GENERAL CONTENTS
All the files on this disk are accessible directly, or may be opened from this table of contents:
- Technical Documentation
- Construction Photos
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 above 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.
Plug in the tubes, which are stowed in the central space, and the line cord. The tubes are numbered on their wrappers for position; circuit balance is slightly better with them installed in the correct positions. I suggest you plug in tubes 2 and 4 (the bottom tubes on the plate; the right channel) first, then the top tubes last, so you don't have to reach around them. Starting the tube pins into the socket holes correctly is not a trivial operation with these tiny devices, be careful . The tubes do not automatically sit nice and straight in the sockets; I fiddle with them until they do, and then they'll stay there.
Each tube has two automotive O-rings around it as vibration dampers. You may certainly upgrade these to boutique items if you wish, but 6AQ5's are somewhat microphonic; the sound is clearer if the tubes are damped and the unit sits on its rubber footies.
Looking from the rear of the chassis, the line cord and fuseholder are now on the far left, and the audio connections on the right. The two sets of RCA connectors closest to the right edge of the chassis are the inputs , right channel on the bottom, left channel on the top. The pair closest to the corner of the chassis are selected by the up position of the switch, the pair to their left (from the rear) by the down position.
To their left and separated a little are the output connectors; again, right channel on the bottom, left channel on top.
GENERAL OPERATION
All controls are on the front panel. On the far left is the source select switch The up position selects the input jack pair closest to the corner of the chassis (input 1), the down position selects the other pair (input 2), the center position is mute. Next to the source select, on its right, is the volume control.
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.
The unit does invert absolute polarity. This is common to all single-gain-stage plate-loaded circuits, and there's no practical way to correct this. If you are certain that your system 'back end' (preamp, amp, speakers) presently does not invert, and that the present preamp does not invert (or there is none), the easiest fix is to switch polarity back to correct at the amp/speaker interface, by swapping the speaker wire connections at either the amp or speakers. I test the whole system, with a device consisting of a battery and a momentary switch, with an RCA male output. This is plugged into the first component after the source (amp or preamp), and the motion of the bass driver observed or felt with a finger while the switch is pushed and released. If the positive battery terminal is connected to the center pin of the RCA, the speaker cone should move outward when the button is pushed, inward when released. I am a proponent of maintenance of correct absolute polarity, but it's actually a rather thorny problem, as correct polarity is not always observed in recording, from song to song or track to track before the mixdown.
Output tubes should last at least 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 RCA outputs, the hum+noise on the output of this amp is a measured 0.15millivolt, so if you can hear any hum or noise, talk to your builder, it may be time for a tune up.
That's it. Plug it in, turn it on, and go. This is not a complex circuit, though it is quite powerful and has reasonable gain and a lot of drive. It has been my experience that adding a lot of signal adjustment to the circuit degrades the sonics; this preamp is optimized for sonics and amp drive, rather than a lot of switching and tonal adjustability.
© Eric Kingsbury; April 12, 2005
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