Tube Amp Archive

To be more precise, the cap that failed was on the output side of first gain stage, from plate to ground. Paul ran the amp without it and it worked just fine, but he did install a new one. I was confusing it with some other circuits I've been studying that toss a cap in between grid and plate. For t...

Holy mackerel, it's mid-August already. THE KALAMAZOO LIVES, in glorious silence at idle right up almost to 9/10. Briefly, Paul went through it, did a lot of the same things I did, verified that I hadn't really screwed anything up, cleaned up a bunch of connections and improved grounding, all of whi...

It's been awhile since I reported, but I have an excuse: I've been busy. Let's see. I did swap out r7. No change. I reflowed many but not all connections. No change. Oh, yeah, the little oscilloscope revealed a lot of RF in the circuits--double-digit megahertz on down--so added grid stoppers. That g...

Step bits yield surprisingly precise and clean holes. Where there is room to use one, the result is better than a tapered reamer, which is all I had for many years.. er, decades. Just an example of what I'm referring to: https://www.homedepot.com/p/General-Tools-1-8-in-to-1-2-in-3-175-mm-to-12-7-mm-...

A 47uF/630v Solen is about 2"x3" according to TubesAndMore specs. Solen sheet shows 10uF/630vdc to be 26x30 mm. So, bigger than the F&T's that I used in the Kalamazoo. A lot bigger but I might could have made room. There's plenty of room in the push-pull Epiphone Constellation, but that's a bass amp...

The Wonderful World of Oscilloscopy is a fearsome place. Pretty clear that I have a lot to learn here. Also pretty clear that a "real" o-scope would be easier to learn on than this little boxful of menus, but I've largely sussed the display and buttons. Less clear to me is what probing different spo...

If you watch it in a dark room you might be able to spot the arcing to confirm a bad OT, you would also likely be able to smell it. A bit more scientific, you can do this if you have a spare neon and a 9v battery. https://www.premierguitar.com/the-super-secret-transformer-tester If you don't have a...

...However, a megger (hi-pot ohmeter) is pretty inexpensive these days. About $100... Although I love an excuse to buy another tool--and I'm aware of more than a few occasions in the past where I could have used such an instrument--I'm also aware that I can get a suitable Hammond for far less than ...

The old OT: https://photos.smugmug.com/Projects/Kalamazoo-/i-GWtGqWS/0/MhxC65xvzbDVdF425cK6KCShS54HhRX9NJ6CKFQh3/X3/IMG_2942-X3.jpg https://photos.smugmug.com/Projects/Kalamazoo-/i-mxnBbLz/0/MQKpGF5Zp7pRMk3nSwwmxWFbBKS5WCPh2pJVT9ZDp/X3/IMG_2941-X3.jpg https://photos.smugmug.com/Projects/Kalamazoo-/i...

Stevem wrote: ↑Fri Apr 18, 2025 3:38 pm A resistance test is meaningless if it's intermitanly arcing.

Yes. Nevertheless, if it's arcing, resistance could also be out of spec and eligible for replacement regardless.

Thanks for the reminder. Truly appreciated.

Your crackling issue I believe is the OT arcing I am willing to bet at this point. Hmmm. It is an ugly old thing. The paper wrapper is wrinkled, almost looks wet, or like it got wet--it's not and it hasn't. More oily or waxy. I'll take a picture later, have to mow the lawn before it rains. I'll che...

Just a quick update: I removed the 6x4 and soldered a pair of diodes across the appropriate terminals. Voltages downstream were 10+ volts higher than with the tube, as expected. Amp was a bit louder, timbre was blangy metallic, sudden attack but pretty clear. Oh yeah, the crackle was muted a bit, bu...

Going over the rectifiers max spec can shorten its lifespan. People do it all the time but better to stay within limits. MOD and nichicon make 40uf caps. This only applies to the first cap off the rectifier, you can go higher with others. Thanks for this. I might have some caps in that range. Antiq...

...Crackling could be a sign of rectifier tube failure. And there it is. (Maybe..) I've thought about a SS rectifier just because 6x4 aren't very common, nor are the substitutes, and that socket is a bowl of ramen. Full-wave bridge rectifiers are cheap and abundant. Well, easy enough to pursue this...

Question: What happens when PS filter cap's capacitance rating is too high? Background: O'Neal's guide to the KM2 suggested increasing the PS filter cap from a 10/10/20 to 20/30/40, also suggesting 40uF was a maximum according to RCA 6x4 rectifier tube specs. Closest I could find were 22/30/47 F&T 4...