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Bias always necessary?

PaulSG

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Dec 10, 2002
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I'm going to be replacing the stock tubes in my Blues Junior with JJ EL84's. Do I need to adjust the bias? I did a search for bias, but the threads I found confused me.
 

les strat

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Not in a Blues Junior, even though they are set a bit cold from the factory. A tech can fix this.
 

PaulSG

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Great, so it should just be a case of plug and play?

Any idea if the Epi Valve Junior is the same way?

Can someone explain what "single ended class A" tube amp really means in laymans terms?
 

sgtJoe

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Oct 25, 2003
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I think it just means one tube, not in the push-pull configuration the 6L6's and EL34's use. The Mesa Boogie site has a very long article on Class A and Class B, I downloaded it and printed it out, it's like a small book but very good reading.

My tech said EL84 amps are "fixed bias" meaning there is no adjustment needed when changing tubes, you can just swap them out although some may run hotter, temperature wise, than others. He also said you may notice a need to change them more often as EL84 amps run the tubes hotter.

I have a Blues Junior and my son has the Epi Valve Jr head, both great sounding amps.
 

audioman

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Nov 23, 2005
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Great, so it should just be a case of plug and play?

Any idea if the Epi Valve Junior is the same way?

Can someone explain what "single ended class A" tube amp really means in laymans terms?
Yes. You should be able to drop tubes in and be fine to play, same with the Epi Junior amp. These are cathode biased and should be good to go. FWIW, NOT all EL84 amps are plug/play, you need to know the circuit they are used in.
Also, Class A does not = plug and play. There are class A push/pull amps with fix bias and Class A/B Cathode biased amps. You need to know your circuit and specific amp.

As for Class A discussion. Randall Aiken has some EXCELLENT description of technical questions and general amp info on his site. Worth the 20 minute read.....some is technical, but there is a lot to learn.
Aiken Amps

Here is his 'general' description of Class A:

Q: I'm still confused about this "class A" stuff - how about a more general explanation?
A: The general idea is this: Picture a sine wave going positive above zero volts and then negative below zero volts. The way the tube is biased and the circuit is set up determines when the tube (or tubes) is on. It gets a bit shaky when you are dealing with push-pull, which uses tubes in pairs (or quartets, or more, always in multiples of two), or single-ended, which uses one tube (or more, but they are in parallel with the first one, not in push-pull).

Class A: All tubes are "on" for the entire sine wave, in either single-ended or push-pull. In single-ended class A, one tube (or two or more in parallel) amplifies the whole sine wave. In class A push-pull, one tube is "pushing", while the other tube is "pulling", but they are both on for the entire waveform. In push-pull class A, one tube amplifies the sine wave, the other amplifies an upside-down version of it (180 degrees out of phase), and the two complete signals are added together in the output transformer to produce a new sine wave of higher power than you could get with a single tube in the single-ended configuration. This class of operation is characterized by high-linearity and low distortion, which makes it ideal for audiophile-type gear, and very low efficiency (DC power in vs. AC signal out), which means you don't get a lot of watts for your money. All single-ended output stage guitar amps operate in class A mode.

Class B: This class is usually used for push-pull only, although you could have a single-ended class B amp, but it wouldn't sound very good for audio. In class B push-pull, on the top half of the sine wave, one tube is on ("pushing"), while the other tube is off ("idle"). Then, on the bottom half of the sine wave, the other tube is on ("pulling") while the first tube is off ("idle"). Essentially, one tube amplifies the top portion of the sine wave, while the other tube amplifies the bottom portion. The two "halves" are added together in the output transformer to make a new, completely-whole sine wave, so there is no clipping caused by the amplification of only half the wave by each tube. This mode is characterized by very high efficiency, so you get more power than you can in class A, and a type of distortion known as "crossover distortion", which is caused by the non-linear overlapping of the turn-off and turn-on times of the tubes.

Class AB: This class is sort of in-between class A and class B, where the tubes are biased higher than class B, but not as high as class A, so one tube still turns off at some point in the sine wave, but not at the exact zero crossing. This allows more "overlap" in the two halves, so there is little or no crossover distortion, but the efficiency is still much higher than class A, so you get more watts for your buck. Most push-pull guitar amps operate in class AB mode.
 

les strat

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OOOPS! In post #2, I meant they are set way too hot at the factory in BJ's. Sorry.
 

PaulSG

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Thanks for all the replies.

Now I just have to wait for the tubes to arrive.

:dude:
 

DannyBoy

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Nov 23, 2003
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You don't HAVE to bias this amp, but it sounds MUCH better if you do a bias mod and set it to appropriate specs for the tubes you put in.
 

dartanion

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Nov 24, 2007
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Generally you should check the bias whenever you change your output tubes as there is no guaranty that the new output tubes will have the same electrical characteristics as the previous tubes. This matters not what type of bias method is used in your amp.

Without getting too technical, cathode biased amps use a resistor between the tube's cathode and ground and a higher value resistor between the tube's grid and ground to set the bias point. Since the cathode resistor is the critical element in how the tube biases, changes in this resistor adjusts the bias point. However, since this is normally soldered in place, bias is not easily adjustable. It is usually acceptable to swap in new tubes without adjusting the cathode resistor.

Fixed bias amps use a dedicated supply voltage applied to the grids of the power tubes to set the bias point. Normally there is a trim potentiometer in the bias circuit so that the bias point can be set easily when new tubes are swapped in.

Class of operation (A, AB, B, etc.) has to do with a bunch variables; but is always determined at full, unclipped output. Thus, the interpretation of Class AB amps are Class A at low volumes is incorrect by definition. They act like Class A, but are not. Class A is not better than Class AB, they are just different. The marketing departments at certain amp companies would like you believe this fallacy though.

Single Ended means that the fundamental topology uses a single output device. Thus, most single ended tube amps use a single output tube. Multiple tubes can be used in parallel to achieve higher output.

Push - Pull means the fundamental topology relies on at least 2 output devices and can also have parallel pairs added for more output power.

All of these characteristics of an amp are independent of each other. This means that not all single ended amps are Class A, not all push-pull amps are class AB, not all Class A amps are cathode biased, etc. In fact, many classic single ended guitar amps do not operate in Class A, but Class AB.

If you all want a more technical explanation, let me know.
 

TBR623

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Dec 5, 2001
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I have a Soldano Astroverb with EL84's. Alot of people are under the impression that all EL84 amps are cathode biased and don't need to be biased. In my case the Astroverb is a fixed biased amp (Classs A/B) and when I tryed a pair of NOS GE EL84's they began to redplate within a few minutes.There is no adjustable bias pot-so any bias adjustments would have to be by changing the bias resistor values. I would say if you are replacing tubes-I'd give tech a call and find out what they say.
 

dartanion

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Nov 24, 2007
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The Astroverb is indeed fixed bias, but the schematic shows a bias trim pot. If this is not affixed to the chassis for external adjustment, then I bet it is inside the chassis on one of the boards. I haven't had an Astroverb on the bench, so I can't verify this. Anyone have a gut shot of one handy? It also makes a lot of sense to keep the bias trim pot inside the chassis as this is not something that the average consumer should be messing with.
 
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