HP 9-Pin User Manual download pdf (Page 7)

GlassWare Audio Design

Jumper J7 connects the PCB’s ground to the chassis through the top centermost

mounting hole. If you wish to float the chassis or capacitor couple the chassis to ground,

then either leave jumper J7 out or replace it with a small-

valued capacitor (0.01 to

0.1µF). Warning: if rubber O-

rings are used with PCB standoffs, then the ground

connection to the chassis is not likely to be made.

Tube Selection

Unlike 99.9% of tube circuits, t

he Aikido amplifier defines a new topology without fixed

part choices, not an

old topology with specified part choices. In other words, an Aikido

amplifier can be built in a nearly infinite number of ways. For example, a 12AX7 input

tube will yield a gain close to 50 (mu/2), which would be suitable for a phono preamp or

a SE amplif

ier’s input stage; a 6FQ7 (6CG7) input tube will yield a gain near 10, which

would be excellent for a line stage amplifier; the 6DJ8 or 6H30 in the output stage

would deliver a low output impedance that could drive capacitance

laden cables or even

high-imp

edance headphones. In other words, the list of possible tubes is a long one:

6AQ8, 6BC8, 6BK7, 6BQ7, 6BS8

, 6DJ8, 6FQ7, 6GC7, 6H30, 6KN8, 6N1P, 12AT7,

12AU7, 12AV7, 12AX7, 12BH7, 12DJ8, 12FQ7, 5751, 5963, 5965, 6072, 6922,

E188CC, ECC88, ECC99… The only sti

pulations are that the two triodes within the

envelope be similar and that the tube conforms to the 9A or 9AJ base pin-

out. Sadly, the

12B4 and 5687 cannot be used with this PCB.

Internal Shields

If the triode’s pin 9 attaches to an internal shield, as it

does with the 6CG7 and 6DJ8,

then capacitors, C11 and C12 can be replaced with a jumper

, which will ground the

shield. However, using the capacitors will also ground the shield (in AC terms) an

d allow

using triodes whose pin-9 attaches to the center tap of its heater, such as the 12AU7.

Cathode Resistor Values

The cathode resistor sets the idle current for the triode: the larger the value of the

resistor, the less current. In general, high-mu triodes require high-

value cathode resistors

(1-2K) and low-mu triodes require low-valued cathode resistors (100-

1k). I recommend

running the output tubes hotter than the input tubes

; or put differently, run the input

tubes cooler than the output tubes.

Interestingly enough, a lower idle current for the

input stage do

es not seem to incur the same large increase in distortion that one would

expect in other topologies (a testament to the Aikido’s principle of symmetrical loading).

For example, 1k cathode resistors for the input tube (V2 and V3) and 300-

ohm resistors

for the output tubes (V1 and V4), when using 6FQ7s or

6CG7s throughout. Thus, the

output tubes will age more quickly than the input tubes, so rotating output for input

tubes can extend the useful life of the tubes.

Capacitor C3 allows the bottom output triod

e’s cathode resistor to be bypassed, when

resistor R8 is replaced with a jumper wire; this arrangement is useful when driving low-

impedance loads, such as 300-ohm or 32-ohm

headphones, as it provides the lowest

possible output impedance from the Aikido amplifier.

If used, C3 should be at least a

1kµF capacitor. On the other hand, if high-

capacitance cable is to be driven, use a

higher idle current and retain the cathode resistor, R8, and leave capacitor C3 off.

Current is more important than the lowest possible output impedance.

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HP 9-Pin User Manual Page 7

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