18Watt.com

I have an 18Watt TMB that I built a few months back and am now using it to teach myself how to bias the tubes. The design is the TMB version from the Weber speaker folks and it is not modified from the schematic. It looks like I have very unbalanced plate current as determined by using the output transformer resistance method. Numbers are as follows:
Primary Center Tap to V4 Plate - 65.1 Ohms, voltage drop of 1.924VDC
Primary Center Tap to V5 Plate - 64.5 Ohms, Voltage drop of 2.4VDC
This works out to 29.5mA on V4 and 37.2mA on V5

Since the plates and cathodes are common, does that leave me with the tubes being the issue?

cuffers65240 wrote: ↑

Fri 07/30/21 12:28 pm

Since the plates and cathodes are common, does that leave me with the tubes being the issue?

If you really want them matched for some reason, you can use separate cathode resistors. But that may not give you the sound you desire. Is there something wrong with the amp's sound, or are you chasing numbers instead of tone?

If the amp sounds good, play it.

If you're concerned about the actual tube bias, what you really want is to calculate dissipation and make sure that's between 75 to 85% for EL84s.

Thanks,
Josh

If the control grids are the same, then it's the tubes themselves. What happens when you swap the tubes? Is the higher current side still higher? Or does it move with the tube?

Jack

Thanks Josh: Yes, I was just chasing numbers. It seems that from your comment the unbalance would not be an issue if it sounds good and I have been pretty happy with the sound. I am undertaking this process to also play with the sound and understand how amp tone works on a deeper level. Having said that, I may increase dissipation a little bit to get to the range you talked about and see what that sounds like.

Thank you as well Jack. Yes, swapping tubes causes the imbalance to move with the tubes.

cuffers65240 wrote: ↑

Fri 07/30/21 5:03 pm

Thanks Josh: Yes, I was just chasing numbers. It seems that from your comment the unbalance would not be an issue if it sounds good and I have been pretty happy with the sound. I am undertaking this process to also play with the sound and understand how amp tone works on a deeper level. Having said that, I may increase dissipation a little bit to get to the range you talked about and see what that sounds like.

Thank you as well Jack. Yes, swapping tubes causes the imbalance to move with the tubes.

If you want to experiment with the sound, start by getting the dissipation in the happy range, and then look at things like B+ and plate voltages. In our downloads section we have voltages for most of the circuits on this site. There should be something close enough to your build to work from. See how you vary from our target voltages, and record before and after samples if you change stuff to see if you can tell the difference, and what you actually like.

Thanks,
Josh