Help: Diagnosing Non-Functional Reverb

[Mango Moon]

Hello All,

I'm no electrical engineer but I've got a functional multimeter here and I'd sure appreciate some diagnostic guidance from the experts on this one:

I've got a vintage tube amp with reverb (2-spring Gibbs/Hammond unit) but the sound of the reverb itself isn't real prominent (i.e., it just isn't there), and I'm told by others who own the same model that the reverb effect should be quite obvious (i.e., deep and lush). So, the question becomes whether the problem lies in the pan itself, the driver (a lone 6DR7 tube) or something else unknown.

The reverb pan itself appears to check out just fine with DC resistance at the two input/output terminals reading roughly 170-175 ohms each, but I have no available means to test the 6DR7 driver tube. I would think (??) I should be able to confirm its output by testing the line-in current (in milliamps) at the male RCA connector (i.e., the input to the reverb pan), right? I've researched the topic as best I can and its supposed to yield a reading of just 2mA, but this is where it gets dicey for me . . . .

My multimeter is a Commercial Electric HDM4100 and I'll admit that I'm not completely confident that I've got the probes connected to the proper choice of plug-ins, nor can I find a suitable operator's manual on-line. However, with the two respective input/output RCA connectors disconnected to/from the reverb pan and the amplifier turned 'on', when I ground the black lead to any transformer stud, and I connect the red lead to the male (input) RCA connector leading to the reverb pan, I get a reading of "zero". In other words, it would appear there's no current.

Have I done something wrong here or is my thinking screwed-up? What's the next step?

[jrfrond]

I've seen problems with 6DR7 reverb drivers several times in some Magnatone amp models.

Use you ohmmeter to check right at the pan inputs and outputs, disconnected from the reverb send and return cables. See if you have continuity. If you do, I'd replace the driver tube.

[Mango Moon]

Thanks John!

Actually, I spoke with Accutronics Reverb about this in some detail and they indicated that the readings of 170-ohms and 175-ohms at the reverb pan's input/output terminals (with RCA to/from cables disconnected) were 'spot-on' for the specific model of Gibbs reverb pan in question. Thus, I have concluded that the pan itself is probably good, and of course, 6DR7 tubes are relatively cheap, so I've got one of those coming also.

But the question remains, what if the 6DR7 tube doesn't solve it? Isn't there a practical way (using a multimeter) to determine whether there is sufficient current (2mA) arriving through the male RCA connector (from the driver) to the 'input' side of the pan? If so, how do I do it? I guess what I'm looking for is guidance on how to use the multimeter for that purpose. Although two milliamps (2mA) sounds like a pretty small value, my multimeter appears to be capable of measuring that low, but I'm not comfortable on how and where to connect the two multimeter leads? Don't want to burn anything up!

[Steve Conner]

You need to set your multimeter to AC milliamps, since the current is AC. Even then, you might not get an accurate reading, since most multimeters are calibrated to read 60Hz AC from a power line, not amplified twangs from a guitar.

You could try hooking a small loudspeaker or headphones to the reverb drive jack. That will help you to judge the quality and quantity of drive signal. However this might not work with a high impedance tank: I've only tried it once with the 8 ohm kind, and it went uncomfortably loud in headphones.

[jrfrond]

I like down-and-dirty testing in general. If you have any doubts about the pan, you can drive it from a guitar amp speaker output, and send it back to a second input on the amp.

BTW- what amp DO you have?

[Twist]

Is this a transformer driven reverb tray?

[Mango Moon]

The amp in question is a twin '62 Magnatone Venus 480 Custom and everything else on the amp seems to work perfectly fine, although the vibrato is also rather punky (i.e., not as prominent as the vibrato on either of my other two Magnatones (280 & 260), neither of which (incidentally) have reverb.

As for 'transformers', its got 'em, but as I said earlier, I'm not an electrical engineer or technician, so I'm afraid I can't tell you whether its "reverb" is "transformer driven" or not.

Thanks for the continued feedback guys.

[jrfrond]

I KNEW this was a Magnatone you were talking about! For everyone's reference, here's the schematic:

Magnatone 480 Schematic

This pan is driven directly from the driver tube with a coupling cap. The reverb is recovered by a single-transistor stage where it is also mixed with the original signal and then sent to the input of the phase inverter on one of the power amps, so reverb emanates only from one speaker. These were budget amps, and the absence of iron to drive the pan was probably a cost-cutting measure.

The last one of these amps I rebuilt needed desperately to be recapped, and that is ALWAYS an issue in older amps. However, the only tube I DID NOT replace was the 6DR7 and guess what? The reverb didn't work!

If I were you, I'd sit tight until that 6DR7 comes in. Once you've eliminated the most likely suspect, you can then proceed to troubleshoot other areas.

[Mango Moon]

John,

Wow! You da man! I had no idea. I've had this same 480 schematic all along but frankly, most of it's gibberish to me. I'm slowly gettin' there though. I think the one thing you've pointed out here that I wasn't aware of, is the fact that the reverb reports to just one speaker?!#&? I did not know that. I knew that the reverb applied to just one channel (i.e., channel two), because channel one is dedicated to vibrato, but based on a block diagram that I recently acquired for it (see attached), I was of the impression that both channels reported to all four (two 12", two 5" speakers), depending solely on channel of input. Oh well, live to learn!

Anyway, I will indeed replace the 6DR7 tube when it arrives (should be here tomorrow), and I'll cross my fingers from there, but if its reporting to just one speaker (as you say), even when working properly, the reverb effect may not be as 'obvious' as I'd hoped. And if that's true, hey, that's what E-Bay is for, right? The fact is, in terms of vibrato and other features, the 480 simply doesn't compare to the 260 or 280. For example, it lacks reverb, but the vibrato on my 280 is so incredibly spacious and multi-dimensional, its nuts by comparison! And, in addition to its two internal 12" Oxfords, the 280 can (and does) simultaneously drive two more external 12" cabs (loaded w/EVM-12Ls in my case) per channel, which the 480 cannot (the 480 can only drive one external per channel). Thus, the headroom is much-much nicer with the 280.

[jrfrond]

The Magnatone 480 has an interesting reverb setup. The Reverb pot is a dual 5K, one side for the pan return and the other for dry signal tapped off before the drive tube, so that the wet/dry mix is turned up in tandem. It then feeds to 3.3K mix resistors before it goes to the transistor amp stage and lastly feeds pin 7 of the 12AU7 phase inverter. However, this is the ONLY place that signal enters the power amp. If you have the reverb turned all the way down, you have NO signal emanating from one speaker, and this is NORMAL.

It is also interesting to note, and I know this from experience working on these, that the two channels are voice differently, which makes for great clean tones. These babies are sleepers.