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.
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.
| 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
``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.
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!
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)
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)
C = C1 + C2
Each capacitor will have the same voltage across it.
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.
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.
How about...
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 actuallt 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 sendin 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 Celesion 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 wether you really want to go with the added expense, weight, maintenance and extra wiring that a 100w amp requires.
-Aletheian-Alex