Weber 6O100 kit build blowing fuses [SOLVED]

[Chief]

Tried searching for this but I couldn't find anything. I'm building a Weber 6O100 kit for a friend. It's my first amp build and I'm having a little trouble. Looking for help narrowing down what I should look into to solve this problem.

Schematic https://www.tedweber.com/media/kits/6o100_schem.jpg

I'm following Rob Robinette's tube amp startup guide and everything works fine. I get all the way through past biasing and no issues. But after that I've blown two fuses. The fuse that's blowing is the 1a slo-blo on the PT center tap. The first time it blew it just happened after I was playing it for a few minutes testing out the controls. I couldn't remember what control I had adjusted when it happened so I checked everything and noticed I had forgotten the wire between R13 and OT yellow. This means there was no feedback loop.

Jump forward to I wanted to clean up the build a little bit so I rebuilt my turret board to make it narrower and cleaner and re-ran all the heater wires to give them more breathing room. Do all the startup tests and it's running nice. No hum, good sound, but then I turned up the Treble and Mids controls and about 50% they introduce oscillation. Can't remember if I had turned them up before it blew the first time. Bass control is clean. Then I turn up the Presence control and the fuse died at about 80-90%. I also noticed that when I briefly turned up the Master volume to max it went into oscillation. Preamp volume control is max-able without issue.

What does this sound like? I can't figure out what to google.

Oh and I'm using KT66 tubes. They all look fine. No arcing or anything and they seemed to measure ok. I'm using only the supplied components, parts, and wire. No shielded wire was provided so I'm assuming that means they think this build is reasonable without it.

Thanks :)

[VMS]

Have you tried to swap the OT primary leads?

[shooter]

The fuse

shouldn't blow because of

no feedback loop.

Jump forward

did both blow after the "rebuild"
if so
how long did you jam before rebuild, how long was it running total

[Chief]

First one blew before the rebuild. It was the reason I rebuilt it. I couldn't see anything wrong other than the missing wire and that didn't seem like it would blow a fuse so I thought maybe since it was so cramped there was some short that happened from vibration while playing or something. So I rebuilt and tidied up.

At the time of the first blow I had had the amp on for a while it taking measurements and stuff before I started playing guitar through it . It blew about 2-3 minutes into playing the guitar.

The second time was also about 3 minutes but it happened as what seemed like a direct result of turning up the presence control. At about 80% it let out a tiny squeal and then the fuse popped. But that could also just be coincidence. I have a couple more fuses on hand. I'll try and replicate.

I haven't tried swapping the OT primaries yet. I'll try that later tonight. I had read about it but it seemed like in the threads i saw it suggested the op's came back and said it started making lots of noise right away so they swapped them back. I just assumed since mine sounds good except when you turn up the treb and mids that it was something else. I'm definitely not opposed to trying though in order to narrow it down.

[VMS]

The fuse

shouldn't blow because of

no feedback loop.

You are right shooter.

I also noticed that when I briefly turned up the Master volume to max it went into oscillation.

but this and the "turning up the presence" made me suggest the primary leads swap


Pictures and voltages could also help us narrow down the problem

[MFowler]

If you go to the Weber kit building forum you will find myself and others talking about our oscillation problems with the OR100.


I do not remember what I did to cure the problem and I gave the amp to my brother-in-law so I do not have it to look at.


I tossed the weber board and made my own much wider then weber's so the filter caps wouldn't hang over the sides.


Mark

[terminalgs]

Maybe a high frequency oscillation your transformer can't put on the secondaries (or your speaker can't produce or your ears can't hear).  Maybe your power section is pushing that freq. At 120W?   


If it is this, you can test it on the bench.  Put a amp meter in place of that 1A slo-blo,  turn presence control off (and all volume controls too) turn amp on.  let it heat up and watch the amp meter. record the amperage.  let it sit a while as-is, then start fiddling with the controls to see how the meter needle moves.


If it s a problem with the design, maybe a small mdf cap from plate to cathode on V2B?   maybe 50pf 1000V ceramic disk cap. (although it seems like people on the weber forum would have discussed that...)

[silverfox]

If it is this, you can test it on the bench.  Put a amp meter in place of that 1A slo-blo,

Seems to me this would be a great way to fry the PT. The amp meter typically will handle more than 1 amp of current.
Is it possibly red platting just prior to the fuse blowing?

silverfox.

[Ritchie200]

I like to post this info from time to time.  It may help in this case to narrow down the issue.  I used to work for Bussman Fuse.  One misconception is that any current over the rated value of the fuse will cause the fuse to blow. Yes, and No. Yes, it will eventually open - but it may take some time. It takes about 200% over-current (to rating) for the fuse to blow instantaneously. Let's say your amp is drawing about 30% over-current. It would take an hour for the fuse to open. The performance graph ramps up fairly quickly, but there is a time factor. Does not matter if the fuse is a fast blow or slo blow, constant over-current performance is identical.  If you can play for a while it is likely drawing between 1-2 amps unless something is arcing.  I am not suggesting that you put in a larger fuse.  There is a problem, but just barely!  I hope this helps.

Jim

[nandrewjackson]

+1 on the OT primary swap. So common for these to cause squeal and oscillation on a fresh build

[Chief]

Sorry to leave everyone hanging like this. After I wrote that post I got kinda sick of looking at the thing for a little bit then I had some other other projects come up, but I finally got around to it today and I don't even want to tell you guys what the fuse problem was. I had the 1 amp and 4 amp swapped... DOH!

Also I registered over at the weber forums so that I could see them and saw a post that said his OT Primary wire colors didn't match the layout sketch so I reversed mine and it solved the whistle on the feedback loop. I think what's happened is weber used to provide weber transformers but they have switched so another brand and the layout was made off of the old ones because my power transformer also didn't match the layout.

At any rate I'm still getting some hum in the preamp section but after prodding around i found the guilty wire so I'm gonna replace that one with shielded and see if it clears it up.

Thanks for all the suggestions guys I really appreciate the help :)