Do you re-bias your amp after changing your power tubes?

Make it loud here.

Do you re-bias every time you change your power tubes?

Yes, every time.
14
64%
I don't, if I use a properly matched fresh pair
3
14%
Never, who cares if the amp still sounds great?
5
23%
 
Total votes: 22

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Re: Do you re-bias your amp after changing your power tubes?

Post by zhivago » Mon May 09, 2022 11:34 pm

Very cool! I will get it today! 8)
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Re: Do you re-bias your amp after changing your power tubes?

Post by JSett » Mon May 09, 2022 11:57 pm

zhivago wrote:
Mon May 09, 2022 11:34 pm
Very cool! I will get it today! 8)
If you feel you need it, feel free to send me a message when it lands and I can go through it with you on a Zoom call or something. I can easily put my Bassman on the bench and do a like-for-almost-like example.

I have a feeling you'll be absolutely fine though - it's pretty foolproof and the instructions that come with it are pretty straightforward.
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Re: Do you re-bias your amp after changing your power tubes?

Post by zhivago » Tue May 10, 2022 11:02 am

johnnysomersett wrote:
Mon May 09, 2022 11:57 pm
zhivago wrote:
Mon May 09, 2022 11:34 pm
Very cool! I will get it today! 8)
If you feel you need it, feel free to send me a message when it lands and I can go through it with you on a Zoom call or something. I can easily put my Bassman on the bench and do a like-for-almost-like example.

I have a feeling you'll be absolutely fine though - it's pretty foolproof and the instructions that come with it are pretty straightforward.
Perfect! This sounds great! Hopefully it will get here over the next couple of days!

I'll update the thread once it's here. :)
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Re: Do you re-bias your amp after changing your power tubes?

Post by zhivago » Thu May 12, 2022 12:58 pm

Ok, the gadget is here!

It all ended up being bit of a pain, as I had to completely remove the amp chassis from the head, but I have some readings.

I first tried the old valves that were crapping out. The had really mis-matched readings:

41.3...

Image

and 36.6

Image

...which I take is not a good thing...so I popped in my matched set:

61.4 and 61.2 was what I got:

Image

Image

So I think a trip to the tech is in order...

The bias pot was right there...daring me to twist it, but I resisted, as ermmm...I don't know what I'm doing! :D

Image

Really fun, as always, to peak around my new (old) amp...the chassis is dated to 2016, so this amp has been around.

Anyways, that's where I'm at. :)
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Re: Do you re-bias your amp after changing your power tubes?

Post by øøøøøøø » Thu May 12, 2022 3:09 pm

You should’ve twisted it!

Watch the number. If it goes up, twist it the other way!

First step: figure out what number you SHOULD be seeing.

Do you have a schematic for the amp that lists the plate voltage?

If so, we can easily figure out what number you’re expecting to see, and you can tweak that little trimmer accordingly.

Oh and the mismatch on the first set isn’t the hugest deal. I’ve got amps whose tubes have similar mismatches and they sound fine. It just ends up that one tube ends up biased a little warm and the other a little cool.

In a hi fi amp I might care; in a guitar amp much less so. Worst case it clips a little asymmetrically which can even sound cool.

Extreme mismatches can be not-good, but something like what that first set is showing here isn’t horrible.

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Re: Do you re-bias your amp after changing your power tubes?

Post by øøøøøøø » Thu May 12, 2022 3:17 pm

Just running some back-of-envelope math, I can say that ~61mA is almost certainly too high, and you’re right to exercise caution and not run your amp like that.

36-40 is probably more in the range

If your amp has a fairly typical 440 VDC on the plates, you’d probably want to see about 35-40 mA plate current at idle.

At a cooler 400V plate voltage, somewhere in the range of 37-44mA is more like it

Under most conditions, aiming for 38-39mA is probably going to get you close enough to not damage tubes and sound pretty good (assuming the output stage of your amp is “pretty much like a 50 watt plexi”)

If you’re already in a place where you’re literally LOOKING at the bias trimmer, I see no reason to not just set it there and enjoy your amp.

If you want to really get it dialed in to a specific plate current/dissipation target, you’d need a way to measure exact plate voltage (but in my opinion/experience, this level of pickiness isn’t usually necessary)

But there’s no normal condition for which I’d expect to see 61mA as acceptable in anything Plexi-like, so take that under advisement!

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Re: Do you re-bias your amp after changing your power tubes?

Post by zhivago » Thu May 12, 2022 11:46 pm

Pretty much spot on with your numbers, Brad...I did some online digging around, and while I can't find the schematic, I found a post by Dave Friedman himself saying 32 is what he recommends per tube, but "anywhere between 30 and 40" should be ok...some people online say 32 is a bit on the cold side of things, so might go for something like 35, or so? :)

I still have the amp out so I am tempted to try and twist the bias pot and get it sorted over the weekend. Just looking at the picture the bias pot is not really near anything and if I make sure I don't touch anything else while I adjust it it should be ok/safe to do?
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Re: Do you re-bias your amp after changing your power tubes?

Post by zhivago » Thu May 12, 2022 11:49 pm

Oh and dumb question - but, I need to twist anti-clockwise to get the number down to 34-35m, right? :-[
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Re: Do you re-bias your amp after changing your power tubes?

Post by øøøøøøø » Fri May 13, 2022 12:26 am

The fact that you’re concerned about being careful allays any uncertainty I may have about advising you to go ahead.

The simple/obvious safety stuff:

1) use a screwdriver to adjust the trimmer (I doubt you were thinking of grabbing it with your bare hand, but just in case!)

2) only one hand in the amp at a time—don’t rest your other hand on the chassis (earth) as you’re doing any kind of work with one hand.

3) be sober—don’t drink (etc) and work on your amp.

To be honest, this is the safest procedure you can do on a live amp. If you exercise a modicum of care, the risk is really quite low. If you’re concerned, have someone nearby to keep an eye on things.

I’ve gotten popped with high voltage only once, and I was being careless and it was 3 AM and I should have been sleeping. I was doing something far, far riskier than tweaking a trimmer.

FWIW, there’s no guarantee that anti-clockwise is the direction you want—just turn it slightly and watch whether the number increases or decreases. Going a few mA higher for a second isn’t going to instantly fry your tubes.

Sounds like 35mA per tube is a fine target. The higher you go, the less crossover distortion you’ll have, and maybe marginally more wear on your tubes. Some like the sound better. The lower you go, you could get some raspier distortion at high levels, or a touch of a harder/more brittle sound.

Good luck!!

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Re: Do you re-bias your amp after changing your power tubes?

Post by JSett » Fri May 13, 2022 12:56 am

Definitely turn that bias trimmer. You're basically finished once that is done! Make sure you use a plastic handled screwdriver and turn it slowly, while watching the meter.

Get it to 32-35mA, let it sit for 10-15 minutes and see if it drifts at all. If it does, just re-adjust and let sit again. If it all seems good....wrap it up and play it!
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Re: Do you re-bias your amp after changing your power tubes?

Post by zhivago » Fri May 13, 2022 1:24 am

Many thanks guys!

I will definitely go slow and careful! :)

I will have a go over the weekend and report...I wish I could do it tonight, but I have too many things on after work.

Stay tuned! 8)
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Re: Do you re-bias your amp after changing your power tubes?

Post by zhivago » Sat May 14, 2022 5:24 am

Success! 8)

I had a 30 min window of spare time this afternoon, so I headed upstairs, positioned the amp, got everything hooked up and went very, very, VERY slowly.

Everything went without a hitch. Turning anti-clock wise lowered the reading down and I got it to 34.5 in the end. I tried fine-tuning a bit more to hit an OCD-approved 35, but my nerves got the better of me and I called it "close enough for rock n' roll", which is of course my roll of choice! :D

I will have a chance to plug in properly tomorrow as I have to head out soon (we have a rare sunny Saturday here in London), but I just wanted to thank you guys for helping me out with this.

8)
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Re: Do you re-bias your amp after changing your power tubes?

Post by øøøøøøø » Sat May 14, 2022 5:54 am

It’s only scary the first time! Congrats, and you did the right thing.

Now you won’t have to pay a tech for this anymore!

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Re: Do you re-bias your amp after changing your power tubes?

Post by zhivago » Sat May 14, 2022 6:24 am

øøøøøøø wrote:
Sat May 14, 2022 5:54 am
It’s only scary the first time! Congrats, and you did the right thing.

Now you won’t have to pay a tech for this anymore!
It was a relief getting it done, and now I definitely feel more confident! Many thanks, Brad! 8)
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Re: Do you re-bias your amp after changing your power tubes?

Post by JSett » Sat May 14, 2022 7:16 am

Whoop! Congrats!... A good hurdle overcome and a skill that'll be with you forever now - the best kind of learning experience.

I've found this useful ability to be quite handy in the help-friends-out-in-exchange-for-beers department...which is one of my preferred departments :)

Rock n roll.
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