Everything POWER TUBES and the C5!!!

Orpheus777

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Guitar amplifiers use 3 basic tube types: 1) Preamp Tubes for Gain/Tone/Inverter circuits; 2) Power Tubes to Amplifier Current and 3) Rectifier Tubes to convert A.C. to the required D.C. voltages needed to run the rest of the amps tubes. (The C5 does not use a rectifier tube) What we're focusing on in this thread is the Power or Output Tubes, which in our case is a single EL84.

Power tubes are fairly larger and Hot. They're usually located around the middle of the amps chassis and away from the transformers to reduce noise. It is common to see Power Tubes in pairs or quads in Larger push-pull style amplifiers; but in many small watt amps, like our Class 5, a single tube is sufficient. The Class 5 is a Cathode biased, single ended 5 Watt Amplifier, operating in Class A.

Most all guitar amp power tubes are Octal base, that is they have eight pins; but the EL84 utilizes a 9 Pin miniature socket just like the 12AX7's found in the pre-amp section of your amp. This popular Tube comes in three essential flavors... the EL84, the 6BQ5 and the 7189. The 6BQ5 is the exact same tube just the American version/designation while EL84 is the English designation. The 7189 however can handle more Voltage then a standard EL84 and an excellent choice for this amp if you can find one but not everyone will like the way this tube sounds... tone is subjective... it will clean up your amp some and the low end is more solid giving you a little more headroom... but I prefer the classic cream of a standard EL84 running in Class A.

Few can afford and rarely do I recommend burning NOS power tubes... they are expensive and only last 1-4 years depending on use (2 yrs is a good avg) and therefor not very cost effective or realistic... I mean go price a matched set of Mullard EL34's or El84's... I only recommend NOS power tubes for studio use....save your NOS power tubes as they become more and more rare and expensive daily!!!...BUT!!!, on the other hand... another great thing about the Class 5 is that it is a single ended class A amp which utilizes a single power tube in other words no need for matched pairs...which translates into huge savings on the NOS market!... This means that most of us can afford to run the Class 5 with NOS power tubes! (Certainly in studio) Yet another reason why I say the Class 5 is an awesome studio amp in a Class of it's own!

Just like when working with pre-amp tubes, I recommended you get your hands on as many as possible and various brands of, NOS and current production Power Tubes (EL84's) and then choose your favorites by ear. If you can't do this then take them to a friend or Tech who has a bunch of tubes to comb thru. Since the C5 is Cathode biased there is no need to re-bias the amp in between tube changes, just swap them in and out, but be careful as they do get hot and you don't want to damage the tube (remember to turn the amp off before pulling the tube)... I usually use a towel or a glove. So why both NOS and New production you may ask... well because they are both good-quality, plus every tube sounds different and Tone itself is subjective. In most cases I prefer the vintage stuff over the new stuff but I'm more of a blues guy and have that luxury.

Lucky for us the EL84 is one of the most popular tubes ever produced and used in many applications outside the guitar world, so we don't have to worry about a limited supply. Also because of there popularity and the subsequent demand, much money is being spent on current production models and some really good EL84's are the result. In fact many will swear by the currently produced Sovtek and others by JJ tubes... I like them both. But what's important here is that you don't have to drain the bank chasing down elite NOS tubes in order to have great tone. But if your trying to find that Magick sweet spot where when you pick soft the amp cleans up and when you pick hard it compresses just right... for this type of Dynamics you may have to try a few different tubes.

Also because the C5 is Cathode biased, you can't easily “fine-tune” your bias voltage... in other words, what you get is... what you get. Your amps “idle” current is set and because of this every tube will operate at slightly different levels, drawing more or less Current and/or Voltage and this of course affects tone. So literally every single tube you try, regardless of brand or when it was produced, will both sound and “operate” differently in your amp... don't believe me, then roll up your sleeves and give it a try...

So how does this all work???

Well, all tubes in your amp, (Pre-Amp, Power or Rectifier), all work upon the same essential principles; that is they take a "cathode" and heat it up to move electrons around inside of a vacuum, which is usually composed of glass. These electrons pass through one or more grids, (depending on function & design), which then control the flow of electron current; the electrons then strike the anode (plate) and are absorbed. In this process, the tube will take a small signal voltage/current and make it into a larger voltage/current, thus amplifying it. With pre-amp tubes the primary function is an increase in gain or signal voltage and with Power Tubes the primary function is an increase in current. It takes Wattage to power a speaker and Wattage equals Voltage times Current. It goes like this... First the preamp tubes give you the desired Gain/Voltage, then Power Tubes amplify the Current, which gives you the correct Wattage, while a Transformer matches the impedance of the speaker to that of the Output or Power tubes... and we have sound. Remember that Power tubes are designed to amplify Current and not Gain and this is just one of the many reasons behind why some may prefer the tone of Power Tube Overdrive to Pre-amp Tube Distortion.

Once again pull out your C5 schematic and notice how similar our Power Tubes are operating in comparison with the pre-amp tubes. (On the schematic the Power tube is labeled as V3A-EL34) If you recall from the Preamp thread we discussed how the (K)Cathodes always ran to ground, the (A)Anodes/Plates always towards the power supply and the (G)Grids are connected to the output signal from the stage before... well the pattern continues as that is exactly what is going on here as well... Look and compare them... Note how the Output Signal from the Pre-amp section (just prior) is connected to the Grid on the Power Tube. The Grids are connected like this across the Amp, from Input to Power Tube, stage by stage, with each tube either 'amplifying' Gain or Current... and the Grids are the control Valve in this process...in the Pre-amp section they control the current within the circuitry to produce the desired Voltage Gain; then the Grid on the Power Tube (which is connected to the Output of the Pre-amp section) determines how much current flows thru the Power Tube to produce the desired wattage.

If you recall the Pre-amp Tube thread then you will know what I mean when I say that a 12AX7 is a dual-triode Tube and how exactly that works (if not go read it now). Similarly the EL84, like most (but not all) guitar amp Power tubes, is a PENTODE; this because it has 5 active elements as opposed to the Triodes 3... but the basic 3 - Grid, Cathode & Anode are common on all Tubes... so what are the extra two elements that give us the 5 found in our Pentode??? Well in order to make Power tubes more efficient and more 'Stable' two more grids have been added over time, so you now have a Control Grid, a Screen Grid and a Suppressor Grid... so we end up with an Anode, Cathode and 3 grids. It was a gradual progression in tube technology that gave us our EL84 or the modern day Pentode we had the Tetrode which was a 4 element tube. The KT66, KT77 and the KT88 are all Tetrode power tubes well known for their clean tone and low end.

So what's going on inside the tube with all these Grids etc...???

So you turn your amp ON and the filament Heaters flash and/or begin to warm up (that's the glow). This slowly warms up the Cathode exciting electrons which are attracted to the Plate/Anode. Simply put, opposites attract so electrons jump from Cathode to Anode. The Plate then, which receives the electron flow, becomes the “Output”. That's the first two elements...now in between the Cathode and the Anode we have our first Grid, whose job it is to control the rate or flow of electrons and thus called the “Control Grid”. This is the Grid nearest to the Cathode and when voltage is applied, it causes the Anode Current to vary resulting in a varying Voltage at the Anode; with proper Biasing this voltage is amplified... and you now have a Triode (3 elements)... To this we add another Grid whose job is to isolate or “Screen” the Control Grid from the Plate. This because a small capacitance naturally appears between the two, much in the same way we saw that Tube Shields do (see pre-amp thread). By isolating the Grid from plate we prevent certain parasitic oscillations, but we also increase Gain because it speeds up the flow of electrons...so in a nut shell... this Grid has both Screening abilities and makes the tube more efficient. With the addition of a “Screen Grid” we now move up to a Tetrode tube (4 elements). To this we then add one more Grid which gives us the Pentode or 5 element Power tube like the EL84 inside our C5. So the job of this 3rd and final Grid is to Suppress secondary-emissions which are simply the electrons that bounce off the plate. This “Suppressor Grid” is placed between the Screen and the Plate and like the Cathode, is kept negative in relation to the Plate thus Suppressing those secondary-emissions... and there you have it... all 5 active elements working together...

What about Tube Biasing???

Well first I should mention that there are varying opinions as what is the proper way to Bias a Power Tube. In a nut shell it comes down to this... that which is technically the “best” is NOT that which sounds the “Best”. There are various methods as to how you arrive at the ideal Bias voltage most of which try to locate that ideal idling current between Saturation and Cutoff.

As already discussed, Current enters the Tube at the Cathode which is attached to the Negative/Ground side of the power supply and leaves at the Anode which is attached to the positive side of the power supply. Since “opposites attract” the negative electrons move from Cathode to Plate while the Grids, which are physically placed between the two, controls how much current flows thru the tube. It does this by varying its' electrical charge in relation to the Cathode. As the Grid goes more and more Negative (Reducing Plate Current) it will eventually stop the flow of Current and move into “Cut-Off”. As it becomes less and less Negative (Increasing Plate Current) it becomes easier and easier to push the Tube into “Saturation”. It is this BIAS VOLTAGE, using the Grid like a control “Valve”, that sets the point between the two that will become the tubes “Idling Current”. Now you generally want Pre-amp tubes to operate as linearly as possible and this is why the mid-point between Cutoff and Saturation is generally accepted as the correct Bias Voltage for pre-amp tubes. This is set using a resistor between Cathode and Ground. Once current is drawn thru this resistor a voltage develops on the Cathode making it more or less positive with respect to the Grid. This is called Cathode Bias or Self-Bias and never needs to be adjusted once set. This is how most all Pre-amp tubes and some Power Tubes are Biased. In fact... in the C5 both the Power and Pre-amp tubes are Cathode Biased and never 'need' adjustment.

However, you should know that when determining the “correct” bias voltage, there is some wiggle room in both directions... The truth is that every Tube has a “safe” zone or operating range, within the given application and as long as you stay within it, your fine. For example in a Tweed Bassman you may set you Bias from as low as around 20mA to as high as around 40 and every Tube type in every Amp will have a different “sweet” spot... for me in a Tweed Bassman I want to burn Tung-Sol 5881's at around 29mA... but everyone has there own ideal. There is however real substantial variances in Gain, Tone and Dynamics within the 'safe operating range'. Because of this fact many Tech's in the “know” will bias their Amps by ear. To do this they simply monitor the Bias voltage while playing, listening and comparing tone at different levels until they find the “Sweet Spot” within that tubes operating range. I like to do this with the actual or same type of guitar that I would want most to match with this particular Amp.

In a fixed biased amp it is easy enough to adjust bias, as you will have some sort of internal bias adjustment POT/screw; so you can fairly quickly and with relative ease, adjust and compare tonal differences. So those of you who really want to learn how to Bias an amp, I recommend you start with fixed bias for this very reason. Eventually you will learn how moving the Bias in one direction makes the Amp louder and closer to saturation while the other direction it becomes cleaner and thinner... with experience you will find your own words to describe this, what's important here is that we learn yet another way to manipulate and fine tune our tone... via Bias Voltage and Tube selection.

But wait... the C5 is Cathode biased... so how do we go about setting the Bias Voltage and selecting the best tubes???

The C5 never 'needs' adjustments... but if you really wanted you could fine tune it a little. I would recommend for most that you simply choose the best sounding EL84 you have... but if you want to take it a bit further... First you need a way to monitor Bias. There are several ways this is done; If you can track one down you could try a Bias tool, some times called a Bias Right. A few people like Weber VST have offered this product or similar products before. It is a simple adapter that goes between the power tube and socket with a couple of leads that you plug into your meter and your ready to go... this is very safe, quick and easy. Unfortunately Weber is no longer offering this product nor do I know anyone that is. All other methods require that you work inside the amp with LIVE and potentially lethal voltages!!!! So make sure you know what you are doing!!! For this very reason it may not be practical or even beneficial for most C5 owners to go much further. Other may feel the need to fine tune the Cathode Resistor and Cap some... we will go into this more later... lets look at picking that tube.

If you are able monitoring bias voltages all the better... in either case simply start plugging in Power Tubes while carefully noting differences. First listen for and focus on drive and dynamics, if monitoring Bias you will find a spot where the Amp is very responsive to your playing style so that when you pick soft it cleans up some and when you pick hard it drives or compresses in the way you want; Then note the mA or mV depending on what your tracking, and you will now know the approximate level at where you prefer your bias to be set regardless of which brand of tube you use. All tubes within a mA or two from this point will respond with the same approximate dynamics so you have an excellent reference point. Now take a listen to as many different brand EL84's that you can get your hands on ( as close your your chosen Bias point if you know it)...Then play, listen and take detailed notes focusing on the differences in tone, including highs, lows and mid-range bark. Carefully describe as many variables and characteristics as you can and with each brand and tube note your results. Usually, with not too much effort, you will find a brand and a tube with the tone you prefer. In time you will intimately know how each tube in your box sounds... then you can “fine tune” any amp you have.

Here are some of my notes: I compared 8 matched pairs for a total of 16 EL84's. Four modern and four vintage... This include: Vintage; Amprex 70's, Amprex Bugle Boy, Mullard Black Burn, Mullard 70's... & Modern; Sovtek, J/J, Tube Amp DR & Ei... So what does a Tube Snob/Guru like myself burn in his C5... I settled on a early square-getter Blackburn Mullard EL84, no surprise here. I felt it gave me the best overall tone both thru the Headphone jack and in normal operation and with loads of classic vibe... very PLEXI... but the standard Amprex was also very good with more sustain, cream and volume then the Mullard and sounded more Modern; really shined when played wide open thru an Greenback. A very good tube and a great alternative was the Tube Amp Doctor EL84 which came stock on mine!!!... I preferred it overall to all the current production tubes though Ei was also very close. Of those two I thought the Dr tube was more modern and tighter but the Ei had a little more vintage vibe and cleaned up much better... I would recommend one of the four depending on your goals and pocket book. In either case find a reputable dealer who you know measures and grades his tubes.

OK if you've come this far and you still want to tinker a bit more... suppose you come to the conclusion that you want to run your amp a little hotter or that you want to use brand 'X' but with just a little more drive and then...then it would be perfect... or maybe you want just a little more or little less low end etc... lucky for us there is a bit more we can do...Both the Cathode Resistor and the Cathode Cap affect Tone; both their values and what there composed of contribute. Now I wouldn't recommend drastic changes but for the Cap (C12 on schematic) you could go one step up or down and compare; make sure to observe polarity here (negative to ground) and do not drop the voltage rating, but otherwise your safe to experiment. With the Resistor much the same... do not drop the wattage rating. (With either Cap or RI you can go up voltage/wattage but never down). The Cathode RI in the Class 5 (R27) is 150 ohm, 3W. After running my tests I think this is a good value. If anything I may want to run my tubes a bit higher so I may experiment staying within a 10-20 Ohm range of the 150. The TONE of both CAP and RI will change and improve in my opinion, by simply switching to better quality components even if at the same rating... for instance I would guess that a Carbon Comp or Film RI with a Mustard style cap would sound better.... try it and see... for now I will leave my board until I address a few other areas. We will get this when we finally get to the board.

Now for some of you, this may seem like way to much work and effort... but don't fret, there is always the easy route... just buy a new tube from a reputable company, plug it straight in and it should work fine... No Bias adjustment needed... but for those of you, who may need a little more... hopefully I given you some food for thought.

Enjoy!!! and get those power tubes humming...
Orpheus
 

Roadburn

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Interesting, but very long read.


B.t.w. Dr Tube is a Dutch amp tech, Tube Amp Doctor is a German company.
 

MartyStrat54

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Also, the JJ 844 is available as a power tube substitute. Russian NOS tubes are available as well.

I also disagree on the comments concerning the price of NOS EL84's. You can pick up a single quite cheaply. How long will they last? That depends on how the tubes will test. Most "good" NOS tubes will last longer than any CP power tube. I won't go into the reasons why since I have posted up on this numerous times.

Thanks for taking the time to write up a knowledgeable thread.
 

Orpheus777

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Also, the JJ 844 is available as a power tube substitute. Russian NOS tubes are available as well.

I also disagree on the comments concerning the price of NOS EL84's. You can pick up a single quite cheaply. How long will they last? That depends on how the tubes will test. Most "good" NOS tubes will last longer than any CP power tube. I won't go into the reasons why since I have posted up on this numerous times.

Thanks for taking the time to write up a knowledgeable thread.


A fellow Tuba Man!!!!...hey Marty nice to see you chime in... As far as the EL84 price...I agree singularly they are affordable that's why I mentioned that most people can afford to run this amp using all NOS tubes which is a huge PLUS... as far as how long do they last it really depends on use and my figures were round abouts based ongoing and regular use there is no exact time frame but I agree NOS USA or European will out last and out perform current production tubes across the Board. I have the luxury of burning nothing but the best in all my amps. As far as Russian NOS, outside of 5881/6L6 style tubes I'm not impressed...Haven't tried the JJ 844... I reviewed the tubes I had in stock, I may try one some day but like you I really only use NOS in my amps....

I'm glad everyone is enjoying this thread... it should be read with the pre-amp thread as the two go hand in hand... also for the record it was the German based Tube Amp Doctor tube that I mention in my review... a very good new production tube.

Orpheus
 

MartyStrat54

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Reviews coming in on the 844 state that it produces a deeper, tighter low end and the top end is warmer than a regular EL84.
 

DirtySteve

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Finally had the time to read this all the way through. I feel like I just took a class ...Awesome!
 

jupiter89

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Great read ! I am gonna have to go through it a couple more times though so I can pass a quiz if it comes along.
 

dread1

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Awesome read..:)Musiccorp just sent me the 3 piece back for my Class 5. Along with the kit they sent me a tube amp doctor el84, gold labeled with the number 10 on it. The stock one was a JJ, red marshall labelled with a number 7 on it. What's the difference in the numbers. Headroom right?
 

Orpheus777

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I am glad you enjoyed...read my other threads for more thought and theory. Yes that is my understanding behind the number system as well but would like to compare are few numbers before I could say for certain, the one I tested and came in my amp is a number 6... I'm going more in depth into speakers and speaker options next... and I will introduce the next MOD...
Orpheus
 

Orpheus777

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Would it be 1 being the most headroom, or 10?

I don't have the tubes to verify for myself nor is the company web site clear on this... but usually a higher number depicts a hotter tube in that case 1 would be the coldest and 10 hottest and then the most Headroom would be around 3-5 but I'm sure an email to the Tube Amp Doctor would clear it up... i mean it's their tubes right.

I am glad some are enjoying my threads...this one should be read with the preamp thread...

Orpheus
 

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