Kalamazoo Model One failure...

[harpidiot]

First off, I don't know what I'm doing. I can be safe inside an amp, and learn easily, but I don't possess any knowledge of my own about amps. That being said:

So I bought a Kalamazoo Model One from a guy who said it was humming real bad. I installed a three-wire power cord. Then I decided to do a cap job. Going from what I read on a harp site (I'm a harp player), for the power supply filter caps, I went with 20uF for all three. After that, the amp sounded great! Then I decided to change all the other caps, and while at it, I upped their values all to .1uF from .0047, .01, and .01, respectively. Then when I turned it on (with the chassis open), I saw what looked like a lightning bolt behind clouds come from the fuse lug that I had connected to the hot mains wire. I immediately turned it off. No blown fuse or any damage that I could see, and no smoke or smell.

But now when I turn it on, I get nearly zero volume at full crank. The preamp tube looks fine, but I think the power tube and rectifier (tube) are both bad. I put a new power tube and tube rectifier in it, and while the power tube survived this test, the rectifier blew immediately. I verified that it no longer works by putting it back in the amp I got it from, and it wasn't working.

So what did I do? I can afford to take it to a tech, but I really love doing stuff on my own, and I have other amps I can play in the meantime. So I'm hoping for a diagnosis that I can use to remedy this problem on my own. I'm not sure what other info is pertinent, so please, ask me questions.

Thanks in advance!

[Enzo]

You did something like installing an electrolytic cap in backwards, you neglected to trim off the excess lead wire from some new part and it is touching the chassis, you created a solder bridge with a solder blob touching two things that ought not be connected, you installed a new part in the wrong spot - classic mis-wire, that sort of thing.

[harpidiot]

The only things that I have noticed are some random fibers (like from q-tips) on socket plates, and I've removed them all. No solder anywhere, and no burnt wires from my soldering tip or anything. I turned the lights off and powered it up and checked for arcs, but I saw nothing. The electrolytic caps are all correctly polarized, and the non-polarized coupling caps aren't touching anything they shouldn't be touching.

I hope it is something simple that I've done incorrectly that can be reversed, so I'm going to check everything again to be sure.

And I have a DMM, so if I need to, I can check stuff that way.

Thanks for your response.

[Enzo]

So does it work with the old rectifier tube in there? If the old tube doesn't blow up, then start taking voltage readings.

[harpidiot]

Yes it does. What readings should I make? I'm clueless as to what readings are important in this matter.

[Jaybird]

http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/
Here is a link that has a schematic to check out the values and voltages. Click on Model 1 and then click on Schematic. There are also good photos on the site.
I have built a few Kalamazoo clones and these schematics have helped a ton.
Good Luck.

[MWJB]

"Then I decided to change all the other caps, and while at it, I upped their values all to .1uF from .0047, .01, and .01, respectively." and "But now when I turn it on, I get nearly zero volume at full crank." - Changing the .0047uf cap to 0.1uf was not a good move, this will bleed a lot of your harp signal to ground...maybe try 0.01uf at the largest or go back to 0.0047uf?

Check voltages at pins 1, 6, 3 & 8 (clockwise from the gap in the pin sockets) of the 12AX7 & check voltages at pins 2 & 3 of the 6BQ5/EL84. All dc, turn the pots fully down, probe with one hand, other in your pocket.

[Jaybird]

- Changing the .0047uf cap to 0.1uf was not a good move, this will bleed a lot of your harp signal to ground...maybe try 0.01uf at the largest or go back to 0.0047uf?

I normally use .1's for coupling caps on my amps. I play harp and have found that the sound is better with the bigger caps. Not so shrill, but deep and hornlike with less feedback problems. I have been using this for a while, as have other harp amp builders I have talked to. For guitar MWJB may be right on, but for harp IMHO go with the .1's or sometimes .022.

[MWJB]

Jaybird, I mod & service (& build if you want) amps for harp players. Yes, .1uf for coupling caps in many cases won't be a problem, in some amps it can hurt dynmamics & high end (my own single ended EL84 amp has .022uf coupling caps - after A/Bing with 0.1uf).

But the crux of the matter is that the 0.0047uf cap is a tone cap, 0.1uf will very likely be too big here.

[Jaybird]

MWJB you are right, the .0047 for the tone cap is best. I was referring to the coupling capacitors. I am sorry that I was not clear in my statement. I still think that the .1 and .022 cap is best for the coupling cap with harp,to my ear, because there is more bass reponse. In my Model 1 amp that I most recently finished, I took the Tone stack out of the circuit and have a Volume control only. I also added a 22uf/50v cathode bypass capwith a 1.5K cathode resistor. the amp sounds very good with harp. However I will try your suggestions on one of my projects and see what I think. It is always good to try different things. I am by no means an expert in this stuff and am willing to learn at all times.

[harpidiot]

Hey, thanks for the replies. I've got some caps and a multimeter, so I'll start taking readings and and switching out caps if need be. I'll report back soon.