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Old 12th March 2012, 07:33 PM   #1
cbdb is offline cbdb  Canada
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From Replace your Amp tubes with - THD Yellow Jackets

How much of this is BS? I know enough about tubes to know swapping them will not change an amp from class A/B to class A, but dont different tubes need different bias?
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Old 14th March 2012, 02:25 PM   #2
Simon B is offline Simon B  United Kingdom
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Hi, I've no personal experience of using these things, and you don't state what amp you're thinking of using them with! Hopefully you've found and read this, by the manufacturers: http://www.thdelectronics.com/pdf%20stuff/yj.pdf Apparently, these things contain a cathode bias resistor, so using them will give you fixed cathode bias, ignoring your amps bias supply. I'm a bit concerned about how much power is being dissipated in that yellow plastic cylinder, there's about 100 v being dropped in the anode (plate) circuit, and some in the cathode bias resistor too - whatever anode current the cathode bias sets on the EL84s, multiply that by 100 and you're roughly at the dissipation in each yellow jacket. Have a look at this page: THd Yellow Jackets schematic - Hoffman Ampifiers Guitar Tube amp forum Look at "Yellow Jacket 100.Jpg" and also "EL34 to EL84s.pdf" for someone else's way of doing this. Amplifier operation class is determined by bias, and an AB amp only goes into AB at higher power levels - at low levels it's usually working in pure class A - it really isn't such a rigid dividing line. It really depends on what amp you're putting this into - hope this helps a bit....
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Old 14th March 2012, 05:49 PM   #3
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Default THD YellowJackets

I had a pair in my Ceriatone Dumble clone. They work!!! I can crank the amp up to "10" at home and get power tube compression. By comparison, with 6L6s in place, I can't turn it up above "2" without having pictures fall off the walls. It gives the amp a cathode-biased, EL84, Vox-y power amp tone to the familiar-sounding Dumble-type preamp.

It appears that THD has discontinued them, but you can probably find them on eBay. Alternatively, I think I may still have a pair.

Another choice (and what I currently have in my amp) is the long-discontinued Groove Tubes Substi-Tube 928PR, which sounds a bit better than the YellowJackets but runs the EL84s at a higher plate voltage (which will, doubtless, shorten their lives). Still, EL84s are cheap.

What I would really like is a fixed-bias/cathode bias switch for the 6L6s so I wouldn't have to swap out tubes every time I go from 6L6s to EL84s and back. Does anybody know how to do that? My tube theory isn't as good as it ought to be.
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