The AM Forum

THE AM BULLETIN BOARD => Technical Forum => Topic started by: WU2D on February 15, 2014, 02:33:18 PM

Title: Linear Refurb
Post by: WU2D on February 15, 2014, 02:33:18 PM
Last big project before returning to the workforce. I knew it could not last...

A couple of years ago I started getting complaints that my workhorse Linear Amplifier was Humming a tune. We started a thread on the old GSB-101 through 201 MKIII series at that time and you can look up that thread that basically talked about creating a virtual center tap with resistors, diodes or ad HryLR Tim suggested, by lifting the ground and strapping the low voltage side of a filament transformer across the existing 6.3 winding and grounding the center tap. Anyway I did nothing and it sat on the shack floor unused.

So I finally took her apart and got to it. Here is what the basic schematic looks like. As you can see it is an old buzzard 4 811A amplifier with TX/RX switching off a 110 VAC external cheater cord key line usually off your TR switch or Rig. You can plainly see that one side of the 6.3 fils is indeed grounded.

So what should one do with an amp like this? After remembering that I normally use the amplifier inline rather than using it as a separate TX/RX setup and that the darned thing pops the breaker about every other time I turned it on and that it has never has to my knowledge had any caps changed out, here is what I came up with as a list of updates.

1. Refresh all electrolytics.
2. Change Heathkit Cord with inline fuses to conventional heavy 3 wire cord and add fuseholders on chassis.
3. Do the Filament mod. with the filament CT to ground system.
4. Change the relay to work in a standard XCVR mode, keyed with an external grounding signal ( grounding internal relay to actuate it and normally bypass mode with no key).
5. Add a step-start to reduce the breaker pop and pop when the tune operate sw is inadvertently left in operate problem.
6. Clean and lube.

I can report that all hum is gone and no more breaker issues. But the hum was not traced to the filament or the HV caps! It was the BIAS cap that was gone. This put 60Hz (not 120 Hz as with the main power supply) right on the grids modulating them nicely.

Title: Re: Linear Refurb
Post by: WU2D on February 16, 2014, 09:33:39 AM
Before I buttoned her up, I wanted to get a hum measurement and Steve QIX and Al VTP had great signals this morning and they measured over the air. The 120 cycles was down 40 dB and the gain reported was 9 dB at AL's and 10 dB at Steve's. No 60 Hz, which was the original problem. This is really all one can expect from a Linear with AM. The exciter was running more juice than proper (see reducing power thread) for good linearity.

Also no tuning circuit was put between the exciter and the amplifier, another excellent idea for improving linearity. I think that a low pass style matchit box with some coupling power control and attenuation is in order! Do I smell another project?

By the way, the original 100 uF 450V caps are large but as you know modern computer grades are capable of putting almost 10 times that mount of capacity in the same size. I was tempted to go for really high value caps but settled for a conservative increase of 50%, using 150 uF, 450V caps. They look silly, they are so small. Thanks to all who gave suggestions on this project. Not sure if going to 1000 uF caps would work with the choke input supply? 5 in series would give 200 uF instead of 25 uF - Comments?

Title: Re: Linear Refurb
Post by: WQ9E on February 16, 2014, 10:03:44 AM
Congratulations on getting it hum free!

I saw the ad for one of the GSB-201 models when I was a new ham and I really like the case style.  I picked up Mk IV version a few years ago and that has been running with a SX-117/HT-44 pair but now I have the matching Hallicrafters amp so the Gonset will get paired with another rig.  I have the earlier GSB-101 awaiting restoration and then it will get paired with a GSB-100/HQ-170 station.

I recall some people have reported problems with the output network of the 201 but mine has never complained.  It certainly is a heavy amp and the power supply is solid.

Title: Re: Linear Refurb
Post by: W3RSW on February 16, 2014, 10:08:28 AM
You've added a step start so that should help taking the first several cycles of surge off all the iron and the initial charging of the caps.  Also the more "C" in T = RC, the more "T."  Heh, heh.  This will help offset the increase in the "Q" in Q= CV.   I'd go for a full 100uf or more equivalent.  Also I'd add another cap/bleed resistor in series to the string since cost is not a factor on your one-off mod.

Be fun to run an analysis in CircuitMaker, tm.

Title: Re: Linear Refurb
Post by: WU2D on February 16, 2014, 11:22:55 AM
People make fun of Gonset and talk about the use of cheap parts, but I do not see that here. I see a clever "what if?" approach to really the first reasonably priced tabletop linear. You sure see a lot of them at hamfests so it obviously was successful as a product.. It is heavy, but portable enough to drag to a field day. In fact this particular unit came from an Army officer who took it to Vietnam in 1968 along with the SR-150. And he shipped it home!

That power transformer and choke are serious iron and the capacitors are good quality. The ceramic switch could be a bit heavier but the coil seems fine. Who can complain about a box that can use 811A's or 572B's? The tube sockets are good quality recessed ceramics and tube locks are employed. This is the germ of the SB-220 and all of those Ameritrons.

Title: Re: Linear Refurb
Post by: WD5JKO on February 16, 2014, 11:57:40 AM

  I too am a proud owner of one of the Gonset Linears, The GSB-201 MKII with 572B's.

I have used it for many years, and for the most part, it has been trouble free. Some observations on my part though:

It has a built in manual "step-start" when in the tune position. This is of little use for me, and the turn on still results in a "bang" when I turn it on when not at zero crossing. I try to time it, 1, 2,3...but getting it down to the Milli-second is pretty hard. Your step start is a better idea.

The filament transformer is grounded on one side resulting in 60 hz hum modulation at about 5%. The fix here is not simple since there are two problems:

1.) Must provide an artificial AC centertap to the filament transformer winding. I have done it two ways, and one works, and the other does not. I used a 10 ohm 25 watt WW rheostat to make a CT. This worked the best. I also used a 6.3VAC CT 2A filament transformer leaving the primary N.C. Bad move, perhaps the unequal winding resistances was a factor, or core saturation from unequal currents was a factor. Had lots of 60 hz modulation with odd multiples thereof.

2.) The filament chokes to the RF tubes is NOT bifiliar wound. There should only be one choke. This gives some strange band to band variation where it may hum on 40M, but not on 20M. Making the choke bifiliar wound then would disturb the neutralization next project in the Que.

The filament choke(s) have too little inductance on 80M, much like the SB-200. Input SWR might be 5:1 or higher on 80M when it is 2:1 or less on 40-10M.

I have seen strange behavior on 10M, and anything more then 100W out on AM eventually heats something up as you watch the RF out drop. 15M is good.

I played with adding bias between the artificial CT and ground. I used a 12V 50 watt zener. This added bias pretty much cuts off the tubes; not suitable for SSB. What it does do however for AM, is to allow you to boost the efficiency about 5%, say 33% to 38%. This is huge. The added bias reduces the gain, so if the drive is increased until the same RF output occurs, then the efficiency improves. The positive modulation peaks also improve. Adding additional bias without increasing the drive will increase distortion.

With my droopy AC power, I can run 140 watts AM RF out all day long, and have room for 140% positive peaks. Adding in the extra bias, along with extra drive, I can do about 160 watts. If I add my old MFJ 100W 'T' network tuner at the input with a short RG-8 coax, I can do 180 watts carrier, and peak 140% modulation.

The Thang needs a tuned input with enough Q to keep the waveform proper during the every other half cycle loading.

Just some rambling here, sorry for being long winded.


Title: Re: Linear Refurb
Post by: W3GMS on February 16, 2014, 01:31:41 PM

Well your report is encouraging!  I have the same amplifier that is cosmetically in good shape.  I never took the cover off so not sure what its like inside but I suspect it is fine.  So a few simple mod's and it will be good to go.  I wonder if there would be any advantage to put 572's in it?  If the iron and other parts are not up to the task, it may be best to just leave it as is with the 811's.  My plan is to use it with one of the modified small screen modulated transmitters such as the DX-60, T-60 and others in the same power class. 

Joe, GMS 

Title: Re: Linear Refurb
Post by: WU2D on February 17, 2014, 07:44:34 PM

The amplifier will support them and you simply have to disable the -6V operating bias. The 572B's are more expensive, have more dissipation and are more rugged and will run more power. But my 811A's are  still humming after many years of use and they do 500 Watts out on CW with an ARC-5 driving it.

Title: Re: Linear Refurb
Post by: Opcom on February 17, 2014, 10:08:24 PM
I didn't realize how compact that amp is. Does it run fairly warm?
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