SB-200 - ground the grids or not?

[W3BH]

Hi all,

I'm rebuilding an SB-200 for 160 meter operation only. I brought it back from Cairo having picked it up with some other Heath equipment at the local flea market. It had been taken care of quite well, but had been disabled, probably by the security fellows, and there was no 120V from the secondary for the bias and relay. The two wires had been trimmed and taped and appeared to have been shorted internally. This did not appear to have affected the 2400 and 6.3 windings as far as load tests have shown. I replaced all rectifier diodes, caps and bleeder resistors, rebuilt the input and output circuits for 160, took out the SWR bridge, replaced and calibrated the meter, upgraded plate choke and filament choke, put in some glitch resistors, beefed up some critical bypass caps and added the soft start unit for the power supply. The full Monty as it were.  Now for the bias and relay circuit?

There were three choices for repowering it. An extra transformer to provide 120VAC on the secondary, tap the 2400 vac through a voltage divider or rectify AC straight off the line (ouch). I managed to find a suitable one to one transformer out of the junk box that just fits inside and which will operate the relay. I rebuilt the rest of the circuit and I'm getting about 130VAC open relay and -1.25 volts on the bias closed. That's pretty close but not the -2 the manual recommends or the 2 to 5 I've seen in SB-200 posts. It will pass and I can tweak it up by fiddling with the voltage divider.... but then I thought, I don't need the relay anyway as I'm always on a separate receiving antenna. So why not just strap the grids to chassis and use a relay (12 volt this time) to unground the centertap of the secondary of the filament winding during receive? I would want this relay controlled rather than controlled by a standby switch.   

Has anyone tried this and if so what should I be thinking about or what am I perhaps missing? Anybody have a schematic or diagram of a grounded grid SB-200 perhaps?

Many thanks to all,

Jim
W3BH

[KM1H]

If its shorted inside it will fry the transformer in good time.

[W3BH]

The secondary of the 120v is open, not shorted as such. I'm well aware of the implications of that. There is as yet, no short from either side of the secondary of the 120v to anything else. The 6.3 v has run loaded at length with no indication of fault. What the 2400v secondary will do under operational load is anybody's guess, but rather than simply junk the transformer at this point, I'm will to take a chance that what took the 120 secondary out 'may' not eventually affect the other two secondaries or the primary. There was no trace of burning or heat on the transformer and no smell of burning. If loosing a secondary in such a case is an automatic death sentence for such a transformer, I'm ready to replace it. I've never taken down my "Tune for Maximum Smoke" sign since I was a novice.

Thanks

[W2VW]

The fil center tap closure should be bypassed with something like 20K. I forget why.

[W3BH]

Thanks - you're correct. The Handbook (1964) edition shows on page 194 a 10K to ground on standby as part of the filament return and meter circuit on a grounded grid amp. It provides 'substantially cut off bias while in standby". I think a diode can also be used.

[KM1H]

The two wires had been trimmed and taped and appeared to have been shorted internally

My reply was based on the info you supplied above. I would think then that whatever was done could be reversed by removing the end bells.

Simply using a resistor to provide standby bias is a recipe for disaster, particularly with Russian and Chinese tubes but even with the original Cetrons. Heath went with -125V of cut off for a good reason as that amp has always been a hair from becoming an oscillator especially above 20M. Watching 572B anodes glowing almost white is a real experience

[W3BH]

KM1H,

Thanks very much.

I misspoke about the internal short in the first post and I agree with you entirely about the chances of the transformer becoming toast over time, as it were, with an internal short. I don't this is a short, but I'm not out of the woods yet as it may crater suddenly under load. It's hard to tell ahead of time how a transformer is going to blow when it goes, but this is rather a junk box project and I have a very minor investment in it so far. I'll see how far I can take it as it stands.

FYI, the tubes are Cetron 572Bs. By the looks of them, they could have just come out of the box. My feeling is that this amp was never put into service in the first place. It was completed, but there were wiring mistakes that I think would have brought it down straight away and this probably happened because of the bias-relay circuit being subjected briefly to 220VAC instead of 120VAC. Then, I believe, the authorities in Egypt put the 'red wax' on the unit and probably confiscated it or it sat on the shelf for many years before ending up for sale in the flea market. when they find something like this that is not licensed all Hell breaks loose and they will usually try and permanently disable the unit - thus the snipped 120VAC secondary leads ending right where they come out of the transformer. The authorities perhaps thought they were cutting the primary leads. LOL. That did shut it down though.

I'm going to bet that the transformer and tubes will hold up - we'll see about that?

If the transformer goes south, you'll be the first to know and I'll very gracefully accept an 'I told you so'.  

Many thanks
 

 

[WA1GFZ]

You need a resistor at the CT to limit the voltage on the heater winding. BTW you could get 120 with a filament transformer backwards on the 6.3 volt heater winding. 

[W3BH]

WA1GFZ,

Thanks - I had not thought of that one and I have a couple of filament transformers that would fit the bill and would be considerably smaller than the 1:1 I installed. That's all academic now as I am going to proceed with the grounded grid solution for better or worse. I've noted the resistor you mention, now it's time to do a little math before I go any further.

[KM1H]

You need a resistor at the CT to limit the voltage on the heater winding. BTW you could get 120 with a filament transformer backwards on the 6.3 volt heater winding

Huh? The heater winding CT is grounded in the original. If he is going to try and blow it up in pure GG then its needs both operating and standby bias as the 572B is not a zeo bias tube at 2400V and can become a great TPTG oscillator in standby when not biased well beyond cut-off. This will have to be fed in thru the CT.

[WA1GFZ]

If you need 100 volts of bias I would go with a 50K in the CT during stand by.
3 mils of current through the resistor would bias the cathode 150 volts above the grid. So that should cut it off. I built a 572 linear about 40 years ago and just don't remember the resistor I used but 50K most likely. If you need some operating bias just put a 10 watt zener or a string of dioles in series with the relay that shorts out the resistor. There are many handbook articles that use this method. 

[KM1H]

That should work fine as long as the relay contacts always do their thing. The nice thing about the SB-200 design is that no relay contacts are involved in bias switching.

[W3BH]

Thanks - from the schematics I've looked at so far anything from a 2K to infinity seems to work. So far I've seen 4K to 50K in published circuits.

W8JI shows this on his site as follows.  http://www.w8ji.com/bias.htm. The simplified circuit shows a single zener, cathode bias resistor and a relay. The relay is somewhat of a worry and I really need a backup shutdown circuit in case the relay fails. 

How to generate sufficient voltage for cut off on standby with 572Bs needs to be addressed of course. It appears that it can - My next concern is that I use a relay that will easily handle the current requirements and that I have glitch protection in the meter and plate circuits. Operating bias to be supplied with a string of zeners. I'll do some more checking the literature, a little math, rig it up and see what happens.

Looks like this is not going to be an SB-200 at the end of the day. I have some new in the box matched Simpson meters which are at least 50 years old for Plate, Grid and HV... hmm. I've also got a nice aluminum front panel already chassis punched that they would fit right into....

I very much appreciate all the comments I've received so far.
Thanks


 

[W2VW]

If you are going to do all that stuff why not just build another amp from the ground up? Better bottles could be used and so on. SB-200s are bringing a good chunk of change even now.

I am the same way. Can't stop coming up with neat stuff to retrofit.

[W3BH]

W2VW-

You're absolutely right. And apart from the power supply, which I just rebuilt, practically every component has been changed out and upgraded. In fact I have another chassis that would take the tubes and transformer straight away and the face plate already has the holes for the three meters I would use. It would give me a lot more room to work in and it's set up for racking as I have a 3/4 size Bud rack that would suit it fine. I really need some more room for my 160 tank circuit as well - working in the confines of the SB-200 case is a pain. I'm not going to sell it to anyone however, it looks like Swiss cheese from all the holes I've drilled and I'm on a retirement budget so I don't want to throw away the 572Bs and transformer till I've had a go at building something around them and see if it works. I thought I might finally get over 100 countries on 160 cw this year, but conditions have been so bad that I gave up and decided to see if I can build something with a little power for next season. I'll get this one running and then consider a second one with bigger bottles if I can get the parts together. I was out in Cairo for quite a few years as SU9AM and found that I spent more time working with old flea market obtained radios than I did on the air. It was like being a novice again and building your first rig. It was a shame I could not ship back all the spares, tubes and chassis that I had acquired. Out there I rebuilt two KWM-2As from the ground up from spares from carcasses of others that had been trashed by the Egyptian Air Force. I even built power supplies for them and used them on the air for a couple of years. That was a lot more fun than being DX. hi

Thanks for the comment...

Jim

 

 

[W2VW]

A pleasure to talk with you.
I think you have a lot of fun left for the future!

[KM1H]

Heath uses around -2.3V operating bias which could be increased a bit for stability as well as require a bit more RF drive, 100W is pushing 2 tubes pretty hard. Dentron used a 9.1V Zener and I usually run around -5V in 6M conversions, sometimes more with Chinese tubes.

[W3BH]

Thanks - that's very helpful at this stage. It's nice to have some ballpark figures from others who have actually run the same tubes in the same configuration. The 572Bs are listed at -2 and I can understand why a little more (negative) would help stability. I've read a lot about how hard it is to stabilize 572Bs at 6 meters - I understand their shape could have something to do with it. I want to set up so that I'm not driving them all that hard. My interest is in keeping the IMD down, running a cool final and having a lot of room to spare as far as stress on components is concerned - conservative. I still have nightmares about the pink slip I once got for running a pair of 1625's at something like a 170 watts, maxed out and gassing a really gorgeous purple. Apparently the chirp was awesome.
Thanks for the tip on the 9.1V Zener also.

[W3BH]

So why did Heathkit settle on -135 volts as the standby cutoff voltage for the two 572Bs in the SB-200? Perhaps since the relay required that voltage, it was simply convenient?

[KM1H]

I guess I'll have to keep repeating myself

The SB-200 is squirrely and on the edge of taking off at times. Putting the tube in way beyond cutoff prevents it becoming a TPTG oscillator in standby.

There are circuit fixes that stabilize the amp, especially on 20M and above.

With Chinese and Russian tubes it doesnt matter what band, its better to be prepared for when you blow up the Cetrons anyway.

[W3BH]

Ok, That's sunk in finally. Thanks


http://www.garant-funk.com/svetlana/appnoteNO.15.html

"W6TC bought a used FL-2100B and modified the bias circuit: Fig 5 shows the original arrangement and Fig 6 the modification, incorporating a voltage-doubler configuration to increase the stand-by cut-off bias. After modification, the amplifier becomes unconditionally stable when fitted with the higher-gain Svetlana 572B.

W6TC modified the stand-by bias in his FL-2100B to increase the bias voltage by using a voltage-doubler arrangement in order to ensure unloaded stability when the higher-gain Svetlana 572B valve is fitted.

The Heathkit SB-200 which uses a similar circuit has 100V cut-off bias and is stable when fitted with the Svetlana 572B. This applies also to the new Yaesu FL-2100Z."


http://www.w8ji.com/fl2100_problems.htm

Cut-off bias

"Diodes D301, 302, and 303 along with capacitors C301 through C301 form a negative voltage tripler for tube bias. Approximately -50 volts bias is supplied through R303. This is adequate to cut off any tube, regardless of slightly lower production mu."


This was helpful also.

[KA2QFX]

I concur with most posters on all their suggestions but offer this.

IF you're worried about relay failure on Stand-by, just leave the 20K-50K ohm cathode cut-off resistance in circuit all the time and only BYPASS it with the relay.

I never liked using fixed bias employing a big Zener to raise the cathode to some arbitrary voltage. Just tossing a coin on the "correct" bias voltage is guaranteed to give you a less than optimally biased tube stage. Similarly, at such low bias (2-7 volts) stacked diodes will exhibit very poor regulation over the rated load range. I've always replaced those circuits with a big (TO-3) transistor set up as a cathode voltage regulator that's adjustable.   Also, being able to adjust the voltage for equivalent bias current from tube to tube will also give you a good indication of the state of your tube's gain and remaining life. 

Mark

[KM1H]

There are several ways to skin the cat if all you are interested in obtaining a bias voltage. However for real world survivability during a tube internal gas arc its real hard to beat some healthy diodes such as 6A10's or even 1N5408's. Regulation is real simple and inexpensive, add a high value electrolytic across the string, about 10K uF is commonly used. Some tubes are more prone to arcing than others and the 572B, which is the theme of this thread, is one of them.

Carl