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jjang1993

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The hum may be the caps getting old but its more likely the bias is off.

 

If that amp has old made in USA tubes they may not be a matched pair and whoever installed them was unable to evenly bias them.

I have to take a look at the schematics on that exact model/year to be sure, but fender used a bias balance pot on many of their amps instead of a bias pot for each tube. The single balance pot "Assumes" the bias is ideal on the first tube but in reality it may be well under or over its rating because the circuit is set to a fixed value. The second tube is adjusted to match the first so it too can wind up being under or over its own ideal current levels.

 

The balancing works out much better when you have a matched pair but the brans of tubes has to be right for them to match the fixed amp circuitry. If you buy tubes too hot or cool you cant tweak the bias of the first tube to make them both work unless you have a tech who can use a bias tester and find the ideal resistors for that particular brand of tubes.

 

I had this issue replacing tubes on my fender last time. I had to try several different sets of tubes till I could find a set that had a power range that matched the vintage Fenders. Some sets like Ruby and JJ were biased far too high for that amps design which was originally set up back in the 60's for most USA tubes. I could do a popular modification and convert the amp to a dual bias setup but so long as I can find tubes that balance its not major priority for me.

 

Personally I'd re-tube the whole amp. You seem to like bright tones. The EH tubes will definitely give you that.

Heres the chart for the EH

 

[img2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/s3.amazonaws.com\/tubedepot-com-static\/tubegrading_eh-12ax7.png"}[/img2]

 

 

 

The Tung Sol are nearly identical except they have lower gain, lower sensitivity, have a little more headroom because the gain is lower, and become microphonic more quickly.

 

 

 

 

 

[img2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/s3.amazonaws.com\/tubedepot-com-static\/tubegrading_ts-12ax7.png"}[/img2]

 

Sino tubes are the brightest but look at the noise levels.

 

 

[img2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/s3.amazonaws.com\/tubedepot-com-static\/tubegrading_si-12ax7a.png"}[/img2]

 

Genelex are the second brightest and have better noise levels. Sensitivity is low and gain is high so these would be better in a high gain amp that needs brightening.

 

[img2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/s3.amazonaws.com\/tubedepot-com-static\/tubegrading_gl-ecc83.png"}[/img2]

 

 

 

A really bad choice for your needs would be these JJ's which are very dark sounding, breakup more easily, and have twice the noise and microphonics.

 

 

 

 

[img2=JSON]{"data-align":"none","data-size":"full","height":"645","width":"1157","src":"https:\/\/s3.amazonaws.com\/tubedepot-com-static\/tubegrading_jj-ecc803g.png"}[/img2]

 

If you're looking for power tubes I have a bran new matched set of JJ's I bought from Tube Depot about 9 months ago I can let you have at half cost. They have no more then 10 minutes use on them. I tried them out and they weren't what I was looking for and bought a matching set of EH instead. The JJ power tubes are much better then their preamp tubes and are preferred by many in Fender amps.

 

 

 

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I don't think you know what a Champ 12 is. The amplifier has a single ended cathode biased 6L6 for an output stage.

 

It also has a very strange Reverb circuit where the springs are driven by the secondary of the output transformer. There is a Reverb Trim control (possibly part of a circuit designed to avoid feedback) and a Tape Input which I wonder about.

 

Interesting amplifier.

 

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The 7581/A tube is an industrial strength version of the 6L6GC which is also said to be the American equivalent to the British KT-66. The ratings 7581 are roughly the same as the 6L6GC while the 7581A has higher ratings. The 7581 has a plate voltage rating of 500 V--35 watts (5 watts more than the 6L6GC) and a screen rating of 450V--5 watts.

 

The 7851 and the KT66 may run in your amp but you wont be getting any more wattage out of them unless you had a higher voltage transformer with a higher plate voltage. Unless you put more juice into a tube you don't get more out. I doubt either will be more suitable for a single ended amp

[h=1]5881/6L6WGB[/h] [h=3]This tube was first developed by TungSol for the US Military. They are in short stubby glass envelopes. There is no difference between the 6L6WGB or the 5881, they are the same tube. The 5881 has a maximum plate rating of 360 volts at 23 watts and a maximum screen rating of 270 volts at 3 watts. DO NOT USE IN AMPS CALLING FOR 6L6GC's. They will likely melt down and damage your amp.[/h]

 

There are 9 variations of the 6L6 tube total. The differences mainly involve the tube build allowing different plate voltages to be run and therefore getting different wattages out. You should ONLY use a tube that's designed for your transformers B+ voltage or run something that can handle a higher voltage. You don't increase the wattage buy doing this without a higher plate voltage, but tubes like the 6L6GC and some of the military versions are much more durable then some of the older 6L6 tubes and can hold up better in combos where vibrations rattle them to death.

 

You should read this so you know what you're dealing with and avoid making stupid mistakes buying the wrong class of tube. http://www.vacuumtubes.com/6l6.html Since this is a Fender and since Fender designed those circuits to run 6L6GC tubes, I suggest you stick with those.

 

 

The 6V6 tubes are not the same tubes as the 6L6. They run on different voltages and current and to make the switch you'd need to do extensive work. The 6V6 are designed for amps that produce lower wattages with each tube producing between 10~15W max. The 6L6GC produce up to 30W and the variances up to 35W when higher plate voltages are applied.

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