Question about mixing power tubes?

[ElUnoAstuto]

I just got a 5150(4 x 6L6's), and I pulled two of the tubes for half power, but I broke one when I pulled it! Pulled too hard on the glass, and then I heard that "psh" sound, like when you open a jar, and the glass flopped around in the plastic base, so I tossed it. I've got three stock Shuguang 6L6's, and one Electro Harmonix 6L6EH. The 5150 is fixed bias, meaning that if I needed to retube it, I would just order a set and pop 'em in.

Edit: found more tubes! So I've got

3x stock Shuguang 6L6's
2x Sovtek 6L6 WXT's
1x Electro Harmonix 6L6EH

What would be better for my amp?

Running two matched tubes, with the increased voltage?

Or running 4 6L6's, with different brands?

Thanks guys.

 

[wixedmords]

Re: Question about mixing power tubes?

EUA, fixed bias means you do need to bias the amp. The amp runs at a specific bias, not within a range. If it has a bias pot, you can use that but if it has no bias pot, you need to get tubes within a specific range or bias the amp by adding or removing resistors. This is also the case for many Mesa's. Cathode biased amps have a little more play in what range of tubes they will accept, therefore most say they don't need biasing. I check even cathode biased amps because I have seen a few run really hot and taking the "change without biasing" theory as gold would have likely caused meltdown of some sort.

You should run matched output tubes, specifically if you have only one bias pot, but if you have a seperate bias pot for each pair, or for each tube you can run something a little different. What you do have going for you is that the 5150's are known to run cold, usually so cold they are in lower-crossover distortion. So if a tube causes you to run hotter, you have a little room for error. But, try to be safe.

If I was to pull tubes, I would want to know the plate voltage and what the bias is.

 

[ElUnoAstuto]

Re: Question about mixing power tubes?

There aren't any bias pots on my 5150, it's stock. Roger Crimm over at the Peavey forum said that for a tube swap, no biasing would be necessary, due to the 5150's fixed bias nature. I know that that's not 100% "correct". There aren't any amp techs in my area, so nobody could bias it for me, and I certainly am not confidant enough, nor do I have the tools to undertake it. I don't want to order a new set of tubes, because by the time they get here, I'll be gone! (leaving December 5th for USAF) At any rate, what would be better, to just run the two stock Shuguang 6L6's until I find an amp tech/can order new tubes? Or run 4 6L6, the kicker being I have different brands? Thanks Wixedmords, I appreciate the help!

Edit: IIRC, stock 5150's have a plate voltage of aprox. 15ma?

 

[wixedmords]

Re: Question about mixing power tubes?

I think all the thanks should go to you EUA, for serving your country.

:bigthumb:

 

[ElUnoAstuto]

Re: Question about mixing power tubes?

Lol, thanks. So I should just live with the two stock 6L6's for now?

 

[ES350]

Re: Question about mixing power tubes?

Just run the two for now, if you need a quad, run a pair of Sovteks as well...as far as matching, nobody used to bother and people are still trying to cop those tones...

 

[brit84]

Re: Question about mixing power tubes?

stock tubes are 6l6gc so if you want to run without having to do anything the get some of those, i think theyre chinese 6l6gc when stock

 

[Lake Placid Blues]

Re: Question about mixing power tubes?

As long the odd ball tube doesn't run much hotter or colder than the other three it should be fine. With the lights turned down, and the B+ running, look at the tube plates. If the odd ball is showing a red/hot spot on it's plate, then that's probably not ideal, but if not, your probably fine. Some people find that mismatched tubes give a more complex tone anyway.

 

[Gearjoneser]

Re: Question about mixing power tubes?

Yeah, just use whatever you have lying around. Ideally, you want a matched quad, but Lake Placid Blues is right. As long as nothing's glowing weird, it's fine.

In the future, it's a good idea to grip the base of the tube and wiggle it upwards in a circular pattern. Same thing when you install them.

 

[kmcguitars]

Re: Question about mixing power tubes?

Edit: IIRC, stock 5150's have a plate voltage of aprox. 15ma?

You might wanna double check this? You maybe confussed w/t Cathode Current. Plate voltage is a much higher # and measured in DCV.

 

[ElUnoAstuto]

Re: Question about mixing power tubes?

That's why I ask you guys!

Problem solved! I got in touch with an old guitar buddy, and he gave me a matched quartet of Groove Tubes for the power amp, and matched quintet of GT 12AX7R's for the pre amp. Pretty cool! Just got back from the practice shed, those Groove Tubes are pretty crunchy!