Fender "Twin Reverb Amp" has no output. - diyAudio
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Old 26th December 2007, 04:49 PM   #1
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Default Fender "Twin Reverb Amp" has no output.

Hello everyone! First I would like to say I know nothing about tube amplifiers & it is all very new to me, but I am familiar with electronics and I enjoy repairing car audio amplifiers, but again, tubes are very new to me. I am interested in learning about tubes and how they operate, as I have been curious about that for a while.

Anyway, on to the amp in question: My dad has played guitar for many, many years & has used his Fender "Twin Reverb Amp" since before I was born! Well it has worked all this time perfectly other than of course he changes tubes when needed & maybe cleaning the potentiometers. This amp has been all over the place when he used it a lot, so it has seen a lot of traveling. The amp was made I think in 1971. The amp uses (4) 6L6 output tubes.

Well, the last time he used it a couple months ago it worked fine, he was using external speakers from another amp with this amp at the time & again, worked with no problems, he then hooked the internal speakers back up before he was done and it worked fine still, so when he was finally finished using it, he said it worked & showed no signs of problems other than some scratchy knobs. The knobs have now been cleaned. Now when the amp is powered up everything seems fine other than absolutely no sound output, nothing at all, not even a hiss or an air type sound, dead quite! He looked at it a little bit, basically just cleaned it out to see if that would help & it did nothing, since I enjoy repairing electronics he asked me to take a look at it, so here I am now.




Here is what I have done so far:

I tested the speakers and they work fine.

I tested the output wires from the amp with my multi-meter set to ohms and it showed a direct short, from what I have read that is common in these types of tube amps so there is always a load on the Output Transformer when the speakers are unhooked. I never heard of shorting the outputs on an amp before, but again, tubes are new to me.

I unhooked the Output transformer to test its secondary Out Of Circuit & it still shows a direct short, or about 0.3 Ohms, which I would still consider a short as my meter reads close to that sometimes when I touch the leads together.

So my question at this point is: Is it normal for the Output Transformer to show a short or 0 Ohms on its secondary where the speaker connects?

And if this were not normal, why would the Output Transformer be faulty after all this time of working fine?

And, can I temporarily somehow bypass the Output Transformer to see if that is the problem?

Ok, thank you to all who reads this, & I hope to maybe get some help so my dad can use this again! Take Care & Happy Holidays. Kyle
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Old 26th December 2007, 10:32 PM   #2
Brion55 is offline Brion55  United States
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Yes the output transformer will read very low on an Ohm meter. Remember the output is only 4 Ohms.

The 70's Fender amps are usually pretty easy to troubleshoot. A Fender with no sound at all would have me checking power supply voltages first. Please remember that the Twin PS supplies over 400 volts to the output tubes and this can be lethal. The man I first learned electronics from (back in the tube days) told me to always keep one hand in my pocket when working on a "hot" tube circuit!

Here is a schematic for a Fender Twin of the era you describe.
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File Type: gif twin_reverb_sm.gif (33.1 KB, 274 views)
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Old 26th December 2007, 10:38 PM   #3
Brion55 is offline Brion55  United States
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Just thought of something else...

Fender amps have 2 speaker outputs. One has a "shorting" jack so that there is a load on the amp if it is turned on without the speakers connected. If the speakers are plugged into the wrong jack, there will be very little or no sound because of the shorting switch on the main speaker jack.
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Old 27th December 2007, 11:46 PM   #4
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Brion55,
Thank you for the reply! Yeah, I was not sure if those type of transformers were suppose to read 0 to 0.3 ohm on the secondary. I will check the voltages in the Power Supply and compare them to the schematic.

I read about the "shorting" jack that shorts the output transformer when no speakers are connected. We are using the normal SPEAKER jack output for the internal speakers; which bypasses the shorting tab, I did check it & the shorting tab is disconnecting normal when the speakers are plugged in. (this is the jack that the internal speaker are always plugged into when in use).

Since the shorting jack shorts the output of the Output transformer I tested the transformer out of circuit (I unsoldered it from the output connectors) and that’s what it showed on my ohmmeters, 0.3 ohms. So I was not sure if that was to low & maybe the transformers windings were internally shorted, but if you say they read low on an ohmmeter, that must be normal, so I will check voltages.

I will be careful while working inside the amp as I do realize it has dangerous power levels inside. You say keep one hand in your pocket while working on hot tubes? I do appreciate you informing me on the danger of working on this, since I am not real familiar with tubes, any knowledge on safety you can share with me would be great. But again, I am very careful in this kind of equipment, as I know it can be dangerous (especially for a beginner).

Thanks again for the info & I will post back with what I find out, though I am working on other stuff as well, so this may be a slow project as I get the extra time. Take Care. Kyle
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Old 28th December 2007, 10:02 PM   #5
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Pictures can be helpful. Its always easier for someone ELSE to spot someones stilly wiring mistake I know from experience

Your primarys might read 150-350 ohms plate to center tap, and yes its normal for your 8 ohm tap to read .3 ohms. The taps on the transformer dont read "impedance" with your dmm, rather DC resistance. Nominal plate load will be about 4k ( cant remember exactly for a twin reverb.) However if you measure them you get a very low DCR, causing you to assume you have a short.

I guess one way you COULD tell if your opt is fried is if you get a SMALL 6.3v transformer and wire it to the 8 ohm tap and see if you get a couple hundred volts out the primary. Oh yeah, make sure its unsoldered from the amp first
Keeping one hand in your pocket has noting to do with the fact that the tubes are hot- it has to do with the fact that there is over 400v sneaking around in there-even when its off since capacitors store their charge for a while. If you accidentally touch something while its on and your other hand is touching the chassis youll get a nasty shock at least, if not death at the worst.

It sounds like its probably something simple, but since you have no experience with tubes I would walk into a good guitar shop and ask them about it. If they dont seem to know much about it DONT let them touch it. Take it to an old fart that knows something about tubes.

Other things to look for are kind of obvious, such as burnt resistors and parts, or bad solder joints. My 5 watt that I built quit after two years due to a bad solder joint on the CT of the power transformer. I went through that entire thing and found out it was that simple. Go figure.
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Old 11th January 2008, 12:25 AM   #6
Brion55 is offline Brion55  United States
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Yep, us "old farts" are good for something!
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Old 3rd January 2009, 08:22 PM   #7
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Brion55 & ThSpeakerDude88,

I have not worked on this amplifier since my last post, has it been a year already? Wow! Where does the time go when you get older, when I was little it seemed as though a year took forever to come around.

I have been busy with other things & my workbench has been a mess, seems like there is always something on it being worked on for some reason or another. The Fender amp has been sitting in pretty much the same spot I put it when I set it to the side.

I am hoping I will get a chance to look at it again soon, & I will post back what I find out, & will try to post some pictures of it for any other help I may need. Again, I will be very careful around the High Voltage when I am working on it too.

I want to thank you both again for replying to my post & for the help, I greatly appreciate it! Happy Holidays to you both & to everyone! Thank you again. Take care.
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Old 20th January 2009, 02:46 AM   #8
rkevans is offline rkevans  United States
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With those symptoms -- Replace the fuses.

Really.

They'll probably look fine, but I'm guessing one of them is bad. That was the solution for both of the guitar amps I've 'repaired' recently.

Rick
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Old 21st January 2009, 05:20 PM   #9
jjman is offline jjman  United States
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If the jewel and/or any of the tubes light up the fuse is good. There should only be one fuse on that model and it's on the primary of the power transformer. Mounted on the back of the chassis. There should be no internal fuses and no “power supply” fuses in that model. Of course this is not the case on many other brands and more modern amps.

It's a very desirable amp for those who need loud and clean volume. If it didn't weight so much it would be even more valuable than it already is.
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