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GENERAL CONTENTS
All the files on this disk are accessible directly, or may be opened from this table of contents:
HOOKING UP
The component adheres to the general conventions for audio gear. As you look at the amp from above and in front, the power supply is on the right and the audio amplifier on the left. The line cord enters on the power supply side, the fuseholder below it. I have included a package of fuses with the amp, but if a fuse blows, find out why before firing up. I certainly do not anticipate this happening, since the amp has been tested and played a total of about 25 hours without incident; but shipping damage could occur..
On the audio side, the right channel is on the right and the left channel on the left. The speaker terminals are 0 (com, green) on the left, and 8Ω (red) on the right for each channel. With the speaker 'plus' terminal to the red amp terminal, the amp preserves absolute polarity The bias adjust jack and potentiometer are located between the two output tubes of the pair for each channel; the left channel to the front and left of the 6V6's, the right channel to the rear and right of the pair. The amp has been adjusted with the output tubes in a particular placement, and may be a bit out of bias if they're switched around. I will number them; install in the positions shown.
A note: the positions of the 7062's (5965's in the picture) don't matter operationally, but they (along with the outputs, to a lesser extent) are the balance control. If you hear a channel imbalance, from left to right, try switching the input tubes for each other, channel to channel. As I write, I can see you rolling your eyes about this (Hambone, man!), but in practice it works very well. This organic amp deserves to have one hippie feature, and this is it.
If you peer over the top of the amp so that you can see the back, you will see two pairs of RCA jacks, under the left channel speaker terminals. These are the line inputs. The pair closest to the corner of the chassis are selected by the upper position of the selector switch, and the pair farther from the corner by the lower position of the switch; the center position is mute. The top jack of each pair is the left channel, and the bottom jack is the right channel.
On the front of the chassis, from left to far right; are the source selector switch, the volume control, and the power on / off switches (standby/heaters on the left, B+ main on the right). The up position of the selector selects the #1 line inputs, the center position is mute, and the down position selects the #2 line inputs. The volume control operates conventionally, except that I have limited it to 6 watts for user safety. The down position of the power switches is off, up is on.
STARTUP AND ADJUSTMENT
You will need a DC voltmeter, which will come in a small box with several other meters; AC volts, milliAmps, ohms, and so on. The kind with a dial and needle (a VOM) is better for this purpose, and can be purchased for $15 - 30 at Lowe's, Home Depot, RatShack, etc. I have provided the test lead required. Put the tubes in the amp according to the numbered stickers, and connect it up normally in your system. Plug the forked end of the test lead into the VOM (red to +, black to COM), and the miniphono into either of the bias test jacks on the amp. Set the VOM function dial to DC volts, the lowest range (probably 2.5 volts).
The amp can be fired up without inputs, but should not be operated without an appropriate load on the speaker terminals, as this can damage the output transformer primary, so hook up the speakers to the amp. The binding posts are a form of 'zero terminal', which refers both to their sonic effect and their cost. Under the finger nut you will see a gold ring terminal, which is the actual transformer lead. Put your speaker spade between the nut and the ring, so that it lays right on top of the ring.
On the right side of the chassis are the two power supply switches. Since this circuit uses solid state diodes which conduct immediately, a 'Human Powered Time Delay' is employed to insure long tube and capacitor life. On the left is the heater supply switch, and to its right is the main high voltage switch; in both cases, down is off, up is on. When powering up; turn on the heaters, wait fifteen seconds or so, and turn on the circuit high voltage. When turning off, I usually reverse the procedure (turning off the B+ first), but this is not actually necessary. Okay, turn the heaters on, wait fifteen or twenty seconds, and turn on the main. Observe the reading on the meter. It should climb up over five or ten seconds and read one volt. If the reading is lower than one volt, gently turn the pot next to the jack clockwise (if higher, turn counterclockwise; you get the idea) until you get a reading of one volt. Plug into the other channel, and repeat. Go back and check the first channel. If it's off, correct, and go back and check the other channel. Repeat until both channels are there; one volt. There's a lick to this tha'ts easier to comprehend than to explain, so just try it out. What you are actually reading is the voltage drop across a 15Ω resistor in the common-cathode tail of each output pair. Since I = V/R, then I = 1V/15Ω, or .067 Amps. Thus, the current in each 6V6 willl be half this, or 33.3mA. Since P = I V, P = 315V * .033A, or 10.5 watts, a nice conservative operating point. By this arithmetic, the 15W limit for your outputs would occur at a reading of about 1.45 volts (the plate voltage having probably sagged to about 310 volts), so feel free to experiment with bias readings up to about 1.2 volts, or 1.3 if you like dangerous fun. A hotter operating point will yield less max power, but more power in class A; it's difficult for me to tell at a remove what will sound best with your speakers. In my system, I did not find that hotter operating points sounded any better, so why?
Yes, the jacks are quite close to the output tubes. There's a reason for this. Yes, you will scorch your fingers a time or two until you get used to this procedure. You an animal, or what?
Put on your favorite album, ask your Favorite Person into the room, have a listen. I invite your comments.
GENERAL OPERATION
After that first album, check the bias again, then put the meter away. The bias supply in this thing would bring Jake LaMotta to his knees, so don't worry. Take a reading every week, or month, or so. Find a pair of input tubes that will give you the tonality and balance you like, and use them. The input tubes will last indefinitely; I change mine just to hear a little different sound. 5965's, E180CC's, 7062's, are computer tubes; their official specs only define full on and full off, and bulletproof reliability. Their sonic goodness is just a happy accident; expect and deal with a good deal of variability in actual plate characteristics. Without a balance pot, this variability is your friend. The orange-label Amperex USA 7062, Mullard/Amperex Holland E180CC, RCA 5965 all sound very good, and a little different to each other. Personally, I prefer the three-mica RCA's, which I have installed in the amp. I think the secret is to get a considerable number of the brand you like, and swap them for balance and tone. If you would like a methodology for doing this, post me and I shall pontificate further.
Output tubes should last 10,000 - 20,000 hours at this voltage and operating point. The determining symptom is that the hum (heater-cathode coupling) goes up when the tubes age; and unequally, so that the hum is not common-moded out of the differential output signal. With my relatively fresh GE or EH outputs, the hum+noise on the output of this amp is .3 - .4millivolt. If it gets up to a millivolt, you can often get it back down by swapping output tubes into different pairs experimentally, to heuristically get equal heater-cathode coupling in the pairs. If you play this game, don't forget to rebias afterward.
The amp currently sounds slightly fizzy when first turned on, taking about one album to settle down. This is probably fresh capacitors, and should subside in a few dozen hours of operation. My example still takes about an album to 'focus', though; I hypothesize that maybe vacuum tube gear, which produces substantial heat, has to approach thermal equilibrium to stabilize sonically.
Different brands of output tubes seem to require differing voltages on the grid to achieve the same operating point; the ElectroHarmonix particularly requiring 3-4 more negative volts on the grid. Any time you put in a different set of output tubes, give the bias pots about an eighth of a turn or so in the counterclockwise direction, and so start with the O.P. a little on the safe side; then rebias with the meter. Do not hot swap tubes. You will see hotrods in repair shops and hi-fi salons do this; it's a very unhealthy practice for the gear. Turn off the amp, and let the tubes cool a few minutes before pulling. Insert and remove the output tubes holding them by their bases, not the glass.
Aloha,
Poinz
© Eric Kingsbury; April 27, 2005
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