Unbalanced Plate Current

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

Post Reply
cuffers65240
Unrated
Posts: 7
Joined: Sun 11/03/19 9:36 am

Unbalanced Plate Current

Post by cuffers65240 »

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?
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3100
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: Unbalanced Plate Current

Post by JMPGuitars »

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
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 545
Joined: Tue 10/16/18 11:19 am

Re: Unbalanced Plate Current

Post by TriodeLuvr »

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
1 x

cuffers65240
Unrated
Posts: 7
Joined: Sun 11/03/19 9:36 am

Re: Unbalanced Plate Current

Post by cuffers65240 »

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.
1 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3100
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: Unbalanced Plate Current

Post by JMPGuitars »

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
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

Post Reply