Simple tube question

[notCardio]

If I want to replace a preamp (or power) tube with an identical tube type (let's say a 12AX7 for a 12AX7, etc.), but with different characteristics (more or less rapid gain), do I have to worry about things like adjusting bias, or can I try different ones just by swapping them out?

I'm sure more tube questions will follow later.

 

[ggunn]

If I want to replace a preamp (or power) tube with an identical tube type (let's say a 12AX7 for a 12AX7, etc.), but with different characteristics (more or less rapid gain), do I have to worry about things like adjusting bias, or can I try different ones just by swapping them out?

I'm sure more tube questions will follow later.

With preamp tubes, just swap them; preamp tubes don't have a bias. For power tubes, it depends.

 

[notCardio]

OK, when you say 'it depends' with power tubes, what does it depend on? The amp I'm specifically thinking about for the purposes of this discussion is a Bugera 5 watt that just has one 12ax7a preamp tube, and one EL84. I've also got a Blues Jr. with 12ax7's that I'd like to try different preamp tubes in. I'm not really dissatisfied yet with any of the power tubes, I'd just like to know for future reference.

Thanks.

 

[rprnj]

OK, when you say 'it depends' with power tubes, what does it depend on? The amp I'm specifically thinking about for the purposes of this discussion is a Bugera 5 watt that just has one 12ax7a preamp tube, and one EL84. I've also got a Blues Jr. with 12ax7's that I'd like to try different preamp tubes in. I'm not really dissatisfied yet with any of the power tubes, I'd just like to know for future reference.

Thanks.

Well some tubes run hotter than others - every tube is rated for a different dissipation . If your power tubes are bias'd incorrectly you run into problems anywhere from sounding like garbage to frying your tubes in a very short amount of time.

 

[stevieb]

Changing pre-amp tubes can make a lot of difference, but same for same would only result in a big diff if you went from, say, cheap China tubes to expensive NOS tubes. Changing to a different tube- 12aT7 in place of a 12aX7, for instance, will make much diff. Google it, lots of guidance on line.

 

[antichef]

The Blues Jr. is a "fixed bias" amp (not sure about the Bugera) - this means you can't easily change the bias.

My Blues Jr. was, in my opinion, biased way too hot by default - the sound was overly crunchy to my ears (and not Fender-y enough) and strained, like glass about to break, if that had a sound. The EL84s were very hot if you put your hand near them. I had some NOS RCAs I wanted to try out, but I was afraid to run that much juice through them.

I found some instructions on the internet and lowered the bias by replacing a particular resistor (after following the well documented safety procedure of draining the filter capacitors so I wouldn't get zapped), and with the new resistor, the amp sounded cleaner and not straining any more. Then I put in the RCAs, and I really liked the sound.

I've read that many (most? all?) Blues Jr. amps are biased on the hot side out of the factory.

Edit: google for 'Billm' - that's where I found my info - he recommends swapping the resistor for a trim pot so that you can easily adjust the bias, but I just went for a different value of resistor. More info than you're asking for, of course, but there's a very good website out there that Billm has put up that you may find interesting.

 

[notCardio]

Well, I assumed that you couldn't change the bias without some tinkering, all I really wanted to know was whether you could swap tubes of the same type without having to change the bias for different manufacturers. Sounds like it's safe to try swapping the 12ax7a's for different 12ax7a's. I actually kinda like the sound I'm getting now from this particular Bugera, I'm just always on the search for better. I really don't like the sound of the overdriven preamp on it's own (sans pedals) of my Blues Jr., so I think I'll try swapping tubes in it. It has the stock GT tubes.

Thanks for all the help.

And I remember reading all of the billm mods, but I'm definitely not a modder type, if there's soldering or drilling involved. I'll swap tubes and speakers, but that's about it. If I have a reasonably strong belief that something will make a big difference to me, I'll pay someone to mod it, but it's got to be worth it to me. On some of those billm mods, I either liked the 'before' version better, or didn't think the difference was worth the hassle or expense. But then, I have unusual tastes.