Trashpicked Vintage Tube Bass Amp!

Thought you all might be interested in this...Just received a gift from my drummer, an inveterate trash picker. He told me he'd found an 'old bass amp', and since he doesn't play and I've done him a few favors, he'd love to give it to me. Of course, as a drummer and a non-tech guy, he has no idea what it is or when its from or ANY info...

This is what shows up on my doorstep:

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As you can see, it's an old Kalamazoo Bass 30. I can tell right off that it's of the 60's to early seventies era. But the first day or two I have it I couldn't take any time to research or open it up. Solid state? Tube? Hrrrrm.

Some quick time online, and I now know that this indeed is a Gibson era Kalamazoo 30w tube amp. Originally it was a closed back sealed cab with two 10" speakers, but sometime in its life someone did a bit of surgery and turned it into an open back cab:

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This makes sense to me, because as we all know, older bass combo amps are basically useless for bass, and much more suitable for guitar. This one is about the size of my Fender twin, but a LOT lighter. When I first lifted it, I assumed it must be a solid state amp because of the lack of poundage!

Where's the KNOBS?!?! Well, they're here:

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On this neat little flip out panel on the back. You'll notice that the controls are minimal at best. Loudness (term used loosely), Treble and Bass. The on/off is controlled by the treble knob. Odd I know. At this point I had not turned on the amp. I know that older amps are best left alone until you get a chance to suss out the guts a bit, even when they come for free.

The speakers are the original 10" Jensens:

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Good news! The speakers are whole and undamaged, unlike many cast-away amps that have been around for decades! Hopefully I'll be able to date the amp by the speakers. They have several markings. The frames are imprinted with: 220550 C10P C8296 3
The speaker itself is stamped:3010B10W1. The amp was in production from 1965 through 1970 from what I can tell, but I'd love to know what year this one is.

OK...some quick time with a screw drivers (the back panel screws are a mis-matched lot of sizes and types, this makes me nervous as to what I might find!), and I've got the back panel off. First I note that at some time a squirrel or mouse may have lived here. There are sunflower shells in here!

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Cool! It is the tube version of the Kalamazoo bass amp! Good news! A closer look:

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Lots of dirt, dust and cob-webs. Two decent looking transformers and 4 tubes. No rust or decay to be found! Also, everything looks to be original!

What do we have here:

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The pre-amp tubes are two 6EU7. I can't tell what brand these are...besides that they're marked USA. Anyone have any ideas? Also, any of the tube people here want to provide some kind of commentary on the 6EU7? Its cool to me that this isn't the usual 12AX7/6L6 or 6V6 amp...

Power tubes are 2 GE 7591's. They look to be original too...but I could be wrong.

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Here's some spec info on this amp:
http://www.netads.com/~meo/Guitar/Amps/Kalamazoo/Bass/

And a schematic of the amp from the same source:



Any commentary on this welcome!

As of now, as it looked like the amp was undamaged, I did plug it in. If I intend on using it regularly, the two prong cord will have to go. If and when that time comes, I'll probably also have the amp pulled from the chassis and have the wiring and caps checked out.

I haven't and probably won't put a bass into this. BUT...it sounds GREAT as a guitar amp. Run 3/4 up and with the bass all the way down and the treble all the way up its warm and fairly dark. A PRS hollow body with 59's in it cranks sweetly with nice breakup and crunch. My Telecaster with single coils sounds even better. That chime/grind that makes you think of Pete Townshend or Page in the early days but without overpowering volume. Its louder than my 65 champ by a long shot but not nearly as loud as my 100w blackfaced/silverface twin.

The speaker code suggests that the speakers were made in december of 1965, meaning that it is probably a 1966 amp!

JimmyM said:

That's a beaut! I wouldn't hesitate to try it with a bass at lower volumes, but you're probably right that it'll be better as a guitar amp. Looks a little shallow for bass.

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Thanks! To me it sounds like it would bottom out, but maybe for low volume around the house stuff it could be fun.

I've now vacuumed up 40+ years of dust and dirt out of it and given the tolex a good wipe down. I've vacuumed out the sunflower seed shells and over all it's a very cool looking amp. It will get some use for recording guitar parts I believe as well...When I powered it up it grumbled at me for a few moments, as old tube amps like to do, but then it operated silently.

Phlatwounds said:

Cool find.
I found a similiar type tube amp in the trash along time ago.
Rushed it home, plugged my guitar in, then was met by one of the worst shocks of my life.
I guess that's why it was in the trash.

I advise anyone that ever finds anything like this, especially with the old two-prong cord be very careful to check for electrocution possibilities. I was about 13 and had no idea that this could happen.

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Yes, found amps can be problematic. Everyone should be VERY careful with any amp that's been out of use, or you might suspect has been out of use for more than a few years. With tube amps, you need to worry that the capacitors have gone south or that the tubes have gone, or that one of the speakers might be shorted all of which (and other things) could cause not only electric shock risk, but also risk of fire. In this case, I didn't see any water damage, but this amp was left outside in Syracuse NY and it could have suffered from rain or snow as well! This is why I did open it up before trying it, but I didn't open the amp chassis as I didn't have the tools to get it out of the amp cabinet.

Never_grew_up said:

I hate you.
The only thing I've been offered lately is every massive old analog TV my friends have all replaced w/HD flat screens.

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Hey, I hear you...I've been lucky over the years. This amp is cool, but I was also given an early 70's Fender Twin Reverb by the guy I used to work for. Had I worked for him longer I would have worked on getting gifted some of his other gear, including old Princeton and Harvard model fenders and SEVERAL 60's Fenders and 60's Rickenbacker guitars! I knew I wouldn't be able to score his 58 Les Paul though!

Not as much of a score, but still cool is my 1965 Fender Vibro-Champ. Back a bunch of years ago, I noticed that everyone was making high end Class A low wattage guitar amps and charging a million bucks for them. It seemed funny, since the classic one...the Champ was easily available for not much money. I ended up buying mine on eBay for under 300$ with shipping. It came, complete with RCA black plate and Mullard tubes! Now they go for more than twice that?!?! Even at that time I said to myself that Fender amps are under-valued while their guitars and basses of the 50's and 60's were going for big money. Now the vintage amp market has started to catch up.

coreyfyfe said:

From here: http://music-electronics-forum.com/t7078/

"The 6EU7 and 12AX7 - as well as the 20EZ7 - are exactly the same "guts" with different heaters and pinout. If you calculate the heater wattages for each bottle you'll see that they're identical."

and

"The only MAJOR difference is that the 12AX7 has dual heaters, one for each cathode. Because the cathode of the 6EU7 share a heater, which is not as close to the cathodes as in the 12AX7, the plate voltage and heater-to-cathode voltage are a bit higher."


As they're uncommon it looks like they're probably pricy. Tube depot has some sovtek 6EU7s for $20 each. And by the way, I hate you with as much envy as possible. Seriously.

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Interesting. To me they looked noticeably different from 12AX7's, but the dual heater of the 12AX7 would explain that.

I read through the thread posted, and I agree that this is a 'dark' sounding amp. With guitar, the treble maxed but the bass all the way down was the most usable tone. I can imagine that this would have been a mud-monster if one bought a Gibson bass with it back in the day! The tone was more balanced with my Telecaster and it's screaming bridge pickup.

Hey, no need to hate...these type of things are out there and everyone has moments of luck.

GlennW said:

The 6EU7s might be Sylvania or Phillips (guessing at the ink color). Is the one on the right cracked? Cool amp, btw.

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Nope it's not cracked, in fact, that's a spider web as I hadn't cleaned the amp before I pulled the tubes.

I've been trying to read up on the 7591 and while fairly rare, I've seen some very cool commentary. Both EH and JJ make versions, with the JJ being closer in size and shape to the originals. I guess that 7591 were only made by one or two manufacturers, Sylvania and Westinghouse (who engineered the tube originally). It looks like GE were rebranded Sylvanias.

The 7591 was also used by Ampeg in their Jet and ReverbRocket guitar amps, and supposedly have better clean headroom than 6L6 or 6V6 tubes. Also, at least one statement said that they're closer to EL34's in their output and tone, and supposedly very sensitive The 'breakup' is described as being very 'Marshall-like', which explains what I was hearing and why it sounded good to me.

fdeck said:

Please get that done right away. The two prong design can be lethal.

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I have no intentions currently on using this amp even really on a semi-regular basis. If I did, this would be in the shop tomorrow. Unfortunately I live in a turn of the century apartment that has no grounded outlets! I know...simply awful. So, as this amp won't be leaving the house, and none of our outlets are grounded, having the cord switched out won't make any real difference. I'm fully aware of the dangers of ungrounded equipment. This is also why I don't use an amp at home when practicing. My old 65 Vibrochamp also still has its original 2-prong for the same reason. When this goes to get it's cord cut, I'll also have the caps either evaluated or replaced and all of the usual responsible upkeep.

Stromrider said:

The 7591a is the octal varient of the 7868 power tube. The 7868 was used in the Ampeg SB12 I think. In fact, looking at the schematic's, the designs look somewhat similar. If you wanted to, a competent tech could rewire the amp to use 12AX7's making replacing the tubes a bit cheaper. I'd hesitate to make any serious mods on an amp like that however.

If you're going to leave it with a two prong cord for awhile at least check and make sure there is no "death cap" on the AC line. Definitely get it into a shop to get checked out and fitted with a three prong cord soon. Like Now.

Cool score!

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Thanks! I think that modding to 12AX7 would be a mistake. Besides the power cord, I believe that the 'charm' of vintage amps (especially off the beaten track type amps) comes from the design differences like the tubes. While reports say that the 12AX7 and the 6EU7 are roughly equivalent, I'm of the belief that every difference is a tonal difference!