking a small metal resistor in the line. To 
be effective you ahve to increase the supply ESR, and the 
resistor has to be able to withstand a glitch (3000V) from 
end to end and not arc or explode.

2.) It needs the grids grounded to the chassis with the 
shortest possible leads.

3.) It should have a negative rail clamping diode to protect 
the m,eters and other components.

4.) You should probably get rid of the 120V relay voltage 
inside the amp by adding an internal transistor buffer.

The rest is just emotional engineering that makes people 
feel good about nothing.


> 1. Is there a procedure to out-gas, or otherwise prep 
> these 'dormant'
> 3-500Zs for operation?  Is it reasonable to hope that they 
> will work after
> all of these years (they were good when put in storage).

One of the worse things to do is store large glass seal 
tubes for years in any environment. It is a bit worse in a 
humid or damp location because moisture deteriorates the 
metallic coating used in seals.

The primary gettering in a 3-500Z is the gray powder on the 
anode, and it is ONLY activated by heat. I recover old large 
glass tubes by running the filaments and applying positive 
grid bias. I run just enough HV to color the anodes a dull 
red. I cook the tubes for several hours.

You can actually temporarily convert the SB220 supply to a 
half-wave rectifier instead of a doubler. You can do this by 
removing the transformer lead to the center of the capacitor 
bank and adding a temporary similar diode bank to convert 
the doubler to a bridge.

This will give you about 1000 volts DC for the anode on CW, 
and 1500V on SSB. A variable negative voltage applied  to 
the filament center tap can be adjusted to make the tubes 
dissipate a few hundred watts per tube. Watch the grid 
current to be sure you don't over-do the grids.

I start out at about 1000 volts and 250mA Ip for a few hours 
and then go up to 1500 volts and 300mA or so for a few 
hours.

This will allow the tubes to safely getter at modest 
voltage. It also bakes out the transformer at lower voltage 
(the HV secondary has no DC in this situation, so it runs at 
about half of normal peak voltages).

> 2. Is replacement of other stock parts recommended for 
> preventative
> maintenance?  In particular, I am wondering if the 
> 5.1V/10W zener, the
> carbon comp. resistor in the parasitic suppressors, and 
> some of the HV
> ceramic bypass caps are good candidates.

Carbon resistors do sometimes go bad from age. The mechanism 
is the leads inside the resistor oxidize and lose contact 
with the carbon substrate. I'd replace the parasitic 
resistors with Ohmite OY series metallic composition 
resistors. The HV multiplier resistors are also subject to 
age related failures. I'd just test them to see if they are 
good.

I would not worry about the zener or any ceramic caps.

> 3. Is it advisable to directly ground the 3-500Z grids, 
> instead of the stock
> arrangement of grounding through a parallel RFC and 
> capacitors?

Yes. Absolutely. Ground the grids.

> 4. Are there other modifications to the stock circuit, 
> such as relay timing,
> parasitic suppression, etc. that anyone would recommend?

No, except buffer the relay with a transistor buffer so your 
exciter is happier.

> 5. Finally, does anyone have a favorite HV DC probe 
> (model, manufacturer)?
> I am going to buy one for this project and future amp work 
> (under 10 kV OK).

I just bought a standard HV TV set probe from MCM 
electronics. It works great, and so far I have not been 
killed while using it.

73 Tom