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Post by genmce on May 30, 2012 11:25:57 GMT -5
Hey all,
I just acquired a fender twin reverb silver face with pull master volume.
Output is low and it sounds farty.
Tubes all check ok.
Filter caps have been replaced at some point.
Output transformer appears ok - friend did a dc resistance check.
Advice on what to look for next?
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Post by reTrEaD on May 30, 2012 13:28:54 GMT -5
Advice on what to look for next? 1 - Test with a known speaker cab instead of the speakers in the combo. Unlikely these are a problem, but it's so damned easy to do that it's worth making this the first step. 2 - Get a schematic that's accurate for YOUR amplifier. "Silverface" encompasses several internal design changes. 3 - Measure DC voltages without a signal applied and compare to voltages noted on the schematic. Small discrepancies (10% or less) are very common and inconsequential. But larger discrepancies will lead you to the heart of any problem. 4 - Attach a resistive "dummy load" to the output jack and sine wave generator to the input. Adjust for about 1 watt output at the load. Use an oscilloscope to measure the signal at each step of the chain. Compare the measured gain of each stage with what should be expected given the tube characteristics and circuit configuration. |
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Post by sumgai on May 30, 2012 16:54:09 GMT -5
Farty? Well, that stinks! ;D I suppose you've already got a schematic, right.....  As usual, my first question is, how do you know the tubes are good? Did you test them in another known good working amp? At this point, stop. Put down your tools and meters and such. Open your eyes and start inspecting for bad solder joints, wires that are barely hanging on (one strand left connected), that kind of thing. More the 90% of the time, it's something you can see that'll be the cause of the problem. Such bad connections can go for years, and suddenly just give up the ghost, so to speak. After that, look to see that the B+ voltage is about right, and don't bother with the filter caps - if they're massivly bad, it'll be visually obvious. If they leak a little bit, there'll be a lotta hum, but the sound (output) won't be farty. Assuming those all pass muster, then start checking for a leaking/bad DC coupling capacitor between the stages. If this goes bad, the plate voltage (very high positive DC) can appear at the grid of a following stage, knocking the bias for a loop (it's supposed to be slightly negative). Have your friend do a voltage check on the grid of each tube section. (With one hand in his pocket - the power will be on, and dangerously high voltages will be present nearby!) If he encounters a positive voltage somewhere along the line, then the coupling cap that is supposed to block the DC from the previous stage has done left the building. HTH sumgai |
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Post by reTrEaD on May 31, 2012 5:58:18 GMT -5
Farty? Well, that stinks! ;D *slaps sumgai with a trout*  |
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Post by genmce on May 31, 2012 8:01:23 GMT -5
Advice on what to look for next? 1 - Test with a known speaker cab instead of the speakers in the combo. Unlikely these are a problem, but it's so damned easy to do that it's worth making this the first step. 2 - Get a schematic that's accurate for YOUR amplifier. "Silverface" encompasses several internal design changes. 3 - Measure DC voltages without a signal applied and compare to voltages noted on the schematic. Small discrepancies (10% or less) are very common and inconsequential. But larger discrepancies will lead you to the heart of any problem. 4 - Attach a resistive "dummy load" to the output jack and sine wave generator to the input. Adjust for about 1 watt output at the load. Use an oscilloscope to measure the signal at each step of the chain. Compare the measured gain of each stage with what should be expected given the tube characteristics and circuit configuration. Have schematic, have not tried the other cab yet... thought about it, forgot about it... Don't have a oscope, signal gen or dummy load. Might have to find someone with those tools. Friend with tube tester, I used to have one but sold it earlier this year, thinking I would not have tubes anymore... That friend does not have oscope, signal gen or dummy load either... |
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Post by sumgai on May 31, 2012 22:50:11 GMT -5
genny, In my oft-spotty career, I used a tube tester only up until the end of WWII. After that, I figured that I might as well practice what I preach - the only valid test is to substitute with a known good part. In the case at bar, I'm just reversing that.... take a known good part out of a perfectly good amp, and substitute with a suspected bad part. Same test, different direction, same results - you'll know if the tube is good or bad. (Of course, this presupposes that there's another amp handy for testing, but as luck would have it, that's a more common scenario than a player having one or more known good spare tubes stashed away nearby. *  ) Now, all of that said, there are times when/where an o'scope is necessary, but for now, not needed. Ditto for the dummy load, it isn't time yet for that either. In fact, I'll caution you only so far as to mention that you should keep a nose out for anything that starts to smell bad, as you test the voltages. Next to your eyes, your nose is a Gawd-send of a troubleshooting tool.  If you don't want to do this yourself, that's good... you're being smart, both for recognizing your limitations and for desiring to remain amongst the living. But if you do wish to make these tests, a simple DMM from any store you can find will do the job - even the 14.99 jobbies down at Checker Auto Parts, if that's all you've got in your town. Online, you can find 'em for less than a sawbuck.  (Although the shipping might sting....  ) Keep us up-to-date here, and don't be shy about asking for further help! ;D HTH sumgai * Of course, if you're the thinking kind, you'll realize that the testable amp also has the same kind of tubes, so why not just pluck one them out, and plug it into your bad amp? Indeed, why not? I only mention this because you can haul a pocketful of tubes over to your buddy's house, instead of making him drag his whole amp down to your joint.  |
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Post by genmce on Jul 10, 2012 7:18:38 GMT -5
Thanks for the advice.
He did not have another amp for me to test in.
So we could only use the tube tester.
I ended up taking it to another guy, who replaced a bunch of caps and found some bad wiring.
His "fix" was to remove the master volume - effectively setting the master to 10. The sound cleaned up and is much louder.
However, I can't get any other sound besides the excellent clean out of it now. I want to get a bit of overdrive but there is none unless I crank it so loud I can't hear anything...
I am at that cross over point.
Keep this big thing or sell it and buy some hybrid amp.
I am leaning toward selling...
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Post by newey on Jul 10, 2012 10:47:21 GMT -5
If he knew enough to know that this jury-rigging would solve the problem, then presumably he knew what the actual problem was/is, and what it would take to fix it right. I mention this because, if you're thinking of selling it, it may be worth a lot more to a buyer if everything works as it should, rather than as it is now, without the functioning master volume. For example, if the fix would cost "x" dollars, but the resultant price difference equals "x + $500", you would spend the money all day, every day. If it's "x + $30", save yourself the grief. A Twin is a wonderful thing if you need a Twin; if you're not using it for gigging and/or studio stuff, there are plenty of other options to get you that Fender clean plus a bit of crunchy goodness when needed. Amps that won't break your back or your bank . . . |
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Post by kurtseifert on Sept 7, 2013 13:41:12 GMT -5
I have a 70's silver face twin reverb. I was playing it last night and heard a pop and the power went out and wont switch back on. Is this a tube problem or something more serious?
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Post by sumgai on Sept 7, 2013 21:31:40 GMT -5
kurt, You blew the fuse. Oh, OK.... your amp blew the fuse, not you. Essentially, you can try one more time with a fresh fuse, I think that's a 2½ amp Slo-Blo, but check the markings on the back of the chassis to be sure - don't take my memory for it. If the fuse blows again, immediately or after a short bit of time, then you've got one or more real problems. Are you adept in the arcane arts of Tube Troubleshooting, or at least willing to dive in and look around first, before taking the amp to a qualified technician? And while I'm at it..... Hi, and  to the NutzHouse! sumgai |
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Post by kurtseifert on Sept 8, 2013 15:53:09 GMT -5
Lol.... Dar... That should have been the first thing i checked!!  Thanx!!! |
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Post by ux4484 on Sept 9, 2013 8:05:41 GMT -5
Whatever you do... DON'T let the smoke out!
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(Although the shipping might sting.... 
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Thanx!!!