#1
hey guys, so ive had may blues jr for about 1.5 years now and havent replaced the tubes once yet. its starting to get obvious that they really need it. this is my first tube amp, so i dont know anything about what to buy for new tubes.
and replacement is pretty simple, right? just yank out the old ones and stick the new ones in? if theres more to it itd help if someone could fill me in there too. thanks guys!
#2
That's about it since the amp is cathode biased. I'd squirt the tube pins with some contact cleaner and run them in and out of their sockets several times, too. Dirty, corroded sockets can cause noise and, in bad cases, major problems.

As for what tubes, you'll need three 12AX7 or similar dual triodes for the preamp and two EL84s for the power amp. I always recommend people go to either Doug's Tubes or Eurotubes. Doug's very knowledgeable and carries a lot of different types and brands of tubes, both new production and NOS, and can get you whatever sound and breakup characteristic you want from the amp at a reasonable price. Bob at Eurotubes only sells JJ brand tubes, but he's a nice guy and will give you a good deal and is also quite helpful in selecting specific tube types and grades.
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#4
Quote by Mike-T93
That's about it since the amp is cathode biased. I'd squirt the tube pins with some contact cleaner and run them in and out of their sockets several times, too. Dirty, corroded sockets can cause noise and, in bad cases, major problems.

As for what tubes, you'll need three 12AX7 or similar dual triodes for the preamp and two EL84s for the power amp. I always recommend people go to either Doug's Tubes or Eurotubes. Doug's very knowledgeable and carries a lot of different types and brands of tubes, both new production and NOS, and can get you whatever sound and breakup characteristic you want from the amp at a reasonable price. Bob at Eurotubes only sells JJ brand tubes, but he's a nice guy and will give you a good deal and is also quite helpful in selecting specific tube types and grades.

pretty much, but only the preamp tubes in the blues jr is cathode biased. power tubes are grid biased...
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#5
Oh, thanks for catching that. I was under the impression they were all cathode biased, as most EL84 amps seem to be.
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#6
yeah, but i think a grid biased power section would be better when switching tubes than cathode biased. if the output section was grid biased, it's still fed with the same grid voltage. if the section was cathode biased, the bias will be different when switching between EL84s with different current pulls.


i think blues jr. uses grid bias because it's more or less fail-safe when switching powertubes. i dont know if this way is cost efficient...

grid bias: needs bias winding on the transformer, diodes, resistors, capacitors... or extra resistors and capacitors for filtering from the main power supply... this would shift as you play i think.

cathode bias: needs a 5w or so resistor, and a nice capacitor.


not sure how much a 5w resistor and a good electro cap would cost them... but grid biasing is pretty fail safe i think.
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#7
Ah that's cool. Does that mean different breakup ratings on power tubes have less effect, though?
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#8
well, tubes don't really have "breakup" ratings. it's just how much current it pulls at a given plate/bias voltage. hotter tubes pull more current, colder tubes pull less. at least for most tube suppliers.

this would affect cathode biased power tubes more than grid biased ones i think.
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#9
thanks guys! especially EC for all the information.
so im looking at the eurotubes page and thinking about just oing with one of the kits for the blues jr there. he asks you to specify whether your looking for max headroom, moderate or early breakup. unfortunately, i dont even know what that means... can someone give me a simple explaination to give me an idea?
thanks a lot
#10
also: i just talked to someone at one of the lcoal guitar shops who said that the 12AX7s really dont need to be replaced normally, unless there was some kind of major problem. should i just do the EL84s or get both kinds?
#11
Two things I want to clear up:

1. Grid biased amps are the ones that have to be rebiased. Cathode bias is usually referred to as "auto biasing." The problem is that your amp is not adjustable on the bias supply (according the schematic I'm looking at). You should measure the current through your tubes and your voltage at the plates. If may turn out to not need a rebias, but I would check everything before I would feel comfortable.

2. The guy at the local guitar shop was right. Preamp tubes last much, much longer than output tubes. However, the third 12ax7 (which is the phase inverter) works almost as hard as the output tubes. I would change this tube while you're at it.


As for brands, I recommend JJ tubes in the power section, and either an Electro Harmonix or JJ 12ax7 for the phase inverter.
#12
both biasing method for power tubes are around the same i think. grid bias amps will need rebias, cathode bias amps will need rebias too, eventually. both to the same extent. and to the same extent, they don't need to be rebiased either. unless an amp is running super hot and if you stick in a little hotter tube it'll explode, it wouldn't need much rebiasing. when you need to rebias, both biasing methods need to be rebiased.

its just that grid bias method is easier to have an adjustable bias. cathode bias has to switch the cathode resistors to bias.

blues jrs are biased fairly hot. it idles about 11w a tube i think, which is nearing class A. some people say it's better to lower the bias voltage to about -13~-14v. some people say that would kill blues jr's character. but running 11w on idle would probably kill your tubes a bit faster.
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#13
Quote by ECistheBest
yeah, but i think a grid biased power section would be better when switching tubes than cathode biased. if the output section was grid biased, it's still fed with the same grid voltage. if the section was cathode biased, the bias will be different when switching between EL84s with different current pulls.

Other way around.

Cathode bias is easier for swapping out new tubes compared to fixed bias (never heard the term grid bias).


Quote by ECistheBest
i think blues jr. uses grid bias because it's more or less fail-safe when switching powertubes. i dont know if this way is cost efficient....

Fender set up their bias system in that amp the cheapest way they could. While making sure the amp cooked tube so then you had to buy more "Authorized Fender Tubes".

Quote by ECistheBest
grid bias: needs bias winding on the transformer, diodes, resistors, capacitors... or extra resistors and capacitors for filtering from the main power supply... this would shift as you play i think.

You don't need a separate tap actually. You can take your voltage right off the HT winding.

Why would "this" shift as you play???

Quote by ECistheBest
cathode bias: needs a 5w or so resistor, and a nice capacitor.

Most cathode biased amps are in the lower power level. For a BJ (15W or so) you can use a 2W-3W resistor.

No signal goes through that bypass cap so it doesn’t need to be good quality cap. Low ESR is nice but not a deal breaker. I would be more worried about the value of it tbh. A 20V cap is fine there.


Quote by ECistheBest
not sure how much a 5w resistor and a good electro cap would cost them... but grid biasing is pretty fail safe i think.

Nah, other way around.


EDIT: The actually grid voltage is a meaningless measurement. That voltage will mean totally different things depending on what tubes you have in. Tubes are +/- 20% electronic devices (worse now then back in the day). So what I am getting at is -13V-14V might bias one set of EL84s just fine while it might bias another set of EL84s a bit on the cold or hot side.
Last edited by kurtlives91 at Jul 22, 2010,
#14
Another clarification:

The reason cathode biasing is more "fail safe" than fixed biasing is that with cathode biasing the actual voltage the tube is seeing is the difference between the cathode and and anode. So if you are running 329v (the voltage fender says the blues jr. runs at) and you are biasing the to -11v, you are actually running the tube at 318v.

The max plate voltage of an EL84 is 300v, so fixed biasing at 329v is already pushing it. Since you are actually running the tube at 318v when cathode biasing, the tube is a little closer to operating at what it was designed to.
Last edited by end_citizen at Jul 22, 2010,
#15
Why would you subtract the grid voltage?

To find the plate voltage you measure from cathode to plate. Or plate to ground then subtract the cathode voltage. The cathode voltage shouldn't exceed around 10V at idle.