First Crack at Adjusting Bias

[PDC]

Today, I took my first swing at adjusting bias - this was on my Fully Loaded 50 Watt Nitro.  I had rolled several sets of tubes through all my amps recently and found that I really liked JJ EL34-II tubes in the Nitro.  I found the EL34-IIs to be pretty tight on the bottom end and have pronounced upper mids and highs.  The Nitro tends to be a pretty bottom-heavy amp and the brighter character of the EL34-II really seemed to compliment the amp.  But I was just swapping and playing ...

So I got ahold of a Bias Master with the 4 Octal probes.  I know - it doesn’t give me plate voltage.  But my thought is that as long as I get the tubes To settle in around 32-33, I would tailor the rest by ear anyway.  Also, I liked the idea of having 4 probes to more easily see how closely matched the tubes are.

My JJ El43-IIs were out of a supposedly matched quad, but the pair in my amp were at least 5-6 ma off from the get go.  I juggled them with the pair from the quad that were on my shelf, and got 2 very closely matched pairs - one higher and one lower.

But the big surprise is that the EL34-IIs were only at 24-25 ma even after letting the amp sit in the ‘on’ position for 30 minutes (with the volume all the way down and my cab hooked up, of course).  I slowly brought them up to 32 ma - waiting about 5 minutes between each half turn of the bias trim pot to let them settle in.  After confirming that they were settled in at 32, I turned everything off - let everything cool down - removed the bias probes - and then got her fired back up.

All I can say is ‘wow’ - what a difference!  You can definitely tell how a ‘colder’ bias vs a ‘hotter’ bias affects the tone.  The amp sounds warmer and has a bit more ‘sag’ and sustain than it did before - even with the gain only around noon.  It was a fun project.  Now I need to check my Metro Mod 5 just to see where its at!

[gtrjunior]

Jul 3, 2020 16:35:06 GMT -5 PDC said:

Today, I took my first swing at adjusting bias - this was on my Fully Loaded 50 Watt Nitro.  I had rolled several sets of tubes through all my amps recently and found that I really liked JJ EL34-II tubes in the Nitro.  I found the EL34-IIs to be pretty tight on the bottom end and have pronounced upper mids and highs.  The Nitro tends to be a pretty bottom-heavy amp and the brighter character of the EL34-II really seemed to compliment the amp.  But I was just swapping and playing ...

So I got ahold of a Bias Master with the 4 Octal probes.  I know - it doesn’t give me plate voltage.  But my thought is that as long as I get the tubes To settle in around 32-33, I would tailor the rest by ear anyway.  Also, I liked the idea of having 4 probes to more easily see how closely matched the tubes are.

My JJ El43-IIs were out of a supposedly matched quad, but the pair in my amp were at least 5-6 ma off from the get go.  I juggled them with the pair from the quad that were on my shelf, and got 2 very closely matched pairs - one higher and one lower.

But the big surprise is that the EL34-IIs were only at 24-25 ma even after letting the amp sit in the ‘on’ position for 30 minutes (with the volume all the way down and my cab hooked up, of course).  I slowly brought them up to 32 ma - waiting about 5 minutes between each half turn of the bias trim pot to let them settle in.  After confirming that they were settled in at 32, I turned everything off - let everything cool down - removed the bias probes - and then got her fired back up.

All I can say is ‘wow’ - what a difference!  You can definitely tell how a ‘colder’ bias vs a ‘hotter’ bias affects the tone.  The amp sounds warmer and has a bit more ‘sag’ and sustain than it did before - even with the gain only around noon.  It was a fun project.  Now I need to check my Metro Mod 5 just to see where its at!

When I first biased my QR I emailed Scott for his advise on it.
I have a 100w with the Drop B+ feature.
He told me to bias at 30mA in the non-drop B+ setting.
Then when I switch to Drop B+, the plate voltage drops and therefore the bias will rise to around 40mA.
Reason I’m posting is because I’m not sure how to bias the 50w, since it is Drop B+ by default.
BUT....I agree that this amp seems to excel with a somewhat colder bias.

Might be worth an email to Scott to see what setting he suggests.
I’d be interested to know too.

[PDC]

I may do that. It’s at 32 right now which is right in the middle of the ‘reasonable’ range for 25 watt EL34 tubes and it sounds really nice. Not a ‘huge’ difference in tone from where it started (25 mA) but certainly smoother and better ‘feel.’

[PDC]

So, I really don’t mind ‘telling on myself’ because I’m finally at an age where: 1) I’m really not trying to impress anyone anymore; and 2) I really don’t care what people think of me... SO - if someone can learn from my mis-cues, I’m all about it! I played my freshly biased Nitro the other afternoon and the longer I played, the less I liked the sound and when I peeked at the tubes, they were not red-plating, but headed that way(!). So I shut her down and let her cool down and re-checked bias. Apparently, the reason I thought this amp was biased so ‘cold’ is because I just didn’t leave it in the ‘on’ position long enough for the bias to rise completely and level off. I let her sit for about 30 minutes before, but this time I let her sit for an hr. (My bias probes do not allow playing through the amp while they are hooked up.)  After an hr, the bias drifted up considerably before settling in well over 40 mA. I slowly brought her back down to 32/33 waiting 5 min between each adjustment. After getting everything buttoned back up, the amp sounds MUCH better - even after playing LOUD for a good while. (The tubes stayed pale orange - no more drifting toward melt-down!)

I also biased my pair of Metro Plexi 12,000 heads - waiting PLENTY of time for everything to heat up. One head sounds best with JJ EL34-II tubes and the other really sounds best with JJ E34-LS. I ended up rolling countless tubes through each head - even allegedly ‘matched’ quartets from Tube Depot and The TubeStore had 1 or 2 tubes that were very close (2 mA) and then one 5-7 mA lower and another 5-7 mA higher than the ‘matched’ pair. So I just rolled and rolled and rolled until I had 4 of each tube that were within 2 mA of each other while fully warmed up. It took a full day, but my amps have never sounded this good.

[gtrjunior]

The first time I biased mine, I waited for what seemed like an excruciatingly long time to make sure the bias was stable.
I also learned (just like you did) that an amp, any amp can and will drift in whatever direction you make your adjustment.
So glad you took a minute to look at the tubes when you realized something wasn’t right.
Once I open an amp up and remove the head shell I like to be 100% certain things are correct before buttoning her back up. It’s kind of a PITA having to keep opening it up. Lol.

[splawndude]

Good to hear PDC.

Also guys, consider just leaving the back off altogether.

[gtrjunior]

That doesn’t offer any protection for a gigging player tho.

[splawndude]

Very very true my friend.

Conversely, if you are in the middle of a gig and a tube goes south you can't see very well and will take longer to troubleshoot.