Tweet
Here is a non-invasive method that I have come to use to assure that the Push-Pull output tubes in my vintage (antique) amplifiers are biased in balance with respect to one another. I do have a complete audio laboratory with all the requisite equipment to make audio amplifier measurements "under the bottom cover". But, it is an effort to do. However, here is an alternative and good way to see if things are fairly well balanced without resorting to bench testing. Laser - Aimed infrared thermometers are now available at very low cost. On my vintage equipment (ones that do not have built in bias meters) I use the laser thermometer aimed at each tube and note the temperature (aimed at the same location on the power output tubes plates). For example, I recently restored an inherited Eico HF-20 mono integrated tube Amp that my ma and pa built in 1957 on their kitchen table (I remember them doing that). Carefully, I went through all the circuitry and fixed the issues that I found (brought it up on a Variac, and problems showed up and I fixed them). After bench testing it, I found a weak KT-66 and replaced both in the P-P pair. After replacement, it's output was 23 Watts as measured with a fixed 8 Ohm resistive load at 1 kHz, so all was good.
Then, I set it up and let it play for 8 hours in the background. To confirm the output tube balance, I then used my A530 (laser pyrometer {non-contact} purchased on Amazon) aimed at the same approximate locations on their plates and got the following readings:
V-5: 287 degrees F
V-6: 282 degrees F
So, that means that the tubes are sharing the power load within around 1.7% of one another.
That means that the power amplifier portion of the test subject is balanced about as good as one can expect. When that is true, the harmonic distortion is quite low (a few percent) - - - if not true, then the harmonic distortion is probably beyond the manufacturers specification. Certainly if there is a 25% difference in temperature between the two output tubes, one of the output tubes is not performing correctly and the %THD will be higher than expected.
Craig
ps - some tube power amplifier circuits have built - in bias meters (with what is called "fixed bias" circuits as opposed to cathode biased circuits) that are adjustable directly via a switch and observation (examples are the Marantz model 8B and Model 9).
Then, I set it up and let it play for 8 hours in the background. To confirm the output tube balance, I then used my A530 (laser pyrometer {non-contact} purchased on Amazon) aimed at the same approximate locations on their plates and got the following readings:
V-5: 287 degrees F
V-6: 282 degrees F
So, that means that the tubes are sharing the power load within around 1.7% of one another.
That means that the power amplifier portion of the test subject is balanced about as good as one can expect. When that is true, the harmonic distortion is quite low (a few percent) - - - if not true, then the harmonic distortion is probably beyond the manufacturers specification. Certainly if there is a 25% difference in temperature between the two output tubes, one of the output tubes is not performing correctly and the %THD will be higher than expected.
Craig
ps - some tube power amplifier circuits have built - in bias meters (with what is called "fixed bias" circuits as opposed to cathode biased circuits) that are adjustable directly via a switch and observation (examples are the Marantz model 8B and Model 9).