Performed my first tube amp rebias last night

[sosomething]

Went perfectly!

My old Splawn Pro Mod had been out of commission for months after a tube failure that took some other components with them to the great turret board in the sky. I'd had the amp repaired by my trusted local tech but opted to not go for a retube at the time since:

1. I didn't need it right away since I have other amps I can use

2. I'd decided I never wanted to have to take my amp in to a tech just because I retubed it, and I didn't want to retube my amp again and not have it operate within the desired range.

So I had to learn how to rebias an amp myself!

My wife had given me a new multimeter for Christmas, so after a fresh set of JJ's KT88s and a handy YouTube video by Bob of Eurotubes, I was in business. The amp is now retubed, back up and running, and sounds great. I even had enough time to play around with some different NOS preamp tubes I've got laying around and found a nice set for the amp.

I feel so empowered.

 

[FuseG4]

Re: Performed my first tube amp rebias last night

I love your concept of a turret board in the sky

 

[King IzzO)))]

Re: Performed my first tube amp rebias last night

You mean you didn't turn off the lights and adjust it by how red the plates got?!!!

 

[xxxplorer]

Re: Performed my first tube amp rebias last night

That video from Eurotubes is how I did my first re-bias as well. Good stuff. I have since gotten a 'bias probe' for my Marshall, but in all honesty, I liked doing it without it better. It is a good feeling!

 

[sosomething]

Re: Performed my first tube amp rebias last night

You mean you didn't turn off the lights and adjust it by how red the plates got?!!!

That's kinda how I used to do it... but I'd just turn the trim pot until I smelled burning ozone.

 

[DankStar]

Re: Performed my first tube amp rebias last night

While I am happy for you, I feel less significant than before I knew this. I'm lucky if I don't plug my guitar cord into the footswtich input.

 

[sosomething]

Re: Performed my first tube amp rebias last night

That video from Eurotubes is how I did my first re-bias as well. Good stuff. I have since gotten a 'bias probe' for my Marshall, but in all honesty, I liked doing it without it better. It is a good feeling!

I swear he's the only guy on the whole damn internet who actually just explains the equation for calculating bias mA, tells you the process, and shows you how to test without extraneous BS. It's a great video but I probably watched a dozen terrible ones before I found it.

 

[dystrust]

Re: Performed my first tube amp rebias last night

+1 for Eurotubes.

I have one of their bias probes, and I've used it on a few amps. I'll have to track down that video and see if I also prefer going without the probe. Before I do that I need to get a better multimeter and have another go at my Jubilee. At the moment the bias is a bit cold, and I don't like how bright it sounds.

 

[crusty philtrum]

Re: Performed my first tube amp rebias last night

It's easy enough to do. Simply adjust until the flames are small and blue and the smoke is at a minimum.

 

[zzmoore]

Re: Performed my first tube amp rebias last night

I do not know anything about Euro-tubes, other than they are into JJ tube sales.
I watched 3 of their "bias videos".
I believer it was for a:
Fender Deluxe
Fender Deville
Some type of Marshall
I was kind of shocked that they just pick a number (based on cathode current) and announce that the amp is now properly biased. They (euro-tubes) have absolutely no idea of B+ voltage. Without that 50% of the equation, I do not understand how they think the amp is biased. I also do not understand how "bob" can claim an amp is biased too hot or cold without the B+ VDC.

In Fairness to Euro tubes, all I did was watch the videos. I did not READ any of their text (not sure I want to), which may explain A LOT of info that is missing from the video.
His analog meter did not come close to jiving with his DMM when he biased the Deville. The DMM showed something like 71mV while his analog meter showed something in the area of 32mA. That adds up to 64, and that is a lot lower than what his DMM showed by about 8mA or so. I cannot rememberer the exact Numbers, but they were pretty far off.
IF you just need to get some new tubes into an amp in a hurry so you can play a gig, you can probably set a pair of 6L6 at 65mA and a pair of EL34 at 75mA, and have fine results.
But that is not really understanding how to bias an amp. It is just reading an accepted "safe" cathode current and getting on with your life.
If somebody really wants to learn how to bias an amp, I would never even buy a "bias meter". Just invest in a good DMM and ask somebody with knowledge to teach you a few ways to bias a tube amp, or read Randall Aiken's sight. He gives a very clear explanation of what it takes
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