AX84 General FAQ

AX84 is a trademark of Doberman Music Products, LLC - used with permission.

These are questions of a general nature that seem to crop up repeatedly in the AX84 forums. Some of the answers are clear facts, others are opinions. I've tried to carefully differentiate between the two, but if you think I missed that boat on a question, please send email to <meoATrru.com> (replace the AT with "@" in your email!) with the details.

If your question isn't answered here, ask it in the AX84 forums.

The Questions

1. I just finished my amp and it's not right...
2. How should I power up my new amp for the first time?
3. My voltage readings are all wrong!
4. My plate voltage is over spec!
5. My wattage is over spec!
6. What size wire do I need?
7. Is 300 volt wire enough?
8. All I care about in resistors is resistance, right?
9. What wattage resistor do I need?
10. May I use AX84 or the project names on my amps?
11. Where should I put my output transformer for the least hum?
12. Where should I put my reverb transformer for the least hum?
13. What is the headphone trick?
14. What happens if I put resistors in series?
15. What happens if I put resistors in parallel?
16. What happens if I put capacitors in parallel?
17. What happens if I put capacitors in series?
18. How much current (mA, amps) does my PT need to be rated for?
19. Should I build a P1 or eXtreme, or a High Octane?
20. How does speaker efficiency affect volume?
21. I really NEED a 100 watt amp!
22. What about silver-plated wire?
23. What do these cap codes mean?
24. How can I tell how much power my amp is putting out?
25. Why don't these amps use tube rectifiers?
26. Can I use a tube rectifier on my build?
27. What is the value of (for example) a 4K7 resistor?
28. What is the value of (for example) a 2n2 resistor?
29. How do I convert between capacitor units (micro, nano, pico)?
30. What type of solder should I use?
31. I think my tubes are too hot. I can't touch them!

Some Answers

1. I just finished my amp and it's not right...
2. How should I power up my new amp for the first time?
   Before asking anything else, go read Paul Ruby's startup guide at http://www.paulrubyamps.com/info.html#FirstPowerUp.

3. My voltage readings are all wrong!
   The schematic voltages are just what one individual observed at one specific point in time with a specific set of components whose exact values we don't know at whatever the wall voltage was right that minute (ambient air temperature and airflow may impact these a tiny bit as well 8^).

   If they're within 5% you should be ecstatic; within 10% is not at all unusual. Since many power transformers for tube amps actually have a 117V primary (in the USA) and the wall voltage is typ. 125-130V, many amps start off with high voltages no matter what.

4. My plate voltage is over spec!
5. My wattage is over spec!
   If the tubes are red-plating, you definitely need to re-bias and/or (if the B+ is above spec) drop the B+. Red-plating usually means, "Hey, you! Imminent Death of Tubes!"

   But if the tubes are not red-plating, higher than spec readings (typ. voltage, current and/or dissipation) are not necessarily cause for alarm. In such cases, you may see somewhat reduced tube life compared to tubes run completely within their specs. Just how much the tube life is reduced depends on the tube brand (design, manufacture, QC, etc), the circuit, how hard you play, and the individual tube itself.

6. What size wire do I need?
   For almost everything inside an AX84 project amp to date, 22 guage is plenty. This size wire will carry at least 7 amps in chassis wiring. Certainly this is plenty for everything besides heaters. For those, add up the heater current for the tubes you plan to use. Add in 20% to 100% for inrush current (depending on how paranoid you are). If the result is less than 7A, you're fine. See table below for other wire sizes. Larger wire sizes may not fit in your tube socket terminal holes, esp. on 7 pin and 9 pin sockets. So use the smaller of the sizes available that will carry enough current. You also care about insulation breakdown voltage (next question).

   American Wire Guage (AWG) [1]	Current Capacity [2]	Nearest British Wire Guage (SWG) [3]	Ohms / Foot [4]	Ohms / Meter [4]
   18	16 amps	19	0.00651	0.00228
   20	11 amps	21	0.01035	0.00362
   22	7 amps	23	0.01646	0.00756
   24	3.5 amps	25	0.02617	0.00915
   26	2.2 amps	27	0.04162	0.01457

   All data from 1978 ARRL Radio Amateur's Handbook or computed from data there.

   Notes

   1. AWG = American Wire Guage, diameter in 1/1000 of an inch
   2. Amperages are for continuous duty current with insulated wire in conduits, bundles, or cables.
   3. Nearest British S.W.G. number, diameter in mm
   4. Max. wire temp. of 212F (100C), max. ambient temp. of 135F (57C)
   5. Estimated, not shown in original table

7. Is 300 volt wire enough?
   If the voltage potential between the wire and everything it could possibly touch (or come extremely near to) is less than 300 volts, yes. But if it gets even close to that, you should consider using wire with insulation having a higher breakdown voltage. Otherwise a power line surge might cause problems.

8. All I care about in resistors is resistance, right?
9. What wattage resistor do I need?
   With resistors, you generally care about the following criteria:
   1. Resistance : this is the obvious one.
   2. Tolerance : for guitar amps, 5% to 10% is fine, depending on taste and availability. If you're really picky, go with 2% or 1%, but expect to pay more.
   3. Wattage : make sure you use a wattage rating higher than the resistor will ever see. I like to have 25% or more power in reserve. For instance, if I calculated a resistor would dissipate 0.9 watts, I would use at least a 2W resistor, not a 1W.
   4. Type : some people insist you need carbon comps for a true, vintage sound, but carbon comps drift more over time and with heat, and are noisier (hiss, popping, etc). carbon film are quieter but some people prefer their sound over metal resistor. Metal film or metal oxide are quieter still. Wire-wound resistors are mainly used when you need a 5W resistor or larger. There are huge discussions about resistor types; feel free to search the AX84 archives for more data.
   5. Voltage rating : as with wire insulation, make sure this will handle the voltages you care about. In most resistors, you only care about the voltage drop across the resistor, but if the resistor is up against, or can possibly touch, another conductor (such as a ground lug!) or another resistor, you have to worry about the maximum potential between the resistor and the other conductor or resistor.

10. May I use AX84 or the project names on my amps?
    Chris Hurley owns the AX84 trademark; the various project names belong to a variety of people. Here's Chris's response to a question regarding the use of the AX84 name on an amp:

    ``It is my intent that there are no finished AX84 amps. To be consistent, Doberman [Chris's amp kit company -Miles] doesn't sell AX84 branded amps either, or any AX84 logo'd items at all- no faceplates, no chassis, no t-shirts.

    ``People say "well, this amp is just for me, so whats the harm?" The harm is that it is extremely rare that any amp stays with its original owner forever.

    ``Please folks- don't use the trademarks on your amps. Make up a cool name for your amp. You probably did something cool and unique when you built it- put your own cool brand on the amp.''

    For more information, please check out the following page: http://ax84.com/bbs/dm.php?id=124792.

11. Where should I put my output transformer for the least hum?
12. Where should I put my reverb transformer for the least hum?
13. What is the headphone trick?
    This is the best way to find the quietest place to put a reverb, output, driver, or any other signal transformer to get the least hum. CAUTION: THIS INVOLVES WORKING ON THE AMP WITH WALL VOLTAGE PRESENT. WALL VOLTAGE CAN BE FATAL.

    First set up the power transformer. If it's not installed yet, all the better, but if it is, this will still help. If the PT is not installed, wire up the primaries to a power cord, but do not wire up any secondaries-- tape the ends of the secondaries. If the PT is installed, fine, just make sure there is no circuit on any of the secondary leads (pull all tubes, and disconnect leads and tape them as necessary).

    Now set up the signal transformer (output, reverb, whatever). Hook one set of OT secondaries (your choice, but I usually go from ground to the highest impedance tap, don't worry about impedance matching here!) to a set of headphones. Tape the primary leads.

    Put the headphones on, plug in the power cord, and start moving the signal transformer around. Try it all over the chassis, and also rotate it. While you might expect the least hum with the transformers as far from each other as possible and at right angles (and that may be the case for you), it might come at an odd angle and/or position. Sometimes the least hum occurs when the transformers are side by side. When you find the quietest spot, use a permanent marker to mark where the mounting holes should be.

    If you have more than one signal transformer, repeat this for each one. When you're finished, unplug the PT. That's it!

14. What happens if I put resistors in series?
    When you put resistors in series, you just add the resistance values. For instance, if you connect a 100K resistor and a 220K resistor in series, you get 320K.

              R = R1 + R2
     

    Each resistor will have the same current through it, but each resistor's voltage drop will be proportional to its share of the resistance.

              Vr1 = Vr * (R1 / R)
              Vr2 = Vr * (R2 / R)
     

15. What happens if I put resistors in parallel?
    This is a bit more complex. The general equation for paralleled resistors (or impedances in general) is the reciprocal of the sum of the reciprocal of the resistances:

                        1
              R = ------------
                  1    1    1
                  -- + -- + --
                  R1   R2   R3
     

    When you are just paralleling two resistances, you can simplify this to

                  R1 * R2
              R = -------
                  R1 + R2
     

    When the values of R1 and R2 are the same, then the result is simply

                  R1
              R = --
                  2
     

    Each resistor will have the same voltage through it, but each resistor will carry current proportional to its share of the total resistance.

              Ir1 = Ir * (R1 / R)
              Ir2 = Ir * (R2 / R)
     

16. What happens if I put capacitors in parallel?
    When you put capacitors in parallel, you just add the capacitance values. For instance, if you connect a .1uF and a .2uF capacitor in series, you get .3uF.

              C = C1 + C2
     

    Each capacitor will have the same voltage across it.

17. What happens if I put capacitors in series?
    This is a bit more complex. The general equation for series capacitors is the reciprocal of the sum of the reciprocal of the capacitances:

                        1
              C = ------------
                  1    1    1
                  -- + -- + --
                  C1   C2   C3
     

    When you are just putting two capacitors in series, you can simplify this to

                  C1 * C2
              C = -------
                  C1 + C2
     

    When the values of C1 and C2 are the same, then the result is simply

                  C1
              C = --
                  2
     

    Each capacitor will carry voltage proportional to its share of the total capacitance.

              Vc1 = Vc * (C1 / C)
              Vc2 = Vc * (C2 / C)
     

    This is occasionally done to get a specific smaller value of capacitance than one has on hand, but in general this is done to increase the voltage rating beyond that of the available capacitors. In this case, it's a good idea to place a resistor in parallel with each capacitor to help equalize the voltage. In power supplies (the main place this happens in guitar amps), 220K resistors is a good value to start with if you aren't sure what to use. If the caps are of different voltage ratings, you should probably use resistors of different sizes, in direct proportion to the voltage ratings of the caps. For instance, if you were putting a 200V cap and a 350V cap in series, you might use 220K and 330K resistors.

18. How much current (mA, amps) does my PT need to be rated for?
    You need to determine this for each of the secondaries.

    For the high voltage secondary, power tubes are the most important factors. You'll have to estimate the peak current draw per tube plate and screen and multiple by the number of tubes. For a single 6BQ5/EL84, the max. plate current is supposed to be 65mA, and the screen is typically 5mA or less, but throw in a few mA for good measure, call it 75mA. (You can get this from the spec sheets). For two 6BQ5s in Class A, double that. For Class AB, the average current is the same, but the instantaneous current is higher, depending on how far toward Class B you bias. At Class B, you'd need a secondary capable of double the Class A current.

    Now add current for each of the other tube sections. Each half of a 12AX7 typically pulls around 1mA, but could pull up to 3mA. If you aren't sure, use 3mA per triode. BUT... if there is any chance you or someone else would ever substitute in a 12AT7 or 12AU7, you'd best figure 10mA or 20mA max per triode. That's a whole new ball game! (You may have to hunt to find the actual maximum, rather than typical, ratings.)

    Adding these up, a basic P1 that will never use anything but a 12AX7 is safe with an 80mA high voltage secondary (75mA power tube + 6mA 12AX7). Any PT made should be be able to handle 1 extra mA. But you might also want to include a safety factor, and get one capable of a bit more current.

    Now you need to determine the main heater current. This data is more readily available. For a 6BQ5 it's 0.76mA. For a 12AX7 with the heaters in parallel for 6.3V usage, it's 300mA. So a P1 needs a 6.3V heater tap rated for at least 1.36A. But again, look at what other tubes you might use; a 12BH7 pulls 600mA with the heaters in parallel.

    Finally, if you plan to use a 5V rectifier, make sure the 5V winding can provide however much current your rectifier will require. Depending on the tube and load the load here can be anywhere from just under 500mA to a couple of amps. If you want to go the old Bassman or Mesa route (multiple tubes) then double or triple the current as necessary.

19. Should I build a P1 or eXtreme, or a High Octane?
    Consider the following:
    1. (a) Highway to Hell (b) Hells Bells
    2. (a) Love Gun (b) Lick it up
    3. (a) Iron Man (b) Miracle Man
    4. (a) Smokin' in the Boys Room (Brownsville Station) (b) Smokin' in the Boys Room (Motley Crue)
    5. (a) Breakin' the Law (b) You've got another thing comin'
    If you lean more to (a), consider a P1 or P1-eXtreme. If you lean more to (b) consider a Hi-Octane.
    -Chris Hurley

    How about...

    1. (a) Train, Train (Blackfoot) (b) Crazy Train (Ozzy)
    2. (a) Dyer Maker (Led Zeppelin) (b) Dyer's Eve (Metallica)
    3. (a) Eighteen (Alice Cooper) (b) 18 and Life (Skid Row)
    -Pete Rittwage

20. How does speaker efficiency affect volume?
21. I really NEED a 100 watt amp!
    I'll give you the obligatory speech about watts VS. volume so that you don't get all caught up in the 100+ watts thing.

    The way that watts translate into decibels is in powers of 10. That being the case, if you want to do twice as loud, you need to go 10x higher in wattage. The difference in volume between 100W and 50W is actually only 12% less or so. HALF the volume of 100W is actually 10 watts, and TWICE the volume of 100w is actually 1,000 Watts. And when is the last time you played a 100W amp on a setting higher then 3 on the volume without everyone screaming to turn it down!

    Now, after you have digested that, we can move along to the next part.

    Speaker efficiency is also key to volume. That curve is also not linear. If you take 2 otherwise identical speakers, one with a sensitivity rating of 103dB, and one with a rating of 100dB, that -3dB drop is the equivalent of sending half the power into the speakers... like moving from a 100W amp to a 50W amp, of about a 12% drop in power.

    Now, if you move from the 103dB speaker to a 97dB speaker that is otherwise identical, that -6dB drop is like moving from a 100W amp to a 25W amp. That being the case, a 25W amp played through 103dB speakers is exactly as loud as a 100W amp being played through 97dB speakers.

    OK, digest that for a second and then we'll move on.

    Last week, I played a 35W amp on full blast (but not clipping) through a Marshall 4x12" cabinet loaded with Celestion Vintage 30's, which have a sensitivity of 100dB. I took out a sound pressure level meter and put it 10 feet away from the speakers. The 35W amp produced a clean power level of 117dB, which is only 3dB quieter than an airplane landing.

    With all that in mind, you should decide on whether you really want to go with the added expense, weight, maintenance and extra wiring that a 100w amp requires.

    -Aletheian-Alex

22. What about silver-plated wire?
    Silver-plated copper wire shouldn't be used in any equipment that may be exposed to humidity over 50%. The silver reacts with the copper and causes the copper to turn into rust ("red plague"). Yes, this can only happen in the presence of oxygen, but it is very easy to "nick" a wire when stripping, even with the best tools and training. I have seen lengths of silver-plated copper wire in aircraft that still had continuity but had increased in resistance to several kOhms per foot. Tinned copper does not have this problem.. Silver lowers the resistance, but I can't believe there's an audible difference.

    More: http://www.corrosion-doctors.org/MatSelect/corrsilver.htm

    and for the really studious types (notice by the dates that this is not a new problem):

    1. Anthony P L & Brown O M 1965, Red plague corrosion, Materials Protection, 4, 3, pp. 8 - 18.
    2. Friebel V R 1969, Corrosion problems on silver-plated copper wire, Wire and Wire Products, May 1969, pp. 39 - 43, 53.
    3. Peters S 1970, Review and status of red plague, Insulation/Circuits, May 1970, pp. 55 - 57.
    4. Peters S T & Wesling P B 1968, Corrosion of silver-plated conductors, Proc. SA M PE 13th National Symposium, pp. 395 -406.
    5. High Temperature Insulated Wire, Report on Copper Corrosion, NEMA Standards Publication, No. HP 2-1968 (reaffirmed by NEMA 1972).
    6. ESA PSS-01-720, Draft, Test Procedure to Determine the Susceptibility of Plated Copper Wire/Cable to 'Red Plague' Corrosion, 9 December 1983.
    7. ESA PSS-01-708, Manual Soldering of High Reliability Electrical Connections
    -Ken Moon

23. What do these cap codes mean?

     European cap material codes:
     MKP = metallized polypropylene
     FKP = metal foil and polypropylene
     MKT = metallized polyester foil
     MKC = metallized polycarbonate foil
     FKC = metal foil and polycarbonate
     MKI = metallized polyphenylene sulphide
     MKS = polystyrene (metallized or with foil) 
     

    -Matthias M

24. How can I tell how much power my amp is putting out?

    " In order to get an accurate measurement of output power, you should use a purely resistive load... if you are just looking for a ballpark figure, a meter and a load resistor will do fine, if you have the amp set just prior to where you start to hear distortion when a speaker is connected.

    Sub a load resistor equal to the speaker impedance, measure the RMS AC voltage, square it, and divide by the measured value of the load resistor. Be *sure* your meter reads in true RMS, or you will get an incorrect result. Use a 400Hz signal as the input source, because this is where most speakers have their ``flattest'', or nominal, impedance."

    -Aletheian-Alex, distilled down from Randall Aiken's site

25. Why don't these amps use tube rectifiers?
26. Can I use a tube rectifier on my build?

    In a push-pull amp, the tube rectifier will drop more voltage as the current through it increases. As you turn up the amp and dig in, the power tubes pull more current. This causes the tube rectifier to drop more voltage, causing an overall drop on the B+, changing the tone of the amp-- adding some mojo.

    Most of the [AX84 amps] are single ended. SE amps pull the same amount of current no matter how loud or hard you play. There is no mojo gained. SS diodes are cheaper, and easier to use for the same result.

    -Bobby Kirbos

    The mojo refered to above is commonly known as "sag", which results in a compression effect, sometimes called a "singing sustain".

    For more discussion on this topic, see the thread Bobby's answer above is from on the AX84 BBS at https://ax84.com/new/bbs/dm2.php?id=207139.

    -Miles (Harrison Ford Prefect)

27. What is the value of (for example) a 4K7 resistor?
28. What is the value of (for example) a 2n2 resistor?

    Many times, engineers will use the multiplier as the decimal point. It harkens back to the days of poor printing when you could easily miss the decimal point (or comma in some euro diagrams) and screw things up royally.

    Examples

     2.2 Megohms = 2M2
     2.2 Kilohms= 2k2
     2.2 ohms = 2r2
     .22 ohms = 0r22
     

    -Aletheian-Alex

    The same thing applies to capacitors. Examples:

     .0022uF = 2n2
     2.2pF = 2p2
     

    -Miles (Harrison Ford Prefect)

29. How do I convert between capacitor units (micro, nano, pico)?

    Randall Aiken's site covers this quite well (as usual!)

        http://www.aikenamps.com/CapConvert.htm
     

    -Miles (Harrison Ford Prefect)

30. What type of solder should I use?

    Make sure you never use solder that says "acid core". Plumbers might use that stuff to solder oxidized metals but it will corrode the $#|+ out of your electronics if used. Avoid that stuff like the plague. Here are three types that are OK to use:

    1. "63% Lead 37% Tin" solder is the easiest stuff to use IMO since it melts cleanly at the lowest temperature. Make sure it says "Rosin Core" if it has flux added - most have it. Then if it says "no-clean flux" it's even easier. The Kester brand works for me. (By far the best -ed)
    2. The 60/40 rosin core stuff melts at a little higher temperature, it's also OK to use but it takes a little more technique since it goes through a transition phase between flowing and setup that the 63/37 stuff does not do. IMO it's more prone to "cold solder joints" if your technique is bad. Google or wiki the term "eutectic" if you are curious about this stuff. The 63/37 stuff is a eutectic solder, all the other percentage formulations of Tin/Lead solder, like the 60/40, are not. If in doubt, go with the 63/37 stuff. (Only if you can't get 63/37 -ed)
    3. The "2% Silver stuff" is OK to use too, IIRC it's 62% lead 36 Tin 2% Silver. There might even be stuff with a little higher Silver content. Some folks think the 2% stuff makes a stronger solder joint than 63/37 and some even think it adds to the mojo or "sound" of the solder joint. No comment on the mojo ;-), but it makes a good joint if you have a good iron and know what you're doing. Rat shack even has some, it's not bad stuff.
    Bottom line:
    I'd go for the stuff in #1, get a small amount of it in a diameter that you hope makes it easy to feed into the solder joint and see if you like it. If you use solder that has too small of a diameter you will find that you have to feed more length in at each joint and that is a pain. Too large and you'll easily get a big blob of the stuff if you're not careful. Just like guitar string gauges, use what you like and fits your style.

    Tech Tip:
    If you get a big spool of solder, wind a smaller amount of in into an empty "solder wick" plastic holder thingy - the ones that are about as big as a half-dollar coin with a hole in the middle. Mount a dowel, ball point pen, or something like that in the chuck of your cordless drill, push the empty "solder-wick" spool onto the post, and then wind solder from the big spool onto the "solder wick" package until you fill it up - this make a handy solder dispenser. The outer flap holds it from unwinding but you can easily pull a few inches out at a time for a joint.

    This is much better than holding a big freakin' spool of the stuff in your hand, or having to cut little pieces of it off the spool each time to use and then dropping these little unused ends around everywhere where the dog can eat them.;-)

    -Jeff S

31. I think my tubes are too hot. I can't touch them!

    The spec sheets often list a maximum bulb temperature of 180 to 200 degrees C for rectifier and power tubes. Some of the horizontal sweep types such as 6DQ6 and 6GV5 list as much as 240.

    That is instant 3rd degree burn territory.

    Preamp tubes on the other hand should not get much over 50 degrees C.

    -Sean Weatherford