SB-1000 Linear Amp Low Power on 75m & 40m, Normal elsewhere

[KD0CQ]

Hey folks, I've recently picked up a Heathkit SB-1000 amp. My first tube amp since I started back in '95. It has a nearly new tube in it, and I received a brand new spare 3-500ZG with it too. The amp is clean inside and out, and everything seems to work as it should except for the following problems on 75 & 40m.

It seems to tune up real well and do everything it should on 20m-10m no problem (yes it has the 10m kit installed) and easily outputs 1 KW.

However, 75m and 40m is a different story. I'm thinking it must be something to do with my antenna on those bands, maybe you can point me in the right direction. On 20-10m I'm using a Maco aluminum vertical tuned to 28.450 (fed with RG8), but it tunes very well all the way down to 20m.

On 75m and 40m I'm using a homemade resonate fan dipole, fed with RG8. Coax run is approx 50' to an air coil choke then to the dipole. SWR is near 1:1 on respective resonate frequencies. I'm only showing around 30-40% of what it should be outputting there even driving it with higher power (60-80w). To get the obvious stuff out of the way, I know how to tune it (Thanks Jim) and the band switches are labeled correctly, doubled checked that. I have the amp plugged into its own 240AC outlet on its own breaker. The High Voltage is at 3,400V (normal).

Is this a problem with the antenna or feed line I'm using on 75m and 40m or could there be something else wrong?

Any advice or tips would be very welcome.

73
Chris - KB0TLW

[W1QJ]

Hi Chris, I am going to tell you the same thing I tell every new amp owner that comes on here with problems. YOU NEED A DUMMY LOAD. There are too many variables with antennas/feedline to determine exactly what your problem is. SO if you had a dummy load we can eliminate an antenna system fault immediately. I suggest since you are an amp owner now that you at least pick up a used cantenna by Heathkit or better yet a NEW gallon paint can job from Ameritron. You don't need anything fancy. Assuming your antenna is OK, check your bandswitch for burned contacts. That would be the first place I'd look after the antenna was ruled out. Lou

[WB2WIK]

Lou's right, a dummy load would be very helpful.

But if the amp works fine on the higher bands and not on the lower bands, I'd suspect that "indications" might be obvious:

For example, does the amp peak for max output on 40m and 75m within the tuning range of the LOAD capacitor? Or does the LOAD cap have to be cranked up to max (which is "0" on the panel markings), so it's not really peaked?

Does the grid current look the same on 40m and 80m as it does on 20m and above? If it's substantially lower, then possibly the input circuit needs re-tuning; if it's higher, maybe the amp isn't LOADing up all the way, which could mean a failed or open padder capacitor.

Stuff like that. There are usually telltale signs when "something's different."

[KH2G]

I run one at this time and since I never trust peak read tunes, I go a bit old fashion - I start with the load down and use the plate current to dip it. I then bring the load up watching my grid current, as the load comes up, keep dipping until you have a shallow dip and less than 200 ma grid current. (I'm usually about half or less than that but max is 200ma. You may exceed it for a few seconds but get it on down. You'll probably show better than a kw. If it won't tune up like that then get out the dummy load even if you have to use some high power incandescent lights. they are better than nothing and give you a good idea what your amp is doing. BTW did you check your filament voltage not that it effects load but does affect the tube life.

[KD0CQ]

I've got a 1KW dummy load borrowed now. Seems to be doing the same thing into the DL.

I opened it up and one side of the 160m/80m tank coil was loose from the solder lug on the switch. I soldered that back into the correct place (looked like it was just barely there in the first place). Put the case back on, plugged it back in and tuned it up on 80m. Seems to be quite a bit better there, but still seeing issues on 40m (of course). I'm going to let the caps drain and do some more inspection on the switches and the 10-40m tank coil, and make sure all the solder joints are as they should be, etc.

I didn't see any burning on the band switches, but it's pretty tight in there. I did clean as best I could with a pencil eraser.

[KD0CQ]

OK, Looks like I fixed it, or at least corrected the symptoms.
It loads great now, full power out on all bands.

Looks like the lead running from the switch to the 40m tap on the tank coil had arced or something and melted the solder and bonded together 2 rings in the middle of the coil, along with making the connection to the solder lug very week. I cleaned up the bonded rings on the coil and re-soldered the lead back to the tap. Searched everywhere else and could find nothing out of place.

Now I'm wondering what caused this? I read somewhere that there was an issue tuning the amp up on 160 that caused some arcing. Something about a anti-corona washer not present on the 160m tuner capacitor padder terminal? I've not even tried 160. Rarely ever key up down there. so if that's the issue I'm not worried about it. That might explain the problem coming off the 160-80m tank coil but not the 40m one...

Any ideas?

To answer KH2G, no I haven't measured the voltage or current on the filament yet. But I'm going to do that next just to make sure. I sure don't want to shorten the life of the tube!


Thanks for the tips guys.

[WB2WIK]

Sounds like a kit that wasn't built particularly well.

[KD0CQ]

Sounds like a kit that wasn't built particularly well.

Everything else looks pretty good. Not sure what happened in these two situations, but top and bottom the rest of it looks fine. Including solder joints.

[W1QJ]

Glad its fixed. Yes, there was a corona washer on the AL-80A but not on the 1000. Many people add it to the 1000.

[KD0CQ]

Glad its fixed. Yes, there was a corona washer on the AL-80A but not on the 1000. Many people add it to the 1000.

Would that effect any other band than 160m? I can find very little information about it. Do you think it a good idea to go ahead and add it anyway?
Thanks