1965 Alamo 2574 Reverb Unit

[savantjk]

Not an amp... But more like an amp than a pedal. Internet searching has returned very very little information.. hoping someone here with more knowledge than me can give some pointers.

Playing through a Harmony H303a (no reverb) and a Silvertone 1484 (nasty reverb).

So I picked the unit up for $75 off Craigslist while out of town. Powered on and tubes lit up, but had no way to test it there - a person without other music equipment had it.

Got home and plugged it up.. annnddd only had sound when the mix knob was turned to bypass the reverb. Not much sound, though. Reverb pan that came in it (which I assume is original) is a Gibbs dated 1965. 17" unit with a C stamped inside... I just decided to try some cheap things and bought a MOD type C replacement reverb pan and replaced the tubes. Looked for loose connections and didn't see any.. cleaned it up a bit. Cleaned the pots. Have more sound now and pushes more gain with the new tubes when the mix knob is turned away from the reverb.. but even with the mix knob all the way toward reverb and the reverb knob cranked as well ( this knob doubles as the power switch ) I have no reverb... Ordered Mallory caps and have them on hand, but I just don't think that's going to get the reverb working... my next thought was maybe the power switch / reverb intensity knob is powering the unit on, but not increasing the reverb as it should and it's remaining at zero... But still seems odd for a pot to fail like that. My next thought is that the footswitch could be failing, the cord is solid, solder point ( factory hardwired ) is solid, and button seem to work... I don't know if default would have it on or off.

I've tested the output on both reverb tanks and they will make spring noise through a mini amp with rca connectors that I have. No real way to test input on them.

So the next thing I can do is the recap - am I wrong in thinking this probably won't solve it?

Thanks in advance for any help

[g1]

Does this seem to match what you have?

[savantjk]

I'm not well versed in electronics at all... so reading schematics is currently foreign to me. Just doing the easy troubleshooting. Local guitar shop doesn't do much amp repair, but will change the caps. So if that doesn't help anything I'd either have to drive about 80 miles or send it off... But I've been having a hard time even finding someone to send it to.

[g1]

Where are you? It's good to add your location into your profile.
Someone here may be able to recommend a good repairer.

[savantjk]

In North Louisiana. Updated on profile, thanks!

[Mick Bailey]

I wouldn't recommend replacing any more components until you (or someone else) knows what's wrong - troubleshoot, then fix. You need to establish that the tray is getting a signal, and establish that anything the tray produces is getting amplified. The drive part of the circuit is a tiny power amplifier that energizes the input transducer. It will also energize headphones. I have an old pair that I can use to listen to the drive circuit. Just unplug the connector and clip-lead or otherwise connect to the headphones and listen to whether there's a signal when you play through the unit. If this is OK the output of the tray can be tested by unplugging the connector an placing a finger on the plug, or by tapping/shaking the tray. Do you hear a buzz, or hear the springs boinging?

Further work needs the use of a DMM and checking that the DC voltages are correct and that depends on whether you're confident enough to work on a piece of equipment that's connected to the mains.

[66 Kicks]

I dunno, guys. I don't think that schematic will work with no DC path for one triode and no ground reference for the grid on another triode. The triode of the 6U8A probably does drive the tank from its cathode, but not like that. The output coil could be directly connected to the 12AX7 grid, but the other end of the output coil would need to be grounded.

I have a mid 60's Gibbs tank with a "C" stamped on it and is equivalent to a modern "4F" type.

[Enzo]

Don't read the schematic so literally.

Consider the reverb part drawn as just a representation of the whole part, not its innards. Draw a vertical coil at each end instead, now how does it look?

Now we have reverb drive coil from 6U8 triode cathode to ground, and other end is reverb return coil between triode grid and ground. Look better? Now both your concerns have a DC path to ground.

[66 Kicks]

Don't read the schematic so literally.

Plans drawn by engineers of any sort are explicitly meant to be interpreted literally. In fact they will fall back on this premise when they make a mistake as they claim that it is not a mistake. They do not claim that the mistake is not a mistake because you can interpret it to be something different than what was literally drawn. The more authority someone has, the less likely it is that they will admit to mistakes.

Consider the reverb part drawn as just a representation of the whole part, not its innards. Draw a vertical coil at each end instead, now how does it look?

That was pretty much the guess I was making in my post. It is outer extreme ridiculous that I should be taking my best guess when reading a schematic, so I commented on it. I don't like the way that he wrote the date, either.

Really I just posted because I wanted to point out that a Gibbs with a "C" stamped on it is not the same as a MOD with a "C" on it. If the input coil on the tank really is setting the bias on the driver triode, then a MOD type "C" is going to set a really, really hot bias.

[Enzo]

OK...


I see it as an example of something similar to drawing a bridge rectifier as a square symbol with one diode symbol in the center.




One can like or dislike such a convention.

[66 Kicks]

It's not the convention that I am objecting to. It's the incorrect use of a convention. Had he drawn his lines so they connected to the outer rectangle, I would have felt a lot more confidant about what he meant. Instead, he connected his wires to the ends of a coil that had its leads protruding from the rectangle. Same thing with that bridge rectifier. I don't get to randomly connect my lines to any of the contacts coming out of that rectifier and claim that the reader should be able to interpret what I meant. You can get radically different results by hooking that thing up in different ways.

You're not going to see things my way and I'm not going to see things your way. I don't care. What about using that MOD type "C" in this rig?

[Enzo]

I would have to look up the MOD series specs, I have no reason to doubt your judgement there. 4F sounds like a winner to me.


I agree that drawing symbols can confuse the novice. I look at it as the context demands certain needs be met. Sometimes I don't even notice drawing errors because the circuit needs to be how it is. Like a crossing line shown as connected but really is just crossing. If taken at face value would say short out the filter cap, but in glancing at the schematic, such a connection wouldn't enter consideration. If any of that makes sense. (I made up the example, don't look for it on the schematic here)

[g1]

I posted that schematic because there is no schematic available, and that guy did everyone a favour by drawing one up. I figured it beats the hell out of working without a schematic. Yes, his reverb tank is imperfect but I think anyone with familiarity would suss it out, as indeed you did.
There is no such thing as a Mod 'type C' tank. He said he used the Mod 'type C replacement' which according to their product marketing is the 4FB3D1B.
You said yourself, "I have a mid 60's Gibbs tank with a "C" stamped on it and is equivalent to a modern "4F" type", so what is the problem?

[66 Kicks]

I don't know if MOD makes a type "C" or not but if they do, it certainly isn't readily available. The aes/tubesandmore/amplifiedparts/cedist site claims each MOD tank is a replacement for a tank with the same number. If I were to say that I had a MOD type F replacement tank, I would mean that I had a tank with the letter "F" as the second digit. But then the MOD 4FB3D1B is characterized as a "Common replacement for a Type C Ampeg". It does not say that it is a replacement for a Gibbs with a "C" stamp, but "Ampeg" and "Alamo" both begin with the letter "A" and the letter "C" is mentioned. By lucky coincidence, the 4FB3D1B does have the necessary input impedance, if this is indeed what the OP purchased.

[g1]

You are correct. My mistake. I wrongly assumed the 'type C' would be the same for Ampeg or for Gibbs, but that does not seem to be the case.
Exact replacement for Gibbs type C is 4FB2A1C according to this: Reverb Tanks
As you say, lucky coincidence the impedances are the same, as the decay time and ground schemes are not.

However, I disagree with your statement "If I were to say that I had a MOD type F replacement tank, I would mean that I had a tank with the letter "F" as the second digit". I've never heard anyone else put that idea forward and don't know why anyone would assume that.