I like using the old RMC stuff. Also, instead of trying to find a 170pf which is extremely hard. Try paralleling 2 x 82pf or 3x 56pf which are common values.
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X-Force 600HD Smoked
- Thread starter Shadetree Mechanic
- Start date
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- Tags
- x-force
I just ordered some capacitors and resistors. Normally I would go to RF parts, but they didn't seem to have a good selection of high voltage disc caps. I found another place called Tedss.com that has a very big selection and a great search page.
https://www.tedss.com/Capacitors
https://www.tedss.com/Capacitors
A few pictures to update the progress. I also upgraded to 4 AWG wire.
I did a little testing with it today, the output match is perfect. Barely moves the needle off of 1. Better than I could have hoped for. The input match is around 3:1 and the variable cap seems to make no difference. Maybe its time to try the input coil idea?
Is it me, or are the mica caps on the input transformers too high in value? I was thinking they need to be 330pf, not 500pf. The 468 trimmer is suitable for its application, unless it got damaged and it's open. Usually, it's best to use the trimmer cap to dial in the input match, then measure the value of the trimmer, then replace it with a proper 1kv mica capacitor of the same value.
~Cheers~
~Cheers~
Input should need 240pf roughly. On the HG2879, it needs a 470pf or 500pf across input transformers. HG have different perimeters than Toshibas. They are not drop in replacements.
It sounds like amp is more stable now. Not going into oscillation or full conduction?
Anymore testing? Should be around 230-240pf for input tune. But with it being biased, may affect input tune slightly. Back in my early days, I put a 4610 trimmer across input transformers and tuned it properly. No guessing on values. Did this across output transformers as well. Tuned the amp for output, while watching current draw. Worried more about input than maximum smoke. Tried to find the happy spot. Not just assembling and calling it good.
I haven't had a chance to do anymore testing, probably get to it one day this week after work. Putting the trimmers on the transformers sounds interesting. Someone here showed an amp that had this on the input, I think it was a Palomar or something. How would a variable cap on each input transformer be tuned so it would still be balanced? By measuring the current draw on each pair and making them the same current? I will look to see if I have a 464 or 465, maybe that's all it needs.
I am still amazed that the output SWR barely moves the needle, its just like my tube amp. And I haven't lost any power with the extra feedback resistors either. After I get this XForce dialed in, I am going to take another look at my Texas Star 350.
With trimmers across input transformers, I adjust for lowest vswr on input. Then I remove and measure. Sometimes when I pull they aren’t the same. So I add the 2 values and then divide by 2. Then whatever that number is, I try to use closest or exact value and solder the fixed value across both transformers.
It seems to me these trimmers can get a little funky at the bottom end of their range...could be just me, but it’s sure seemed that way at times.
The 464/465 will put you more towards the middle of the range.