Donkey Stomper 2x8 pictures and discussion

[loosecannon]

Hi all,

I wanted to post some pics of the amp i am currently working on, since i don't think we have any donkey stomper amplifier pics on the forum yet. (i could be wrong)

so the first few posts will just be pictures of the amp that people can use for their own purposes, but there are a couple of things that caught my eye, and i would like some opinions on.
The pictures illustrating the areas in question will be included in this first post.

I haven't started investigating too much yet, but the amp came in with the complaint that it would only show a reasonable output SWR on one channel, and the SWR would rise quickly as you moved away from that channel. i believe the customer said it was channel 40.
I have not tested the amp yet to confirm this on a dummy load and my antenna.

the first thing that caught my eye was the 'free floating' ends of the input transformers for the 8 pill section.
standard practice in building these types of amps is to leave the input side floating above the copper board by 1/8" or so. as can be seen in the pics, these are laid right on the board.

my first question is does anyone else see a problem with mounting them this way?

next take a look at the output side of the input transformers for the 8 pill section. they should be soldered on top of the 'pill strip', but at least two of them are soldered with the transformer sitting on the copper main board.

again, does anyone see a potential problem here?

the last thing i wanted to get some opinions on is whether or not it would be worth it to turn the 2 pill driver section around. i mean put the input where the output is and vice versa.
seems to me you save a lot of stray wire length doing this, but those of you who have built a bunch of these amps can probably tell me whether im wasting my time or not.

for those that don't feel like following the play by play, just enjoy the pics!
LC

 

[loosecannon]

 

[loosecannon]

 

[loosecannon]

 

[loosecannon]

 

[nomadradio]

whether or not it would be worth it to turn the 2 pill driver section around. i mean put the input where the output is and vice versa.

What the Fred?

If that part ain't broke, why fix it?

The "free-floating" input splitters are just a labor-saving measure. Rather than fabricating and mounting separate splitters with three solder connections each, they take the wire used to wind a transistor pair's input transformer and make it long enough to wind into one side of the splitter.

Looks a bit sketchy, but it does provide one advantage. Fewer wire-to-wire connections. And connections coming loose are a big factor in the bigger picture of electronic failures and breakdowns. Fewer connections equals fewer connections to fail.

OK, maybe that's a bit lame, but it's literally true.

Your wacky high SWR could be caused by the negative feedback caps. Noticed one says "103", or .01uf

Should be zero point one, or "104". Those caps are ten times too small. Tends to make the amplifier unstable.

Probably got the builder another 50 Watts on his meter to build it that way. But at the expense of keeping it stable.

But a .01uf cap is just wrong for that job. At least you only need ten of them.

73

 

[loosecannon]

thanks for the reply nomad.

i know the 2 pill section works as it is, i was just wondering if anyone else thought it was dumb to face it the way they did.

the thing that got me about the input transformers was that they go to the trouble of winding a wire up in ferrite in order to make that connection to ground, so my mind said why bother trying to keep RF off of that connection if you're just going to lay them right on the board.
at least one of them is actually touching the board enough to short it to ground without that wire attached.

your negative feedback theory might just be why that output tuning cap is extended pretty much all the way out.
hmmm...

thanks for the input.
LC

 

[543_Dallas]

thanks for the reply nomad.

i know the 2 pill section works as it is, i was just wondering if anyone else thought it was dumb to face it the way they did.

the thing that got me about the input transformers was that they go to the trouble of winding a wire up in ferrite in order to make that connection to ground, so my mind said why bother trying to keep RF off of that connection if you're just going to lay them right on the board.
at least one of them is actually touching the board enough to short it to ground without that wire attached.

your negative feedback theory might just be why that output tuning cap is extended pretty much all the way out.
hmmm...

thanks for the input.
LC

I would have flipped the driver section around. I do everything I can to keep all lead lengths as short as possible. It should be easy to flip it around.

Those chokes to ground are to dc ground the bases of the transistors without giving rf a path to ground. Not the same as just shorting the base to ground. To bias one of these amps lift the choke from the board and connect it to your bias supply.

Use some pill strips or cut a piece of circuit board to slide under the input transformers if you think it's going to cause a problem.

I would break apart the driver section and the two 4 pill sections and make sure they are all tuned for 50 ohms input and output. Then make sure the splitter and and combiner circuits are tuned properly. The output tuning cap is probably all the way out because there was one in the amp he copied and he didn't know what else to do with it.

 

[loosecannon]

thanks for the reply 543,

i will be getting into it this weekend to see just how deep into it im going to have to go.

i heard from someone that this guy was being taught by BBI, but i don't have any reliable info on that.

it would stand to reason that he knew how to construct the amp, but not how to tune it.

i know im going to raise the input transformers off the board at the very least.
LC

 

[Shockwave]

Way too much unshielded wire around that power variable. If it's carrying RF and more than an inch or two long, we should be thinking about coax to prevent RF feedback and oscillations from getting into the driver stage. That output cap is also acting like an antenna inside the amplifier. Look at how long the unshielded wire feeding it is and how much of the inner tube with RF on it, is not inside the shielded outer tube. If it needs to be set like that, cut most of the inner tube off so it's not radiating inside the amp.

If the increase in VSWR is not proportional to the change in frequency and goes up suddenly, you are almost certainly dealing with a self oscillation. Dummy loads are not your friend for seeing this issue since they provide a flat VSWR at the oscillation frequency also. This test must be done on a resonant antenna or through an antenna tuner into the dummy load.

 

[loosecannon]

thank you shockwave!

when it comes to conductors carrying RF, my mind says shorter is better, and to be honest, i would like to remove the whole variable circuit, but it's not my amp.

my attitude is that the more people look at a design, the better it gets.
this amp was built well but not by someone who understood what it takes to make it resonant.
LC

 

[nomadradio]

Just brings to mind "1-800 Toll-Free" and his attitude about putting a driver in the same cabinet with eight RF transistors. He'd say that anyone who builds a driver into the same cabinet was a moron. Too many problems with stability. He was never one to sugar-coat his advice.

Short wires are smarter for RF circuits, Shielded wires reduce the risk of unwanted feedback.

Tricky part seems to be putting enough RF filtering in the DC power wire to the driver. Can't remember how many of them we had to add filtering in that exact spot.

Most, would be the likely bet. If the one ferrite bead in that wire is sufficient, that's great. If not, the thing will oscillate.

I'm not so sure you'll shorten any wires that are terribly important by turning around the driver. I would definitely use shielded wire for all the input wiring that leads to the driver.

73

 

[Shockwave]

Good DC power filtering between an internal driver stage and final stage is not done with one choke and one bypass cap. It's usually followed by a second ferrite core choke and another bypass cap to provide more isolation. At one point I had a customer request a single amplifier be constructed with one pre-driver 2SC2879 driving 6 more in the driver stage and 24 in the final stage.

DC filtering is not the only problem when drivers are in the same case as the final stage like this. You need to add more shielding between each stage. This is easy to do with copper roof flashing soldered vertically, between stages to the PC board ground plane surface. With this layout, the amplifier was perfectly stable under all operating conditions.

 

[Highway Man]

why didn't the box just go back to Donkey?

 

[Shockwave]

why didn't the box just go back to Donkey?

Most builders have no problems spotting defective parts in their amplifiers but when the problem may be an oscillation related to the layout of that circuit, it can take much longer to learn how to eliminate those issues. Some never even have a chance to see the problem on the bench because they only test them into a broadband dummy load that hides the tell tale signs of increased reflected power that takes place with oscillations. Even with a spectrum analyzer and the dummy load, you may not be able to reproduce the oscillation into a purely resistive load. Take it into a real world situation where there may be some reactance in the load and it could start misbehaving.