Troubleshooting AL-800H amp

[Moleculo]

Tonight my trusty AL-800H with dual 3cx800A7 tubes went out. It just happened kind've all of the sudden, too, and I wasn't even working it hard (I really never work it hard)

I can't get any kind of power out unless I set the plate and load both to "0". That doesn't seem good to me.

Before I go buy new tubes, can someone tell me if how I can test them to see if they're OK?

Anything else I should look at before trying to find a tech to repair?

 

[Captain Kilowatt]

How much power can you get out of it and is it the same on all bands? Are you seeing any grid current and if so how much when driven normally?

 

[Moleculo]

OK, I just ran some tests:

On 75 meters, I have to drive the amplifier with about 40 watts to get 400 out. Normally 40 watts in would drive this thing to about 1800 watts. The grid current is almost nothing, nada, zero. When it went out, I was using it on 75m with a 4 watt drive putting out about 500 watts. I was also doing a digital mode at the time, but this is well within specs.

On 40 meters, I can get 1800 or so watts with the same 40 watt drive. Keep in mind that full power is about 2500 watts, which I still seeing if I drive the amp with about 55 watts. It see about 10-15mA of grid current with this. This all seems OK based on past useage

On 20 meters, I can also get about 1800 watts with the same drive, although keeping the grid current in check is a little touchier, but that's normal also.

Driving this amp with 40 watts is nothing, as it will handle 100 watts in. Like I said, I was driving it with 4 watts when this happened. I guess the good news is that it's not a tube if I'm still getting full output on other bands other than 75/80. Any suggestions on how to go about troubleshooting this?

 

[Captain Kilowatt]

Hmmmm......(scratching head) I'll go to Ameritron and download a schematic and have a look. Might be tomorrow before I get back here again.It's pushing midnight here already.:bored:

 

[Moleculo]

I'm guessing this has something to do with it: The LF coil is badly warped. I wondered why it seemed like I needed a lot less load than normal on 40m. It's hard to see in the picutres, but the glass rods holding the tube are completely warped and curled in on the inside. I wish I knew how this happened. I never run the thing hard. Maybe it's a ventilation problem with the design of this amp?

Anyway, this sucks because as I look at the amp, it would be very easy to replace this part IF you could get at the underside. But to get at the underside you have to disassemble 1/2 of the amp, which is a bigger job than I'm willing to undertake. I guess it's time to call Ameritron.

 

[Captain Kilowatt]

OUCH! That is not good at all Mole. I was looking at the diagram and was thinking it had to do with the output network in order for the tune and load to be off. I was thinking about one of the padding capacitors for the lower bands being blown but it looks as if you have found the problem. It sure looks as if there was some serious RF current flowing in there to cause that kind of heating.

 

[TonyV225]

OMG Mole thats it I just looked at one that did the same and a guy out here redid it he offered the Amp to me and after that coil revamp its flawless and has been. Thats Ironic I was just talking about this lastnight crazy I guess a few of them have done it out here. You should send that pic to Ameritron or hell I will that shouldnt be happening but it is. I ll ask again what this guy used out here to replace it but its perfect now and is guaranteed to not happen again

 

[AudioShockwav]

I missed this post....are you sure the rods are glass? It looks just like the ones the MFJ uses to hold there tuner coils with, and they are some kind of plastic materal. Have you contacted Ameritron?
I think if you can fix the coil, it will solve the problem, but then you have to wonder why?
If you were not pushing the amp, what caused the heat, that warped the coil?
Seems that it would be better to fab some spacers out of heavy fiberglass pc board by cutting slots in it to hold the coil if the stuff they are using can not take the heat......

73
Jeff

 

[TonyV225]

Jeff thats crazy I agree I couldnt believe it when I read this. As I said Im looking at buying an Ameritron that did this very same thing but is now fixed better than new. All in all this should (NOT BE HAPPENING REGUARDLESS) And Im curious as to what Ameritron has to say about it. The amp Im looking at was also treated good and not abused and did this very same thing so you know there has got to be others out there and my guess is alot of others. Would be interestin to see what Ameritron has to say. I know my older one never had this issue nore have I ever seen this until Moles and the one Im looking at.

 

[Moleculo]

Well, I got it back. Of course they didn't say what caused it. I'm guessing it's just that glass-looking rod spacer material they use can't handle the heat. They also said the band switch was burnt. Well no duh...that's the least I would expect to happen with the LF coil attached directly to the switch melts down.

 

[W5LZ]

Coil 'warping' is sort of common with coil stock using plastic spacers (they all do) and the most common cause is too much heat. Usually that heat is from inadequate cooling, not necessarily too much current. The Ameritrons with a 'buck/boost' transformer are particularly susceptible because of the fairly common practice of 'bumping' the voltage for more output, sort of. As to the burned switch, sooner or later... etc, etc. Clean them! NO lubricant except on the bearings. And then there's the thing with them little green men thingys...
- 'Doc

You might be surprised as how much a sheet of that AC filter over the air input grill will help.