M.O.F Metallic Oxide Film resistors have the resistive film on the outside?

[N3DRB The Derb]

recently obtained Gonset amp #2 "for parts or repair" unit ( snicker ) had these resistors as parasitic suppressors. I know you can use these   but I read that the film is actually on the outside of the body. Correct or no?

A parasitic suppressor wound in the normal manner coiled on the body of the resistor as one would on a carbon unit would have some interesting results, no?

[W3SLK]

Derb, my old buddy Eric, WB4VVI, (SK) told me that some of these film type resistors were 'spiral wound.' That is to say that the resistive material would be coiled on the form giving them some inductance. He told me how he had problems troubleshooting a radio until he final shot gunned the R's. He place them back one at a time to see which caused the problems. He found the offending component and called the manufacturer to find out why. He talked to a person in tech support, (remember when you could talk to a real person?), who told him how the part was manufactured. They both concluded it was from inductance caused by how it was deposited on the form.

[N3DRB The Derb]

the fact that they were not soldered together couldn't have helped much. 

[The Slab Bacon]

MOF resistors are usually made of glass rods with the resistive film deposited on the outside. If you break one in half, you can see this. I have used then for just about everything BUT parasitic supressors and never had a problem. Even in RF applications, they seem to work fine. IIRC they were originally touted as "flame proof" resistors. I have even stacked them to make small dummy loads with no adverse effects, even at VHF and UHF frequencies.

                                                             The Slab Bacon

[WA1GFZ]

some film resistors are spiral wound but most of them are deposited on a rod as Frank says. All film resistors have limited peak power ratings. Carbon composite have the highest. I used a 1 ohm one watt resistor in a transient with almost 200 amps peak current. It has run for hours as part of a 1600 watt peak transient generator. When we first built it we tried this fancy large film current sense resistor good for at least 10 watts. It blew open in a few minutes. The one ohm carbon was used and removed from our old transient generator that was built about 30 years ago.

[WBear2GCR]

Derb,

Best to use a carbon composition resistor for the parasitic suppressor, if you are going to use a parasitic supressor.

Imho a spiral cut film resistor would be inductive in the "range of interest" so would be questionable in this application - the trick would be to know in advance what the internal construction of a given film type resistor happens to be. 

There are three schools of thought on the parasitic supressor:
- the traditional
- the Rich Measures
- the W8JI (or it that W8AI? - have to check...)

Pick one.

Whatever you do Derb - beware of the latent sonic effects of capacitors!! 

               _-_-bear

[N3DRB The Derb]

 I will, bear.

 I am just insanely interested in how cum these GSB 201's have such a problem with the bandswitch contacts. I now have 2 and each one has come in with the 40-20-15-10 contacts fried to a crisp. It's now personal, I'm either gonna solve it and swing the munky or die trying -  - I think it's a combo of things and Slab identified some of them and from all the reading I am doing I am picking up some others.

I am fixated on making these things work for me. The 'new' one I bought offa ebay for 150 is in better shape than I thought it was. I'm going to confer with the slab + JN + JJ G-G amplifier brain trust and work out a consensus on how to tame these things and make em work for me, but for now it goes back on the shelf so I can do the link coil on the tuna in some manner I'm going to try Skip's 7Yoohoo's  hi temp hot gun glue idea tonite. If you hear me on it was a success.



 

[Jim, W5JO]

Your problem is probably a parasitic.  If you can find them, get two 100 ohm 2 or 3 watt globar resistor and parallel them.  It may be difficult to find them but since that thing has burned bandswitch contacts, the is probably where the problem lies.

[N3DRB The Derb]

actually Jim, I have 2 of them and both of them have burned the bandswitches up in he exact same places. I found 2 new wafers down in Richmond for each one but I's sure like to track down the causes before I put in my new wafers n have one of them burn and sizzle on me.

Slab Bacon pointed to the plate choke being wimpy and it is - single wound little thing about 3 " high at best. I am getting back the first one this weekend, Slab has it but I think his schedule is too busy these days so I'm going to take it back over. Hopefully I'll find enough goodies at this show coming up where I can get it cooking and use it as a model to restore the other one I just got.

I'd love to find 2 National 152 chokes as a starting point, along with what I need to make new suppressors and stock up on HV bypass caps. Lots and Lots of junkbox stuff.

[k4kyv]

I use a pair of commercially made parasitic suppressors, circa 1938, made by Bud IIRC.  They look like ordinary 20w wirewound resistors with a few turns of #14 wire wound round them.  I'm not sure if the resistors are non-inductive or not.  I have seen non-inductive wirewound resistors made with two layers of wire, each wound in the opposite direction from the other.  I have them in the grid leads of the HF-300's and they work.  Before acquiring them I had a pair of 5w carbon composition resistors wound with wire, but they always ran hot.

[w3jn]

Parasitic supressors can be kinda touchy.  I have a virtually mint Hallicrafters HT-20 that apparently was never used due to the parasitic suppressor burning out.  I replaced the resistor, using the same value and the same coil that was on the old one.  Tuned the thing up into a dummy load on 10 M and I starting seeing copious amounts of smoke being blasted out by the fan     Yep, the new supressor crapped out also.

Since the thing uses a 4D32, and knowing for a fact that Art Collins was the best RF engineer ever, I copped a pic of a 32V parasitic suppressor, copied that exactly, and havne't had a problem since.

You may wanna check out the 30L1 parasitic supressor design and see if it differs from the Gonset.  AFAIK none of the Collins Collectors Mafia have complained about parasitic problems in 30L1s.

[W3SLK]

When it comes to parasitics in amplifiers, Rich Measures is the man. He advocates using nichrome wire in a hairpin configuration to tame UHF/VHF parasitics. I know Duane, KK4AM used them to get rid of VHF spurs on his 813 amp. Here is his website: http://www.somis.org/ Scroll down to the section on amplifiers. Check out the subject that matches your problem. Don't be afraid to send him an email and communicate with him directly. He will get back to you pretty promptly. I hope this helps.

[WBear2GCR]

Some disagree with Rich Measure's methods:

http://W8JI.com

I think '8JI has worked it through more thoroughly.
No doubt that the Measure's method may work fine in some situations though.

I suspect that beyond the blowing of the contacts due to parasitics, it is entirely possible that hammy hot switched the bandswitch too...

                _-_-bear

[N3DRB The Derb]

It's just wild that 2 of the same amps had exactly the same problem in the same place. I'm doing a lot of reading on the G-G subject. One of the sites recommends grounding the grids directly for DC and RF at the same time, using ribbon and a rf bypass cap in parallel on the same grid terminals.

I think Slab has hit on some points that need to be addressed in the design. the plate and grid wires in the same wiring harness for instance. I see others, such as long ribbon striplines to reach the tuning and loading controls from the tank and bandswitch area. I think both of these ribbons may need to be broken up themselves with some type of suppressor. The one from the tuning cap to the plate choke is over 5" long+ and so is the one to the loading cap. Something inside tells me this is trouble. When you have 1 amp that fails in his way, it can be dismissed. 2, not so easy to do.

I'm going to lick this and use these. I still like these amps, when debugged they will give me 225 watts carrier with the Yeasu's. They are dirt cheap, even I can afford them. Gonset paint quality was is crap, which means they usually look like shit as found. Thats' ood for me, because I'm going to re do everything anyways like I always have.

KS3L used to use one of these amps with he 811's and he never had a lick of trouble with his. I see plenty of problems with other amps on all these sites - the kenwood 922's seem to be a problem.

[WBear2GCR]

another thing - back in the day we ran them at W2DSC (college station) a ton on 40m - no probs other than blowing holes in the plates of 811As... so I suspect that they get a wee bit squirrely on 15m and 10m. If you don't run 'em there, probably they'll just play fine. Guessing, of course.

                     _-_-bear

               

[W3SLK]

Tom Rauch is a complete A$$hole! I wouldn't believe anything that came out of his craw even if I saw him get it from the Burning Bush!

[WA1GFZ]

Measures is a better source of information

[Todd, KA1KAQ]

Heh, yeah - I was thinking the same thing when I read that post: would the 'some people' be Tom Rauch? If so, he disagrees with everything Rich Measures says, as a matter of routine. That 'exchange' has been going on for as long as I can remember, on the amps list and elsewhere. And I agree completely with you Mike(y). 

BTW - he (W8JI) does not like being referred to as Chucky or Chas.

Along with parasitic suppressors, isn't there another area that MOF resistors should be avoided? I'm thinking audio, but that may be cap types. Just seems like there was someplace else....

[WA1GFZ]

I thought MOF resistors were better at audio?

[w4bfs]

I used MOF resistors to make 50W 300 Ohm load for G2DAF amplifier.  By arranging them physically as a 'brick' and ASSuming that their spiral resistive bands were aligned consistently with the exterior color code banding, I put them in parallel with bands 'flipping' with their neighbors as well as the next layer.  It seems to have worked as it formed a 300 Ohm load with very little reactance 160 thru 10 meters.  By the way, the G2DAF amp will work with low distortion if the voltage multiplier time constants are kept in the proper ranges.   73   John

[W3RSW]

Uh oh!
Well anyway, no smoke yet.....
..MOF R's in many years service in the HB 8875's amp.

No problem using them as parasitic suppressors. The resistors even have little vent holes in the ends. - three each 100 ohms/ 2 watt MOF's, paralleled under 2 turns 3/8" Cu strap.

From all I've read it looks like par. supp's are almost a "one-off per amp." art, almost witchcraft.  I have to admit that after one post I popped the cover of my 32V2 to see what Art did! Some basics must be observed but suppressor design plus the plate RF choke seem to be almost taylored on a one-off basis with so many other individual factors to be considered.

Even if the plate choke falls inbetween resonances using a dipper, by the time it's installed in a HB amp, (cabinet capacitance, coupling and tube cap's,. strap run cap's etc.), and with so many bands now available between the ol' 160, 80, 40, 20...etc. it's now almost pot luck if suppressors or plate chokes work the first time.   And with some people's luck, anytime