6900 Kingsbury Road | Templeton, CA 93465 | 805-461-4100 | http://www.demeteramps.com/
VTHF-300M Power Amplier User’s Guide
Congratulations on your purchase of the VTHF-300M, one of the nest sounding all tube ampliers in the world today. To insure that your unit provides you many years of listening pleasure
please follow the directions included in this guide.
USER ADJUSTMENTS
Bias Controls: each output tube has it’s own bias adjustment. Properly adjusted bias insures
lower distortion and noise and insures a longer tube life. Although the bias is adjusted at the
factory, tubes will age differently over time and will need adjustment to insure optimum performance. The bias should be adjusted when you rst use your amplier, a week later, and monthly after that. This will insure long tube life (allow 15 minute warm up time before adjusting).
Procedure
1. Connect your amplier to the proper speaker load.
2. Turn on the amplier and let it warm up for 15 minuets.
3. Remove amplier controls cover plate on front panel by turning the thumb screws at the ends
of the plate counter-clock wise.
4. Connect voltage meter to the amplier by connecting the black lead to the black ground input
on the right side of the panel. Then connect the red lead to tube 1 red input connector on the
left side of the panel.
5. Set your voltage meter to millivolt scale (MV) and adjust the trim pot with a small at head
screw driver until you read 300 millivolts plus or minus 5 millivolts. In some meters this will
read .300 volts.
6. Repeat steps 4 and 5 for tubes 2 through 8.
7. Replace cover plate.
Feedback Control: turning up and down the feedback on the amplier affects the sound in a
number of ways. With the feedback on high the sound is tighter and the distortion and gain are
lower. The feedback set lower will increase the gain and make the sound smooth and loose. Set
the feedback control to suit your own listening preferences.
Output Mode: this switches the amplier’s output to either pentode (full power ultralinear
operation) or triode (half power triode operation). Each mode has different sonic attributes: the
pentode mode has more power and dynamic bass, the triode mode has a warmer and more silky
midrange. WARNING: adjust this switch only with the amplier turned off, the switch noise
could damage your speakers.
Fan: this switch controls the speed of the cooling fan. The fan is optional and only included in
ampliers specially ordered with this option, as fan noise can disrupt the listening environment.
6900 Kingsbury Road | Templeton, CA 93465 | 805-461-4100 | http://www.demeteramps.com/
VTHF-300M Power Amplier User’s Guide
GENERAL INSTRUCTIONS
1. Make sure your speakers are connected to the proper impedance speaker terminal 8 or 4
ohms. If your speaker is a different impedance, connect to the one closest to your speaker’s
impedance.
2. Make sure your volumes are adjusted for the proper stereo perspective.
3. If you hear hum coming from your speakers you may have a ground loop and have to lif t
grounds to eliminate the problems. Consult your dealer for the solution if this occurs.
4. If the amplier is used in a bi amp situation and is used to operate high frequencies the feed
back should be set on low and 40 ohm 20 watt resistor needs to be attached to ground and the
8 ohm tap.
5. WARNING: Tube ampliers need to see a load to operate. Please make sure speakers are connected before turning amplier on. Failure to do so will result in damage to the output transformer and void your warranty.
OUTPUT TUBE SELECTION AND INTERNAL ADJUSTMENT
The output tubes on your amplier have been carefully selected and matched for your amplier.
When replacing tubes or if you have a tube that fails or goes out of the adjustment range you
can order either a replacement set of tubes or a single tube by calling your distributor or Demeter Amplication for an exact replacement.
If you cannot do this and the tubes you buy fall out of the adjustment range of the front panel
controls there is an internal master bias adjustment. WARNING: THIS SHOULD BE PERFORMED
ONLY BY QUALIFIED SERVICE PERSONAL. THERE ARE 600 VOLTS DC PRESENT AND INJURY OR
DEATH COULD OCCUR!
Procedure: unplug amplier from mains. Hook up meter to bias adjustment terminals (ground and
tube one). Set bias controls to mid position. Carefully turn the amplier over and remove the
bottom cover. Plug the amplier in. Turn the amplier on with the proper speaker load attached.
Let it warm up for two minutes. Very carefully adjust the trim pot found on the power supply
board until the meter reads approximately 300 millivolts (.300V). Unplug the amplier and wait
5 minutes. Replace the bottom cover and turn the amplier back over. Adjust the bias for each
tube as previously shown.
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