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Biasing Your Peters Amplifier

These instructions apply to all Peters amplifiers except: 10W models and other cathode-biased models (those do not require biasing). If you are in doubt, look on your Peters amplifier for the bias test points and bias pot opening; if it has these provisions, then the biasing instructions here apply to it.

These are not instructions for how to bias any other amplifier made by any other manufacturer. Please read the entire document before attempting to bias the output tubes in your Peters guitar amplifier.

Leave the amplifier plugged into the speaker cabinet (properly matched to the amp’s impedance selection) for the bias procedure. Also, turn the Master volumes all the way down, counterclockwise.

If the amp has a Response or Modern/Vintage switch, set it to M or Modern. (In V or Vintage mode, the bias reading will be reduced; this is normal. Just bias the amp for the Modern setting, and don’t worry about the fact the output tubes show a lower reading in the Vintage setting.)

Turn the amp off, wait for the output tubes to cool, then carefully remove them. Remove the rubber plug from the bias adjustment pot opening on the chassis if your amp has one (older models only). The bias adjustment pot opening is located on the back of the current models. Some older models had the bias adjustment pot opening on top of the chassis near the back.

Turn the pot clockwise until it reaches the end of its range with a small standard slot screwdriver (flathead type). It’s a 15-turn pot, so don’t be surprised if it takes several full 360-degree rotations. Once you reach the end of the pot’s travel, don’t worry–it has a “ratchet” action so the pot won’t be damaged. You’ll hear and feel it “click” once you’ve gotten to the end of its travel. (Depending on the specific potentiometer, you may not hear or feel a “click”. In this case, just turn the pot a full 15 turns clockwise and it’ll be set to the end of its range.)

Turning the pot clockwise makes the bias “colder”, which is safe. You want to start the biasing from a colder position, and gradually turn it “hotter” (counterclockwise) while watching the voltmeter. That’s the general idea.

Install the new tubes. Turn the power on and let it warm up for a minute. Then insert a voltmeter (or multimeter that can read DC voltage), set to DC mV, into the bias test points. (You’re going to be measuring possibly up to 90 mV DC, so choose an appropriate range on the voltmeter if necessary. 200mV DC is a common suitable range on meters, for instance, to allow the reading to be taken accurately. The meter might have a DCV setting, and “200m” within that section. Or the meter might be auto-ranging. Consult your meter’s documentation to find its proper setting.)

Again, you will be reading voltage, not current (amps). If you set the meter to read current, you will not get readings that make sense.

Use one red bias test point (as the positive connection) and the center black test point (as the negative connection) for your meter. (Testing is always done between one red test point and the center black test point, one red bias test point at a time.)

In amplifiers with two output tubes, the red bias test point on the top is for the tube on the left and the red bias test point on the bottom is for the tube on the right.

In amplifiers with four output tubes, the red bias test point on the top is for the pair of tubes on the left and the red bias test point on the bottom is for the pair of tubes on the right.

(This explains the different ranges in which the 50W and 100W amps need their tubes to be biased. 100W bias test point readings are generally twice as high as readings for 50W models, as you’ll see in the chart below.)

In the case of some Peters amplifiers, the bias points are arranged horizontally; in this case the one on the left is for the tube or pair of tubes on the left, and the one on the right is for the tube or pair of tubes on the right. Each red test point measures with respect to the black connection in the middle, since that’s “ground.

I’m checking the bias reading for the two output tubes on the right in the example below (the next 2 pictures), while biasing the amp (a Professional Series 100W amplifier with four EL34 output tubes). To check the reading for the tubes on the right, I’m using the lower test point.

bias_psdc_1

bias_psdc_2

I’m checking the bias reading for the output tube on the left in the example below, while biasing the amp (a JP-50 50W amplifier with two EL34 output tubes). To check the reading for the tube on the left, I’m using the upper test point.

bias_jp50

With the voltmeter set to DC mV and turned on, then turn standby “on” (“Play mode”). Watch the display on the meter. Since you initially turned the bias pot to the “coldest” setting, you’ll notice the meter will probably read very low (around 1mV perhaps). If the voltmeter reads well over 40mV for one tube (for amps with two output tubes) or 80mV for a pair of tubes (for amps with four output tubes), turn the standby switch to “off” again and make sure you actually did turn the bias pot to the end of its range in the clockwise direction. (If the voltmeter doesn’t read below 40mV for one tube or 80mV for a pair of tubes, and the bias pot is in the most clockwise position at the end of its range, then it’s quite possible the power tubes are bad and should be replaced.)

With the amp on and standby in “Play mode”, you’ll want to turn the bias pot slowly counterclockwise while watching the voltmeter. The reading will slowly increase, and you’ll be trying to get it between the following readings:

20W models (using two EL84 or 6V6): 15mV to 23mV per test point

40W models (using four EL84 or 6V6): 30mV to 46mV per test point

50W models (using two EL34 or 6L6): 25mV to 40mV per test point

50W models (using two 6550 or KT88): 30mV to 45mV per test point

100W models (using four EL34 or 6L6): 50mV to 75mV per test point

100W models (using four 6550 or KT88): 60mV to 90mV per test point

(The amplifiers usually ship with bias settings in the middle of the above ranges.)

The bias voltage takes a couple seconds to “catch up” as you adjust the pot, so be patient while adjusting the bias pot. You’ll notice the first few full counterclockwise rotations probably won’t make much change on the meter reading. Keep slowly turning the pot counterclockwise while watching the voltmeter, and you’ll start seeing an increase in the mV reading.

Once the voltmeter reads close to half of the reading you’re aiming for, make sure you’re turning the bias pot even more slowly and allowing the bias reading to “catch up”. Once the reading is within the appropriate range for your amplifier model, stop and wait a few seconds. If the reading increases, turn the bias pot slowly clockwise to bring the reading back down to within the proper range again. Once it’s settled and you don’t see a significant change in the voltmeter’s reading (seeing the reading climb up and down 1mV or 2mV is ok), then you should be done. The last step is to check the reading of the other bias test point, to see if it’s giving a consistent reading.

Unplug the voltmeter’s red probe from the right (or top) bias test point and plug it into the left (or bottom) bias test point, so you can test the tube (in models with two output tubes) or pair of tubes (in models with four output tubes) on the right. You can leave the voltmeter on while you do this. This way you’ll know if the tube (or pair of tubes) on the right is matched well for current draw with the tube (or pair of tubes) on the left. If the two red test points, when each are individually measured with respect to the common (black) test point, read within a few mV of each other (within 5mV for a 50W model, within 10mV for a 100W model), they’re ok. If they’re out further than that, it’s recommended to get another set of output tubes put in the amp instead (since if the matching is significantly “off”, the sound can be negatively affected). If the readings are very different (considerably more than the numbers mentioned above), do not run the tubes in the amp at all. They’re too mismatched and will most likely sound bad and/or fail quickly.

Once the bias is set and you’ve tested both red bias test points to see if they’re reading relatively the same, then you can turn off the voltmeter and remove the voltmeter’s leads from the test points. Replace the rubber plug that covers the bias pot (if the amp has one). The biasing is complete.

Adjusting to the “optimal” bias point

The optimal bias point is subjective, and you may find you prefer the bias set somewhat “hotter” or “colder” depending on your tonal preference and how loud you play. You may not notice much difference biasing the output tubes differently, but some people have their preferences and there isn’t one setting (or percentage) that applies in all circumstances. Start with a bias reading right in the middle of the range appropriate for your amp model (and tube type) as shown in the list above, and adjust up or down from there based on personal preference as long as you stick within the guidelines given here and adjust the bias carefully (and always have your speaker cabinet attached, as well as having the master volume turned all the way down while biasing). If you want to bias your power tubes “colder” (lower mV reading)–that is always safe, however you’ll find the amp will start sounding more abrasive and less “friendly” all around as crossover distortion becomes more apparent. Biasing the output tubes hotter (exceeding the mV readings given here) will put your output tubes’ life in jeopardy.

Testing bias over time

Some output tubes’ bias will “drift” over time. It’s a good idea to check the bias again after the first week of using a new set of tubes, to ensure the bias is set optimally. If the amp ever sounds odd to you, use that as an excuse to check the bias again. It shouldn’t require regular adjustment but it’s possible for tubes’ bias to drift over longer periods of time.

6CA7 / KT77 tubes

These are generally considered EL34 “variants”. Use the same bias range as EL34 for these tube types. Do not use in 20W models.

6550 and KT88 tubes

For 6550 and KT88 tubes, the bias point is generally considered to be a bit higher: 35mV to 45mV per test point on 50W models (70mV to 90mV per test point on 100W models), although setting it lower can also be fine. The bias range is adequate for most 6550 and KT88 tubes, but you may notice when starting the bias procedure the reading is already about 50% or higher of what you’ll be biasing the tubes to (it won’t start at 1mV for instance). This is normal. Do not use in 20W models.

JJ 6V6 tubes

JJ 6V6 tubes can also be used in Peters 50W and 100W models. Do not use any other kind of 6V6 in Peters 50W and 100W models. A range of 16mV-23mV per test point (32mV to 46mV per test point on 100W models) is recommended. Doubling the impedance is recommended when using JJ 6V6 tubes. This means for instance to connect a 16 ohm speaker cabinet to the amp’s 8 ohm connection.

6L6GC / 5881 / KT66 tubes

When using 6L6GC , 5881 or KT66 tubes in 50W or 100W Peters amplifiers*, doubling the impedance the amp sees is generally recommended. This means for instance to connect a 16 ohm speaker cabinet to the amp’s 8 ohm output. This will give a more proper representation of what the amp was intended to sound like with 6L6GC or 5881 tubes. Remember that you can adjust the damping control for a more aggressive sound (the damping control will work slightly differently in its range for 6L6GC or 5881 as compared to EL34). Use the same bias range for all these tube types (treat them all as 6L6GC).

* Exception to this guideline: there are a few custom/one-off Peters amplifiers which are specifically meant for 6L6GC/5881/KT66 tubes (such as the Super Hawk).

If you have any questions about the bias procedure after reading this document, please contact me (James Peters). If you are unsure of how to proceed, it’s best to not bias the amp yourself. Once again, these instructions are only for biasing Peters guitar amplifiers. Do not follow these instructions for biasing any other amplifier. Doing so may cause damage to your amplifier and/or tubes.