Build Log - #102 - So it Begins

[martin manning]

19V is fine. You just want to drop half of it across the drain resistor.

[PicknStrum]

19V is fine. You just want to drop half of it across the drain resistor.

Thanks for clearing that up for me Martin! I realized I was massively screwing up that bias the initial night I was trying it - apparently I can't think past 8 p.m. Not only was I concerned with getting exactly 18v VDD on the drain resistor but I realized later that I kept checking the source pin of the FET for 9v instead of the drain

Think I've got it straightened out now.

Should be getting transformers, relays, and missing parts in within the week so updates to come.

[PicknStrum]

PT is in! Holy crap, I was not expecting iron this big! I knew it would be big but it's gotta be like 30 pounds. Still have to wire it up but was hoping for some clarification on questions I have.

1. I ended up going with a Hammond 290FX - here are the specs: https://www.hammfg.com/files/parts/pdf/290FX.pdf. Based on some layouts and a quick search, I just wanted to clarify that the orange wire goes to ground and gives the PT a Faraday-type shield - I assume to keep noise out. Is that correct?

2. Also ended up going with the Hammond 166JA12 transformer for the relays - I liked that it had a 1A current rating, which should be plenty for my relays. I know I read in a thread on here (can't find it now) that the relay transformer CT should just be capped off and NOT grounded. I'm going to assume that's correct but I'm wondering why that is the case. Is it because it has the potential to produce noise? EDIT: I did another search and found a post by talbany explaining that a grounded CT on a FWB will cause the transformer to fail. Can someone elaborate? Why is a CT required on a full-wave rectifier but not needed on a FWB nor voltage doubler?

Thanks for helping me out guys!

[martin manning]

PT is in! Holy crap, I was not expecting iron this big! I knew it would be big but it's gotta be like 30 pounds.

Oh come on, it's only a bit over 10 lb!

I just wanted to clarify that the orange wire goes to ground and gives the PT a Faraday-type shield - I assume to keep noise out. Is that correct?

Yes, ground it with the HV CT

Also ended up going with the Hammond 166JA12 transformer for the relays - I liked that it had a 1A current rating, which should be plenty for my relays. I know I read in a thread on here (can't find it now) that the relay transformer CT should just be capped off and NOT grounded. I'm going to assume that's correct but I'm wondering why that is the case.

In a FWB rectifier the AC inputs are alternately connected to ground as the polarity reverses. If there is a CT, it will sit at about half the rectified DC voltage, so you don't want to ground that.

[Guy77]

Hi PicknStrum. I have down several builds with the Hammond 290fx and they have worked great. One thing to note it will run hot, that is just the way they are. As you noted they are quite heavy at 11.6 lbs so they can take all that heat without any problems at all!

Cheers
Guy

[PicknStrum]

PT is in! Holy crap, I was not expecting iron this big! I knew it would be big but it's gotta be like 30 pounds.

Oh come on, it's only a bit over 10 lb!

I just wanted to clarify that the orange wire goes to ground and gives the PT a Faraday-type shield - I assume to keep noise out. Is that correct?

Yes, ground it with the HV CT

Also ended up going with the Hammond 166JA12 transformer for the relays - I liked that it had a 1A current rating, which should be plenty for my relays. I know I read in a thread on here (can't find it now) that the relay transformer CT should just be capped off and NOT grounded. I'm going to assume that's correct but I'm wondering why that is the case.

In a FWB rectifier the AC inputs are alternately connected to ground as the polarity reverses. If there is a CT, it will sit at about half the rectified DC voltage, so you don't want to ground that.

Haha - I can't believe it's only a bit over 11 pounds! I was guessing at least 20 lbs. This proves I'm very bad at guessing weight! It must just feel so heavy because it's so dense.

I guess I've never really had to think about full-wave vs. FWB in terms of grounding CTs as this amp includes 2 of my first solid state rectifiers. After your response, it clicked that I'd almost never use a CT with a FWB and always use a CT with a full-wave. Although, I definitely still haven't wrapped my head around why grounding CT with FWB would blow the tranny. Maybe I should rewatch the Uncle Doug PT videos.

Hi PicknStrum. I have down several builds with the Hammond 290fx and they have worked great. One thing to note it will run hot, that is just the way they are. As you noted they are quite heavy at 11.6 lbs so they can take all that heat without any problems at all!

Cheers
Guy

Thanks for the heads-up! I'll plan on roasting some marshmellows over the PT while I attempt to rock! Just to verify, you're referring to temp moreso that B+ voltage being a little hot right?

[Vertigo]

I guess I've never really had to think about full-wave vs. FWB in terms of grounding CTs as this amp includes 2 of my first solid state rectifiers. After your response, it clicked that I'd almost never use a CT with a FWB and always use a CT with a full-wave. Although, I definitely still haven't wrapped my head around why grounding CT with FWB would blow the tranny. Maybe I should rewatch the Uncle Doug PT videos.

Rob Robinette has an informative section on rectification. Unfortunately, it's on a huge page and I don't know how to link to the specific part of the page it's on. You can find it https://robrobinette.com/How_Amps_Work.htm, and if you do a ctrl+f for "full wave" you can find it.

[PicknStrum]

I guess I've never really had to think about full-wave vs. FWB in terms of grounding CTs as this amp includes 2 of my first solid state rectifiers. After your response, it clicked that I'd almost never use a CT with a FWB and always use a CT with a full-wave. Although, I definitely still haven't wrapped my head around why grounding CT with FWB would blow the tranny. Maybe I should rewatch the Uncle Doug PT videos.

Rob Robinette has an informative section on rectification. Unfortunately, it's on a huge page and I don't know how to link to the specific part of the page it's on. You can find it https://robrobinette.com/How_Amps_Work.htm, and if you do a ctrl+f for "full wave" you can find it.

Will do Sean, thanks! I've read tons of stuff on Rob's website but don't think I've seen the rectifier section. I ended up finding it through your link.

[Guy77]

Hi Picknstrum. Yes just referring to the temp of the transformer not the B+.
Excited to hear how it all turns out when its done!

Cheers
Guy

[PicknStrum]

Alright, so I think I got my wiring all finished. I ran through the boards as I went and just started the startup procedure.

With power on and standby set to standby, there is a fairly significant buzz. Sounds like a bee buzz (how's that for reference). I can tell it's coming from around the filter board and/or the PT. I'm getting 120v between the PT primaries. I decided to just chopstick right around the 2 large (220uF) filter caps and I noticed when moving the negative end of the 2nd cap in series (i.e. the one that is directly attached to ground), that the hum did stop briefly and then came back shortly thereafter.

So I shut down the amp and went to discharge the caps. I discharged the first cap and then checked with my meter. I then jumped over to the 2nd cap with my meter and had a massive pop / spark; at this point I hadn't actually discharged it, I was just checking DC voltage on it - I assume the large pop was the cap discharging but I don't know for sure. I then used my "discharge jumper" with my 20k @ 5w resistor and finished discharging. When I went back to check with my meter, the cap was fully drained and it didn't spark this time.

So I reflowed my solder joints at the filter board and checked continuity again to see if maybe something wasn't connected through those first 2 filter caps. Again, I'm still on standby so voltage shouldn't be going past those large series caps. This time I made sure to discharge the caps with my "discharge jumper" b/f checking with the meter. I still had a very tiny spark on that 2nd large cap but nowhere near the pop from the first time I checked with my meter. I know something is up due to the buzz but I'm also thinking something's up b/c shouldn't I be able to check voltage on that second cap in series without shorting it through my meter?

Any thoughts? I'm going to leave her off for a bit.

[xtian]

Photos of the filter cap area, please!

[PicknStrum]

Okay - got some pictures attached of the filter board itself. I can get other pictures up if you'd like, just let me know the area you're thinking.

I just took a break for lunch and went back to double-check everything - I'm getting continuity where I should be and everything seems grounded properly...

IMG_4884.JPG

IMG_4885.JPG

IMG_4886.JPG

Edit: I also took a couple DC readings at the large series caps:

I've got 458DC rectified coming in from the rectifier board and confirmed that I'm also getting that at the first filter cap. The second cap is getting roughly 280DC:

Filter Voltages.jpg

[xtian]

Can you point to the layout you're using? I looked in the #102 thread, but yours is different.

[artguitar]

The green ground wire does not seem welded to the eyelet

[PicknStrum]

The green ground wire does not seem welded to the eyelet

If you're talking about the green wire at the top of the filter board, it is not. I just have it running through the eyelet. That wire is connected under the board to the upper filter caps on the right and is going to ground.


So, after finding a loose ground connection off my mains plug and reflowing solder joints on the filter board, the noise has quieted down. Although I still hear what I would now call hum (not sure if it's 60 or 120) the hum is much much less.

I am still getting some fairly large pops when discharging the large capacitors with my "discharge wire" - like I said, I have a 20k resistor @ 5watts on this. Is the pop normal for these caps? I've just never had anything pop/snap that loudly, especially when using my discharge wire. The only other amps I've built were 15 watters so maybe it's just the size of the caps? I thought the resistor was supposed to keep that pop from happening.