What is this for?

[Reep]

I just bought a used Peavey Series 300 bass head to use at the band's rehearsal room. As expected with an older amp, I had to open it up to clean the controls which were crackling.

I'm mystified by something I found inside. On the back of the chassis there's a three pin Din socket that looks like the Peavey footswitches should plug in. However, the wires inside terminate into a molex connector with green, black and blue wires. There's no sign of a mating receptacle on either of the circuit boards. What the heck is this thing for?

Also, one of the two plastic brackets on the back that's designed to store the AC cord for transport is gone....I'm guessing it broke some time in the past and was removed because of a broken plastic hazard. Is there a source that I can order a replacement from?

[Enzo]

Like the wires are not long enough to reach a circuit board. I myself am not aware of any footswitch functions on that amp. But it does look as if it belongs there. Is there any print around it on the rear panel?

[Reep]

No, there's nothing anywhere on the panel that looks like it's designed to have that molex connector plugged in.

[Enzo]

I mean outside on th rear of the amp. Usually it says like 120vAC or amps by the fuse holder. Or some number of watts by the speaker jack. I wondered if anything was written near the DIN jack? Yeah inside the wires don't reach anything

[Reep]

Ah, now I understand. The amp's at our rehearsal room so I can't look now. I'm picking it up tomorrow so I'll post then.

[Reep]

I just checked the back panel of the amp, but there's no labelling pertaining to the din jack. I've been wondering if that chassis shared by a different Peavey amp that does use a footswitch, but it's just a way to fill the hole in the bass amp.

I'm also wondering if there's a parts source to replace the missing bracket that's made to wind the AC cord to the chassis for transport.

[Enzo]

On PV amps of the era, the front panel was the preamp and the rear panel the power amp with power supply. SO they did use the same rear panels with several other models. But give them credit, they wouldn't just stick an unused jack in a hole for filler.

It could be something someone added. In any case, you can ignore it. Or print a small label near it calling it Quantum Interface or something mysterious.

I am sure someone has cord wrapping ears. If original isn't your deal, a piece of steel strap bent up and then back down to make a sort of Z shape, and screw that in place.

[Reep]

Thanks, Enzo. I guess my curiosity about the din jack was more academic than need-to-know. It just puzzled me that Peavey would have bothered with installing a jack and harness rather than using a snap-in metal blank-out cover.

But, your comment to me about the cord-wrapping ears is interesting - I hadn't a clue what to go looking for if I was going to search online. If that's a common term for that piece, it gives me a starting point. I haven't got any tools or a vice to manufacture anything to replace what's missing, but maybe if I find a company that sells parts to people who are scratch-building amps, I can look for something now that I know what they're called.

I've only used this amp once since I bought it, and I'm not convinced that I want to invest a lot of dough on this.....The amp seems relatively weak-sisterish for something that's purported to be 150 watts. I had to set the volume over the twelve o'clock position to get an adequate level for a band rehearsal that wasn't particularly loud. My 85 watt Twin Reverb blows it out of the water, so I'm wondering if somethings not right. The Peavey appears to work okay, but after the power's shut off it makes a couple of significant noises, almost like a cap is unloading or possibly an output IC's failing. I remember my Peavey 400 Bass and Mk IV head's being a lot ballsier than this one.

[Enzo]

Do you know that power isn't loudness. Power is logarithmic. 300 watts is only 3 decibels louder than 150 watts. If the preamp doesn't fully drive the power amp, it will sound weak.

This amp is over 50 years old. And from experience I can tell you the small electrolytics they used did not age well. Specifically the 2uf ones. I expect to have to change ALL of them, when I see thee amps. I love the old stuff, but 50 years is 50 years. Those small caps are all through the preamp, and could well be your problem.

Do you have a hardware store around? Like an ACE or a Home Depot or Menards? A Z bracket ought to be found. If you got a flat strap bracket, they might be willing to bend it for you. Or even a garage, they might do it for nothing, of if they want a few bucks, so be it.

[Reep]

I sent the amp to my technician who diagnosed bad caps on the preamp board, He replaced eleven 2.2 caps, and he also found that two of the large power supply caps were bubbling...He said they weren't bad yet, but he replaced them to head off any future issues. The amp sounds much better and the power output picked up appreciably. The schematic, which he found online with some difficulty, states that the amp is rated at 120 watts RMS at 4 ohms.....Not 150 as I had surmised. After changing the preamp caps which had drifted and weren't driving the power amp section properly, he measured the amp's output at 132 watts RMS at 4 ohms, 74 watts at 8 ohms.

There was a secondary problem which I ignored before the amp went for service, thinking that it would probably disappear in the course of the first service. I'd basically forgotten about it, but when I tested the amp last night, it's still there.....The amp works fine when it's powered up. However, after it's shut off, after about two minutes, there's a loud pop through the speaker. Over time, the amp will continue to produce a pop or snap through the speaker every minute and a half to two minutes. Pulling out the AC power cord doesn't stop it from happening.... The only way to stop it is to disconnect the amp from the speaker. It sounds like a capacitor discharge to my uneducated ear, but I'm guessing. I just don't know whether to go on chasing this or just give up on the amp and put it aside as a bad buy. I've already decided it's not enough power for my needs and had planned to sell it and look for something with higher wattage. Now, I've got 165.00 invested and don't know whether to keep trying to get it corrected or just give up on it. Any thoughts?

[Enzo]

Itis just a noise, won't hurt anything. As the system discharges, it goes through moments of instability. Try this, As you flip the power switch of, play a loud chord through the amp to drain it. Any help?

[Reep]

Thanks for the reply, Enzo. Yes, your suggestion parallels what my technician told me just a short time ago. He says thatin older amplifiers, there's no protection circuit to mute the output when the amp is shutting off. And, he suggested, as you did, that if a player players a loud note or chord when the amp is being shut off that it will drain the power supply capacitors faster and eliminate that noise.

When he was working on the amp to change the preamp capacitors, he noticed that the power supply caps were bubbling and although they hadn't failed yet, the bubbling was an indication that they were on the way out and had a limited lifespan, so it was advisable to change them. He didn't have exactly the same values of capacitors in stock, so upgraded them to something larger, and that may has exacerbated the amount of noise, but he says the noise doesn't represent a problem.

[acscomps]

I have this amp, bought new about 1970. It will blow the doors off any building you play it in. Mine is out at the moment, haven't opened it but suspect it has a bunch of bad capacitors and such.

[Reep]

My amp sounded much better, tonally and power-wise after the preamp caps were replaced. However, the symptom of the major pops and crackles after it was turned off got a lot worse after the power supply caps were upgraded. If I hadn't had any options, I probably would have stuck with it because it really sounded not bad. However, I had a chance to sell it and get the purchase cost and service cost back, and another 75.00 put me into a 400 watt Yorkville Sound XS400, so I took the opportunity to upgrade. I do like the older Peavey amps though....For years, I had a Mark IV bass head biamped with a 2x10 Scorpion cabinet with a built-in 200 watt slave amp and two 1 x 18" Black Widow cabinets, and it was as good sounding as I needed - Just a bit too big to move around.

[OldJohn]

Got one of those and been working on rebuilding it when I have time to spend on it. Ditched the amp board because the transistors are all obsolete and substituting is too much work. The original voltage regulator for the preamp is on the amp board and it was fried. Rail voltage is +/- 39/v so I have a Chinese amp board to replace it when I have the time. Built a +24v supply for the preamp board and replaced all the electrolytic caps. Like you, I have no idea what the back panel socket is for. There is no apparent function in normal use and the schematics I've found don't even mention it. You mention the tech had replaced the original power supply electrolytic caps that were "bubbling" and "upgraded" them. That shouldn't cause the noise problems you mention, but check the values. The original caps were 4500uF, 50v.