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Old 7th March 2011, 03:17 AM   #1
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Default Tube amp help!

I just bought my first tube amp, an early 70's Garnet Deputy 50w combo. It has a 1 Hi and Low input and a 2 Hi and Low input, plus one input that joins all four into one for Marshall distortion.

When I bought it, it was a little worse for wear. input 1 was a little quiet, and input 2 didn't work. I know nothing about tube amps, but I decided to open it, and I found a loose wire. I resoldered it, and input 2 worked. Then I realized that input 1 ran at about half the volume of input 2, and I have no idea why it would do that. Can anyone help me?

Also, tubes are still original, it that helps you out at all. (2 6L6, 2 12AX7, and 1 12AU7)
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Old 7th March 2011, 04:13 AM   #2
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Even though guitar amplifiers are designed for guitar pic-ups and such, inputs 1 & 2 are naturally going to be different levels. My Fender 5E3 has multiple inputs labeled 1, 2, 3, & 4. 1 & 2 are labled "low", and 3 & 4 are labled "high". Low inputs are for guitars and guitar pick-ups, and high inputs are for something like a microphones.
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Old 7th March 2011, 11:11 AM   #3
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People tend to focus on the tubes as any source of problems, partly because they come in sockets and are easily replaceable, and partly because fifties televisions burnt up so many. After forty years, it is much more likely that the electrolytic capacitors in the amp are dried up and blocking or shorting out the bass depending on configuration. These all need to be changed just because of the passage of time. They contain water, which leaks out through the rubber seal around the leads. Electrolytics can store voltage with the power off, so before touching any metal inside you need to read tube high voltage safety thread at the top of tube forum. Changing 2 electrolytic capacitors in my 1968 organ doubled the volume. 4 more doubled it again and made the attack feature work. 64 more made it sound like new with good treble and bass. A guitar amp will have fewer electrolytic capacitors. These look like aluminum cans with a plus on one lead or a minus near the other. They have a cardboard sleeve, or plastic, or are tall cans with circles moons and squares indicating different sections. They come in whole mf after 1960. mf stood for microfarad until about 1980, and has been changed to "uf" after 1980 by some illiterate academic who couldn't tell his mu's from his u's. mf offended the fans of the metric system that though it might stand for millifarad, which capacitors would be the size of D-cell flashlights or hand lanterns if they existed at all. Caps have dates like YYWW where YY is year. Any electrolytic caps after 95 might be totally okay, unless they were cheap 500 hour or less versions.
The one tube sure to go based on high hours, not years of life, is the rectifier tube. Some guitar amps "overdrive" some tubes for second harmonic distortion, and that tends to burn them up, cutting output current. This shortening of life is also based more on hours of use ( or abuse, as we organ people call it). You may find more experts on which tube in your circuit is the overdrive, on the musical instrument forum. The 6l6's have a life of maybe 3000 hours, and a pro might have worn them down to a lower power state. All the tubes you listed are amplifying tubes, none are rectifiers. Either you have another tube, or you have solid state rectifiers, which might last forever if they didn't do any of 6 cheesy workarounds that designers did in the late sixties to save money. One SS rectifier in my 1975 tractor battery charger shorted out last year due to a stupidly cheap design and burned out a $100 transformer. You measure worn out power tubes by checking the current in the cathode resistor idle, to make sure it is high enough. 100 ma is about enough for 6CA7, I think 6L6 is a more high power tube with more current, but I am no expert on it. Tube datasheets are at
However, after forty years I would concentrate on the capacitors before worrying much about tubes. Electrolytic capacitors near the fransformer can short out, burning the transformer out, or even boil and explode, drenching everything in conductive borax water and aluminum foil shreds.
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Last edited by indianajo; 7th March 2011 at 11:40 AM.
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Old 7th March 2011, 05:21 PM   #4
DF96 is offline DF96  England
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Millifarad would actually be mF, not mf, and they are quite common nowadays although usually marked in thousands of microfarads. Look in any solid-state amp.

Most academics know the difference between u and mu, but not all fonts have Greek so u is often used instead of mu. There is little scope for confusion, as u is not an SI unit multiple.

For some reason the US was slow to adopt the SI system, so what everyone else calls pF (picofarads) they sometimes called mmf (micro-microfarads).
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Old 7th March 2011, 09:22 PM   #5
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The point is, caps that could be measured in millifarads are huge enough they have a lot of room to print the 000's of microfarads. No confusion about the m. I don't know why they went to uf, but you can't buy mf caps online except at antique radio stores, or if you do you might get something 1000 times too big.
I view the metric system as a way for manufactures to make sure you buy every nut and bolt from the dealer. There are German metric bolts, different Itaiian metric bolts, different Japanese metric bolts. If you want to rethread a Mercedes oil pan stud, you have to drive to a different state or 200 km to the state capitol to get a die; nobody in 2 counties stocks German metric dies. Nobody in the USA has bolts to put a scoop on my Fiat tractor. (it said it was an Oliver, made in Chicago previously) When I was in FRGermany, I couln't buy anything like DOT 3 brake fluid, which is in every grocery store and mini-mart in the USA. There was Mercedes brake fluid, and Porche brake fluid, and Audi brake fluid where our army truck ran out of brakes, but no generic brake fluid. No wonder everybody in Europe rides the train or bus. Their cars are useless after 10 years, nobody can afford to fix them so they flog them off to Africa or Palestine, where I see them on BBC news.
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Last edited by indianajo; 7th March 2011 at 09:32 PM.
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Old 7th March 2011, 09:49 PM   #6
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Grab a copy of this:
This is the schematic for the 100 Watt version but all of the preamp section (page 1 of that document) and most of the power amp section are the same.
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Old 7th March 2011, 11:17 PM   #7
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I looked at the two can caps in it, and I didn't see any bubbling near the seals, but they might very well be dying. I also noticed, when I play a low note at relatively high volumes, I get a harsh loud BUZZ sound, that lasts only as long as the note I play. I told my father that I would like to replace some of the caps in it, and he told me that It's one of the dumbest things he has heard me say, and that there is no way in hell I could do it. The closest old tube repair guy to me lives over 400km away. I don't think I really have a choice.

Also, the amp is a Deputy G100B, the bass model I believe. It should still have a very similar schematic. I did my best following that schematic that I printed out, and from what I can tell, it's very close.
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Old 8th March 2011, 12:00 AM   #8
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And to clarify, to discharge caps, all I would have to do is hook one side of an alligator jumper style clamp to a grounded source, and the other end to the positive lead of the cap?
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Old 8th March 2011, 12:30 AM   #9
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It really depends how big the caps are as to how much trouble you need to take. Have a quick look here for some tips on general capacitor discharge techniques. Your Dad seems to be speaking out of his (auto-censor won't let me put that, so I'll say 'posterior' instead). If you can hold a soldering iron, know which way the capacitors go in and have some new ones of the right values ready it shouldn't take you long at all! If something doesn't work then you can be almost certain it's your soldering to blame as it's very unlikely one of the new capacitors will be faulty. Take some digital photographs if you're stripping a lot out at a time so you know where the new ones go if you forget...

And read that "Tube Learning for Newbies" as has already been mentioned. If you're used to poking around in low-voltage circuits then a valve one could hold some nasty surprises. It's not a good idea to poke things with your fingers to see if they're connected, for example!

It should be a project that is relatively quick 'n' easy with a massive difference at the end - even if the capacitors don't look awful they're pretty old.

Good luck!
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Last edited by Kaidanovsky; 8th March 2011 at 12:36 AM.
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Old 8th March 2011, 12:42 AM   #10
Structo is offline Structo  United States
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Always use a resistor to discharge caps.
The old screw driver trick is for amateurs that will damage components and themselves.

A 100K 2-3w resistor is a good value to use.
Too low of resistance and you will get a spark. Too high and it will take a long time to discharge.
Take a insulated test lead with alligator clips at each end and cut it in two.
Solder the resistor in the middle then put heat shrink tubing over the resistor and solder joints.

Clip one end to chassis ground (a bare metal spot on the chassis) and clip the other end to the + end of a cap.
If you clip it to the last filter cap in the supply it will drain all the caps.
Should take a couple minutes.

Always verify that the voltage is below 10v before touching anything inside.
Good idea to leave the discharge lead attached while working to prevent any charge from building back up.
Remember to remove it before powering up!

Remember that tube amps often use voltages upwards of 450v DC.
Plenty enough to kill you in a split second or certainly enough to throw you across the room and use words not suitable for mixed company.
Keep that smoke in the amp!
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