Delta Blues reverb op amp

[Plunker]

I am new to working on a guitar amp, but have 30 years of electrical/electronic maintenance experience. I have a Peavey Delta Blues that the reverb has stopped working on. I have visually inspected both ends of the wiring from the pc board to the tank. I will check with meter tonight. My question is where is the op amp chip located. I have the chassis out of the cabinet, but don't see it. Maybe old eyes. A pic or description would be appreciated.

[Jazz P Bass]

Peavey Delta Blues

The reverb opamp is marked U1. NJM4558 dual, 8 pin.
These amps are a real PITA to work on.
Verify the reverb tank first.
Each transducer should have resistance. The input will be lower than the output. If any are open circuit, check the red & black wires, sometimes they break if the tank tray gets overextended.
If you touch the return plug with the amplifier on it should make the amp hum.

[Enzo]

There is only one IC in the amp. It is up on the control panel board next to the reverb control.

But as Jazz said, check the reverb pan transducers for continuity. Unplug the cables to it first

[ReginaldBisquet]

On a related note - the stock reverb pan for this amp is an Accutronics 9EB2C1B (800 ohms input / 2575 ohm output).

Question: I have been looking over the schematic of this amp and I can not find the values of the input / output noted in the circuit. Peavey sent me a board layout as well as a schem... but I'll be darn if I can find the notation of the reverb values.

Any help?

[Enzo]

I have never seen any schematic that actually spec'd the impedances of the reverb pan, if that is what you mean. You know the stock pan is an EB type, so there you go. ALL Accutronics reverb pans have the same output impedance more or less. REverbs driven by a little transformer tend to be the AB types - low Z input. The ones driven by an op amp or something tend to prefer the High Z input pans, like the EB or sometimes FB.

Maybe i am missing what it is you want to know.

[ReginaldBisquet]

I have never seen any schematic that actually spec'd the impedances of the reverb pan, if that is what you mean. You know the stock pan is an EB type, so there you go. ALL Accutronics reverb pans have the same output impedance more or less. REverbs driven by a little transformer tend to be the AB types - low Z input. The ones driven by an op amp or something tend to prefer the High Z input pans, like the EB or sometimes FB.

Maybe i am missing what it is you want to know.

Indeed - the rationale behind my question is that I also have a Peavey Backstage Plus from 1985 that has sentimental value for me. However, it doesn't have a reverb tank. This amp is solid state, 35 watts output.

I have been searching all over the Internet and have made some calls to a few stores in search of a replacement. The tech from Peavey sent me the schematic and said the original pans were made by OC Electronics and were two spring types. He said the Peavey part number for the pan is 71011109... which is noted on the schematic.

So... I was thinking I could find the input and output values of the reverb circuit in order to buy a replacement. This is now obviously not the case.

The question is: how do I source a compatible reverb tank with this amp?

[Enzo]

Use the rule of thumb in my earlier post.

Yes, PV used some OC pans in earlier amps, after all, they were "manufactured by beautiful girls in Milton, WI under controlled atmosphere conditions."

If you stock just two kinds of reverb pans, you can cover almost every need.

First, a little research into the PV line will reveal that the older amps with the OC pan had a certain drive circuit, and later amps using the Accutronics pan used the exact same circuit. ANd those expected the 4EB2C1B. Or the 8EB... or 9EB... Peavey has pretty much stuck with the drive circuit for years - the output of a 4558 IC directly drive the pan input. This mean the pan must have a high Z input, the IC would not be happy trying to drive 8 ohms. The highest ACCutronics input impedance if the F, with the E right behind it. PV uses the EB pans.

At the other end of the spectrum are the pans driven by a transformer. Fenders for all time as an example. The little drive circuit is really nothing more than a low power power amp, and the low impedance output drives a low impedance pan. That would be something like the 4AB2C1B or 8AB... or 9AB...

The 4 and 9 pans are the full length pans, the 4 having two long springs and the 9 having 3. The 8 pan is the shorter one used in smaller amps lacking room for the larger one.

SO if you keep a 4EB2C1B pan and a 4AB2C1B pan on the shelf, you can pretty much cover any reverb repair.

[ReginaldBisquet]

I can always count on Enzo for help!

I found the old OC reverb tank... it's got rust and dust all over. I checked the DC resistance of the input and output using the two holes next to the RCA input jacks and both measured zero. Good, right?

Went back and measured a few of the resistors on the board and noticed that R37 (within the reverb circuit) was more than half below it's stated value. Bad, right?

And speaking of reverb with the Delta Blues... man, that's one noisy circuit!

Use the rule of thumb in my earlier post.

Yes, PV used some OC pans in earlier amps, after all, they were "manufactured by beautiful girls in Milton, WI under controlled atmosphere conditions."

If you stock just two kinds of reverb pans, you can cover almost every need.

First, a little research into the PV line will reveal that the older amps with the OC pan had a certain drive circuit, and later amps using the Accutronics pan used the exact same circuit. ANd those expected the 4EB2C1B. Or the 8EB... or 9EB... Peavey has pretty much stuck with the drive circuit for years - the output of a 4558 IC directly drive the pan input. This mean the pan must have a high Z input, the IC would not be happy trying to drive 8 ohms. The highest ACCutronics input impedance if the F, with the E right behind it. PV uses the EB pans.

At the other end of the spectrum are the pans driven by a transformer. Fenders for all time as an example. The little drive circuit is really nothing more than a low power power amp, and the low impedance output drives a low impedance pan. That would be something like the 4AB2C1B or 8AB... or 9AB...

The 4 and 9 pans are the full length pans, the 4 having two long springs and the 9 having 3. The 8 pan is the shorter one used in smaller amps lacking room for the larger one.

SO if you keep a 4EB2C1B pan and a 4AB2C1B pan on the shelf, you can pretty much cover any reverb repair.

[Enzo]

Resistors very rarely drop in value. FAR more likely you measured that resistor in circuit, and paralel circuit paqths made it appear lower in resistance. Lift one end and measure the part alone if you suspect it.

Unplug the reverb pan. Turn on the amp. Turn up the reverb some. Now touch the tips of the reverb cable plugs witha finger. One should hum. That is the return cable, and plugs into the OUTPUT jack of the pan. No hum, bad return, and most likey the 4558 is bad. If you get hum, then it is probably working fine.

And the remaining plpug is the drive output. Got signal there?

[ReginaldBisquet]

When I turned up the reverb, the amp gave out a white noise type sound. While the stock op-amp is a 4558, the only spare op-amps I have are the LM4562NAs and a few AD823s. I used the LM4562NA in the reverb circuit {U3} and tried out your advice.

It works! Beautiful clear reverb is back in my Backstage Plus... but only to position 3 on the dial. After that, the amp emits a static noise.

?

Resistors very rarely drop in value. FAR more likely you measured that resistor in circuit, and paralel circuit paths made it appear lower in resistance. Lift one end and measure the part alone if you suspect it.

Unplug the reverb pan. Turn on the amp. Turn up the reverb some. Now touch the tips of the reverb cable plugs with a finger. One should hum. That is the return cable, and plugs into the OUTPUT jack of the pan. No hum, bad return, and most likely the 4558 is bad. If you get hum, then it is probably working fine.

And the remaining plug is the drive output. Got signal there?

[Enzo]

The ICs in sockets in that amp? You got other 4558s in there somewhere? Swap a different 4558 from some other socket into the reverb position. Try your fancy op amp in some other spot, like maybe the input stage.

[ReginaldBisquet]

Went to the local uber-geek electronics store and picked up three 4558 op-amps. They also had the 4558DD chips... but I didn't know anything about the DD series... so I opted for the standard. Placed the chip in the reverb circuit... and it freaking works! I also oiled the pots with contact cleaner just in case.

The amp works just like it should and the reverb works on all values too! Man, I can't thank you enough Enzo.

The ICs in sockets in that amp? You got other 4558s in there somewhere? Swap a different 4558 from some other socket into the reverb position. Try your fancy op amp in some other spot, like maybe the input stage.

[ReginaldBisquet]

New question in regards to the amp of this topic. I may have posted this already elsewhere... but I have converted my Delta Blues 115 combo into a stand alone head:



I did this for several reasons, mainly because I got tired of the incessant tube rattle (Tom's Tube Tamer didn't quite work for me) and I was curious about a 2x10 set up with this amp. And... quite honestly... because I thought it would be cool to convert it.

To complete the conversion, I would like to make the external speaker jack 16 ohms instead of 8 ohms. I am thinking that I will need to switch the wires from pin 1 to 2 on J13 to dedicate the 16 ohm winding to the external jack.

TRUE?

If so, is pin 1 located at the top of the row of pins or the bottom? I also unplugged the Molex connector at J12/J14 and have it tucked in my drawer of parts - just to keep it out of the way.

Please let me know if I'm about to blow up my amp... and home.