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Old 30th December 2010, 06:21 PM   #11
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Jim -- Set your meter for the lowest ohms scale. The ohms symbol look similar to a "n", being the Greek symbol for omega. Unplug the reverb pan cables from the amp and and place the probes across the pins of each plug -- one to the center pin, and one to the outside connector. It makes no difference which lead goes to which plug connector. One of these cables should read less than 20 ohms, while the other could read 200 ohms or more. This may require going up to the next higher scale to cause a reading to be indicated. As long as you are getting a reading anywhere around these numbers on the respective cable, (other than a completely shorted reading), the reverb pan and cables are good.

If your readings are good here, the next test is to connect a speaker to the reverb output jack on the amp, and see if a signal exists there.

Dave
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Old 30th December 2010, 10:24 PM   #12
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Here's what I got. RCA Cables plugged into the reverb spring unit. Ohms has only one setting on Micronta 22-193 multi meter. one side was .20 the other side was 2.4. Then I plugged a powered monitor speaker (has RCA connectors and low impedance) into the RCA cables coming out of the amp. Reverb turned all the way up, wife listening on the speaker, nothing.

So, would that lead you to think the transformer? Or something else.
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Old 30th December 2010, 10:31 PM   #13
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If transformer. I found one from an eBay retailer Triode Electronics pn# 40-10834
$16. From what I'm understanding here. (like following the flow of water I guess) the cables and tank are ok. The tubes are functioning, so unless it's something in the switching blocking the output, it would then me a power problem i.e. transformer? Hey I'm just guessing here, but looking at the condition of that transformer is what pointed me to that. Ok well what are your thoughts? Oh and if I haven't said it before, thank you so much for your guidance and expertise.
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Old 31st December 2010, 02:17 AM   #14
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Jim -- here's a couple of quick easy things that can help clue us in on the reverb driver transformer's condition:

1. Use your ohm meter to see if you get any continuity at the jack on the amplifier that provides the drive signal to the reverb pan. Connect one probe to the chassis, and one to the inner portion of the jack. If your probes can't make a good connection to the center connection, just plug in a known good RCA cable to the jack and then make your measurement at the other end of the cable. You should get a reading that is virtually a dead short, because it is connected to the output winding of the driver transformer. If you do, that is good. It doesn't indicate so much that the winding is shorted, as it does continuity of the winding. I'm guessing this exercise will test out ok.

2. To test the primary of the transformer, take the shield off the the 12AT7 reverb driver tube. Now turn the amp on and let it run a while -- long enough that the tubes get good and hot. 15 minutes should be more than enough time. Now grasp the 12AT7 driver tube to determine it's temperature. It should be noticeably hot (can't keep fingers on it). If it is only warm (can hold onto with fingers), it likely indicates that the primary winding of the driver transformer has opened, and needs to be replaced. There is a resistor that could open inside the amplifier that is associated with this tube that would cause the same symptom -- but with the obviously poor storage environment this amplifier has endured at some point in it's life, my money is on the transformer for sure.

If the tube is getting hot, and you get a good reading of continuity at the drive output jack, then obviously something else has failed inside the amplifier, that will take a schematic and signal tracing to determine where the fault has occurred.

Dave
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Old 31st December 2010, 02:19 AM   #15
Enzo is offline Enzo  United States
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I don't want to lead you in over your head. At this point, you could have a bad transformer, but they are still down my list. I'd be inside looking to see if plate voltage was present on the reverb tube, and something other than zero on the cathodes.

Power off, plug an RCA cord into the reverb jack on the amp and measure resistance between tip and shield. One jack should read very low resistance, the other jack should read 220k ohms or so.

Borrow a set of patch cords from your stereo, and connect the reverb pan to the amp with them. ANy change?
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Old 31st December 2010, 03:24 AM   #16
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http://www.ampwares.com/schematics/the_twin.pdf

Don't you just love technology. Simple reverb circuit and they have to make it complicated. Remind me not to put in channel witching in my amp.

From footswitch, through some IC's that light up some leds to change the resistance value of photoresistors, then there is the reverb switch and the channel switch on the amp to contend with. Oh what a wicked web we weave.
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Old 31st December 2010, 06:40 PM   #17
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Tested the RCA's coming out of the head amp off. They tested basically the same numbers coming out of the spring unit 2.4 and .200. Held the reverb tube (not the phase inverter) and though it was lit and bright it wasn't very hot. I could have held onto it for several minutes. This was after about 15 minutes with the amp on, playing the guitar and power setting to high. I think I'll go ahead and order the transformer.
Another question. Is there any good product for cleaning up that green stuff and keeping this lubed and protected in the future? I'm comfortable with marking and removing the tubes, removing the head from the chassis and spraying something if its available. I have no problem removing and re-soldering the transformer, or a resistor (if I knew which one) But, big But... I don't want to mess with anything that could shock me. I know where the capacitor's are and won't touch them. I will also unplug the amp for a least a day before doing any service. Any other thoughts or "tricks of the trade" (I'm an offset printer by trade and part time musician) before I start?
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Old 1st January 2011, 04:58 PM   #18
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I ordered the transformer last night. I'll let you all know how it turns out. Thanks again for your guidance and patience. Happy New Year.
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Old 2nd January 2011, 08:40 PM   #19
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That tube should have been quite hot after 15 minutes -- much too hot to hold onto, so it is a very good bet that the primary of the driver transformer has opened. Since the amplifier otherwise plays normally, it's also a safe assumption that the B+ for the reverb driver tube is available, and the tube itself has been eliminated, so a bad transformer is a likely suspect.

Dave
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Old 3rd January 2011, 02:47 AM   #20
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I've been lurking through some threads here watching amplifier talk I see the term B+ used a lot. What does it mean?
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