Flames1950 wrote:The only other tube amp specific tool you'll want is a way to check bias; you can do it with your meter the "transformer shunt" way, but many people prefer a bias probe-style setup that plugs between the power tube sockets and the tubes themselves; it uses a 1-ohm resistor inside that you measure mV across, which translates directly to bias current (Ohm's Law tells ya so!! Current=Voltage/Resistance, and if your resistance is exactly 1, then current = voltage.)
I got my bias probe on eBay, the guys that made it always have them on there. They have single models or a dual so you can switch back and forth between the power tubes on a 50 watter easier. The dual was $45, the company was Amp-Heads or something like that.
Andy
I just bought a probe:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 3776082153
The 2 pages of paperwork say noting about measuring mV just mA. Was I supposed to get a Bias Probe with a 1 ohm built in, or do you have to put a 1 ohm across the leads
I'd like to avoid buying another meter.
Here's the 2 pages of instructions and disclaimers that came with the probe:
BIASTOOL INSTRUCTIONS:
To find the reading for a tube type in your amp:
1) First you must know the cathode-to-plate voltage of your amp. This DC voltage is measured from pin8 (low) to pin 3 (high) at the tube sockets. The reason it is measured at the sockets is because the potential between the cathode (pin8 ) and the plate (pin 3) is what’s important. Not between ground and plate. In a
fixed bias amp, where pin 8 is grounded, they will be the same. The difference will be noticed with the
cathode bias topology, where a resistor is inserted between pin 8 and ground to develop the bias. We
recommend that a qualified technician make this measurement.
2) Now that you have the plate voltage for your amp, select the tube type you are using from the chart, find
the max plate dissipation number across from it, and plug that number into the formula.
3) The answer to the formula will be in milliamperes DC and will correspond directly to the readout you
get on the meter.
4)The formula is designed fro Class AB1 amps. It will bias the tubes on the low side of the midpoint for
Class AB1 and is a good starting point for a new set of tubes. A they age this current draw may go up or
down, depending on the tube brand and other factors. To use the formula with Class A, change the 500 to
900 and do the calculations.
5) The bias point of any amp is somewhat subjective. Within a certain safety range, where the tube will not
exceed the maximum plate dissipation or pass too much current through the output transformer, any setting
that SOUNDS good to YOU is acceptable. We recommend the lowest setting that still sounds good. This
will increase tube life and reliability.
FORMULA:
(max plate dissipation X 500)/ plate voltage = mA meter reading
EXAMPLE
Using the max plate dissipation for a SV6550C in an amplifier with 460V on pin 3:
(35 Watts x 500)/ 460V = 38 mA on the Meter
MAX PLATE DISSIPATION CHART:
(most brands of tubes should be reasonably close to these values)
6V6GT………………………...13.0 Watts
5881…………………………...20.5 Watts
6L6GC/WGB………………….20.5 Watts
SV6L6GC……………………..30.0 Watts
EL34/6CA7……………………25.0 Watts
SV6550C………………………35.0 Watts
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Notice – Proper use of this instrument requires the use of a VOM capable of reading
DC Current in mA. Although an anolog meter is acceptable, a digital readout is
much easier to read, and with few exceptions, much more accurate. A digital VOM
may be purchased at any number of local stores that sell electronics and electronics
parts, such as Radio Shack…
Connection of the probe to the meter is simple- the black lead goes to the common
ground jack of the meter, and the red lead goes to the mA jack.
It is not recommended to operate any tube amplifier with the bias tool installed for
any longer than is necessary to adjust the bias current- in other words, once you
have finished adjusting your bias, remove the BiasTool and reassemble your
amplifier before returning it to service.
The use of an insulating glove or oven mit is recommended for the handling of a hot
Tube in a BiasTool- the pin sockets are a tight fit- use great care when installing and
removing any tube from the socket.
This instrument was manufactured with the amplifier technician in mind- if you are
not comfortable working with high voltage devices, please enlist the services of a pro
for any internal adjustments and repairs to your tube amplifier.
There is no liability expressed or implied as to the use of this instrument. Although,
It was manufactured and tested to the highest quality standards, we will not be
responsible for any damage directly or indirectly related to it’s proper use is at
your own risk, depending solely on your own knowledge of tube amplifier circuits.
Do I need a solder wick? I have read the "How to service your own tube amp" a few times. Are there any other good books I should check out? I can't wait to see the JTM45 and 50Watter kits when they come out! Thanks for a cool place to learn about building amps George! Oh, and thanks for the CD too!
