ADDENDUM TO SE/PP 300B "RETRO" OWNERS MANUAL
All Manley amps have
individual trimmers that are used to
adjust each output tube's bias voltage. "Bias" is a negative
voltage applied to the grid of each output tube which controls the
quiescent, or standing, current draw of each tube. It is like setting
the idle on your car. When we "measure the bias" we actually
measure the voltage drop across the cathode resistor of that tube. When
we "set or adjust the bias" we actually adjust the amount of
negative voltage going to that tube's grid. But don't worry about all
that if you don't want to... we just need you to take a reading with a
meter and turn a trimpot to maintain optimum
conditions for the tubes and verify that all the tubes are
working fine.
1) Optimum bias provides
the best balance of tube life and sound quality. Manley
Lab's amp designs operate the tubes cool at low current so that the they
typically last 5 years or longer if the bias is correct. Although,
sometimes tubes have a short life and others may be fine for 20
or 30 years.
2) Individual bias trims eliminate
the need for expensive matched sets of tubes or the need to replace all the tubes if one tube has a problem.
We do recommend that tubes with "reasonably similar"
characteristics are used and we batch and label them at the
factory. When you need tubes, give us a call
and let us know the number that is hand written on the tube as we have
some special tube batching jigs we've been using for years. Yes, we do sell
tubes - the good ones. After all, we're only as good as our worst
tube....
3) The test points allow
the user to check that each tube is working and that the amp
is performing well (even if you don't intend to trim).
Sometimes if one or two of the 10 tubes is dead the amp may
be operational and sound pretty much OK, checking bias
voltages will tell you more than your ears will sometimes.
It also will tend to show a tube on its way out. You would
see excessive voltage drift or a tube be out of trim range.
This means replace that tube.
1) Definitely, when output
tubes are changed.
2) On a regular schedule,
you should at least check every three months but we
recommend once a month.
3) Not every day and even
once a week is probably excessive.
4) With new amps, the bias
has certainly been factory adjusted but with all new tubes, it's a good
idea to keep an eye on these newbies for the first month or so. If you replace some
tubes that have not been "burnt in" or pre-tested or batched we also
recommend you check the bias every few days for a few weeks watching out
for "infant mortality".
5) If you see a red glow on the plate structure of the tube (inside the tube) then you need to check that tube's bias.
6) On the Retro SE/PP 300B amplifiers, if you hear an increased hum through the speaker, check the bias.
Bias Adjustment Procedure For "Retro" SE/PP 300B
NOTE: You will need a meter capable of measuring 0-1 Volt D.C. in order to make this adjustment. A low-cost Digital Multimeter (DMM) is the tool of choice, and can be obtained from many of the electronics retailers as well as auto parts stores and home centers. (The cheap & cheerful Radio Shack #22-810 variety is the easiest, and it is only $15 bucks.) Set the meter to read 'millivolts' DC or use the 2 Volt scale. The voltages you are looking at are under one volt DC. You will also need a small flat blade screwdriver.
1. Switch the amplifier to "Push-Pull" mode, and make sure there is no music playing through the unit while adjusting the bias (disconnecting the preamp and substituting a shorting RCA plug or cable is best). Note: an open input to an (high impedance) amplifier will pick up buzzing noise. A shorted input is great. Leave the speakers connected as usual. With all tube power amps an output load is absolutely required. Easiest, is just leave your speaker connected to the amplifiers. If you have your amplifier on a test bench or away from the speakers you can use a 5ohm to 10ohm dummy-load resistor (5 or 10 watts is ok) attached across the binding posts. If you are using a low wattage dummy load resistor, do not play any tunes into the amplifier while it is hooked up this way! If you don't know what we're talking about, call for assistance or just leave your speakers hooked up.
2. Connect the black or (-) probe from the meter to the black
(negative) speaker terminal, or directly to one of the screws on the
chassis (ground). All of
the exposed stainless steel and gold parts of the chassis and the negative speaker
binding post are at ground. Get your negative meter probe on ground. It
will stay there for this whole procedure. Connect the red or (+)
terminal to the either of the "Read Bias" test points.
3. If you are installing new tubes in the unit and this is the first
time they have been used, turn both bias trimmers fully clockwise. Then
turn the power on and allow the amplifier to warm up for about 2
minutes (30 minutes for tubes that were already installed; leave bias
trimmers where set).
4. You are now ready to make the adjustments. The left side
trimmer corresponds to the left side tube and test point, and vice
versa. However, the adjustments are interactive; raising the bias
voltage on one tube will lower the bias voltage on the other tube. It
will therefore be necessary to make these adjustments slowly and to go
back and forth between the two
trimmers and test points in order to obtain the proper reading. Set
amplifier for Push-Pull mode. Adjust each trimpot so that each tube
reads between 0.450-0.500 VDC (450-500 Millivolts D.C). If these are
new tubes, allow them to stabilize for 30 minutes and then re-adjust.
Skip this step if your serial number is
KSEPP199 or above
5. Switch the amp to "Single Ended" mode;
the bias reading should be in the range of .600-.800 VDC for each tube
(if either tube reads more than 0.850 VDC, or is steadily rising,
immediately attempt to re-adjust; if unsuccessful, turn the unit off
and contact our service department for further instructions). Now
adjust so that each tube reads 0.680-0.700 VDC on each test
point.
6. Again switch back to "Push-Pull" mode.
Listen to the speaker connected to this amp for excessive hum. It is
recommended that the input of the unit be disconnected from the preamp
for this test (better still if the input is shorted as outlined
in Step 1). Now adjust EITHER (but not both) of the bias trimmers to
minimize the hum present. This amount of adjustment required should be
very small.
Skip this step if your serial number is
KSEPP199 or above
7. Switch back to "Single Ended" mode and
confirm that the reading is still between .680-.700 VDC. A small
compromise may be required.
8. A new pair of tubes should have the bias
re-checked after the first 5-10 hours of operation and once a month thereafter.
OTHER NOTES:
If you remove the 300B tubes for any reason, make sure to replace them into the same socket. Otherwise, you will have to re-set the bias. Failure to do so can damage the tubes. Also, even though the tubes are designed to be inserted into the socket in only one orientation, it is possible to insert them the wrong way. This will cause damage to the tubes or the amplifier; always confirm that the large tube pins go into the large socket holes!
The bias was factory set on this amplifier, and the tube locations noted. Variation between the line voltage at the factory and the final installation location can cause variation in the tube bias. Therefore, it is important to check this upon receipt of the amplifiers, or any time the units change location (other than within the same building).
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