Curious little bargain tube amp that I got to bring back from the dead..... a 1940's Kalamazoo KEA.
This is one circuit that I'll admit I have little idea of how it works. It has a single 6SJ7 driving a pair of 6V6 tubes. How this is achieved I do not know. It's not parallel single ended. Look at the schematic!
The other challenge was finding a 10" speaker that is shallow enough to fit in such a tight space. This project sat around and almost made the 'sold for parts as-is' pile. Nothing fit! But fortunately I was able to find an old CTS speaker in the bowels of Southside Guitars deep in the basement under discarded case parts...... It was dead so we had it re-coned. Speaker had the perfect holes to mount the output trasformer.
In the end it's a fantastic little amp. Not a fuzz box, very clear and defined. These are financially not worth the effort, you'll likely lose money doing what I did. But we were all so blown away by it's charming tone that an employee bought it and will be tracking with it! I have an affinity for the 6SJ7 tube. My favorite preamp of all time so it's not surprising how much I like this amp.
Did the usual, changed the caps etc. Fortunately both transformers were good. I do not think this amp would work well without that odd asymmetrical output transformer. So make sure that part is happy before you commit to one of these.
J
Nice. I was thinking, hey that looks easy to build until I saw your note about the output tranny.
ReplyDeleteBTW, just heard about you on the Amp podcast. Any tips for a 5e3 that's too bassy :) I am going to try your mods.
Best,
Yann
Ha! Bottom heavy 5E3. That may be a first!
ReplyDeleteWell first thing is always work back to front. In other words speaker, output transformer, output tubes / stage, phase inverter, preamp etc. This goes for trouble shooting as well.
My first question would be what kind of speaker is in there? Some speakers are balanced nicely for guitars, some are bass heavy or treble weak and some are weaker on the low end.
After experimenting there what kind of output transformer is it?
What value cathode cap is on that 250 ohm bias resistor?
If all of that checks out then try some different signal cap values. Rather than .1 in the phase inverter try .022 and see what you think. Each stage will make some difference. Let me know what you come up with.
Hey sorry I never saw your reply!
DeleteSo I after listening more of the podcast I decided to gut the thing and use all old parts I had been saving. I didn't think it would make a difference but it did. I can actually hear the "interactive" controls do their thing and what I do now is crank up the volume and tone then use the unused channel's volume to dial my actual volume.
My circuit is completely stock. In the past I tried every mod on Rob Robinette's site and started over when my bass issue persisted. Now that the amp is dialed I am going to try your 3.3 meg trick.
I am going to use your back to front approach on a Princeton Reverb I just built which has the opposite problem. I used old parts again but the amp is ice picky! Such is my luck.
It's amazing how much more than a schematic is involved in these things.
Anyway I liked your second guest appearance on TAVA. Look forward to the next.
Hi! One question about the KEA tranny: what made you think it is asymmetrical?
ReplyDeleteWhat I meant by this was one side was about 100 ohms higher than the other which I found unusual.
ReplyDeleteWe had given up on this amp, the field coil speaker was long gone and there was nothing that would physically fit inside it's shallow box so we were about to scrap it for parts. I needed a period correct transformer for the Fender Model 26 I was restoring and figured this would be a decent fit for looks and sound. But it hummed badly. I took some readings and figured maybe something was wrong with it. So I found another transformer NOS for the 26, sounded amazing and looked great. I installed this one back in the KEA and it was dead quiet and sounded terrific. So I came to the conclusion this was by design.
I didn't have the opportunity to install a different transformer in the KEA as an employee snatched it up the day after I got it going. A truly lovely sounding amp in the end.
J