Biasing question

[studiodunn]

I have just built a 50w amp and installed my first set of test points. In doing this I realized that I often see this same set up with a single bias pot for the pair and individual test points.

Would it be sufficient to set the bias of the OT pair as a set ( mV x 2), i.e. one test point? I always buy matched sets and have a bias rite to double check, but I can't see the advantage of 2 test points on a shared bias adjustment unless I'm buying from a suspect tube dealer and just want to see the difference.

[pompeiisneaks]

The reason there are two test points is so you can see which one is the 'hottest' and adjust accordingly. Each time you put new tubes in, that can flip/flop. You adjust to the one that has the highest current, so that it won't exceed the desired current for your preferred biasing. i.e. current tubes may have one at 42mA on left and the other at 45mA on the right... so you set your bias via the right one. The next set of tubes you buy you might end up with 44 and 41mA when you put them in, etc. so you just need to find the hottest one of the two and base it on that.

Also, you could adjust the bias to have dual pots and one per side to equally balance independent of the tubes so they both run at about the same current.

~Phil

[studiodunn]

Thanks Phil.
I definitely understand the reason for two test points. Since I've seen 2 test points for 100w amps I was just wondering why the same idea of averaging a pair isn't used on 50w amps.

This all stems from me wanting to put a meter in my next build. Now I could easily put a switch in to select individual tubes, but wondered how / if I could just average the set and read that with a meter.
This is the meter I'm thinking about using - https://www.globaltestsupply.com/pdfs/c ... asheet.pdf

I will definitely be trying dual bias pots on the near future.

[pompeiisneaks]

I'd think that averaging could be problematic, as you don't know what your 'worst' is at that point. What if you have a really imbalanced tube setup for some odd reason? One's like 30mA and the other 50mA and you want around 40, that's going to look 'right' but you'd be way high on the top tube?

Otherwise that seems like it makes sense.

~Phil

[studiodunn]

I feel like I'm shoving a square peg in a round hole.....switches are cheap and individually biasing is the only real way to get the tubes balanced and not red plating.

[pompeiisneaks]

Yeah that sounds like a more logical way about it. Are the bias test points more expensive than the switch? If so, then there you go.

To me the dual bias points just seems 'easier' and 'less incident prone'.

~Phil

[studiodunn]

Unfortunately the printed board(s) I am working off of don't implement and I'm not savvy enough to make that circuit change.
On my next turret build I will certainly use dual pots

The test points are cheaper, but I've got some small mV panel meters that want to put in some builds....just to say I did.

Thanks for your help Phil.