Just acquired this last week from a chap in Wisconsin. It's an Alamo 2574 Reverb + spring reverb unit.
I'd been hunting around for one for a while to go with my Alamo Fury amp. Despite the mismatched logo badge and knobs, cosmetically I'd say they look pretty damn good together.
It's a tube driven unit with only mix and level controls. From what I've found out, later issues of this model also had a dwell control, but some were solid state. It has a long spring tank for a nice wet verb and lengthy decay. The tubes in it are a 12AX7, and an oddball 6U8A. I believe the 12AX7 is a driver and recoverer for the reverb. From what I've read online, the 6U8A is a triode/pentode tube similar to the rare 7199, but with a different pin out. My guess is that the triode half serves as preamplifier and the pentode serves for output. Anyone familiar with this tube? Here's the schematic...
The reverb itself sounds very nice when in action. The footswitch for it is still hardwired to the level controls, but I'm probably gonna modify that for a detachable cable and a switched jack so the verb can function without the footswitch if wanted. A quick little demo...
The problem for me is that the unit seems to be doing two non-harp friendly things. A: it seems to be cutting signal...I have to run my tube preamp into it first to get any appreciable sound out of the Fury, and then turn the volume of the Fury up to the max levels. If I bypass the unit and run straight through the amp, it's loud and ballsy. And B: it's putting out a nasty buzz when I do have it running, and it's very noticeable with the amp turned up so high. I don't think grounding is at fault on this since both the amp and the tank are fitted with three-prong power cords.
Some thoughts I had on these problems...chime in if you concur or have other thoughts...
- output impedance on the reverb tank is too low to carry across a healthy signal to the amp - 6U8A tube pentode half is not healthy - old capacitors probably need changed (it sat dormant in a garage for an indeterminate period of time)
Please share your thoughts. I'm planning to take her to a local tech on Friday, but if I have an idea of where to start looking it'd be helpful.
The dry signal comes off the 12AX7 cathode, not great for gain. The 6U8A pentode takes the signal from the input, then feeds it to the triode stage, which drives the tank off the cathode again. I'm not sure whether I'd expect a boost in signal with this...try Rick's suggestion of tube swaps, but it may just be the lay of the land? Fender stand alone reverb tanks eat a little signal too.
Hum with the 2 hooked up together - this could well be precisely because both have 3 prong AC cords, ground loop hum. An interconnect cable with a ground lift adaptor is an idea (safest), or a ground lift on the reverb tank's AC plug/1:1 AC isolation transformer? (Both the amp & tank will still be connected to ground via the connected chassis if the tank's wall AC is lifted).
Check any numbers on the tank itself, they have specific input & output impedances, it's worth checking the tank hasn't been changed out for another model due to broken spring/transducer etc.
Mark...I know Alamo was pretty much the chinsiest of the cheapies back in the day. But what the hell is with the lack of a reverb transformer on this thing?! ----------
I had always wondered what the quality of Alamo Amps was like. They were built in San Antonio and shared a building with a lumber company that is still around today. There are a few techs here in San Antonio that work on them as you can imagine there are a bunch here in town.
---------- Greg Jones 16:23 Custom Harmonicas greg@1623customharmonicas.com 1623customharmonicas.com
Greg, many of Alamo's products sounded great. But just like a lot of the other smaller amp manufacturers back in the day, they built their amps and instruments with the most inexpensive components they could get their hands on, and some of the circuitry they developed could be downright silly. They didn't really document much of what they built either, so now Alamo amp owners are kind of on their own in terms of drawing up schematics and finding replacement parts. ----------
@Hawkeye "Mark...I know Alamo was pretty much the chinsiest of the cheapies back in the day. But what the hell is with the lack of a reverb transformer on this thing?!"
Not everyone used a transformer to drive the tank, Ampeg's '60's Reverberocket is one example of another reverb driven off a tube (though the plate, not cathode in this instance).
After a little further examination last night, I've found that the schematic is not identical to my unit. As you can see, the schematic lists a 1.25A fuse. After taking a closer look at the innards last night, I made the startling discovery that my unit HAS NO F***ING FUSE! Which makes me feel oh so happy that I've used this thing in combat twice now.....both times in venues with screwy or no electrical grounding. Mine also has a pilot lamp on it, and that's not listed on this guy's schematic. So I guess I'm gonna need to get on CircuitLab and redraw this thing after I have it looked at. ----------
Hawkeye Kane
Last Edited by HawkeyeKane on Dec 04, 2013 8:03 AM
Took it by my local tech today. As Mark suspected, it is indeed a cathode driven tank and output. Nothing much can be done for that. My tech suggested running the output through another stage of amplification to add a bit more signal gain, which is doable for me by way of my ART Tube MP. I'll give it a shot.
I tested the tubes, and both registered between 70 and 80 percent, so it's nothing to do with the health of the present tubes. Mark, are there any other tube types I might try to boost overall signal? Maybe a spring coil 12AX7?
The power transformer appears to be a replacement, as it is veritably newer than the rest of the components. Wondering if the fellows at Mercury might have something better that'd work? My tech said if it'd been him who gave it the initial once-over, he'd probably have replaced the rectifier diode. He said it looked ancient, and that he doubted it was even reaching half-wave at this point in time. The unit may have had a fuse at some point, but it doesn't now. So I may get a 1.25A inline breaker to put on the power plug. I really don't wanna risk a surge in current on this thing considering it may wind up being my last stand at the Alamo so to speak.
One spot of good news though....I've confirmed that despite all of Alamo's cheapness in this build, they didn't skimp on the springs. It's a Hammond 1122 tank in there, and it's in good condition. ----------
No idea on replacements for the 6U8... 12AX7s will just be a suck & see situation, the spiral filament 12AX7s are not usually recommended for cathode follower applications.
A 1:1 AC transformer to isolate you from the mains, with a breaker, would kill the fuse & ground loop issues in one hit.
You'd really need to know design voltages vs what you actually have to determine what would be a better PT. ---------- www.myspace.com/markburness
I tried running my ART between the tank and the amp. It delivers up to 60dB of gain to a signal. The results were negligible. So now my thought is resting in the impedance department. Is there any effective way of testing the unit's output impedance other than hooking up a multimeter to an instrument cable off it? If I can get a bead on the current impedance, maybe a small impedance transformer would work? ----------
I thought you used to use it with a line out from a Kalamazoo?
Your output impedance from the reverb unit will be the value of the mix pot at the most (depending then on the relationship of the input it goes into), set fully wet/dry (schem says 250K, so be sure to plug it in the 1M input on the Fury) any difference in signal level if set fully wet, or fully dry?.
I'd blow a few notes through your mic, straight into a AC RMS meter, note voltage developed on a couple of 3 blows. Try again but this time measure the output from the tank...I'd expect hundreds of millivolts to several whole volts? ---------- www.myspace.com/markburness
http://www.facebook.com/markburness
Last Edited by 5F6H on Dec 10, 2013 1:04 PM
I thought you used to use it with a line out from a Kalamazoo?"
Yes.
I did...and still do. But there's only so much gear I wanna sling and haul from gig to gig, not to mention the onstage real estate factor. So I've just been trying to keep it to the tank and the Fury.
There's no apparent difference in signal strength between fully wet or fully dry. Can the RMS measurement you describe be done with a simple multimeter?
Would lowering the value of the mix pot help any? ----------
"There's no apparent difference in signal strength between fully wet or fully dry. Can the RMS measurement you describe be done with a simple multimeter?" Yes, so long as you use the same meter for both measurements (it may be peak, not RMS, but thats not essential right now), we're not looking for a set value, just comparitive measurements. How do these readings compare to the output from the mic? & how does the Fury sound with the mic straight in?
The tank may not be capable of boosting the mic signal, whereas the Kalamazoo can.
"The tank may not be capable of boosting the mic signal, whereas the Kalamazoo can."
Funny how that little amp is becoming more and more of a fix-all item. LOL! Well....I have a head cabinet I'm building for the Zoo right now, and a ceramic resistor for the speaker load. Maybe I'll finish that up, and use all three in my rig. Still have to test that route though. ----------
