How to improve Gibson Kalamazoo Reverb 12 tube guitar amplifier reduce noise fix tone circuits

Yes a real challenge. Did my best to correct the mass of problems. Unfortunately, it became more than the value of the amp. So, the owner and I agreed to quit and he will use it as is. Although we all love the vintage appeal of these amps, you have to draw a line between repair and total redesign.

[@glenwhatley4125]

If you knew the impedance of the original tank, the correct tank would most likely resolve the weak reverb issue. It's always nice to have a variety of tanks around for that purpose. As another tech mentioned if the issue is the tiny wires from the actual transducer to its solder lugs, it is possible to repair them, but that all depends of there's enough length to do that and your dexterity. At the very least you might be able to measure the DC resistance to get an idea if the tank you have is a match. Great idea duplicating the Fender tone stack. Love the videos.

[@electrolytics]

I was thinking that too(the proper impedance for the reverb send and return). But how come the tone generator seems to give a good response in the reverb?

[@DaveHilts]

you hit the nail on the head. I've worked on a ton of Gibson amps and usually the reverb transformer is bad. In this instance the reverb tank (fender) is the wrong tank to use. You should replace it with a Gibson style tank and all will be good (4FB3H1B). I like MOD tanks.

[@60retired]

I bought my amp new in early 67, it was my first amp, its a 66 Kalamazoo Reverb 12. I do all my own amp work. I still have all the original tubes in it, they actually sound better than any of the new replacements. I have heard nothing but good compliments on the way my guitar sounds through that amp. The only components not original are the filter, cathode caps, plate load resistors and ground 3 prong AC cord. Yes the phono jack and input jack are really loading down pin 2 on V1. If you stick a dummy 1/4” plug into input jack 2 while playing through input 1 this ungrounds it and increases gain or you could wire it like a Princeton and remove the shielded wire from the phono jack to pin 2 on V1 for twice the gain like you did. That 2.2K resister on Input 1 that you pointed out as being replaced looks original and is identical looking to the resister in my amp. If you look at the color of the carbon composite, the color and width of the colored bands, most likely the physical appearance matches the other stock resistors in the amp, it’s a good indication its original. Replacement carbon composite resistors stand out, they look much different than the vintage ones. That reverb tank also looks like the original tank to that amp, its a Gibbs Hammond tank and it even has the same 66 date code stamp on the outside like mine. Pin 8 of V1 should be grounded, and one side of the heater secondary should be grounded as well or it can induce more noise, thats why pin 8-9 are bridged. Every stock Kalamazoo Reverb 12 has the black filament line soldered to pin 8 and jumped to pin 9 on V1 from the factory. Mine has the two filament lines with the two resistors raising it above ground stock from the factory, I was surprised to see that, its not on the schematic. The two AC wires, one black and one yellow that run under the potentiometers to the on/off switch in your amp are not shielded. Mine along with quit a few other Kalamazoo Reverb 12 amps that I have seen came from the factory with these two wires inside a spiral metal sleeve to shield them. My amp has no hum and never has. So the hum your hearing could be from the factory not shielding those wires. I would think relocating them would have had better results with noticeably less hum. I agree theres a lot of ground points but its not causing any unwanted noise in my amp like your experiencing. Every riveted terminal strip has two ground points to the chassis. Fender didn’t solder all their grounds in the later amps and I have never seen any real issues with that. I would say if you check the grounds just like you do with solder joints you shouldn’t have major issues. One thing that amazes me is that I’m seeing 6.3V on my filament heater wires with 120V line supply. Its more like 6.8V on my Fender amps. 6.3V is right were it should be which is very unusual. And with a 110 ohm 2W cathode resistor the plate dissipation is 39.7mA, 11.8W = 98% which is perfect for 6BQ5/EL84 tubes. Max 100% safe dissipation is 40.5mA for a 6BQ5/EL84 push pull cathode bias amp, its at 39.7mA. Its safe to run this amp at 100%. Keep these great videos coming Terry, they are greatly enjoyed and appreciated by all.

[@DeathCapAmplification]

The Kalamazoo Two might be their best sounding amp circuit. Of course I haven’t heard them all. But I really like that little Kalamazoo two. Enough that I cloned one in a Layfayette Ham receiver, DLab style! Nice work on getting this amp sounding good. Very gold choice to do the Princeton tone stack. Love this walkthough and troubleshooting!

[@mackfisher4487]

I was thinking about converting a guitar ramp to a general coverage receiver. (Don't take my remarks seriously) I'm just an old RF guy.

[@jjsant3250]

Some people think reducing the values of “input” resistors lets more signal thru. That’s why you sometimes see that “mod” on resistors in that vicinity. They don’t understand that some of those resistors go to ground and there is no current flowing into the tube. No current means no flow to increase. All the current flows to ground. Instead, one can increase the signal voltage (not/no current) at the tube by eliminating voltage dividers and increasing the impedance to ground as Terry has done here by Fenderizing.

[@kendavis8046]

We're going to be arctic (for us, anyways) in North Texas over the next couple of days. We're supposed to be around a high of 16 on Monday. That's okay, it'll all average out when we get to 115 this summer. (For your foreign subscribers, these temps are quoted in F, not C!) In any event, thank you, sir. Still enjoy your channel.

[@matthewf1979]

I like what you did here. These late 60’s to early 70’s Gibson amps are a mess. The decision to install couplets in the tone circuit, with no identification of the values within was and is a bad idea. Gibson never had future service in mind(coupling caps underneath eyelet board)for any amps they ever made. The Norlin era went 5 steps further.

[@choctawhatcheekid309]

Upgrading the speaker would improve that amp immensely.

[@hookydoo]

You should consider tuning your guitar to an open tuning. It would make testing much more pleasant. Great job btw! Love your channel.

[@hookydoo]

@@newscotlandtv Em11 is what it is called.

[@paulhartfiel6722]

I'm a novice amp hobbyist at best...I stumbled upon your channel very recently after purchasing a silver face champ that needed some work...coincidentally, I found an untouched Reverb 12 at a garage sale a few years ago and have done a few things to get it up and running (thanks to some great, but limited, information on internet forums on this specific amp) but it's sat unused for the majority of time I've owned it. When I saw this video pop up on my feed I was thrilled. Very informative - thank you! Look forward to changing the tone circuit as you've discussed here...reverb in mine still works quite well with original tank (same tank as you removed here)...will be a very much improved amp!

[@172turtla]

Terry this is a great video, thanks for explaining the problems with Kalamazoo amps. I have a Bass 50 which seems pretty rare, the previous owner has modernized so it sounds better but the tone circuit and the power switch on the treble, man that is bad! As always keep those videos coming!

[@scottcentilli2401]

Thank you for all of your help with this one, Terry!

[@dhpbear2]

5:06 - My first encounter EVER with a shorting RCA jack!

[@edwardhannigan6324]

Great fix and tips Terry..Certainly sounded like a headache with all it's little problems. Thanks for sharing your knowledge again..Awesome channel as usual..Ed..U.K..😀

[@arnebroxleirnes418]

Originalty schmoriginalty! If a change is an improvement - why not? Good job, D-lab!

[@preiter20]

Friday night with D-Lab! Great video as always Terry. Snauzeramous is back! My take away from your video? Just buy or build a Fender amp, lol

[@deasename5839]

Yea next time i see one at a yard sale or thrift shop for sale I'll just walk away and wait for a fender.I love the new fender PCB's too.

[@studiomarand]

I think you did a good job with the Fender mod. Originality is good when its working ;-) Cheers

[@electrolytics]

That was a good video. Lot of areas covered.

[@Finom1]

Another amazing educational video.

[@stevelark9840]

Gibson/Kalamazoo really did some oddball maneuvers with tone circuits back in the day. I have a Gibson GA-5 Crestline that I just rebuilt. The tone circuit is preventing the amp from sounding great, but I battle with modifying it from the original circuit.

[@jman1428]

Great video terry, i found with my reverb 12 the ac filaments being grounded on pin 9 have lower noise than adding the 100 ohm resistors to ground.

[@sski]

Greetings from Artic Wisconsin. Nice job, Terry. I don't know why they designed the tone circuit that way either. Absolute noisy garbage. Like what you did with it. Cheers!

[@dhpbear2]

16:34 - That hum sounds like 120Hz power supply ripple!

[@ColeWheeler4Lyfe]

Nice Job!!

[@pneumatic00]

Terry, I would say that 3 out of 5 bad reverb cans can be repaired if you examine them carefully. In most cases, the teeny wires are just broken at the RCA jacks and resoldering them fixes the issue...but I have also repaired ones where the input wires are broken at the little tabs on the coils. Still, reverb cans are pretty cheap..but not free!

[@scottbc31h22]

Does the Fender reverb tank you installed have the same input and output impedance as the original?, If not, why not replace the tank with one of proper spec. New tanks cost around $25. If the original tank does not have code numbers, impedance can be determined by the color of the color of the plastic in the bobbin. sound-au.com/articles/reverb.htm www.accutronicsreverb.com/main/?skin=sub05_04.html

[@kiarabrandt6724]

Nice input with the 7 advertisements. Lol

[@rolandbedell6195]

That was real good.

[@hallanvaara6106]

Awesome repair! 1 amp switch! It turns on one amplifier. Terry, your voice is a bit more echoy than it used to be. Maybe a little microphone adjustment might help?

[@taylorfusion]

I love your channel Terry and especially guitar amps. I have a 1955 Gibson GA 20 that desperately needs a handle! I can’t for the life of me find one on the Internet that would fit this particular model. I’ve given up trying to get the period correct one any handle would do at this point. Can you or your viewers recommend a resource I could locate for this sonic treasure?

[@BluegillGreg]

Mine got a lot quieter when I put a power switch on the back of the chassis and got rid of the direct from wall power going through the reverb, tremolo and tone circuits' area. That and a properly installed 3 prong plug was all I did and mine sounded pretty good.

[@telefunkian]

Great video as always Terry. I have a question for you about an older video (last year or so) where you showed off a new LED illuminated magnifier, I believe it was rectangular, but I can't find the video. Do you still like it and would you recommend it? I don't think I see it in your recent videos?!? I need a new one for my bench! Thanks for all the great content.

[@ferraridinoman]

Lets be honest here! Leo got it right most of the time!

[@robertw1871]

Haha yeah.... probably would have been cheaper to toss that POS and get a Princeton...

[@hestheMaster]

Fender yes. Reverb no! Quite the humming mess here. It should be dead quite. Pretty snauzuramous fix Terry !

[@bobrowlette4143]

Great Job, Terry! I remember these amps from the late 60s. My friends and I thought they were junk then and far inferior to Fender amps. Not even remotely in the same ballpark. You made a valiant effort here, but as the old saying goes, you can't make a silk purse...

[@duster71]

That was the first amp I had,either in 68 or 72 forget what year I got it

[@jman1428]

I put a 8eb2c1b tank in mine and reverb is strong, the Fender tank input is 8 ohms to low for that driver trany.

[@danielsaturnino5715]

Great, a fenderazoo princetaroo :D come on Dlab, give us one chord. Just one. How hard can it be? :)

[@adotbeck]

The tone circuits may be bad, but a sine wave is not good for judging things like that. That would be like trying to adjust the color balance on your tv or monitor by just displaying a pure red screen.

[@dhpbear2]

I'm curious how this amp worked even with the original configuration per the schematic. (?) The input resistance using J1 or J2 would be 136K; nowhere near 1MEG!

[@jimbucket2996]

I don't know about those stock tone controls. There must be something wrong with the way those guys from Michigan think.😉

[@jutukka]

That amp looks like a rat's nest but in fact, if the layout is designed well, the wiring can be kept much shorter than in those amps which use turret board and quite long wires to tube sockets.

[@jimotranto546]

Terry, where does the white wire come from or go to that is soldered to the milk dud cap after the tone stack upgrade?

[@dhpbear2]

12:59 - The filament wiring really rots, to boot!

[@jman1428]

Wouldn't the 100 ohm resistor be correct for a cathode bias?

[@GIBKEL]

Who did they contract these out to? Did Gibson build any of there amps or were they all sub contracted out?

[@GIBKEL]

I’m looking for 2k pots for a 65’ Falcon. Any chance you would sell them?

[@DAVIDGREGORYKERR]

ALUSOL Solder works with Aluminium.

[@jts3339]

Gibson for guitars. Fender for amplifiers. Things just work better that way.

[@83roadstar]

Gibson's new amps will slay Fenders amps , It shouldn't be long before Gibson's /Mesa Boogie amps are on the market!

[@jts3339]

@@83roadstar I saw that! In that case, I’ll break the rule. (But I believe they’re going to continue with the Mesa branding)

[@66g0yy]

Maybe, more like a pedal, than amp,

[@_-_Michael_-_]

Why Gibson amps are designed so badly, compared to rest vintage stuff?

[@Jah_Rastafari_ORIG]

Tune that guitar, you _monster_ ...

[@tjsogmc]

that's why I never buy stuff from reverb.

[@jameswilburn5406]

Kalamazoo amps had nothing to do with Gibson.

[@bobfellows6974]

What makes you think that?

[@60retired]

READ: en.wikipedia.org/wiki/Chicago_Musical_Instruments