[Heath] More SB-200 joys...long...

[Kenneth G. Gordon]

On 14 Jan 2012 at 6:02, Guy Giacopuzzi wrote:

> 
> See below...
> 
> On 1/13/2012 4:25 PM, Kenneth G. Gordon wrote: 
>     Today, I replaced the original parasitic chokes with new ones, following a design by Carl 
>     KM1H. 
>     
>     After all was finished, I now have 100 more watts output than I had with the originals, even 
>     with well-worn Chinese tubes. 550 watts output on 80 whereas I was just making 450 watts 
>     output previously. Input plate and grid currents appear to be identical with what they were 
>     before.
>     
>     Measuring the original 47 ohm resistors, I find one is now 78 ohms and the other is 210 
>     ohms.
> 
> 
> How can this be?

Which? The original resistors being so far out of tolerance, or the increased output?

> Can you explain?

I think so: IMHO both are related. 

First of all, a little history. I bought this amp from a friend and fellow ham, a blind lady who is 
a master traffic handler, and who lives in a small town about 75 miles north of here.

When I got it, I found several problems with it. The first being that only one of the 572Bs was 
working. This was caused by a bad socket, but since Charlotte was blind, and her husband 
was not a ham, it was never discovered.

I fixed that and all seemed well, until I tried it on 40 meters. On 40 meters (only) keying the 
amp would often result in a blown breaker in the AC line to our clothes washing machine (!).

At the time I attributed this to some resonance in the wiring, but kept it in mind. I was 
suspicious.

I also discovered two different settings of the main tuning cap that resulted in peak output on 
my wattmeter on both 40 and 15 meters, even at 1:1 SWR or into a dummy load.

Later, it became quite apparent for yet other reasons that there were instabilities in the amp 
itself.

This manifested itself by two very clear occurrences: 1) maximum output never occurred at 
the minimum plate current dip on any band except 80 meters, and 2) more than a few 
moments of full key-down operation would result in the plates of at least one, and sometimes 
both tubes turning bright red. Even a somewhat "long" session at 14 wpm would result in the 
same occurrence. By "long" I mean the preamble to a CW net on which I was NCS on 80 
meters. This seemed unusual to me.

Furthermore, the fact that it was almost always the RIGHT tube that turned red first made me 
VERY suspicious.

Thirdly, but similar to 2) above, when I tried to use a pair of NEW Svetlana 572Bs, the plates 
would slowy turn red, and if left long enough, yellow, after a short period of the amp being on 
...with no drive signal input. In other words, the amp was "running away".

This, and 1) above both clearly proved to me that the amp had some serious instability 
issues, and was in need of neutralization...at the very least.

After some long discussions with Carl KM1H who has had extensive experience with the SB-
200 and is a very knowledgable, imaginative, intuitive and competent tech, I decided that my 
first step would be to examine the circuit in detail, and I also would examine everything others 
have discovered about it. 

Unfortunately, there is much misinformation and misdiagnoses on the net about this and 
similar amps, one of the clearest is the decision by some to increase the grid-to-ground 
capacitor value, ALONE. 

The original design of that part of the circuit was developed by Collins.

The 200 pfd grid cap is supposed to resonate with the length of the tube's internal leads to 
suppress VHF parasitics. Unfortunately, this is frustrated when the capacitor value is 
increased to 0.01 ufd or above as some have suggested. However, there is another problem 
with doing this: modern Chinese tubes are built differently inside, the lead lengths are 
different, and thus 200 pfd is no longer the correct value needed. Furthermore, by using 0.01 
ufd caps, SOME of the VHF parasitics which occurr with new tubes ARE suppressed, giving 
a false idea of what is actually taking place.

So, I have read often of folks using the Chinese tubes and having their amps "run away". 
Most of those to whom this happens blame the tubes, when, IMHO, the problem is not so 
MUCH the tubes as that the circuit was designed around American tubes which were built 
differently and exhibited different charateristics.

I suspect, but don't know, that when these folk's amps ran away, the parasitic chokes 
became overloaded, changing the values (cooking) of the resistors, and in some cases, 
allowing the amp to work semi-properly when they cooled off. My experiences appear to 
support this suspicion.

The Collins-designed grid circuit, while clever and adequate at the time, is not the best, 
especially with modern versions of the 572B.

Grounding the grid either directly or through a high value capacitor is much better, but must 
be combined with OTHER means to properly stabilize the amp.

So, after learning as much as I could, and after quite a bit of experience with the amp, all the 
while using it daily, I began by building and installing Carl's-designed parasitic chokes.

My result says to ME that the amp is now obviously far more stable, and that the new chokes 
killed some sort of VHF parasitic which Heathkit never anticipated since they never had the 
new 572Bs to deal with.

BTW, the Svetlana versions are NOT the same tube as either the original 572B nor the 
Chinese versions and have their own issues to deal with that are separate but similar to 
those I have outlined. Once the proper methods are followed, the Svetlana tubes are fully 
capable of full power output as reliably as are the other 572Bs.

My next step is to install the neutralization method Carl taught me, which, by the way, WAS 
used by B&W, GE, and some other commercial manufacturers of GG amps which used 
813s, but is, apparently, no longer used by any modern commercial amp builder.

>From what Carl tells me about his experiences, I suspect that after I have finished the 
neutralization process, 1) I will no longer experience red plates under normal operation as I 
have up to now, and 2) I will be able to use the Svetlana versions with no problems. Also, 
tuning will become more consistent, and maximum power output will require less drive, 
allowing grid current to be kept lower, extending tube life.

His methods, obviously, make the amp, and any similar amp, much less dependent on tube 
characteristics, i.e., they make the amp unconditionally stable...which any amp SHOULD be 
anyway.

I'll keep you informed.

Ken W7EKB